11111 I r I - r

Based on Material Provided

By

R. V. SHEPHERD and H. D. SNIVELY Large Motor and Generator Engineering Division General Electric Company

Revised By D. H. PRITCHETT Product Marketing Manager U.S. Electrical Motors Division Emerson Electric Company

4032-2 VVHAT THIS TEXT OVERS .....

General A~C Motor Classifications 1

This section discusses the general types, applications, and ratings of a-c motors, The theory of the rotating magnetic field is explained, Power factor, efficiency, and torque as related to a-c motors also are discussed,

Induction Motors 23

In this section polyphasc primaries and secondaries arc discussed, Phase­ wound and squirrel-cage rotors are discussed in relation to the application and operating characteristics of induction motors, Also covered are special types of polyphase induction motors,

Synchronous Motors 49

The operation, ratings, and construction of synchronous motors arc explained in this section, Also, the starting, synchronizing, and synchronous character­ istics of the synchronous motor are included, The use of a synchronous motor for power-factor correction is discussed, In addition, special types of synchro­ nous motors, their exciters, and methods of control are discussed.

Single-Phase Motors 87

The common types of single-phase motors and their principal characteristics are discussed in this section,

Appendix 1 : Solutions to Practice Problems 91

Appendix 2: Answers to Check Your learning 93

4032 General A-CMotor Classifications

TYPES AND APPLICATIONS

1 · Scope of This Text

This text discusses the principles of operation of a-c (alternat­ ing-current) motors and also the ratings and applications of such motors. The text builds upon previous understanding of electromagnetic principles and relates those principles to their use in a-c circuits. First the basic a-c motor types will be briefly discussed to acquaint you with a-c motor terminology.

2 • Induction Motors

Induction motors, both single-phase and polyphase, commonly consist of a primary, such as that in a transformer, to which alternating current is supplied and a secondary to which no electric current is supplied from an outside source. The pri­ mary winding is usually stationary and is therefore called the stator. The secondary winding is usually the rotating part and hence is known as the rotor. In this text the terms primary and secondary will be used when induction motors are being dis­ cussed, since they describe the electrical functions of the parts more clearly than the terms stator and rotor do.

3 • Synchronous Motors

The common type of synchronous motor used for industrial power applications consists of a stator (or armature) like the primary of an induction motor to which alternating current is supplied and a rotor (or field) with coils to which direct cur­ rent must be supplied. The rotor usually has salient (inwardly protruding) poles and field coils similar to the stationary poles and field coils of a d-c (direct current) motor. Other special types of synchronous motors will be discussed in later articles.

4 • Single-Phase Motors

The various types of single-phase motors are also described in this text. The majority of single-phase motors are of the small­ power class, called fractional-horsepower motors, designed for such uses as electric drills, vacuum cleaners, refrigerators, washing machines, hydraulic pumps, mixers, fans, and small wood- and metal-working tools. Millions of these motors are manufactured yearly.

5 • Application of Motors

Each type of a-c motor has its particular useful application. Because of its simplicity, economy, and durability, the induc­ tion motor is more widely used for industrial purposes than any other type of a-c motor for the higher speed ratings. Under varying loads the speed of the induction motor is not absolute­ ly constant, and the power factor is rather low at light loads. Those characteristics are objectionable in many applications. When constant speed, low speed, or high efficiency is desired, the synchronous motor is used. The synchronous motor can also be used to improve the power factor of the a-c circuit to which it is connected. That use is valuable because the cost of electric energy is generally based indirectly on the power factor and not wholly on the actual energy consumed When a synchronous motor is used solely for the purpose of power­ factor improvement and the shaft does not drive a load, it is called a synchronous condenser.

2 PRINCIPLE OF OPERATION

6'" Rotating Magnetic Flux and field

The mechanical effort, or torque, produced by every a-c motor depends on the fact that magnetic poles are created by the alternating current in the primary windings and revolve about the center, giving the effect of a rotating magnetic field. If somehow the rotor, or secondary, of the a-c motor is magnetized with the same number of poles as the rotating field of the pri­ mary, the north and south poles of the rotor will tend to be attracted respectively to the south and north poles of the pri­ mary field. If that attraction is strong enough, it will cause the rotor to revolve in order to keep the poles of the two fields lined up. For example, assume that the frame holding the field poles of a d-c motor is rotated with the field excited. The flux in sweeping over the armature conductors induces an emf (elec­ tromotive force) in them. H the brushes are lifted from the commutator surface and all the commutator bars are short­ circuited by a metal ring, currents are induced in the armature conductors. The reaction between those currents and the rotat­ ing magnetic flux causes the armature to rotate in the same direction as the frame holding the field poles. It can be shown that those currents create a magnetic field around the arma­ ture with the same number of poles as the rotating field and that the two fields tend to grip one another and revolve to­ gether. Exactly the same effect occurs in an induction motor, but without any movement of the primary coils; there is simply a revolution of the magnetic flux. In Fig. l is shown the principle of the rotating magnetic flux as it applies to induction motors. Conductors are shown short-circuited by end pieces that represent the secondary of an induction motor mounted on a shaft, and N and Sare two magnetic poles revolving in the direction indicated by the curved arrow. Those poles do not represent a revolving part

MAGNETIC POLE MAGNETIC POLE Fig. 1 . As the north and south magnetic poles rotate, induced currents cause the conductors to follow the poles.

3 of the machine; they are simply north and south magnetic poles of the primary rotating field. Recall from previous studies that if a conductor is grasped with the right hand so that the fingers encircle the conductor in the direction of lines of force, the thumb points in the direc­ tion of conventional current. Therefore, if the right-hand rule is applied and conventional current flow is assumed, the loop consisting of two conductors and two end pieces can actually be considered to be a single-turn coil. Currents are induced in the conductors in the directions indicated by the straight arrows. Those currents induce a south pole on the face of the conductor-end piece loop adjacent to the revolving north magnetic pole and induce a north pole on the face of the loop adjacent to the revolving south pole. Thus the motor secondary is drawn around by the magnetic attraction between the in­ duced secondary poles and the revolving primary poles.

7 • Rotating Field Principle

In the arrangement shown in Fig. l the primary rotating magnetic field is produced by the two permanent-magnet poles N and S being physically rotated. However, in a prac­ tical induction motor the revolving magnetic field is produced by two- or three-phase currents being applied to the primary windings as necessary. The rotation of a magnetic field by two-phase voltage can be illustrated by referring to Fig. 2. Coils A and Bare mounted at right angles to each other, and they form the primary, or stator, of the motor. The windings are connected to a two­ phase current source. Inside the coils is shown the same con­ ductor-end piece loop as shown in Fig. l. Thus if the resultant

CONDUCTOR-END PIECE ROTOR

Fig. 2. A rotating magnetic field can also be produced by two­ COILA phase voltage applied to two stationary coils at right angles to each other. The rotor will follow the rotating field.

4 field produced by coils A and B can be electrically rotated, the loop will be drawn around, or follow, the rotating field. To consider how the resultant field rotates, let us refer to Fig. 3. There the two primary coils A and B at right angles to each other and curves A' and B' represent the two phase cur­ rents applied to the coils. It should be understood that the currents in the coils vary as represented by curves-A' and B' and thereby produce varying conditions of polarity in the two coils as indicated at position l to 8. The coils do not move; the different positions l to 8 merely indicate different current and polarity conditions; they can be referred to as electrical positions. At position l, as is shown by the curves A' and B' in Fig. 3, the current in Coil A is zero and that in coil Bis maximum negative. The polarity of the coil B is indicated by short arrows, and the resulting polarity of the two coils is indicated by the longer arrow; in this case the arrows agree in direction because coil A carries no current. In position 2 each coil is carrying the same current, that in coil A being positive and that in coil B negative, as is shown by the curves. The resulting polarity, as indicated by the longer arrow, is now 45° (degrees) counterclockwise from the first Fig. 3. The combined magnetic fields produced by two out-of­ position. phase currents electrically In position 3 the current in coil A is maximum positive, rotate in a counterclockwise that in coil B is zero, and the resulting polarity is 45° farther direction.

TWO- PHASE CURRENTS

COILA A A 1 2 3 4 5 6 7 8 1 ELECTRICAL POSITIONS

5 counterclockwise. The remammg positions show that, as the two-phase currents continue through the cycle, the resulting polarity of the two coils continues to rotate counterclockwise and one rotation is completed per cycle. According to this prin­ ciple, either two-phase or three-phase currents in the primary of an a-c motor cause a rotating magnetic field. The preceding discussion explains how a stationary arma­ ture, the primary, can produce a rotating magnetic field just as mechanically rotating a d-c magnet frame can produce one.

8 • Direction of Rotation

The direction of rotation of an a-c motor is the same as that of its rotating magnetic field. The direction of a three-phase motor can be reversed by interchanging the connections of any two supply leads, and the direction of rotation of two­ phase motors can be reversed by interchanging the two leads of one of the phases. The reversal action can readily be illus­ trated by noting that, if the current in coil B of Fig. 3 is re­ versed, the polarity of winding B will be reversed in each po­ sition and the resultant field of windings A and B will rotate clockwise instead of counterclockwise as before.

g • Synchronous Speed

The primary windings of an a-c motor are always connected for an even number of poles such as two poles, four poles, and additional multiples of 2. Since each pair of poles represents one complete magnetic cycle per revolution, the rotating mag­ netic field will make one complete revolution around the motor frame in as many cycles as there are pairs of poles. The number of r/min (revolutions per minute) of the rotating magnetic field, called the synchronous speed, can be shown by the formula

fX 60 N =-----''------number of pairs of poles fX 60 JX 120 L p 2

in which N = synchronous speed, in revolutions per minute f = frequency, in hertz or cycles per second P = number of primary poles

6 When the rotor is turning at exactly the same speed as the rotating magnetic field, that is, at the value of N given by the formula, the motor is said to be in synchronism. That means the motor is in synchronism with the alternator that furnishes the operating current to produce the rotating mag­ netic field for the motor.

Example Problem

At intervals throughout this text you will find one or more example problems solved to illustrate clearly the appli­ cation of a principle, rule, or formula. Read each problem carefully and study the solution until you understand it thor­ oughly.

Problem. For a four-pole 60-Hz (hertz) a-c induction motor, deter­ mine the speed of the rotating magnetic field, or the synchronous speed. Solution. By applying the formula given in Art. 9, the syn­ chronous speed is found to be /X 120 N p

= 60 ~ 120 = 1800 r/min Ans.

10· Rotor Slip

When the rotor, also called the secondary, revolves owing to the rotating field, it does not quite attain the same electrical speed as the field, or primary. The rotor speed is always a little lower than the synchronous speed. The difference be­ tween the synchronous speed and the rotor speed, called the slip, is expressed as a percent of the synchronous speed. For example, a motor with a synchronous speed of 1800 r / min and a full rotor speed of 1710 r / min operates with a slip of

1800 - 1710 X 100 = 5% (percent) 1800 °

If the rotor caught up with the rotating field and rotated at exactly the synchronous speed, the rotor conductors would always be in the same position relative to the lines of force and would not cut any lines of force. Because there would be no relative motion of the rotor and the field, there would be no induction.

7 The slip varies during motor operation. It is very low when the motor is running idle, that is, without mechanical load. It can be as low as only a fraction of l %. At the start, when the rotor speed is zero, the slip is 100%.

Example Problems

Problem I. What is the rotor speed of a motor with a slip of 2% and a synchronous speed of 3600 r / min? Solution. A slip of 2% represents 0.02 X 3600 = 72 r/ min. The rotor speed is 3600 - 72 = 3528 r / min. Ans. Problem 2. What is the rotor slip if a motor operates at a rotor speed of 1195 r / min and a synchronous speed of 1200 r / min? . 1200-1195 Solution. Slip = 1200 X 100 = 0.4% Ans.

11 · Power factor

The power factor of ?.n a-c motor is defined as the power factor of the current the n_.:,tor draws from the line. Rated power factor is that which exists when all rated conditions of load, voltage, and (for synchronous motors) field current exist. Power factors of a-c motors cover the entire range from al­ most zero leading for synchronous condensers and lightly loaded synchronous motors to almost zero lagging for lightly loaded induction motors. Power factor, abbreviated pf, is usually expressed as a percent; it is the ratio between the true power PR and the ap­ parent power P:

Power factor = true power (l) apparent power PR or pf=pX 100 (2)

in which pf= power factor, in percent PR = true power P = apparent power

In the common form of the power factor formula the true power is given in kilowatts, kW, and the apparent power in kilovolt amperes, kV A. Thus the power-factor formula becomes kW pf = kVA (3) in which pf= power factor, as a decimal Power factor will be discussed in more detail in later articles.

8 Example Problem

Problem. If the apparent power applied to a motor is 100 kV A and the true power consumed by the motor is 75 kW, what is the power factor'? Solution. By applying formula 3 in Art. 11, the power factor is found to be

kW pf= kVA 75 100 = 0.75 Ans.

" Efficiency

of an a-c motor is the ratio of the mechanical. power output to the electric power input expressed as a per-· cent.

_ 0.746 X horsepower output . 1 Efficiency. in - k"l . X 100 (,) ·1 owatt mput

For an induction motor the input is calculated from the formula

kW= kVA X pf (2)

For a synchronous motor the 1s

field volts X field amperes kW = kV A X pf + WOO (3)

Practice Problem

Practice problems are included in this text to test your ability to apply a rule or formula. Work each problem carefully and ch.eek your answer against the answer given. If your an­ swer is wrong, compare your solution with the solution in the section "Solutions to Practice Problems" in Appendix l at the end of the text.

If a 12-hp motor requires an input of l O kW, what is the efficiency of the motor'? Ans. 89.5%

9 13" kV A Input

Fo:r a two-phase motor such as that illustrated in Figs. 2 and 3, the kilovoltampere value used in cakulating the kilowatt input is given by the formula

kVA=2XEX/ ( 1) 1000 and for three-phase motors the kV A used in cakulating kilo­ watt input is given by the formula

kVA=V3XEX/ 1.732 EX/ (2) 1000 1000

in which E = line-to-line voltage, in volts I= line current, in amperes Formulas I and 2 may be used when the voltage E and the current I are known. However, motors are usually rated in horsepower, and the rated kV A can be determined from the rated horsepower, power factor, and efficiency, without know-­ ing voltage and current, by the formula 0.746 X rated horsepower Rated kVA =------"---- (3) (rated pf) X (rated efficiency)

Example Problem

Prohlem. What is the kV A input of a 20-hp (horsepower) induc­ tion motor with a rated power factor of 850,f, and a rated efficiency of 89%.? Solution. The power factor is 85 JOO= 0.85 The efficiency is 89 JOO= 0.89 and . 0.746 X 20 Rated kV A mput = 0_85 X 0_89 19.72 kV A Ans.

14" Torque

Torque, the turning effort of the motor, may be cakulated from the formula 5250 X hp output T = ------=-----=-- (l) r / min in which T = torque, in foot-pounds

10 Rated or full-load torque is found from formula I by in­ serting rated horsepower and speed. Percent torque is the percent ratio of actual torque to rated torque; that is actual torque Percent torque d X 100 (2) = rate torque

Practice Problem

If a 20-hp motor rotates at 1750 r / min, what is the torque? Ans. 60 ft-lb (foot-pounds)

15• Starting Load and Accelerating Torque Values

The torque at zero speed on a torque-speed characteristic curve is called the starting torque of the motor. The load driven by a motor will also have a torque-speed characteristic which is known as the load torque. If at a given speed the motor torque is larger than the load torque, the excess torque is known as the accelerating torque; this torque will cause the motor and the load to speed up. When the motor torque exact­ ly equals the load torque, that is, when the motor and load torque-speed characteristics intersect, there will be zero ac­ celerating torque and the motor will continue to operate at that speed.

16• Starting Current

When an a-c motor starts and accelerates, the current which it draws from the line changes with the speed, and a curve of this current in relation to the speed is called the current-speed characteristic. The current at zero speed is known as the start­ ing, or locked-rotor, current. It should be noted that the line current drawn by a motor at any given speed during accelera­ tion is dependent only upon the voltage at the motor terminals; it does not depend in any way on the amount of load.

RATINGS

17 • Frequency

The standard a-c frequency in the United States and Canada is 60 Hz (hertz, or cycles per second), but a frequency of 50

11 Hz is common in European countries. Many countries that are developing new electric power sources use the 60-Hz frequency because of the effect of speed on the performance of pumps and blowers (ventilation systems). The capacity of a centrifugal pump will vary directly with the speed, and the horsepower will vary as the cube of the speed. Other frequencies, such as 400 Hz for aircraft electric motors, are sometimes used, or a variable frequency is used to drive a-c motors in pump and blower applications and thus provide a variable output capacity.

