Industrial Alternating Current Motors

Study Unit Industrial Alternating Current Motors

By Robert L. Cecci Technical Writer This study unit will cover the construction and operation of single- and three-phase or polyphase alternating current (AC) motors. In the first section, you’ll learn about AC motor basics through examples of how coils create magnetic fields w e i v e r P

when they’re supplied with AC electricity. Also, w you’ll e see i how v e r P electromagnetic induction allows the rotor of a motor to have a current flow and, therefore, its own magnetic field. Next, you’ll see how single-phase and split-phase motors operate. This information is then followed by a description of the capacitor motor and a brief presentation of the repulsion- induction motor. The next two sections cover polyphase motors and AC control systems.

When you complete this study unit, you’ll be able to • Explain how AC electricity creates a changing magnetic field in and around a coil • Discuss the principles of electromagnetic induction • Explain why a motor needs a system for starting the rotor and how this is performed with a shaded-pole, split-phase capacitor, and repulsion-induction motor • List the possible problems with single-phase motors and the steps taken to troubleshoot these problems • Identify the components of a polyphase motor and describe its operation • Explain how to troubleshoot polyphase motor systems • Identify the basic motor starter systems used in single- phase and three-phase AC motors

iii BASIC AC MOTOR THEORY 1 Alternating Current and Coils 1

Magnetic Induction s t n e 3 t n o C s t n e t n o C A Basic AC Motor 4 Getting the Motor to Turn 5

SINGLE-PHASE AC MOTORS 7 Uses of Single-phase Motors 7 The Rotor 8 The Shaded-pole Motor 9 The Split-phase Motor 12 The Capacitor Motor 17 The Repulsion-type Motor 19

POLYPHASE MOTORS 21 Polyphase Motor Basics 21 The Delta-connected Motor 22 The Wye-connected Motor 24 Multiple-speed Operation 25 Motor Nameplate Information 29 