18• Enclosures

Motors are usually designed with covers over the moving parts. The covers, called enclosures, are classified by NEMA (National Electrical Manufacturers Association) according to the degree of environmental protection provided and the method of cooling. The two basic categories are the open motor and the totally enclosed ,notor. The most common type of motor is the open motor. It has ventilating openings which permit the passage of external cooling air over and around its windings. If the openings are limited in size and shape, the motor is called a protected, or guarded, motor. That type of motor is used where large pieces of material might o-ccasionally fly through the air, enter the motor, and damage its internal parts. The protected motor is also often used where there is danger of a person touching

Fig. 4. The open-type motor, such as the one shown here in a partial cutaway view, is commonly used in many applications. Courtesy cd U.S. Electrical Motors Division, Emerson Electric Co.

12 the rotating or live parts of the motor. Dripproof and splash­ proof motors are so constructed that drops of liquid or solid particles cannot enter them. A totally enclosed motor is designed to prevent free ex­ change of air between the inside and outside of the enclo­ sure, but it is not sufficiently enclosed as to be airtight. The various modifications of the two basic types are dis­ cussed as follows: Open and guarded motors, which are available up to 100,000 hp (horsepower), are so constructed that successful operation is not interfered with when drops of liquid or par­ ticles strike at an angle of 0° to 15°. Guarded machines have screens to prevent contact with rotating parts. A partial cuta­ way view of an open motor is shown in Fig. 4. Weather-protected type I motors are open-type machines with ventilating passages so designed as to minimize the en­ trance of rain, snow, and airborne particles. Also, the ventilat-

Fig. 5. Weather-protected type 1 motors have specially designed ventilating passages which protect the electric parts by minimizing the entrance of rain, snow, and airborne particles.

PROTECTION SCREEN

Courtesy oj U.S. Electrical Motors Division, Emerson Electric Co.

13 ing openings are so designed as to prevent the passage of a cylindrical rod ¾in. (inches) in diameter. Such motors, like the one shown in Fig. 5, are generally for outdoor applications. Large horizontal motors and high-thrust motors, those with high driving capability, are usually specified with this type of enclosure. Weather-protected type 2 motors are specified where protection from hostile environmental contaminants is re­ quired. This type of enclosure, shown in Fig. 6, is constructed with ventilating passages so arranged that high-velocity air and airborne particles blown into the enclosure can be dislodged without entering the actual ventilating passages of the motor. Totally enclosed motors can be nonventilated, pipe­ cooled, or fan-cooled. The totally enclosed motor is so con­ structed as to prevent free exchange of air between the inside and outside of the actual motor housing. It is used where Fig. 6. A weather-protected hostile environmental conditions and the application require type 2 motor has a unique maximum protection of the internal parts of the motor. The ventilating system to prevent totally enclosed fan-cooled motor shown in Fig. 7 has, under high-velocity air and airborne the shroud, a fan which blows air over the cooling fins, but particles from entering the internal ventilating passages of the internal portion of the motor is actually enclosed to pro­ the motor. tect it from the outside air. Courtesy of U.S. Electrical Motors Division, Explosionproofmotors, as shown in Fig. 8, are constructed Emerson Electric Co. similarly to totally enclosed motors except that they are de­ signed for operation in hazardous locations. These motors have internal restrictions such that explosive gases inside the enclosure must follow a long flame path before coming in con-

Fig. 7. Thetotallyenclosedfan­ cooled motor features a fan which is part of the motor but blows air over the external fins. Courtesy of Westinghouse Electric Corporation

HOOD SCREEN

14 tact with the outside atmosphere. The maximum temperature rises and external surface temperatures of these motors are restricted. Explosionproof motors are listed by UL (Underwriters' Laboratories, Inc.) if they meet certain specified test condi­ tions. Since they are so unique in design, any repair work on them must be performed by an approved facility, usually the factory itself. Some large induction motors used in hazardous areas may be filled with an inert gas such as helium to provide pro­ tection.

19• Horsepower and Voltage Classifications

Alternating-current motors can be grouped into three basic classes: fractional horsepower, integral horsepower, and large-apparatus a-c induction motors. Fig. 8. The explosionproof motor is similar to a totally Fractional-horsepower motors, often called small motors, enclosed motor but is are available in single-phase or three-phase designs. Single­ specially designed to prevent phase voltages are 115 and 230 V (volts). Three-phase voltages ignition of the gases or vapors are 200, 230, 460, and 575 V. surrounding the enclosure. The enclosures used for fractional-horsepower motors Courtesy of U.S. Electrical Motors Division, are normally of either the open or the totally enclosed type. Emerson Electric Co. Most fractional-horsepower motors are manufactured for a definite purpose or restricted to a particular application such as electrical appliances. Integral horsepower motors can be of the single-phase or three-phase type and are designed for voltages of 115, 200, 230, 460, and 575 V. The enclosures for integral horsepower motors are open, drip-proof, guarded, totally enclosed, and explosionproof. large-apparatus a-c induction motors are usually con­ sidered above 200 and up to 100,000 hp. Their operating volt­ ages may be as low as 460 V, but 2300, 4000, 6900, and 13,200 V levels are common. Enclosures for this category of motors are open-guarded, weather-protected types 1 and 2, totally enclosed, and explosionproof.

20° Temperature Ratings The continuous horsepower rating of any motor is usually limited by the temperature at which the insulation can safely operate. Insulation materials are subject to thermal aging and degradation that strain and weaken their dielectric capability until a breakdown between conductors can cause a machine failure. Since the heart of a motor is the winding, consisting

16 of coated, or covered, wire and insulation materials, the wind­ ing must be operated within specified temperature limits. In practice, the insulation materials or combinations of materials used in a motor are usually referred to as an insulation system. To ensure reliable operation, NEMA has defined four classes of insulation systems: A, B, F, and H. The classes and the highest temperature at which each can safely operate are as follows:

Class Temperature A l05°C B uo 0 c F 1ss 0 c H l80°C

Since certain classes of insulation systems are preferred for particular size motors, it is recommended that both the manufacturer's specifications and NEMA specifications al­ ways be consulted to ensure proper application. A vital term of a machine's temperature rating is called the temperature rise, and it is based upon a 40°C (degree Celsius) ambient temperature. The temperature rise is deter­ mined from the difference in resistance of windings with the motor deenergized when cold and then again after the motor has been running at full load for several hours. For example, Class B insulation is used in most motors of standard design. The insulation has a capability of allowing a 90°C rise over the 40°C ambient (130 - 40 = 90°C). Class F insulation is popular for extended life applications. For instance, where a Class B rise (90°C) is specified, a Class F insulation, if used instead, would extend the life expectancy by 2½ times as compared with Class B insulation.

21 · Service Factor

Also shown on a motor nameplate is the service factor, which indicates the reserve margin designed into a motor. A service factor greater than l .0 indicates the motor is designed for use under unusual conditions such as momentary overloads or oc­ casional high or low voltage levels.

2 2 • Horizontal and Vertical Mounting of Motors

Most applications require that the motor be horizontally mounted, that is, that the shaft be in a horizontal position. A

16 horizontal flange-mounted induction motor is shown in Fig. 9. Large horizontal motors are usually furnished with pedestal­ type bearings rather than end-shield bearings. On many pump applications it is necessary to mount the motor with the shaft in a vertical position. Open vertical motors for outdoor service are shown in Figs. 5 and 6. When the load should turn at a speed lower than the motor speed, gears are used and are enclosed by the motor frame. Only the low-speed shaft is outside the frame. The mounting method is critical to the life of a motor and the driven machine. Design details of the motor vary with Fig. 9. Horizontal flange­ horizontal or vertical configuration. mounted motors are designed Horizontal mounting. Horizontal motors have dimensions for mounting on the side ofthe specified by NEMA that are standard for all motor manufac­ driven machine. turers. They allow for interchangeability without changing Courtesy of Allis-Chalmers Corporation shafts, heights, and mounting holes. The base of the mounted motor must be level. For some applications, the motor is mounted on a fabricated steel base, as in Fig. 10. Larger motors require either a heavy steel or a concrete foundation that provides sufficient mass for rigidity and will minimize deflection and vibration. The method of connection between the motor and the load varies from a belt-and-sheave (pulley) arrangement to direct coupling, as shown in Fig. 11 (a) and (b). Sleeve bearing mo­ tors must be connected by the coupling method, since they are not capable of radial load. Ball and spherical bearings are used for belt-and-sheave arrangements. For application of V­ belt sheaves it is always best to refer to the manufacturer's information. Vertical mounting. In many pump applications it has be­ come economically feasible to mount the motor vertically to save space and provide for convenient replacement. The verti-

DRIVEN MACHINE

Fig. 10. For reliable operation, the motor and driven machine require a sturdy foundation to ,_ - .., 4 • - • f , .. I If" p .A • • ti p 'O,. ,- • "t. , I I".( • f FOUNDATION~ ·;•',A•: , ~---· · • .4 • • -~ · , • ..s ·/ •• ,. ;.••• ensure alignment and reduce .ca • ~ • -. "" • ' ". ., r " , t • " """ "" • • • • ~ • • ~ • •, • 4 "'• .".J. , vibration.

17 Fig. 11. In certain applications VERTICAL MOTOR a vertically mounted motor may be preferred over the conventional horizontally mounted motor.

(a) Vertical in-line process pump

CENTRIFUGAL MOTOR PUMP

(b) Horizontal close-coupled turbine pump

cal in-line motor, shown in Fig. 1 l(a), and close-coupled centrifugal pump, shown in (b), are applications of these space-saving techniques.

2 3 • Protective Devices

Various devices are used to provide for protection of motors. A protective device can be considered a form of insurance, since the cost of the device on a large motor is very small in comparison to the costs involved in a complete machine failure. Usually the selection of a protective device designed into a machine is based upon the cost of the device versus the cost of loss of motor and production time. In the accompanying table the general types of protec­ tive devices, their primary function, and the type of protec­ tion provided are listed.

18 Applications of Motor Protective Devices

Device Purpose Protection

Bearing Senses high temperatures If bearing failed, the temperature indicating problem and rotor would rub stator and detectors possible failure. Sensor cause both to fail. triggers alarm or cuts motor off line.

Winding Senses high temperatures; Protects windings against temperature triggers alarm or cuts clogged air passage detector motor off line. effects, overload, and low voltage.

Vibration Senses vibration of motor Vibration caused by broken detectors or the driven load. couplings, damaged driven (load) equipment, or bear- ing problems.

Surge capacitors Limits magnitude of surge Protects against winding and lightning voltage at motor failure caused by high- arresters terminals. voltage surges in power system or induced by lightning.

Space heaters Used to raise internal Protects against excessive temperature of motor moisture condensation on 5°C to 8°C above the windings. ambient temperature to prevent condensation.

24· Noise Considerations

In many applications such that people must work close to motors the noise must be considered. The Occupational Safety and Health Act (OSHA) of 1970 specifies how long a person can work under various noise levels. Noise in electric motors consists of windage, bearing, and electrical noise. Normally windage must be reduced or contained within the enclosure. In certain frame sizes, it is common to redesign the fan to lower velocity; This design change usually increases temperature rise and may necessitate winding with Class F insulation. In large machines, noise reduction is accomplished by eliminating the direct line of airborne noise. This elimination is accomplished by having the noise pass over many surfaces or

19 through baffles. A top-hat design is illustrated in Fig. 12(a). As shown by the arrows in (b), the airflow passes over many surfaces. If further noise reduction is required, special fans and acoustical material may be necessary.

2 5 • Use of Industry Standards NEMA standards describe the various guidelines for selecting electrical equipment. The key to electric motor reliability is a standard which will define the application conditions, enclo­ sure, and insulation. Factors in addition to the temperature (a) Complete enclosure ratings, service factor, horsepower, voltage level, and en­ closures previously mentioned include altitude and frame designations. The normal altitude operating limit for electric motors is 3300 ft (feet). For higher elevations, design consideration must be given to the lesser cooling effect of the thinner air. Another design consideration is the frame and its mount­ ing. NEMA standards include a system of standardized motor mountings to simplify replacement.

Check Your Leaming

"Check Your Learning" is a test of your ability to retain the information presented in the text. It is a simple yet highly (b) Partial cutaway of enclosure effective test. Study each question carefully, and then answer it as best you can. ALL ANSWERS AND SOLUTIONS WILL BE FOUND IN APPENDIX 2 AT THE END OF THE TEXT. DO Fig. 1 2. Noise reduction NOT SEND YOUR ANSWERS OR SOLUTIONS TO THE techniques include the use of SCHOOL. Check each of your answers with that given. If your baffles, acoustical materials, answer does not agree, review the text material to find out why. and special fans, and the deflection of noise over many surfaces. Courresy 1!{ U.S. c/ecrrica/ Morors Division. I. The rotor of an induction motor is similar to what winding of a Emerson Electric Co. transformer? 2. In order to reverse the direction of a three-phase motor, how many supply leads must be changed? 3. In an induction motor, by what basic principle is the rotor bar caused to rotate? 4. If an induction motor could rotate at synchronous speed, what percent would the slip be? 5. What are the four classes of insulation and their temperature values? 6. What term is used to indicate the turning effort of a motor, and in what units is it measured? 7. In a synchronous motor what form of current is applied to the field coils?

20 8. If the right-hand rule is applied to a conductor, the thumb indi­ cates what factor? 9. When the true power of a circuit is compared to the apparent power, what is the relation called? JO. What type of motor is used for a constant-speed application?

21

Induction Motors

POL YPHASE PRIMARIES

26• Core

The induction motor stator, or primary, shown in Fig. 13 has a laminated iron core with slots which contain coils. The frame of the stator assembly has a connection box for connecting the terminals to the power line. The stator windings are sup-

Fig. 13. Stator coils firmly held in core slots develop the rotating field in an a-c induction motor".

Courtesy oj Westinghouse Electric Corporation

23 fig. 14. Large motors use preformed coils. Careful assembly practices ensure reliable motor operation.

Courtesy of U.S. Electrical Motors Division, Emerson E/ee1ric Co.

ported in frames made ~"""=""' cast iron, or steel. The core laminations are welded together and pressed into the frame. In aluminum frames a shrink is usually used for maximum contact and heat dissipation. In larger motors, such as the one shown Fig. 14, the thin laminations are assembled air duct spacers between packets of laminations. That allows for maximum cooling through ventilation passages in rotor stator. Iron cores are held together by steel rings or fingers at the ends of a stack of laminations.

2 " Coils

The coils in Fig. 13 are of the random-wound type which is common for certain machine sizes. The coils are held in the slots by wedges or sticks. In addition, coils of different phases have separators. the complete stator assembly, which consists of the winding, core, and insulation materials in the slots, phase separator, and top sticks, must be unified into one insulation system with an electrical varnish or epoxy resin. The operation protects the assembly against moisture and movement of the coils. Coils for large motors of medium voltage (2300 V or greater) are made from rectangular wire. The coils are wound in an oblong shape and properly sized for the motor. It is

24 common practice to wind each coil with a mica tape. The coils are inserted in the open slots and are then laced and blocked to prevent movement Next the stator assembly is put into a vacuum pressure tank and impregnated with exposy resin. By that method the winding is sealed, which is the most critical requirement in medium-voltage (2300, 4000 V) motors for eliminating voltage tracking across insulated surfaces.

28'" Squirrel~Cage and Wound Rotors

Induction motor secondaries, or rotors, are of either the squirrel-cage type or the wound-rotor (or phase-wound) type. Either of these rotors could be used in the same stator as­ sembly. A motor is designated, according to the type of rotor END RING used, as a squirrel-cage or a wound-rotor motor. A squirrel-cage rotor uses bare conductor bars instead of coils, and the wound rotor has insulated coil windings simi­ lar to those on the stator.

29'" Construction of Squirrel-Cage Rotor

A squirrel-cage rotor for a motor is shown in Fig. 15. The bars Fig. 15. The squirrel-cage are short-circuited by end rings. There are no actual electrical rotor consists of bars short­ connections to the rotor. The assembly of bars and end rings circuited at each end of the has the general form of a squirrel-cage wheel, which is the rotor. The stator's rotating basis for the rotor's name. magnetic field induces currents in the bars. These The a A construction of squirrel-cage rotor is very simple. currents develop magnetic laminated iron core is assembled on a cast-iron or fabricated fields which react with the steel spider, which is pressed and keyed on a shaft. Among stator field, causing the rotor the methods which have been used to fasten the squirrel-cage to tum.

DIE-CAST END RINGS

Fig. 16. A die-cast squirrel­ cage rotor in common use consists of a one-piece casting. The bars and end rings are cast either from aluminum or from a copper alloy. Courtesy of Westinghouse Electric Corporation

25 bars to the end rings are bolts, bolts and solder, solder only, keystone construction, welding, die-cast end rings, and braz­ ing. Of these methods, only the latter two are used extensively in manufacturing modern squirrel-cage motors. A brazed squirrel-cage secondary with die-cast end rings, widely used on small and medium-size motors, is shown in Fig. 16. The rotor bars, end rings, and fan blades are cast in one piece. The cylindrical ball- or roller-bearing cartridges are shown mounted on the shaft.

Fig. 17. The common lamination for a squirrel-cage rmor uses one row~ conductors, but some applications require two circumferential rows to produce specia I characteristics.

(a) Ordinary lamination (b) Double squirrel-cage lamination

Typical laminations for squirrel-cage secondaries are shown in Fig. 17. The common squirrel-cage motor uses lami­ nations like the one shown in (a), and the double squirrel-cage secondary has laminations like the one shown in (b). The double squirrel-cage rotor has two circumferential rows of conductors, and the purpose of such a construction is to ob­ tain special torque and current characteristics that will be dis­ cussed later.

30· Characteristics of Wound Rotors

The phase-wound, or wound, rotor shown in Fig. 18 uses an insulated coil instead of bars. Each coil occupies approxi­ mately the same arc around the stator assembly as a primary coil, and all coils are so connected that the resistance of the

26 Fig. 18. The phase-wound rotor has coils instead of bars. The coils are connected to slip rings so that the resistance of COIL the secondary circuit can be Courtesy of Westinghouse Electric Corporation changed to vary the speed. secondary circuit can be varied by the addition of external resistance in series with the motor secondary winding. In­ creasing the resistance in the secondary circuit of an induc­ tion motor reduces the starting current for a given torque. Also, increasing the resistance reduces the speed when the motor is operating in much the same way that adding resis­ tance in series with the armature decreases the speed of a d-c motor.

31· Construction of Wound Rotors

The coils of a phase-wound rotor are always connected for the same number of poles as the primary, and usually in a three­ phase Y-connection ( or wye) regardless of whether the pri­ mary is connected three-phase or two-phase. The free coil lead of each of the phases in Fig. 18 is connected to a slip ring on the shaft. Stationary brushes (not shown) sliding on the rings furnish a means for adding external resistance in series with the" secondary windings. Usually the resistance is con-

EXTERNAL RESISTANCE IN CONTROLLER

Fig. 19. The controller for a phase-wound rotor consists of SLIP RING AND a "ganged" three-arm BRUSH ASSEMBLY adjustable resistance.

27 nected to the slip ring by means of a controller which can be adjusted to add external resistance ranging from the full amount to zero. A diagram illustrating this connection is shown in Fig. 19. The three-phase motor is cut away to show the slip rings and brushes which connect to the external resis­ tance. The three leads from the motor connect to the line. The three arms of the adjustable external resistance are me­ chanically joined (ganged) as shown by the broken line. The ends of the secondary coils projecting beyond the slots are usually supported against centrifugal force by bands made from special insulating tape, or nonmagnetic binding wire, similar to that used on d-c machine armatures.

Check Your Leaming

I. Name the two types of rotors used in induction motors. 2. Which type of connection (Y or A) is used for the coils of a wound­ rotor? 3. How is the current limited in starting a wound-rotor motor? 4. Why are two circumferential rows of conductors used in certain squirrel-cage rotors?

APPLICATION AND OPERATING CHARACTERISTICS

3 2 • Applications of Induction Motors

Induction motors with phase-wound rotors are preferred to those with squirrel-cage rotors when low starting current or speed regulation is necessary. The speed of phase-wound in­ duction motors can be varied under load; the starting current can be regulated; and the starting torque can be adjusted to any value within the capacity of the motor. Those changes can be obtained by simply varying the amount of resistance con­ nected in the secondary or rotor circuit as shown in Fig. l 9. Specific applications of the phase-wound induction motor with an adjustable external rotor resistance for producing variable speed include cranes, elevators, and punch presses. Beyond a certain size of motor, depending on the char­ acteristics of the circuits to which the motor is connected, the large starting current of the squirrel-cage motor would prove to be objectionable because of the poor voltage regulation and excessive voltage drop in the lines or leads to the motor. Under such conditions an induction motor with a phase­ wound rotor should be used; in fact, in many cases the electric

28 power utility supplying the energy to the motor insists that induction motors connected to their lines, if above a certain horsepower, be the phase-wound type. The poor voltage regulation caused by the starting current of large-horsepower squirrel-cage motors would affect any lighting circuits inter­ connected with the motor circuits by causing an objectionable lamp flicker. Furthermore, the poor voltage regulation may so reduce the voltage at the motor that the motor will not start, since the torque of an induction motor decreases with the square of the voltage.

33 • Purpose for Slip in Induction Motors

As was stated in an earlier article, the slip of an a-c motor is the difference between actual speed and synchronous speed. The slip occurs during the operation of induction motors. The current in the primary or stator winding sets up a rotating magnetic field of flux. When the field rotates at a speed dif­ ferent from that of the rotor, flux will cut the conductors on the rotor and thereby induce voltages and cause circulating currents proportional to the speed at which the conductors are cut by the flux. For instance, at standstill the slip is 100%; thus as one pair of stator poles electrically passes a rotor con­ ductor, the rate of conducting will produce a current of 60 Hz X 100%, or 60 X LO = 60 Hz in the rotor. However, as the rotor speed increases to perhaps 2% slip, the current developed in the rotor will be 60 Hz X 2%, or 60 X 0.02 = l .2 Hz. The torque developed by the motor is proportional to the product of the primary flux and the secondary current. Thus if the secondary rotated at the same speed as the primary flux, the conductors on the rotor would not cut any flux; there would be no secondary current and consequently no torque. Therefore, the motor secondary must rotate at a speed less than synchronous speed but just enough less to allow suffi­ cient voltage and current to be induced in the·secondary wind­ ing to react with the primary flux to produce the required torque. The full-load slip of general-purpose induction motors ranges from possibly 8% in small-power motors to 1% or less in motors of 100 hp and larger.

Practice Problem

What is the frequency of the rotor current for a 25-Hz motor at 4% slip? Ans. 1.0 Hz

29 34 • Variation of Slip with Torque

Since the torque depends on the current in the secondary con­ ductors and that current depends on the rate of cutting the lines of force, the slip, while the motor is running, automati­ cally adjusts itself according to the torque required. If the load increases and thus demands greater torque, the slip in­ creases accordingly; if the load decreases, the slip decreases. The slip at no load-that is, with the motor running idle-is very low because the only mechanical torque required is that necessary to overcome the windage and bearing friction. Con­ sequently, at no-load conditions the secondary rotates at very nearly synchronous speed.

35• Effect of Secondary Impedance on Slip

The currents in the secondary conductors vary directly with the emf (electromotive force) induced and inversely with the im­ pedance of the conductors and their interconnections. There­ fore, to produce a given secondary current and rotor torque, a high-impedance secondary must generate a larger emf than a low-impedance secondary; in other words, the high-impedance secondary must have a greater slip than the low-impedance one. That relation assumes that the motors being compared are similar in all respects except their secondary impedances.

35· Starting Current and Torque

Current and torque conditions encountered while starting a squirrel-cage motor are shown in Fig. 20 (a) and (b). The abscissa, or horizontal line of each group of curves, which is the same for both views, represents the percent of synchronous speed; in both (a) and (b) the ordinates, or vertical values, represent the percent of full-load current on the curves shown in (a) and the percent of full-load torque on the curves of (b). The upper curve in Fig. 20(a) shows the current-speed characteristic with full-line voltage. At the instant the line switch is closed, the line current is 870% of full-load current, or nearly 9 times normal. As the speed increases, the current decreases at first gradually and then more and more rapidly. The current curves do not decrease to zero at 100% synchronous speed because of the magnetizing current that causes the ro­ tating field. In Fig. 20(b) the upper curve shows the torque exerted by a motor starting on full line voltage; it is about 180% of full-

30 9DO

800

(a) Current.. speed CLrves Flill LOAD, oOO ij

f-:if;. 20. J\s shov,m in t!-1e c,~nrcs fo,· c: sq:.ii:rrel-cage r'.'"lotor, : 1w starting current is al)Out nirx) times the normal tulJ- road current but t!-:G sta;-ting torque is relatively /ow. 0 10 20 2;0 ~-8 50 50 70 80 90 "'.::)Q PE9Ci.=:NT C:: SYi\iCHi~ONO~S s;->EE~

(b) rorque-speed curves Ioad torque at the start, incnzses to more tb2.n 5 ti::11es _("uH­ load torque at ;:bcul 83% of syncr:ronous speed, 21L1,d tl-.icn :alls to fatl-kiadl Vb,1ue at about 98% of sync;1roncus speed.

3 7" !::-ffect of Volta;e c::;n Starting Cu7rer-t

Trre strength of 1he rotating magnetic field of ,rn a-c motor is directly proponional to the voltage impressed on the primary winding of the motor. Therefore, at a given slip the secondary current of 2.n. induction mo~cr ·win alsc be proportiona.1 tc the n1otm p::-imary vo1t[~ge. -:'bus if a three-phase-: :nductim: motor we:::e h"J he operated at s line vohage equal tG 60% of n_:)rma],

31 its current-speed characteristic would be as represented by the middle curve in Fig. 20(a). Everywhere on that curve the cur­ rent values are 60% of those on the full-line-voltage curve. For example, refer to Fig. 20(a) and observe that at zero speed and at full line voltage the starting current is 870%, whereas at zero speed and at 60% full line voltage the starting current is 0.60 X 870 = 522%. Therefore, at a lower line voltage the cur­ rent drawn from the line is lower. The bottom curve in Fig. 20(a), labeled autotransformer voltage, will be discussed in a later article.

39· Effect of Voltage on Torque

The torque of an induction motor is proportional to the square of the motor voltage because, as previously discussed, both the strength of the rotating magnetic field and the magnitude of the secondary current vary directly with motor voltage. The relation of line voltage and torque is given by the formula

(1)

in which T = torque at full line voltage, in foot-pounds T1 = torque at actual line voltage, in foot-pounds E = full line voltage, in volts E1 = actual line voltage, in volts

- E12 T or T I - -,- (2) £· also E1 =E/fj (3)

The torque T1 at any reduced voltage E1 may be deter­ mined as a percent of the full-voltage torque T by applying formula I. For example, if the voltage falls from 110 V (nor­ mal) to 100 V, the reduced torque is E/ 100 2 7: = -T = --T= 0.826T I E 2 110 2 or 82.6% of full voltage torque. In other words, given a reduc­ tion of 9% in voltage (that is, from 110 to 100 V), the torque decreases by 100 - 82.6, or 17.4% provided the speed remains constant. If the motor load torque ·remains constant, however, the speed will decrease enough at the lower voltage to give the required torque unless the torque is beyond the motor capacity, in which case the motor will stop. At the lower speed, the

32 motor primary current will very likely be higher than it was at 1W V, and overheating of the primary winding will result. Higher than normal voltage increases the torque and de­ creases the slip. It is also objectionable because it produces greater magnetic or iron losses, which may result in overheat­ ing the motor. Therefore, it is good practice to maintain a uni­ form, normal, line voltage to protect the motor against stalling or overheating. By using formula 3, the starting voltage for any torque within the capacity of a motor can be predetermined as a percent of the full voltage provided the torque of the motor at full voltage is knovvn. Both torque values may be expressed in percent. For example, the motor considered in the curve of Fig. 20 will develop 180% of full-load starting torque on full line voltage at zero speed. If its load is such that it is re­ quired to furnish only 45% of torque at zero speed, then its starting voltage need be only

E1 == E~= Ey0.25 = 0.5£ or 50% of rated voltage. In Fig. 20(b) the lower curve shows the torque-speed characteristic of the motor when operated at 60% of full line voltage. Observe that the torque at each speed on the lower curve is approximately 0.6 X 0.6 X WO, or 36% of the torque at full line voltage. For example, at zero speed the full-line­ voltage torque is about 180%, and the corresponding torque value at 60% of full line voltage is 0.36 X 180, or 65%. Observe in the right-hand portion of the lower curve in Fig. 20(b) that the speed at which the motor will deliver 100% torque at 60% full line voltage is the same speed as that at which the motor will deliver WO + 0.36, or 278% torque ac­ cording to the full- line-voltage curve. In both instances, the speed is approximately 96% of the synchronous speed. Also observe that 100% torque occurs on the 60% of full­ line-voltage curve at approximately 44% of the synchronous speed. Normally the motor would not operate continuously at this point because abnormal overheating would occur due to the large slip at this speed. Thus, normal operation of the motor would be in the right-hand portion of Fig. 20(b).

Example Problem

Problem. If a 115-V motor develops a 5-ft-lb (foot-pound) torque, what voltage is required to develop a 125-ft-lb torque?

33 Solution. By applying formula 3 in Art. 38, the required new voltage E1 is determined as follows: rr:­ E1=Ev~ T = 115/nl-= 115$ = 115x 5 5 = 575 V Ans.

39· Pull-Out Torque If the load on an induction motor is increased sufficiently, a point at which the motor will pull out of step and stop will finally be reached. The torque required at that point is called the pull-out torque. The limit of momentary overload capacity is reached when the motor develops a torque just below the pull-out torque. The three-phase induction motor for which curves are shown in Fig. 21 is rated at 100 hp and 440 V and has a pull-out torque about 375% of full-load torque. That value may be estimated by noting that just below the pull-out point (the point at which the speed curve bends inward) the horsepower is approximately 270 at a speed of 72%. Therefore, the pull-out torque is 270 1 lO0 X 0.?2 X 100 = 375%

100 ci 0 I­ 90 fa I- z a: Ow 80 ~ 400 <( 0 LL a: a: w 70 ~ 350 w a.. 60 ~ 300 ~~ a.. - 50 zt-=' 25 0 40 w >--ww a: 0 a.. a: Zen 30 ::> ~o 0 Qz 20 w LL<( z LLw 10 :::::i Fig. 21. The curves for a three­ 0 phase induction motor show that this type of motor has high efficiency and a relatively constant speed characteristic. OUTPUT, IN HORSEPOWER

34 After the load has passed that point, the current increases very rapidly and all the other characteristics decrease, as is shown by the sharp bends at the ends of the curves. General­ purpose motors have pull-out torques which usually range from 1-½- to 3 times full-load torque. Those maximum torques are of use only when starting or for very intermittent service, be­ cause they are far beyond the continuous thermal capacities of the motors. The motor whose characteristics are represented in Fig. 20 will have a pull-out torque of about 500% at full line volt­ age and 180% at 60% of full line voltage.

4 O • Typical Characteristics for Squirrel-Cage Motors

By properly designing the type of squirrel cage, a wide variety of performance characteristics can be obtained from squirrel­ cage motors. In Fig. 22(a) are shown typical torque-speed 300 w :::> 250 LL 0 0 a: I- 0 200 z I- WO U< 150 a: 0 w ...J ll. I 100 ...J ...J :::> LL 50 0 (a) Torque-speed curves 700 I-z 600 w LL 0: 500 0 a: I- :::> zU 400 WO U< a: 0 300 w ...J Fig. 22. Two rows of bars, ll. I ...J one of low-resistance bars and ...J 200 :::> the other of high-resistance LL bars, can be used to produce 100 special speed and torque characteristics in a squirrel­ 0 10 20 30 40 50 60 70 80 90 100 cage motor. PERCENT OF SYNCHRONOUS SPEED (b) Current-speed curves

35 characteristics for three different types of squirrel-cage motors, and in (b) the corresponding current-speed curves are shown. Curve l is for a normal-torque, low-starting-current motor; curve 2 is for a high-torque, low-starting-current motor; and curve 3 is for a high-slip motor. Among the different kinds of squirrel cages is a single cage with low-resistance bars in. the rotor laminations like the one of Fig. l 7(a), and that construction will result in a motor having characteristics like those of curve l. A double squirrel cage with high-resis­ tance outer bars and low-resistance inner bars of special alloy metals in secondary laminations like the one of Fig. l 7(b) will result in motor characteristics like those of curve 2 in Fig. 22.

41" Typical Characteristics for Wound-Rotor Motors

By changing the amount of external resistance in series with the secondary winding of a wound-rotor motor, the torque­ Fig. 23. Varying the amount speed characteristic of a typical wound-rotor motor can. be of external resistance produces different current changed as shown in Fig. 23(a) and the current-speed curves and torque characteristics in a as in (b ). Curves l to 8 respectively represent increasing phase-wound rotor. amounts of external resistance from zero (as represented by

700 650 UJ ~ 600 :::J w 0 220 a: a: 550 ~ 200 a soo ~ 180 <(O 450 g 160 g 400 ' ' ::J 140 ::J 350 :::J :::J I.I.. u.. 300 I.I.. 0 250 I- ~ 200 ~ 150 UJ o... 100

0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100 PERCENT OF SYNCHRONOUS SPEED PERCENT OF SYNCHRONOUS SPEED (a) Torque-speed curves (b) Current-speed curves

36 curve 1) to a maximum (as represented by curve 8) just large enough to give rated torque at standstill.

4 2 • Maximum Torque of Wound-Rotor Motor

Curves 1 to 3 in Fig. 22(a) illustrate a fundamental point with regard to the torque of a wound-rotor motor; namely, that the maximum value of torque is independent of the secondary resistance and depends only on the motor reactance. That is evident from the fact that the maximum-torque points of curves 1 to 3 are equal. The maximum-torque points of curves 4 to 8 also would attain the same value if the curves of view (a) were extended to the left to include negative speeds, that is, if the secondary were rotated in a direction the reverse of that of the primary rotating magnetic field as occurs during plug­ ging. Plugging is used to break the speed of a polyphase motor; it consists in interchanging two leads to a three-phase motor or the leads of one phase of a two-phase motor while the motor is running. The change of connections results in reversing the direction of the primary rotating magnetic field and consequently reversing the direction of the motor torque. From Fig. 23(a) it is also seen that the secondary resistance does determine the speed at which the maximum torque will occur. As a matter of fact, the slip at which maximum torque occurs is proportional to the total secondary resistance ( exter­ nal resistance plus winding resistance). For example, in Fig. 23(a) the maximum of curve 2 occurs at 50% slip and the maxi­ mum of curve 3 at zero synchronous speed, which is 100% slip; therefore, the total secondary resistance for curve 3 is twice that for curve 2. In Fig. 23(b) are shown the corresponding current-speed curves for the same eight resistance steps shown in (a).

Check Your Learning

I. What is the electrical speed of the secondary of an induction motor under no-load conditions? 2. If in an induction motor the load remains constant but the volt­ age decreases, what is the effect on the speed? 3. What is meant by the term plugging? 4. If the load on an induction motor is considerably increased, what condition will occur? 5. Why are large squirrel-cage motors not recommended for use on lightin~ circuits?

37 STARTING SQUIRREL-CAGE INDUCTION MOTORS 4 3 • Basic Starting Requirements

When an induction motor is first connected to an a-c source, and before the secondary begins to rotate, the slip is 100%. As a result, the momentary current taken from the line would be several times full-load current unless some method were taken to limit it. Fractional-horsepower squirrel-cage induction mo­ tors and the lower-rating large-power motors are usually started by switching them directly onto the line, but larger sizes are often provided with some form of starting device. That device may be a resistance starter, a reactor starter, or an autotransformer starter.

44· Resistor Starting

In the resistor starting method, a variable resistor is connected in series with each phase of the motor primary and is mechan­ ically so arranged that the resistance can be cut out in steps, either manually or automatically, as the rotor accelerates. The addition of the variable resistor in series with the motor impedance produces a voltage drop between the line and the motor terminals, and thus the voltage at the motor ter­ minals is lower than the line voltage, which in effect reduces the current drawn from the line. Occasionally a resistor is added to only two of the phases of a three-phase motor. That technique slightly unbalances the phases, but not enough to cause trouble with the smaller motors on which such a starter would be used.

45 • Reactor Starting

The reactor starter method is similar in most respects to re­ sistor starting except that a series reactor rather than a resistor is used in each phase. For a given value of series impedance, in ohms, the reactor is more effective than the resistor in re­ ducing the motor terminal voltage because the voltage drop across the reactor is more nearly in phase with the line voltage than is the voltage drop across the resistor.

4 6 • Autotransformer Starters

Autotransformer starters, also called autostarters or compen­ sators, consist of autotransformers with switching devices so

38 IL----- AUTOTRANSFORMER 302% IL-----

IM__. A-C UNE

(a) Basic autotransformer connection (b) Example problem values

arranged that reduced voltage is applied to the motor primary Fig. 24. Less iine current is during starting. By the autotransformer principle, shown its drawn from the line when an basic form in Fig. 24(a), the line current his less than the motor autotransformer is used to reduce the applied motor current lM by approximately the same ratio as the motor voltage. voltage EM is less than the line voltage EL, That relation is shown by the formula

in which fa= line voltage, in volts EM = motor voltage, in volts h = line current, in amperes IM = motor current, in amperes

The relation of line and motor values gives the autotransformer starter an advantage over either the resistance or the reactor starter, the advantage being that it draws less line current h for the same motor current IM, For example, refer to the current-speed curves in Fig. 20(a) and observe how the motor current IM at full-line volt­ age starting would be reduced to 60% of its value if an auto­ transformer were used to lower the motor voltage to 60% of normal. Similarly, the line current h is reduced below the motor current IM as shown by the lower curve in (a). Those values are as shown in Fig. 24(b ), and when they are substituted in the formula, the previously described ratio is maintained. EL IM EM= h or 100% 522% =--- 60% 302% Thus 1.66 = 1.66

39 In such an application the line current includes the trans­ former magnetizing current. Also, since an autotransformer is designed for intermittent duty, it should not be left connected to the line after the motor has started. If it is, it will overheat.

4 7 • Autotransformer Starter Provisions

Three autotransformer taps are generally provided in each starter; ·and after a motor is installed, the tap that will give the best results with it should be selected. The taps are generally so spaced that one tap will give from 40% to 50% of line voltage on the motor, another from 60% to 65%, and the third about 80%. The lowest voltage at which the motor starts freely should be used; in the majority of cases that is the middle tap. The highest starting voltage is useful only when the motor must start with high torque, as in starting a long line shaft or a machine with heavy rotating parts. The motor should start its load promptly and accelerate to full speed in about l min (minute) after the switch is closed in the starting position. If too low a starting voltage tap is selected, the starting period will be too long and the auto­ transformer may be overheated, especially if the motor is started frequently. On the other hand, if the starting voltage is too high, the starting current will be higher than is necessary and the motor will start with a violent jerk and accelerate very rapidly. In any case, the switch should be left in the start­ ing position until the motor attains very nearly full speed and then thrown quickly to the running position. If the change is made too soon, an unnecessary rush of current will result when the change is made; if it is delayed too long, the auto­ transformer may be overheated.

Example Problems

Problem. If a 460-V wound-rotor motor is to be started with an autotransformer starter set on the 80% position, what will be the line current if the motor is expected to draw 25 A (amperes)? Solution. First, the motor voltage EM is determined as 460 X 0.8 = 368 V. Then, by applying the formula shown in Art. 46, the line current h is EM h= IMEL = 25 X !: = 25 X 0.8 = 20 A Ans.

40 Practice Problem

If a wound-rotor motor is to be started by an autotransformer that supplies 345 V at the 60% tap, what is the line voltage across the autotransformer? Ans. 575 V

4 8"' Effect of Starting Method on Torque-Speed Characteristics

In Fig. 25 are shown five torque-speed characteristics of a typical squirrel-cage motor. The middle curves show the start­ ing characteristics under the three types of starting condi­ tions previously described: resistor, reactor, and autotrans­ former. The upper curve shows the torque-speed character­ istic under full-line-voltage starting. The resistor starting and reactor starting curves are plotted for a constant series im­ pedance, whereas in an actual starting cycle the series resistor or series reactor may be progressively reduced in steps. The reactor cur.ve is at 50% standstill motor voltage con­ ditions; the resistor curve is at 65% standstill motor voltage conditions; and the autotransformer curve is at the 65% tap condition.

260 240

~ 220 ~ 200 0 I- 180 0 (5 160 ..J ...J 140 ..J i°t 120 LL 0 100 1- zw 80 ~ 60 w a.. 40

20 Fig. 25. Torque curves show how full-lir,e-voltage, reactor, 0 10 20 30 40 50 60 70 80 90 100 resistor, and autotransformer starting affect torque-speed PERCENT OF SYNCHRONOUS SPEED characteristics.

41 The lower, broken-line curve is a load-torque curve for a centrifugal pump which will be discussed in the following article. The intersection of this curve with the appropriate motor-torque curve determines the maximum speed which the motor will attain.

4 g· Part-Winding Incremental Starters

In some applications in which induction motors are used to drive pumps or fans, a two-thirds part-winding starter is acceptable by the electric utility. This type of starter consists of one four-pole and one two-pole magnetic contactor (switch­ ing device); each contactor is selected for one-half the horse­ power rating. A time-delay mechanism included in the con­ tactor assembly is normally adjusted to complete the closure in 3 sec (seconds). Although this type of starter has cost advantages over other starting methods, it has poor torque characteristics as shown by the lower curve in Fig. 25. Therefore, it is not suit­ able for high-inertia loads but it is acceptable for pumps and fans, since very little horsepower is required until 50% speed is attained. Motor controllers will be discussed in greater detail in other ICS texts.

Check Your Leaming

1. What three basic methods are used for starting squirrel-cage motors? 2. Referring to Fig. 25, which of the three basic starting methods produces the lowest starting torque? 3. For what applications is the part-winding incremental starter acceptable? 4. Why _is a compensator-type starter used for a squirrel-cage induc­ tion motor? 5. What starting method is usually used for fractional-horsepower motors?

STARTING WOUND-ROTOR MOTORS Fig. 26. The starter for a wound-rotor motor, also 50• Resistor Starting caHed a phase-wound motor, consists of an adjustable resistor connected through the slip Starters for wound-rotor induction motors consist basically of rings to each leg of the 1) a series resistor, commonly called a rheostat, connected secondary winding. to each phase of the secondary winding, and 2) switching· de-

42 vices to adjust tile connections to vary the amount of resis­ tance in the secondary circuit. A diagram of connections for the resistor starter to a slip-ring motor is shown in 26. The three phases of the secondary are in a Y-connection; three re­ sistors are at Rn, R2, and R3; and the switching device arm can be adjusted as shown. The switch is indicated in the off posi­ tion, an resistance being in circuit; turning it clockwise, as in­ dicated by the curved arrow, cuts the resistors out of the cir­ cuit in steps. The switching is usually done manually for small motors and automatically for large motors.

Fig. 27. As the phase-wound motor is brought up to synchronous speed, the series resistance is cut out of the secondary circuit section by section.

0 1. current with all resistance in (a) Current-speed curves 2. torque with all resistance in 3. current with one section of w 500~6;__----- resistance out ::) 4. torque with one section of LL 0 0 a: resistance out fr- 0 5. current with two sections z 1- 300 of resistance out W Q torque with two sections of 0 ~ 6. a: 0 resistance out w ...I 200 il. _'.i 7. current with all resistance ..J out ::) 100 torque with all resistance u.. 8. 2 out 0 9. current changes; resistance 0 10 20 30 40 50 60 70 80 90 100 cut out in steps PERCENT OF SYNCHRONOUS SPEED 10. torque changes; resistance cut out in steps 100 90 80 70 60 50 40 30 20 10 0 11. current with resistance PERCENT OF SUP cut out in all steps 12. torque with resistance (b) Torque-speed curves cut out in all steps

43 51" Starting Characteristics of Wound-Rotor Motors

In Fig. 27 are shown the current and torque changes in a wound motor with three sections of resistors; the curves in (a) show current changes, and the curves in (b) show torque changes. Curves l and 2 show current and torque with all the starting resistance in the circuit. Curves 3 and 4 show the current and torque conditions with one section of resistance in each phase cut out. Curves 5 and 6 show the current and torque with two sections of resistance out in each phase; and curves 7 and 8 show the same conditions with all the starting resistance out of circuit. The heavy zigzag lines 9 and 10 show current and torque changes while the starting resistor is cut out of circuit in steps. According to the lower curves 1 and 2, the motor starts with 150% of both full-load current and full-load torque. Both the current and torque decrease as the motor speed increases until the first section of resistance is cut out, when the current increases to 240%, a point on curve 3, and the torque to 375%, a point on curve 4. When the second and third sections of re­ sistance are cut out, other increases of both current and torque occur and cause increased speed. In each case the values of the current and torque decrease as the speed increases until the motor is operating at full speed, which is 98% of syn­ chronous speed (2% slip) with both current and torque at 100%, or full value. If the starting resistance were cut out in very small steps, the current and torque would change more nearly as shown by the broken-line curves 11 and 12. The magnetizing current for the rotating field causes the current curves to end above the zero line at synchronous speed, as in (a).

Check Your Leaming l. How is the speed of a wound-rotor motor increased? 2. Compared to the various a-c motors described in this text, what unique mechanical feature identifies a wound-rotor motor? 3. How can smoother torque and current changes be produced when a wound-rotor motor is started?

SPECIAL TYPES OF POLYPHASE INDUCTION MOTORS 52" Multispeed Motors

If only a few speed changes with simple ratios such as 2 to 1 are required of a squirrel-cage motor, they can be made by

44 changing the number of poles in the primary. Recall from an earlier article that synchronous speed N is given by the for­ mula /X 120 N p

Thus if the number of poles is doubled, the speed is decreased by one-half. To understand how the number of poles can be changed, first consider that the basic winding in a three-phase primary consists of six coils. In Fig. 28(a) the six coils are shown in a Y-connection when the four-pole double-throw switch in (b) is thrown to the left or in a ~-connection as in (c) if the switch is thrown to the right. The terminals T1 to T6 are standard des­ ignations found on most motor connection boxes. The ter­ minals L1 to L3 connect to a three-phase a-c source. With the switch to the left, the six coils are parallel-Y- connected. The instantaneous direction of current in the coils causes adjacent poles around the stator assembly to have the same polarity as in (d). Thus the N1 and N poles combine into one north pole and the S1 and S poles combine into one south pole; consequently, a two-pole motor is produced. If the switch is to the right, the six coils are series-con­ nected. The current flow produces pole polarities as in (e); in

(b) Four-pole double-throw (a) Y-connection switch (c) ~-connection

-Fig. 28. The number of poles can be changed by rearranging the connections to the six coils in the primary winding. The speed is inversely proportional to the number of poles. For example, if the number of poles is reduced, the speed is (d) Equivalent two-pole motor increased. (e) Distinct four-pole motor

46 effect, each pole remains distinct and a four-pole motor is formed. Since the speed varies inversely as the number of poles, closing the switch to the right gives one-half the speed obtained if the switch is closed to the left. On a 60-Hz circuit, for example, a two-pole primary gives 3600 r/ min, or synchronous speed, and a four-pole primary gives 1800 r/ min. Two windings can be placed on a primary similar to the one just described, and they can be arranged for connection by means of a drum-type switch to form four ar­ rangements of poles giving four speeds. The ratios in four­ speed motors are usually 1, j , ½, and+. On 60-Hz circuits for example, 4, 6, 8, and 12 poles give synchronous speeds of 1800, 1200, 900, and 600 r/min; on 25-Hz circuits, the same numbers of poles give synchronous speeds of 750, 500, 375, and 250 r/ min. The secondaries of the multispeed polyphase motors are of the squirrel-cage type. Owing to the special nature of the design, a multispeed induction motor is always larger than a single-speed motor of the same high-speed rating. The increase in size ranges from 30% for a motor with a constant torque rating at all speeds to 100% or more for a constant-horsepower-rating motor.

5 3• Adjustable Speed by Brush Shifting

The rotor of a polyphase adjustable-speed motor shown in Fig. 29 has slip rings, commutator, ventilating fans, and rotor windings. A speed of approximately 4 to I is obtained by shifting the position of the brushes around the circumference of the commutator. This motor may be compared to a wound­ rotor induction motor having its primary windings in the rotor and its secondary on the stator. In addition, this machine has an adjusting winding in the rotor similar to a d-c armature wind­ ing, which is connected to a commutator. The motor is pro­ vided with two brush-holder yokes arranged to shift in op­ posite directions. One end of each phase of the stator (secon-

VENTILATING FANS SLIP RINGS

Fig. 29. A polyphase adjustable­ speed motor uses both slip rings and a double-yoke commutator-brush assembly. COMMUTATOR ROTOR WINDINGS

46 BRUSHES

ADJUSTING WINDING SECONDARY WINDINGS

------TO ------THREE-PHASE ~-1-----+--;-i+--t--r--- A-C LINE

SECONDARY Fig. 30. As the commutator WINDING brushes are moved farther apart, the speed of the polyphase adjustable-speed motor is decreased. dary) winding is connected to brushes on one brush yoke. The end of the winding connected to that yoke is shown by a black dot. The other end of each phase of the secondary is connected to brushes on the other yoke. The diagram of an adjustable-speed motor in Fig. 30 shows the secondary winding, the adjusting winding connected to the commutator, the brushes, and the slip rings. Polyphase current is supplied through the leads to the rings which feed the primary winding. Any pair of brushes may be shifted to move farther apart or nearer together. When the brushes, to which each end of a secondary is connected, are on the same commutator segment, the adjust­ ing winding is idle, the secondary winding is short-circuited, and the motor runs as an induction motor with a speed cor­ responding to the number of poles and the frequency of the supply circuit. As the brushes are moved apart, a section of the adjusting winding is included in series with the secondary winding. That causes the secondary winding to generate a voltage to balance the voltage impressed upon it by the ad­ justing winding and thereby causes the motor to change its speed. When the brushes are in the low-speed position, these motors at starting produce from 140% to 250% of normal

47 torque with 125% to 175% of full-speed line current. The maxi­ mum torque at low speeds is from 140% to 250% of normal torque; it increases for the high-speed position to from 300% to 400% of normal torque.

Check Your Leaming

L Name two methods of adjusting the speed of polyphase motors. 2. If a three-phase winding has six coils, how are the terminals usually identified? 3. What designations are used to identify the terminals that are connected to the a-c source? 4. If the six coils of a three-phase induction motor are first .:l -con­ nected and then Y-connected, what is the effect on the speed? 5. In a brush-shifting adjustable-speed motor, what is the effect if the two brushes of any pair of brushes are on the same commuta­ tor segment?

48 0 s

OPERATION AND RATINGS

5 4 • Basic Operation

Any motor which runs at a constant speed that is exactly pro­ portional to the frequency of the current supplied to it is called a synchronous motor. Synchronous motors for industrial power applications always have three windings, namely, a stator winding, a field winding, and a squirrel-cage winding. The motor is started by applying a-c power of the cor­ rect voltage, phase, and frequency to the stator winding. Cur­ rents induced in the squirrel-cage winding cause the rotor to come up to approximately synchronous speed as an induction motor. The motor is then synchronized by applying d-c excita­ tion to the field winding, which causes the rotor to accelerate to synchronous speed and magnetically lock into step with the rotating field developed by the supply frequency. When the motor is locked into step, it is said to be in syn­ chronism, and the motor will continue to run in synchronism for any value of load up to the maximum value that the motor can carry. If the maximum value of load is exceeded, the motor will pull out of step and corrective measures must be taken immediately to prevent damage to the motor.

49 55 • Ratings The data given earlier in this text apply to synchronous motors also. However, small-power synchronous motors are seldom manufactured. Excluding the special types of synchronous motors such as the reluctance and hysteresis types, which do not ha~e a field winding, all synchronous motors for industrial power application are of the large-power class. They are rare­ ly manufactured for less than a 20-hp rating and seldom for less than 100 hp. They are always polyphase and are usually three-phase, but sometimes they are two-phase. When synchronized, the synchronous motor operates at exactly the speed N determined from the synchronous speed formula. Motors of 514 r /min (purely an arbitrary value) and above are classified as high-speed, and those below 514 r / min are classified as low-speed. Most low-speed motors built today are for speeds over 200 r / min, although some applica­ tions require motors for 100 r/ min and lower speeds. Power factor is an important part of a synchronous motor rating. Synchronous motors are always built for either unity, or 100%, power factor or for a leading power factor. The stan­ dard leading power factor is 80%, but synchronous motors can be built for lower power-factor ratings.

5 6 • Applications In general, a synchronous motor can be used for any applica­ tion for which a squirrel-cage induction motor might be used. Three factors generally determine the choice between a syn­ chronous motor and an induction motor for a specific applica­ tion. The factors are cost, efficiency, and power factor. The cost of the higher-speed low-horsepower squirrel­ cage induction motor and control is lower than the cost of the corresponding synchronous motor. For higher horsepower and lower speeds the synchronous machines are less costly. The full-load efficiency of an induction motor is generally 1% to 3% lower than that of a unity-power-factor synchronous motor and about equal to that of an 80% power-factor synchronous motor of the same horsepower and speed rating. Since synchronous motors are designed to operate at unity power factor or at a leading power factor, they can be used to offset the lagging power factor of induction motors used in the same industrial plant and thereby improve the overall power factor in the plant. That often reduces the cost of power, since many electric utilities charge less per kilowatthour for higher­ power-factor loads.

50 CONSTRUCTION OF SYNCHRONOUS MOTORS

57• General The dimensions, proportions, and construction of the synchro­ nous motor vary greatly with the rating and use of the motor. In addition, the provision for vertical or horizontal shafts with different bearing arrangements and the provision for various methods of ventilation-such as open, drip proof, splashproof, totally enclosed, explosionproof-cause a wide variation in the appearance of the synchronous motor.

58• Stator Assembly

The stator of a synchronous motor corresponds to the primary Fig. 31. The synchronous of an induction motor. In fact, an induction motor primary motor can be recognized by the collector rings which feed d-c FIELD POLES excitation to the field poles and by the amortisseur winding.

~--AIR DUCT DISCHARGE

ROTOR ----- COLLECTOR RINGS

\~---COIL GUARDS

STATOR FRAME PIPE SUPPORT

---AIR DUCT DISCHARGE

FOUNDATION CAP

STATOR TERMINAL LEADS

51 can be used as a synchronous motor stator if the squirrel­ cage rotor is replaced by a synchronous motor rotor. The cores of smaller-horsepower, high-speed synchronous motors are constructed in the same manner as the cores for the primaries of induction motors. Synchronous motors of higher horsepower ratings and low-speed ratings have their laminations stacked on dovetail keys welded or otherwise held to the stator frame. Steel fingers at each end of the core ex­ tend to the tips of all the teeth of the punchings and transmit the clamping pressure of the frame, or clamping plates, to the entire core. The stator coils are assembled like those on an induction motor. Machine-formed coils are generally completely in­ sulated before they are installed in the stator slots. In Fig. 31 is shown a 600-hp 180-r / min open-ventilated synchronous motor for driving an air compressor. Such a motor is usually furnished without a shaft or bearings for mounting between bearings on the compressor shaft. In this design the rotor has a split hub and solid rim, and it would be overhung on the end of the compressor shaft. The stator frame is bolted to foundation caps set in concrete foundations. Conductors for carrying current from the collector rings to the field poles are fastened to the rotor hub. The brush rigging is bolted to a pipe support. The stator terminal leads are shown. In opera­ tion, the fan action of the rotor forces air past the end portions of the stator coils and through the air ducts to cool the stator. Air passing through the air ducts discharges from the frame as indicated. Coil guards ( or end shields) protect the winding from possible damage.

5 9 •Pole Construction Except for a few 3600-r/ min pump and blower motors, in which cylindrical rotors are used, all synchronous motors are of the salient-pole type; that is, the pole projects toward the stator. High-speed motors commonly have dovetail-type poles as in Fig. 32. In (a) are shown the individual punched lamina­ tions and the four tubular rivets used to hold the laminations together. The pole tip has holes for the squirrel-cage bars. In (b) is shown an assembled pole. Before the winding is wound, the stack of laminations is wrapped with insulation (not shown) and the insulating collars are installed. The squir­ rel-cage bars, one of which is clearly shown, are installed in their pole tip holes and soldered or brazed to the end-ring seg­ ments. A terminal is located at each of the poles. The assembled pole is for a six-pole 40-hp unity-power-factor 1200-r/ min motor.

62 Fig 32. The field pole of a synchronous motor consists of both a coil winding and squirrel-cage bars. The squ inel-cage assembly enables the motor to be brought up to speed as an induction motor. Some synchronous motors use a double row of bars in the cage assembly.

HOLES IN POLE TIP FOR SQUIRREL-CAGE BARS Unassmnbled

COIL W!NDING INSULATING r::_ND COL.LARS RING

END RING sou BAR COIL TERMINAL

(b) Completed pole assembly

6 ., In Fig. 33 is shown a cross section of an eight-pole synchronous motor · h the method of attachment the poles to the rotor spider. The spider is made of thin punchings which are stacked and riveted together (the rivets are not shown). The external dovetail of the completed pole is slipped into the internal dovetail of the spider, and a key is driven and secured to hold the pole in place. When the spider is finished, the shaft hole is bored and the key is seated! to receive the shaft. This illustration also shows the stator, the pole punchings, and the squirrel-cage holes. Lower-speed machines generaHy have bolted instead of dovetail poles. The construction ot a bolted pole, such as that in Fig. 3 l, is similar to that of the dovetail pole except that the pole punchings do not have a dovetail; instead, the base is punched with a radius closely corresponding to the radius of the spider on which it win rest.

53 Fig. 33. The cross section of an eight-pole synchronous motor without windings shows the dovetail-key arrangement to hold the field coils securely POLE PIECES in place. SPIDER

The rotor spider may be of cast iron or cast steel, or it may be fabricated from steel plates. Rotors of slow-speed synchro­ nous motors for driving air compressors are usually split in two sections for ease of installation. A split rotor, such as the type used with the stator of Fig. 31, is shown in Fig. 34. In this de­ sign the hub is longer than usual in order to serve as a coupling to two compressors located one on either side of the motor. The poles are skewed or spiraled to reduce magnetic noise by avoiding sudden changes in flux as the pole passes the stator teeth. Each pole has its own individual section of squirrel­ cage winding and end rings, so that any pole can be removed for repairs. The end rings in this motor are of the closed type. Leads for connecting the field winding to the collector rings are shown, and each pole is held to the spider by two bolts. The d-c excitation for the field winding is supplied through collector rings which are mounted on the rotor or on the shaft. A 3000-hp 11,000-V 0.8-pf motor for steel mill service is shown in Fig. 35. Air enters from the pit into air chutes at both ends and is discharged through grill-covered openings. By covering the discharge openings, removing a plate, and mounting a surface air cooler at this point, the motor will dis­ charge the air into the cooler, where it will be cooled and then recirculated through the motor. The motor ventilation is then

64 Fig. 34. For a low-speed . synchronous motor, the split­ hub construction provides for easier assembly.

Fig. 35. The steel mill synchronous motor shown uses pedestal-type bearings.

55 called a totally enclosed, recirculating system. The base sup­ ports pedestal-type bearings. One half of a flexible-type coupling is shown mounted on the shaft.

61· Direct-Connected Exciter

In Fig. 36 is shown a cutaway view of a high-speed general­ purpose synchronous motor with a direct-connected exciter. In this type of motor, the end shields, also called end bells or brackets, are bolted to the frame and carry the bearings, only one of which is shown. The motor armature coils, which are mounted in the frame, the field coils, and the squirrel-cage bars are identified. The exciter armature, armature field coils, and commuta­ tor are mounted opposite the drive end of the motor. Air openings are provided in the exciter end shield and the motor end shield. Air entering the motor is directed into the rotor fans, where the air divides. Some of the air flows over the armature coils to the back of the stator core. The remainder flows between the field coils and then outward through air ducts in the core, where it is discharged through several openings. In Fig. 37 is shown a diagram of a typical large-horse­ power low-speed vertical synchronous motor such as is used for driving a vertical-shaft centrifugal pump. The weight of the Fig. 36. Some synchronous rotating parts and the thrust of the pump impeller are trans­ motors, such as the high-speed mitted through the thrust collar to the thrust block, which ro­ type shown in this cutaway tates in the upper guide bearing and on the babbitted surface view, feature a direct­ connected exciter. SQUIRREL-CAGE BAR END SHIELD FIELD COILS

ARMATURE COIL

BEARING

56 ~- DIRECT-CONNECTED EXCITER

UPPER GUIDE THRUST COLLAR BEARING ----+R------THRUST BLOCK

STATIONARY THRUST BEARING ---1PPER BEARING BRACKET

STATOR CORE

LOWER GUIDE BEARING

LOWER BEARING BRACKET

. '-----._FOUNDATION SHAFT

Fig. 37. Vertical shaft motors, of the stationary, spring-supported, thrust bearing. Babbitt is such as used for pumps, require elaborate thrust bearings to a soft, lead-like metal that is used as a bearing surface. These ensure satisfactory operation. parts are in turn supported by the girder-type upper bearing bracket, which transmits the load through the stator frame to the concrete foundation. A lower bearing bracket carries the lower guide bearing. Field coil leads run through a hole in the shaft to the direct-connected exciter and to collector rings

57 mounted at the top of the shaft. Water in the cooling coil re­ moves the heat from the bearing oil.

Check Your Learning

I. What is the purpose of the squirrel-cage winding in a synchro- nous motor? 2. Why are the field poles on the rotor skewed or spiraled? 3. Name two applications of synchronous motors. 4. Why is d-c excitation applied to the field winding of a synchro­ nous motor? 5. Which motor has the higher efficiency: synchronous or induc­ tion?

STARTING CHARACTERISTICS OF SYNCHRONOUS MOTORS

62 • Types of Characteristics

The performance, and subsequent application, of synchronous motors can be attributed to three major groups of character­ istics: l. Starting characteristics 2. Synchronizing characteristics 3. Running, or synchronous, characteristics Different motors with, say, the same running characteristics can have widely different starting and synchronizing char­ acteristics. In the following articles each of these characteristics and their importance to the application of synchronous motors will be discussed. Since synchronous motors start as induction motors, simi­ lar types of starting windings are used in both types of motor. However, there are several important differences in the start­ ing windings of synchronous motors as compared with those of induction motors. Briefly, the differences are as follows: l. The starting windings are located in the pole faces of the salient-pole motors; therefore, they are not uniformly spaced around the rotor as in an induction motor. 2. After the motor is synchronized and field excitation is applied, the starting winding carries no running load current in a polyphase synchronous motor, which has a balanced load on all phases. 3. Since the starting windings carry no load current dur­ ing synchronous operation, there is no / 2R loss in those wind-

58 ings; as a result, they have no effect on the running efficiency of the motor. 4. The exciting field winding is short-circuited through a resistance, and that winding develops some torque during the starting period.

6 3 • Construction of Starting Winding

The starting winding (also called squirrel-cage, amortisseur, pole-face, or damper winding) consists of uninsulated bars snugly fitted into holes in the top of each pole and brazed to short-circuiting end-ring segments on each side of all the poles. The segments ( or end rings) may be bolted together from pole to pole to form a complete closed circuit. In other designs the end rings are not connected between poles. Most motors have a single-layer squirrel-cage bar assembly such as that shown in Fig. 32. When special starting characteristics are required, a double-layer squirrel-cage bar assembly is used. The squirrel-cage winding bars are usually round, al­ though rectangular, diamond, and other shapes are some­ times used. The bars are generally made of copper, brass, or other nonmagnetic alloy. The specific material used for the

w :::, 0 ~ LL 0.... di (.) a: w a.

Fig. 38. In the design of a squirrel-cage, or amortisseur, winding for a synchronous motor, different metals and bar 0 20 40 60 80 100 groupings are used to produce PERCENT OF SYNCHRONOUS SPEED required torque characteristics.

59 bars depends on the desired starting characteristics because, like that of the induction motor, the torque curve is greatly affected by the resistance of the squirrel-cage bars. This relation between torque and bar resistance is illus­ trated in Fig. 38, which shows the torque curves for the same synchronous motor depending on the type of squirrel-cage winding used. The curves are for a high-resistance brass winding, a low-resistance copper winding, and a double-cage winding having high-resistance brass bars in the top cage and low-resistance copper bars in the bottom cage.

6 4 • Squirrel-Cage Heating

All the torque produced by the synchronous motor during starting, except for a small amount contributed by the short­ circuited field winding, is caused by currents flowing in the squirrel-cage winding. Those currents, flowing through the resistance of the winding, heat the squirrel-cage bars at a rate proportional to the rated horsepower times the slip times the percent torque at that slip. The maximum heating therefore occurs at zero speed, when the slip is 100%, and the heating decreases as the machine accelerates to synchronous speed. If the machine does not start, or if it operates at less than synchronous speed for too long a time, the winding can be badly damaged by the heat generated in it. Protection against that situation is provided by synchronous motor controllers.

6 5 .. Voltage Across Field Winding During Starting

When an a-c voltage is applied to the stator of a synchronous motor during starting, transformer action between the stator and field winding can induce very high voltages across an open-circuited field winding. The induced voltage is maximum at zero speed and decreases with increasing speed. At zero speed the induced field voltage- in some instances may be as high as 5000 or 10,000 V. Since the field winding is not usually insulated to withstand such high voltages, the winding may break down if it is not short-circuited during the starting period. To protect the winding during starting, a field discharge resis­ tance is connected across the slip rings. The maximum in­ duced voltage is then limited to the product of the induced field current and the discharge resistance value. The field winding, short-circuited through a resistance, actually forms another closed circuit on the rotor, and as a re­ sult it also produces torque during starting. Since the field

60 winding is a high-reactance low-resistance circuit, the shape of the torque curve produced is similar to that of the low- re­ sistance copper curve in Fig. 38, although it is much smaller in magnitude. By choosing the proper value of discharge re­ sistance in the same manner as choosing the secondary re­ sistance of a wound-rotor induction motor, the torque produced by the field of the synchronous motor can often be made a maximum near synchronizing speed and thereby help synchro­ nize the motor. When a discharge resistance RDR is used, the maximum voltage across the field winding durirtg starting conditions is given by the formula

Vr= !rRDR in which Vt·= voltage across field winding, in volts 1/ = maximum value of field current induced in the winding during starting, in amps RDR = discharge resistance, in ohms

The field windings are normally insulated for a 2500-V mini­ mum, but RDR is usually chosen to limit the maximum value of V1 to half that value or less. Thus the value of discharge resistance is chosen to produce the best synchronizing condi­ tions without exceeding a reasonable value of voltage across the resistance during starting. That resistance v~lue usually lies in the range of 2 to 20 times the ohmic resistance of the field winding itself, depending on the design of the motor.

Example Problem Prob/em. If a motor has a 3-ohm field winding and is to be protected by a discharge resistance five times that value, what will the induced field current be if the voltage is to be limited to 1200 V? Solution. By applying the formula given in Art. 65, the current is found to be Vt ft= RDR = 1200 = 1200 = SO A A 3 X 5 15 ns.

Practice Problem

What value of discharge resistance should be used to limit the induced field current to 40 A if the field winding is in­ sulated for 2400 V and a 50% limitation is applied? Ans. 30 ohms

61 6 6 • Starting Torque and Current and Power Factor

The general information regarding a-c motor torque and starting current previously discussed applies to synchronous motors also. The power factor at zero speed usually varies be­ tween 15% and 40% depending on the type of motor, the shape of the torque curve, and the amount of torque the motor can develop.

6 7• Starting Methods Full-voltage starting is commonly used on motors even up to sizes as large as 5000 hp if the power supply system is large compared to the motor. Reduced voltage at starting can be obtained by means of the autotransformer and reactor starting previously discussed for squirrel-cage induction motors. The methods may be used on large high-speed motors, particularly on synchronous con­ densers and frequency changers, and whenever low starting torque and low starting current are required. Since the syn­ chronous motor starts as an induction motor, the formulas for reduction in current and torque given in preceding articles apply to synchronous motors also. Lower starting torque and current can sometimes be ob­ tained by using part-winding starting. In such an application, only a part of the stator winding is initially energized; the re­ mainder is subsequently energized in one or more steps. When it is possible to use this method, a motor of special design is required. Incremental starting is sometimes used. By this method, several steps of part-winding starting are used or successively higher voltage is applied to the winding by one of the reduced­ voltage methods. This method increases the starting current in successive steps and thereby allows the voltage regulator of the a-c supply generator to bring the line voltage to normal value between steps.

Check Your Learning

I. What are other names for the starting winding of a synchronous motor? 2. List four methods used for starting synchronous motors. 3. When a synchronous motor is in synchronous operation, does the starting winding produce an I2 R loss? · 4. Referring to Fig. 38, which type of squirrel-cage winding would be recommended for application requiring high initial torque?

62 5. Why is a resistance connected across the slip rings of a synchro­ nous motor during starting?

SYNCHRONIZING CHARACTERISTICS

6 8 • Synchronizing Action

The starting and accelerating torques of a synchronous motor must be sufficient to accelerate the load to synchronizing speed, which varies from 92% to 98% or more of synchronous speed depending on the load torque and the flywheel effect of the load. When the motor has reached its synchronizing speed, the discharge resistor is disconnected from the field circuit and the rated excitation is applied to the field winding. This application causes the field poles to become electromagnets of alternate north and south polarity. The poles try to lock in­ to step with the respective alternate south and north poles of the primary rotating magnetic field. If the torque produced by the attraction between rotor poles and stator poles of op­ posite polarity is great enough to cause the rotor to accelerate from its subsynchronous speed to synchronous speed, then the rotor poles lock into step with the stator poles and the machine has been synchronized. If the machine does not synchronize, it will develop less torque with the field excited than it did with the field short-circuited and will lose speed; field excitation must then be quickly removed to prevent pos­ sible damage to the motor because of the large torque pulsa­ tion resulting when a synchronous motor is operated out of syn­ chronism with d-c field excitation on the field winding. Auto­ matic control can be arranged to protect the motor in the event of such out-of-step operation.

59· Pull-In Torque During synchronization two factors retard the acceleration of the rotor to synchronous speed; they are the 1) load torque and 2) the rotational inertia (flywheel effect) of the load. The synchronizing ability of a motor is defined by the term pull-in torque, which is the maximum amount of·load torque that can be synchronized with a specified amount of load inertia. It is sometimes called actual pull-in torque to distinguish it from what is called the nominal pull-in torque. The nominal pull-in torque is the induction motor torque at 95% speed. Nominal pull-in torque is sometimes used to compare the torque level of several motors, but since no account is taken of load inertia,

63 it does not indicate, and should not be used to specify, the synchronizing ability of a motor.

7 O • Application of Field Current

At synchronizing speed the rotor poles are traveling slower than the stator poles by the amount of slip. When field current is applied, it creates alternate north and south poles on the rotor; if the current is applied at the instant that the north rotor poles are directly opposite the south stator poles, nearly maximum synchronizing torque will be developed because un­ like poles attract. If the field current is applied when the rotor south poles are opposite the south stator poles, less synchroniz­ ing torque will be developed because like poles repel. Con­ sequently, there is a best and a worst time for applying ex­ citation with respect to the relative positions of the stator and rotor poles. Some automatic field application control systems are designed to apply field at the proper time to obtain maxi­ mum synchronizing torque.

71· Reluctance Torque and Slipping a Pole

Recall from previous studies that it is much easier for mag­ netic flux to pass through iron than through air. Also, mag­ netic members tend to move in such a way that the maximum amount of flux is permitted to flow. For those reasons, when a salient-pole motor ( one with poles projecting toward the arma­ ture) is operating at very nearly synchronous speed with no d-c excitation, there is more of a tendency for the stator pole to line up with the center of a rotor pole than to line up with the air space between poles. That tendency develops a torque called reluctance torque. The condition cannot occur on cylin­ drical rotor machines, which are machines with poles that do not project toward the armature. A salient-pole motor operating with very little load may have enough reluctance torque to synchronize without appli­ cation of d-c excitation. However, the reluctance torque developed is small, and it is generally of no practical impor­ tance in large-power motors. The principle, however, is used for small synchronous motors, which are discussed in a later article. When a motor pulls into step by reluctance torque, the rotor poles are neither north nor south poles. When excitation is applied, the polarity of the field poles may be such that the

64 rotor poles are in line with stator poles of the same polarity. U that is the case and the field current is increased, the rotor will fall back one pole so that the rotor poles are in line with stator poles of opposite polarity. The action is called slipping a pole.

Check Your Learning

l. When d-c excitation is applied to the electromagnets in the field winding of a synchronous motor, what effect is produced? 2. Briefly, what is meant by the "best position" for applying d-c excitation'?

SYNCHRONOUS CHARACTERISTICS

7 2., Load Angle Effect

When a synchronous motor is operating in synchronism at no load with rated field current, the axis, or magnetic centerline, of the stator pole and the axis of the rotor pole are in line. H an observer were able to ride on a rotor pole face to see what happens as load torque is applied to the shaft, he would see that the axis of the rotor pole falls behind the axis of the stator pole by a small angle. As the load increases, this angle, which is called the load angle, also increases. If the flux lines con­ necting the stator pole and the rotor pole could be seen, they would appear very much like rubber hands or steel springs that stretched as the load angle increased. If the load angle in­ creases to 60 or 70 deg ( electrical degrees; there are 180 electrical degrees between the axis, or centerline, of any two adjacent poles), the lines of flux will be "broken," the maximum synchronous torque will have been exceeded, and the motor will pull out of synchronism. The load angle of average synchronous motors at rated load, voltage, and excitation usually varies between 18 and 24 deg for motors rated with an 0.8 pf and between 30 and 45 deg for motors rated with unity power factor.

73"' Pull-Out Torque

Once a motor has been synchronized, it will continue to run in synchronism if its maximum synchronous torque is not ex­ ceeded. The maximum synchronous torque Ts, also called

65 maximum load, that can be developed can be expressed for practical purposes by the formula V 11 Ts=uX-lXTPo I' r 'fr in which Ts= maximum synchronous torque (maximum load), in percent V, = rated armature terminal voltage, in volts V = actual armature terminal voltage, in volts /1, = rated field current, in amperes Tpo = rated pull-out torque, in percent

The rated pull-out torque of industrial synchronous motors is the maximum load torque, or horsepower (since the speed is constant), that the motor will hold in synchronism for l min with rated armature voltage and rated field current ap­ plied to the windings. It is usually expressed as a percent of the rated torque or horsepower of the motor. The pull-out torque of industrial synchronous motors is not less than 150% for motors rated at unity power factor and not less than 200% for motors rated at 0.8 pf. Higher pull-out torques are possible by special design.

Example Problem

Problem. If a unity-power-factor synchronous motor rated at 2300 V has a rated field current of 30 A and a pull-out torque of 150%, determine a) the maximum load the motor can carry for l min if the line voltage drops to 1800 V with the rated field current maintained, b) the maximum load if the field current is reduced to 23 A with the line voltage at 2300 V, and c) the maximum load if both field current and line voltage reductions occur simultaneously. Solution. a) By applying the formula given in Art. 73, the maximum load Ts is determined as

V iJ Ts=-v, X,X TPo r lfr = 1800 X 30 X l50o/c 2300 30 ° = 0.783 X l X 150% = 117% Ans.

b) By applying the same formula as in a), the maximum load Ts is 2300 23 Ts= 2300 x 30x 150% = l X 0.767 X 150 = 115% Ans.

66 c) By applying the same formula as in a and b, the maximum load Ts is _ 1800 23 Ts - 2300 X 30 X 150% = 0.783 X 0.767 X 150 = 90% Ans.

The example problem shows that a pull-out torque of 150% permits an appreciable momentary drop in armature voltage or field current without loss of synchronism. However, if both the volt­ age and current decreases should occur simultaneously, as in (c), the motor would not be able to carry rated load and would con­ sequently fall out of step.

74· Open-Circuit and Short-Circuit Saturation

If a synchronous motor is driven as a generator at rated speed with the armature winding open-circuited, the generated armature voltage, or line voltage, will depend upon the field current value. A graph of such data produces what is called an open-circuit saturation curve as shown in Fig. 39. On most machines normal voltage occurs near the knee of this satura­ tion curve. The knee of the curve may be considered as the region where the curve begins to bend appreciably, as it does at about 7500 V in Fig. 39. The straight line is called the air­ gap line. The difference between the air-gap line and the

10,000 w C, ; 8000 0 > w w a: a: :::l Cl) 6000 ::) Cl) I- I- < ...I ~~ ::E 0 ::E w a: > a: a. ~ ~ 4000 20oi ~ ::) sz (.) a: ~i-: 9 100 U ~ z ._!. a: Fig. 39. Driving a synchronous w a: a: a. 0 ::) motor as a generator with 0 :c (.) terminals first open-circuited 0 Cl) and then short-circuited produces 0 20 40 60 the open- and short-circuit FIELD CURRENT, IN AMPERES saturation curves.

67 open-circuit saturation curve at a given voltage is the amount of saturation. If the synchronous motor is driven at rated speed with the armature winding short-circuited at the terminals, the short­ circuit armature current, also called line current, will depend on the value of the field current. A graph of such data pro­ duces a short-circuit saturation curve (a straight line) as shown in Fig. 39.

7 5 • Excitation Characteristics

Synchronous motors differ from practically all other types of a-c machinery because magnetizing current (direct current in the field windings) is supplied from a separate source rather than from the a-c power lines. For example, trans­ formers and induction motors take from the line magnetizing current in addition to energy current, the current required to drive a load. The magnetizing current produces no power; it lags the energy current by a 90-deg phase angle, and so it is called the lagging reactive current. It magnetizes the iron and produces the necessary magnetic flux for operation. For a given line voltage and load on a-c motors, transformers, and other a-c power-consuming equipment, the magnetizing cur­ rent is a fixed amount and cannot be changed. In a synchronous motor the magnetization (magnetic flux) is supplied by the field winding and the magnetizing cur­ rent can be changed by simply changing the field current. The effect is shown in Fig. 40 by the characteristic curves (often called V curves because of their shape) of a typical 0.8- pf synchronous motor at rated voltage. The three broken-line curves, called compounding curves, represent 0.8-pf load, l .0 (unity) pf, and 0.8-pf lag. The three V curves show the varia­ tion in armature current for different values of field current at constant loads: no load, 50% load, and l 00% load. Each compounding curve is for a constant power factor; it shows what field current must be used to obtain a given power factor for a given armature current. For example, sup­ pose that it is desired to operate an 0.8-pf-rated motor at 100% rated load and unity power factor. What field current will be necessary for such an operation, and what will be the resulting armature current? The intersection of the l .0-pf compounding curve and the I 00% load V curve in Fig. 40 shows that a field current of approximately 70% of rated value is needed and that 80% of rated armature current will flow. Observe that, in Fig. 40, the LO-pf curve intersects all V curves at their lowest points. Thus for a given load, adjust-

68 Fig. 40. The characteristic and compounding curves showthat l- the power factor for a certain m100 armature current can be varied a: a: by adjusting the field current. :::, (.) a:w 80 :::, ~ ~ 60 < 0 w a:~ 40 LL 0 1- zw 20 (.) a: w a. 0 20 40 60 80 100 PERCENT OF RATED FIELD CURRENT ment of the field current to give minimum armature current corresponds to operation of the synchronous motor at unity power factor. The minimum armature current on the no-load V curve is the small amount required to supply the losses in the motor.

Example Problem Problem. If a 0.8-pf-rated synchronous motor of the type shown in Fig. 40 is operated at 100% rated load and a 0.8-pf-lagging condition, what is the approximate percent of rated field current? Solution. Referring to Fig. 40, the 100% load condition V curve intersects with the 0.8-pf-lag compounding curve at approximately 46 %. Ans.

7 6 • Power-Factor Operation

If the field current in a synchronous motor is greater than that required to produce unity-power-factor operation for a given load, the motor is said to be overexcited at that load condi­ tion, and it will operate at a leading. power factor. Also, it will furnish excitation current to the a-c supply lines. On the other hand, if the field current is less than that required to

69 give unity-power-factor operation for a given load, the motor is said to be underexcited at that load condition, and it will operate at a lagging power factor. Also, it will draw excitation current from the a-c supply lines. From the preceding discussion, it can be said that, simply by varying the field current, a synchronous motor can be made to operate between a leading power factor and a lagging power factor. Synchronous motors that are rated at unity power factor have just enough field current supplied to the field windings to develop the required magnetization (mag­ netic flux) and to offset the demagnetizing effect of the arma­ ture current. However, if the synchronous motor is rated at 0.8 pf, additional armature and field winding capacity is pro­ vided in the design so that a field current can be supplied not only to offset the demagnetizing effect of the armature cur­ rent but also to furnish excess magnetization to the stator. This excess magnetization is fed back into the a-c supply line to furnish excitation to other electrical machinery. Both unity-power-factor and 0.~-pf synchronous motors, when operated with rated field current, become overexcited at partial load conditions and thus supply leading kVA to the line. This capability of a synchronous motor supplying kVA to an electrical system, called power-factor correction, will be discussed in further detail in the following articles.

7 7 • Power- Factor Correction Recall from previous studies that the power factor in an elec­ trical system is the ratio between true power and apparent power. The ratio is often illustrated by the right triangle in Fig. 41, which shows the components apparent power, true power, and reactive power. The power factor angle 8 is the phase difference between the current wave and the voltage wave of a circuit. From the triangle it can be seen that the power factor is the cosine of the phase angle, which is the angle between apparent power and true power. An electrical utility supplies apparent power, and thus the generators, transmission lines, and various equipment

REACTIVE POWER, Fig. 41. The power triangle IN KVAR shows graphically the relation between apparent power, true power, and reactive power. TRUE POWER, IN KW

70 must be able to carry the apparent power demanded. How­ ever, in a circuit, resistance loads consume true power and inductive loads consume reactive power. Since the common watthour meter registers only true power, the ratio between true power and apparent power must be kept as high as pos­ sible. Therefore, the reactive power must be kept at a minimum in order to maintain an economical electrical system. Inductive loads develop a lagging power factor in an electrical system. Therefore, in such a system the addition of unity-power-factor synchronous motors raises the power factor of the system because the energy current (apparent power) is measured without an increase in the reactive current. The addition of motors of leading power factor increases the sys­ tem power factor even more, since the use decreases the lag­ ging reactive current that must be supplied by the system. To illustrate the improvement of the power factor by add­ ing a synchronous motor, let us consider the following ex­ ample problem.

Example Problem

Problem. An industrial plant has a total load of 2000 kW at 0.8-pf lag. Expansion plans call for the addition of a 1250-hp motor. What will the resulting plant load and power factor be if the new motor is a) a synchronous motor rated at unity power factor, b) a synchronous motor rated at 0.8-pf lead, and c) an induction-type motor rated at 0.8-pf lag? d) Which of the three motors will pro­ vide the best improvement of the power factor? Both synchronous motors and the induction motor operate at a 93.5% efficiency. Solution. a) Before the addition of the 1250-hp motor, the initial power-factor triangle components are determined as power factor (cos 0) = 0.8 By applying the rule of squares, from trigonometry, sine of 0 is

sin 0 = .J l - pr2

= .JI - 0.82 = VI - 0.64

= y'o.36 = 0.6 Next the apparent power, kV A, is determined, according to Art. 11, as

kVA= kW pf = =20;;..:0c..c.0-"'k'-'-W-'-- 0.8 = 2500 kVA

71 1000 KW

1500 KVAR

2000 KW 2000KW (a) initial load conditions (b) Addition of 1.0-pf synchronous motor

750 IKVAR 1000---, KW '-- f 750 '--, IKVAR 1500 KVAR

2000KW

( c) Addition of 0.8-pf synchronous motor (d) Addition of 0.8-pf (lag) induction motor

Fig. 42. The initial load and Then the reactive power, kV AR, is determined as power factor of an electrical system and the improvement in kV AR= total kV AX sin 6 power factor can easily be = 2500 X 0.6 displayed by power triangles. = 1500 kV AR, lagging If the given and determined initial conditions are plotted, the re­ sulting triangle is as shown in Fig. 42(a). To add the 1250-hp unity-pf synchronous motor to the system, the motor pf, sin 0, kV A, kW, and kV AR are determined as follows: power factor ( cos 0) = 1 sin 0 = VI - pf2 = 0 Motor kV A, according to Art. 13, is

kVA == 0.746 X 1250 1.0 X 0.935 = 932.5 0.935 = 997.3, or 1000 kV A Motor kW= motor kV AX cos e =lO00Xl = 1000 kW Motor kV AR= motor kV AX sin (J = lOO0X0 = 0 kVAR

72 After addition of the motor, the result- ant values are Total kW initial kW + motor kW = 2000 + IOOO = 3000 kW Ans. Total kV AR::.::: initial kV AR+ motor kVAR = 1500 + 0 = 1500 kVAR, Ans. Total kV A "'" V kW" + kV AR2 "'' v 30002 + I 5002 = y9,000,000 + 2,250,000

=--0 v' I l ,250,000 = 3354 k "\/ A Ans. - total kW Power factor= ·total kV A

= 0.894, or 89.4% Ans. The effect of the addition of the unity-power-factor motor to the initial conditions is shown in Fig. 42(h ). Observe that the load increases to 3354 kV A and 3000 kW and the power factor improves from the initial 0.8 to 0.894. Expressed as a percent, the new pf is 89.4%. h) For a I 0.8-pf synchronous motor, the motor pf, sin 8, kV A, kW, and kV AR values are Power factor (cos 8) = 0.8 sin 0 = J l - pf1

I ~ = v l - 0.8" = VI - 0.64 = y0.36 = 0.6 Motor kV A, according to Art. 13, is

kV A= 0.746 X 1250 ,cc:J..32.,.,5-_ 0.8 X 0.935 0.748 = 1246.6, or 1250 kV A Motor kW"" motor kV AX cos 8 = 1250 X 0.8 lOOO kW Motor kV AR= motor kV AX sine = 1250 X 0.6 = 750 kV AR lead

After addition of the 1250-hp 0.8-pf motor to the plant load, the resultant values are Total kW= initial kW+ motor kW = 2000 + 1000 = 3000 kW Ans.

73 Total kVAR = initial kVAR + motor kVAR = 1500 Jag+ 750 lead = 750 kV AR lag Ans. Total kVA = JkW2 + kVAR2 = J30002 + 750 2 = J9,000,000 + 562,500 = v9,562,5oo = 3092 kVA Power factor= total kW total kVA _ 3000 - 3092 = 0.970, or 97% Ans. The effect of the addition of the 0.8-pf motor to the initial condi­ tions is shown in Fig. 42(c). Observe that the plant load increases to 3092 kV A and 3000 kW and the power factor is improved from the initial 0.8 to 0.97. c) For adding the 1250-hp 0.8-pf-lag induction motor, the motor pf, sin 8, kV A, kW, and kV AR values are Power factor (cos 8) = 0.8 sin 8 = VI - pf = JI -0.82 = JI -0.64= \f'o.36=0.6 Motor kV A, according to Art. 13, is

kV A= 0.746 X 1250 0.8 X 0.935 _ 932.5 - 0.748 = 1246.6, or 1250 kVA Motor kW= motor kVA X cos 8 = 1250 X 0.8 = 1000 kW Motor kV AR = motor kV A X sin 8 = 1250 X 0.6 = 750 kV AR lag

After addition of the 1250-hp induction motor to the plant load, the resultant values are Total kW= initial kW+ motor kW = 2000 + 1000 = 3000 kW Ans. Total kVAR = initial kV AR+ motor kVAR = 1500 + 750 = 2250 kV AR Ans. Total kV A= -VkW2 + kVAR2 = J 30002 + 22502

74 - \/9,000,000 + 5,062,500 == y' 14,062~500 - 3750 kV A Ans. total kW Power factor == total kV A = 3000 3750 =-0 0.8, or 80% Ans. The effect of the addition of the 0.8-pf induction motor to the initial conditions is shown in Fig. 42(d). Observe that the plant kV A load increases from 2500 kV A to 3750 kV A and the plant kW load in­ creases from 2000 kW to 3000 kW. Note also that the power factor remains the same, or 0.8 pf. Thus the 0.8-pf induction motor has not lowered the power factor of the system. Ans. d) In any application the selection of which type of motor should be used is determined by an evaluation of such factors as initial cost, efficiency, and expected savings due to improved power factor. In this problem the best improvement of power factor for the plant load is made by adding the 0.8-pf synchronous motor. Ans.

Practice Problem

If a 4000-kV A 0.9-pf-lead synchronous motor is applied to a system which has a 2000-kV AR inductive load, determine a) the new kV AR of the system and b) whether the new kV AR results in a leading or a lagging pf. Ans. a) 256 kV AR; b) lagging pf

79· load Angle and Maximum Torque

As previously explained, the angle ·between the axes of stator and rotor poles, called the load angle, increases as load is ap­ plied. When the loa~ angle increases to about 70 deg, the synchronous motor will pull out of step. For a constant load, the load angle increases as the excitation decreases. In the same manner, for a given load, the load angle is increased if the line voltage is decreased. As a result, the maximum syn­ chronous torque decreases with a decrease in field current and with a decrease in line voltage.

7 9 • Hunting and Flywheel Effect

If a coil spring that is fastened at one end has a weight at­ tached to the lower end, the spring will deflect a certain amount, depending upon two factors: 1) the stiffness of the spring, that is, the spring constant, and 2) the amount of the weight. In a synchronous motor the load angle corresponds to the deflection of a spring and is a measure of the motor's

75 "spring constant." On the other hand, the flywheel effect of the motor and its load corresponds to the weight attached to the spring. If the weight attached to the spring is pulled down and then released, it will oscillate about its original position and finally come to rest at that original position because of fric­ tion. The frequency of oscillation is determined by the spring constant and the amount of weight. In a similar manner, a sudden change in load on a syn­ chronous motor will cause the revolving rotor to swing ahead and behind its steady load angle. The swinging is called hunting, and the rate of swinging is called the natural fre­ quency of the rotor. The swinging of the rotor induces a current in the amor­ tisseur (starting) winding because flux is moving back and forth across the pole face. The current causes a loss in the winding which, if the change in load does not occur again, will stop the hunting in the same way that the friction losses in the spring stop the weight from oscillating. If the weight, when oscillating, is pulled downward each time it starts to move downward, the amplitude of oscillation will increase and may become great enough to break the spring. A similar condition may .exist in a motor driving a reciprocat­ ing load such as a plung~r pump or a compressor. Such equip­ ment has a variable load; for example, the load of a single­ cylinder equipment varies once each revolution if single-acting and twice each revolution if double-acting. If the rate of load change is too close to the natural frequency of the rotor, ex­ cessive swinging or hunting will result. When the rotor of a synchronous motor swings in the hunting manner described, each swing produces a variation in line current, or a current pulsation, approximately propor­ tional to the amount of swing. If the line current changes are excessive, they may be a source of line disturbance that causes lights to flicker and may even cause the motor to drop out of synchronism. The possibility of such a disturbance must be carefully considered when synchronous motors are used for the applications mentioned or for similar applications, and a limit of current swing not to exceed 66% of normal full-load motor current is usually required. Since the natural frequency of the rotor depends on the spring constant of the motor and on the flywheel effect, either or both of these factors may be changed to avoid ex­ cessive hunting and hence excessive current variations. When a relatively small increase in flywheel effect is required, bal­ last rings or flywheels can be bolted to the rotor spider rims. Alternatively, large flywheels can be mounted on the shaft.

76 so· Dynamic Braking Some applications require rapid stopping of the driving motor. For example, rubber mill motors are always provided with dynamic braking to provide rapid stopping if an operator should become caught in the rolls. Although synchronous motors can be stopped quickly by plugging, as described earlier in the text, they can be stopped even more quickly by dynam­ ic braking, and that method is generally used. To dynamically brake a synchronous motor, the armature is switched from the power line and the armature terminals are short-circuited through a three-phase braking resistor. Field current is maintained, and in some cases increased, dur­ ing braking. The machine then acts as a short-circuited gen­ erator whose frequency constantly decreases as the machine slows down. The rotational energy of rotating parts of the motor and load is dissipated in / 2R loss in the braking resis­ tor, which must be designed for such service. There is an optimum resistance value for the braking resistor to achieve the fastest stop. With the optimum value of resistance and normal-load flywheel effect, the number of revolutions required to stop usually varies within plus and minus 0.5% of the rated r / min, depending on the design of the motor.

Check Your learning l. If the field current supplied to a synchronous motor is less than that required to produce l .0-pf operation, what excitation state occurs? 2. What type of power is measured by the common watthour meter? 3. Does a transformer take leading or lagging current from the line? 4. Briefly, why is a synchronous motor used in the electrical system of an industrial plant? 5. Under a certain load condition what basic change is necessary to vary the power factor of a synchronous motor?

SPECIAL TYPES OF SYNCHRONOUS MOTORS

81 • Reluctance Type High-speed motors of -½ to 15 hp that operate on the reluc­ tance torque principle already discussed are available for use in applications requiring constant speed, especially when direct

77 current is not available for excitation. One form of such a motor can be made from an induction motor having a cast rotor if a number of wide axial slots equal to the number of poles are machined to the proper depth. The unmachined por­ tion of the rotor forms the salient poles, and the machine will synchronize and run in synchronism as a reluctance motor. Since such motors have no field windings, they take their excitation from the line and therefore operate at a low, lagging, power factor.

8 2· Synchronous Condensers

Synchronous condensers are synchronous motors used solely to furnish excitation to the supply system; that is, they do not drive loads. Since their speed is not fixed by the requirements of a mechanical load, they are built for the speed which gives lowest cost, lowest loss, and minimum space requirement. They are usually located near large load centers to supply excitation to the a-c line and thereby reduce losses in transmission from the generating station. They are also used to regulate voltage. Synchronous condensers are rated in kVA; when fully excited, they deliver to the system corrective kV A equal to their rating. Since they have no useful power output, losses rather than efficiencies are the measure of performance. In Fig. 43 is shown a typical no-load V curve for a syn­ chronous condenser. The curve is a plot of armature current, on the vertical scale, versus field current, on the horizontal scale. The lowest point on the V curve is the field excitation

UNDER EXCITED OVEREXCITED LAGGING LEADING ~ OPERATION OPERATION ~ 100 ZERO-PF a: OPERATION ~ 80 z .,.: z 60 w a: a: ::, 40 (.) w Fig. 43. The no-load V curve a: 20 of a synchronous motor shows ::, I- that by varying the field current < the motor can be made to ~ 0 produce either a lagging or a a: FIELD CURRENT _...,. leading power factor. <

78 required for unity-power-factor operation. At that point the synchronous condenser is neither giving nor receiving excita­ tion from the d-c supply system and the armature current is at a minimum, just enough to supply the machine losses. A vertical broken line is shown beginning at the lowest point on the V curve. To the right of that broken line the power factor is leading; to the left of that line the power factor is lagging. With an increase in field excitation the synchronous condenser current becomes leading; and when the maximum field current value is reached, the rated armature current value is reached at zero power factor leading. On the other hand, if the field excitation is decreased below the low point of the V curve, the armature current becomes lagging and reaches a value, usually 40% of normal armature current, with zero field excitation. Thus the synchronous condenser can produce from lagging power factor to leading power factor by merely changing the field excitation. Synchronous condensers are normally run overexcited; but with a long, high-voltage, lightly loaded transmission line (which acts as a condenser and takes leading current), the synchronous condenser may be operated underexcited to take excitation from the system and hold the voltage down. The air friction or windage loss of synchronous conden­ sers is relatively high because of the high speed at which condensers operate. In order to reduce the loss, large syn­ chronous condensers are enclosed in a gastight shell or tank filled with hydrogen. Hydrogen is used because it is the light­ est gas and causes only approximately one-tenth of the windage loss in air. It is also relatively inexpensive. Because hydrogen mixed with air forms an explosive mixture, it is kept in the shell above atmospheric pressure to prevent air from entering the shell. Not only are losses avoided by the use of the gastight tank; the tank is also watertight and can be installed outdoors and thereby reduce building costs. Hydrogen also has the advantages of better heat removal from the machine, re­ duction of fire risk, less windage noise, and longer life of the insulation. The heat loss of the machine is removed by water run through surface coolers inside the machine. Machines of this type have been built up to 60,000 kVA.

93• Rotating-Armature Motors

Some small, high-speed synchronous motors having a station­ ary field and a rotating armature are built to use d-c machine parts. Such machines, when designed for three-phase power, have three slip rings mounted on the shaft to carry power to

79 the rotating armature. The slip rings must be insulated for the same voltage as the armature winding. The insulation neces­ sary to provide that voltage is one of the limitations on the size and voltage ratings possible with the type of construction used for synchronous motors.

8 4 • Supersynchronous Motors

The supersynchronous motor is so arranged that the stator and the rotor can each rotate on its own set of bearings. The rotor shaft is connected to the load and is stationary during starting; the stator, however, accelerates and synchronizes when power is applied through slip rings. After synchronizing, the armature (which represents the stator) is braked to a stop by a brake band around it; and as the stator slows down, the rotor speeds up until, when the stator is finally stopped, the rotor is turning at synchronous speed. This design feature allows the use of a low-torque, low-starting-current motor, because the heavy load is started with the rotor and the stator in synchronism and full synchronous torque can be developed. The supersynchronous type of motor is presently seldom built because of its high cost and also because of improve­ ments in the starting characteristics of the conventional motor. A variation of the supersynchronous motor is a conventional motor with an electric dutch. When that motor is synchronized, the clutch is energized and the maximum synchronism torque is available for starting the load.

8 5 • Synchronous Motors in Frequency-Changer Units

Occasionally an interchange of a-c power between supply systems of different frequencies is desired. The most common transfers are between 25- and 60-Hz systems. A frequency changer consists of a motor driven by one system coupled to an alternator supplying power to the other system. A syn­ chronous motor is most often used for the application. For synchronous-motor-driven frequency changers con­ nected to 60- and 25-Hz systems, the highest common speed is 300 r /min. Such a speed means 24 poles for the 60-Hz ma­ chine and 10 poles for the 25-Hz machine.

8 6 · Round-Rotor Machines Practically all two-pole synchronous motors are built with the so-called round rotor or turbine-generator-type rotor. The

80 Fig. 44. The round rotor for a synchronous motor shown without windings is used for most two-pole machines.

motors are constructed like turbine generators except that an amortisseur winding is installed at the top of the field coil slots as well as in slots machined in the pole face. The rotor of such a motor, without windings, is shown in Fig. 44. It is part of a 4500-hp 3600-r/ min two-pole synchronous motor.

8 7 • Synchronous Motor with Phase-Wound Amortisseur Windings

Some synchronous motors having an insulated three-phase amortisseur winding (starting winding) built into the rotor pole faces have been constructed to obtain variable starting char­ acteristics similar to those of the wound-rotor induction motor. Such motors can be built with a rotating field and stationary armature or with a stationary field and a rotating armature.

8 8 • Ship Propulsion Synchronous Motors

Many large synchronous motors have been used for ship drives. The motor is coupled directly to the propeller, and speed is varied by varying the supply frequency. The power source is usually a steam-turbine-driven generator and, by changing the turbine speed, both alternator and motor speeds can be changed.

81 8 9 • Multispeed Synchronous Motors Multispeed synchronous motors with a single stator and single rotor, and with speed combinations similar to those of multi­ speed induction motors, are built for some special applications such as mixers in rubber mills. The stators are similar to those of multispeed induction motors, but the rotors are so arranged that one half of the winding can be reversed with respect to the other half in order to change the polarity in the manner shown in Fig. 28(d) and (e).

Check Your Lsarning

I. What kind of synchronous motor is not used to drive a mechani­ cal load? 2. A reluctance-type motor would be recommended for what kind of application? 3. Why are some synchronous motors built with a phase-wound starting winding?

EXCITERS AND SYNCHRONOUS MOTOR CONTROL

9 O • Exciters

A source of direct current must be available for exciting the field of the usual synchronous motor. On high-speed syn­ chronous machines, motors, converters, and generators, a direct-connected exciter is often mounted on the shaft of the machine to furnish d-c excitation. On low-speed machines, excitation is commonly supplied by a high-speed belt-driven exciter from the synchronous motor shaft or by a separate high-speed, motor-driven exciter.

91 · Synchronous Motor Control

A synchronous motor controller must control both the a-c and d-c power to the motor during its starting, synchronizing (pull­ ing into step), and synchronous operation in such a manner as to obtain the best performance of the motor and at the same time protect the motor against damage. The requirements of large-power synchronous motor control and the usual methods of performing the necessary functions are briefly described as follows: 1. Provide for starting and stopping. This requirement is

82 usually met by a push-button control scheme which energizes a closing coil on the line contactor or on the circuit breaker. 2. Provide for short circuits. The line contactor or circuit breaker should be designed to interrupt the currents that can flow during short-circuit conditions. 3. Provide for overload protection. The primary line (or the secondary of a current transformer connected in the pri­ mary line) is connected in series with a thermal-type or induc­ tion-type relay, which is connected to take the motor off the line when the relay operates because of a continuous over­ load on the motor. 4. Provide for protection against pull-out. If a motor is suddenly loaded to the point of pull-out, the overload relay will not operate quickly enough to protect the motor; there­ fore, another type of relay is used to perform that function. The additional relay operates on the high current and lagging power factor which exist at the point of pull-out. If the motor has sufficient torque to resynchronize at full load, the pull-out relay can be so connected that it removes field excitation only and thereby makes possible immediate resynchronizing of the motor with the a-c supply if the load should permit. If the motor is not designed to synchronize with its full load, the pull-out relay will cause the motor to be disconnected from the line. 5. Provide starting winding protection. If the motor stalls or is operated for too long a time as an induction motor, the amortisseur winding may be damaged. That fact was explained previously in connection with possible squirrel-cage heating under starting conditions. Protection against that possible damage is usually obtained by a thermal-type relay in parallel with an inductance coil, the combination being connected in series with the motor field circuit. At zero speed (rotor with field winding stationary) most of the induced field current passes through the thermal relay because of its low impedance as compared with that of the parallel path through the in­ ductance coil under zero-speed conditions. As the motor speed increases, however, a larger and larger portion of field cur­ rent passes through the inductance coil because its reactance, and therefore the impedance, decreases as the slip frequency decreases. In other words, its impedance decreases as the rotor approaches synchronous speed. The design characteristics of the thermal relay and induc­ tance coil are so proportioned that the heating of the thermal relay corresponds closely to the heating of the amortisseur winding in the pole faces. If the permissible heating is exceed­ ed, the relay causes the motor to be disconnected from the line. 6. Provide for field application and removal. The impor-

83 tant function of the field application relay is to automatically apply excitation to the motor when the motor reaches syn­ chronizing speed. Relays for that function can be classified as angle application, speed application, and time application. They are discussed in the following articles.

9 2 • Angle Application Relay The angle application type of field relay does not operate un­ til the motor has reached synchronizing speed; at that time it applies field excitation depending on the angle between the stator and rotor poles. Some relays of this type avoid applying the field at the worst angle; more sensitive types apply the field at, or very near, the best angle. The more nearly the relay can apply the field at the best angle, the more synchronizing torque the motor will develop and the less disturbance there will be on the a-c supply lines. For those reasons, the angle application type of relay is recommended for field application. It can be operated from the variation in armature current and power factor or, more commonly, from the induced fieid cur­ rent. In Fig. 45 is shown an oscillogram of the line or armature volts, the armature current, and the field current of a 60-Hz synchronous motor taken during synchronizing when an angle application type of relay was used. The lower curve Fig. 45. The oscillogram for shows the best position for field current, which occurred when a synchronous motor shows the induced field current was zero and increasing in the posi­ the best position to apply the tive direction, that is, in the direction in which the d-c field field excitation. / ARMATURE VOL TS, 60 HZ

FIELD CURRENT

BEST POSITION TO APPLY FIELD FIELD CURRENT APPLIED

84 current was to flow subsequently. That point on the curve cor­ responds to the best angle of field application. The worst angle at which the field of the motor could be applied is shown to the left of the best angle point. Initially, when the motor is starting, the frequency of the current induced in the field winding is the same as the supply frequency, 60 Hz, for example. As the rotor comes up to speed, the frequency in the field winding decreases. For the oscillo­ gram shown in Fig. 45, the one cycle of induced field current just prior to the best angle point compares to about 22 cycles of armature current. Thus the slip is 1+ 22 = 0.0454, or about 4½%- g 3 • Speed Application Relay

The speed application type of relay causes the field current or excitation to be applied when the rotor has reached a definite speed for which the relay has been set. Although the speed application relay insures that the motor has reached synchronizing speed, the angle at which field is applied is largely a matter of chance. Such a relay usually operates from the induced field current.

9 4· Time Application Relay

The time application type of relay causes field current to be applied when a definite time has elapsed after starting the motor. It is the least desirable of the three types; it takes no account of possible changes in accelerating time due to changes in load or voltage, and it applies field at no definite angle. It is a timer relay that is energized when the motor is started. g 5 • Brushless Synchronous Motors

As previously discussed, d-c excitation for the field winding of a synchronous motor is supplied by a direct-connected, belt­ driven, or motor-driven exciter. In any case, commutation prob­ lems occur because of the use of brushes. and current-collect­ ing devices: commutators or collector rings. To eliminate commutation problems, a brushless exciter system, or brushless synchronous motor, has been developed. In the brushless synchronous motor there are, of course, no brushes, no collector rings, and no commutator assemblies. The only "connection" between the stator and the rotor is

85 Fig. 46. A brush less synchronous EXCITER MOTOR FIELD motor eliminates the problems CONTROL FIELD CIRCUIT of brushes, slip rings, and commutator assemblies.

o=> "'~{0 en

LINE CONTACTOR RECTIFIERS FIELD DISCHARGE RESISTOR

the magnetic fields between them. In Fig. 46 is shown the basic circuit of a brushless synchronous motor. The com­ ponents mounted in the rotor assembly are shown enclosed by broken lines. In this system, d-c voltage is applied to the sta­ tionary exciter field. The resulting magnetic field induces a three-phase a-c voltage in the rotating exciter armature. Re­ call that a synchronous motor is started like an induction motor by applying three-phase voltage through the line contactor to the motor terminals L1 to L3. During starting, the field dis­ charge resistor is connected across the motor field winding. The a-c exciter output voltage is converted into d-c volt­ age by a group of semiconductor rectifiers, called a bridge rectifier. Also included in the rotor are automatic control circuits. At the correct moment, the discharge resistor is dis­ connected by one control circuit and the d-c excitation is ap­ plied to the rotating motor field by the other circuit to produce synchronous operation.

Check Your learning

1. What three types of relay applications are used for applying field excitation? 2. If a synchronous motor stalls, what type of relay is usually relied on to prevent damage to the amortisseur winding? 3. How is the source of d-c excitation for a high-speed synchronous motor usually furnished?

86 • I I - otor

g 6 • Comparison of Polyphase and Single-Phase Motors

The single-phase induction motor is essentially the same in construction as the polyphase motor. The principal differences are in the method of starting and the manner in which the revolving flux is set up after the motor is running at operating speed. If an induction motor primary is provided with only one winding and is connected with a single-phase circuit when the secondary, or rotor, is at rest, the magnetic poles in the primary core do not rotate but simply increase in strength, decrease, and reverse their direction continually in 11;nison with the single­ phase a-c current. Their location remains constant, and no torque tending to start a squirrel-cage rotor is developed. But if such a rotor is somehow started in either direction, it will accelerate in that direction to a speed very nearly in synchronism with the primary current and will continue to run as a single­ phase induction motor.

87 The simplest explanation of those facts is that the result­ ant effects of the primary current and the induced current in the moving secondary establish a rotating magnetic fidd. At standstill the magnetic field is stationary; as the speed increo.scs, the Hux rotation becomes more nearly like that of the poly­ phase motor until, at operating speed, the flux rotation of the single-phase motor is practically identical with that of the polyphase motor. The general principle of starting single-phase motors is to use two or three windings and to supply those windings with displaced, or out-of-phase, voltages. The result is a rotat­ ing magnetic field similar to that of a polyphase motor. The displaced voltages are obtained by the use of either resistance and inductance or resistance and capacitance in the windings.

g 7 • Application of Single- Phase Motors Unlike polyphase motors, which are constructed in any size, single-phase motors are built almost exclusively in small sizes commonly called fractional-horsepower ratings. ff poly­ phase current is available, polyphase motors are preferred and used with few exceptions. When large motors are involved, it is always economical to provide polyphase current. For that reason, although single-phase motors larger than 10 hp are occasionally built, very few such motors larger than 3 hp are manufactured in quantity. Approximately 95% of aH single­ phase motors are rated less than l hp and are intended prin·· cipally for use on residential circuits; they arc known as frac­ tional-horsepower motors.

9 8 • Special Requirements of Single-Phase Motors The use of single-phase motors on residential circuits requires that such motors be not only efficient and reliable but also that they be quiet in operation and free from radio interference. The last two requirements are largely responsible for the development of single-phase induction motors of the split-phase and capacitor types. The superiority of these induction motors has led to their universal use on such appliances as washing machines, refrigerators, and oil burners and for other residen­ tial applications.

g g • Classifications of Single-Phase Motors

Single-phase motors may be divided broadly into three groups: induction motors, commutator motors, and combina-

88 tion commutator-induction motors. Each group is built in more than one fo1m, as follows: Induction motors include shaded--polc, permanent split-ca­ pacitor, split-phase, capacitor-start, and two-value capaci­ tor motors. Cornmutator motors include series and repulsion motors. Commutator-induction motors include repulsion-start in­ duction and repulsion-induction motors. In addition to these broad classifications, there are other special types of singie-phase motors, including, for example, the so-called hysteresis motor. That motor runs at its true synchronous speed and is therefore used for such applica­ tions as electric clocks, phonograph turntables, and timers. Single-phase fractional-horsL'povver motors are discussed in greater detail in othl'r tt>xts.

Check Your Learning

l. What type of single-phase motor is usually used in a timer device? 2. What basic scheme is used to start single-phase motors?

89

Appendix1

SOLUTIONS TO PRACTICE PROBLEMS

Article 12

. . . 0. 746 X horsepower output Efficiency, m percent = k . tt . t X 100 11 owa mpu = 0.746 X 12 X lOO = 8.95 X lOO 10 10 = 89.5% Ans. Art. 12, formula 1

Article 14

5250 X hp output Torque T= ----=-----='--­ r/min

52 ~~~ 20 = 60 ft-lb Ans. Art. 14, formula I

Article 33

Rotor frequency= 25 Hz X 4% = 25X0.04= l.0Hz Ans. Art. 33

91 Article 47 EM EL= % 3 345 Art.47 = _:= 0.6 =575V Ans. 100

Article 66

RDR=-Vr 11 2400 X 0.5 1200 = 40 -:ro-= 30 ohms Ans. Art.65

Article 77

a) kW= kVA X pf = 4000 X 0.9 = 3600 kW kVAR = JkVA2 - kW2 = J 40002 - 36002 = J16,000,000-12,960,000 = J3,040,000 = 1,743.56, orl744 kV AR Thus 2000 kV AR - 1744 kV AR = 256 kV AR. Ans. b) Since the initial pf was lagging and the synchronous motor supplied only a leading 1744 kV AR, the resultant is still lagging. Ans.

92 11111 I

ANSWERS TO CHECK YOUR LEARNING

Article 25

1. The rotor is similar to the secondary of a transformer. Art. 2 2. Two of the three leads must be interchanged. Art. 8 3. Magnetic attraction caused by induced currents in the rotor bars results in the rotor being drawn around by the rotating field. Art. 6 4. At synchronous speed the slip would be zero percent. Art. lO. 5. Class A, l05°C; Class B, l30°C; Class F, 155°C; Class H, 180°C Art. 20 6. Torque; it is measured in foot-pounds. Art. 14 7. Direct current is applied to the field coils. Art. 3 8. The thumb indicates the direction of conventional current flow in the con­ ductor. Art. 6 9. Power factor. Art. 11 10. Synchronous motor. Art. 5

Article 31

1. Squirrel-cage rotor and wound rotor. Art. 28. 2. Y-connection. Art. 31 3. A variable external resistance is connected in series with each motor sec­ ondary (rotor) winding. Art. 30 4. To obtain special torque and current characteristics. Art. 29.

93 Article 42

1. The secondary rotates at very nearly the synchronous speed under no-load conditions. Art. 34 2. The speed will decrease enough to produce the required torque. Art, 38 3. Plugging is the operation used to brake the speed of a polyphase motor by interchanging leads while the motor is running. Art. 42 4. The induction motor will pull out of step and stop rotating. Art. 39 5. Large squirrel-cage motors may cause lamps to flicker if connected on the same circuits. Art. 32

Article 49

l. Resistor starting, reactor starting, autotransformer starting. Art. 43 2. Reactor starting. Fig. 25, Art. 48 3. Pump and fan applications. Art. 49 4. The compensator starter, known as an autotransformer starter, reduces the applied voltage to the motor while keeping the line current at a lower value than if the motor were simply connected across the a-c lines. Arts. 43, 46, and 47. 5. Fractional-horsepower motors are simply connected directly across the line. Art. 43

Article 61

I. By decreasing the external resistance. Art. 51 2. Slip ring assembly. Art. 50 and Fig. 26 3. By cutting out the external resistance in smaller steps. Art. 51

Article 63

l. Changing the number of poles. Art. 52 Shifting the brush positions Art. 53 2. T, to 76. Art. 52 3. L,, L2, and L3. Art. 52 4. The speed doubles. Art. 52, Fig. 28 5. The adjusting winding is idle, the secondary winding is short-circuited, and the motor runs as an induction motor at a speed corresponding to the number of poles and the a-c frequency. Art. 53

Article 61

1. Induced currents developed by the rotating field cause the rotor to come up to synchronous speed as an induction motor. Art. 54 2. To reduce magnetic noise by avoiding sudden changes in flux as the pole passes the stator teeth. Art. 60

94 3. Power-factor improvement. Art. 56 Constant-speed applications. Art. 54 4. To accelerate the rotor to synchronous speed and lock it into step with the rotating field. Art. 54 5. Synchronous motor. Art. 56

Article 67

1. Squirrel-cage, amortisseur, pole-face, and damper winding. Art. 63 2. Full voltage starting, reduced voltage starting (autotransformer or reactor method), part-winding starting, incremental starting. Art. 67 3. No, because the starting winding does not carry load current during syn­ chronous operation. Art. 62. 4. A high-resistance brass winding. Fig. 38, Art. 63 5. To short-circuit the field winding and thus prevent the otherwise induced high voltages from damaging the winding insulation. Art. 65

Article 71

I. Alternate north and south poles that lock into step with tlie rotating mag­ netic field are produced. Art. 68 2. When the rotor poles are traveling at almost synchronous speed and the rotor north poles are opposite stator south poles. Art. 70

Article 80

1. The motor will be underexcited, operate at lagging power factor, and draw excitation from the a-c supply lines. Art. 76 2. True power. Art. 77 3. Lagging. Art. 75 4. The synchronous motor develops a leading power factor which counter­ acts the lagging power factor caused by inductive loads. Art. 77 5. The field current must be varied. Art. 76

Article 89

1. Synchronous condenser. Art. 82 2. Constant speed at a low, lagging power factor. Art. 81 3. To produce variable starting characteristics. Art. 87

Article 94

1. Angle application, speed application, and time application. Arts. 91 to 94 2. Thermal relay. Art. 91 3. By a direct-connected exciter. Art. 90

95 Article 98

I. Hysteresis-type motor. Art. 99 2. Use two or three windings and supply those windings with out-of-phase voltages produced by resistance and inductance or resistance and capaci­ tance. Art. 96

96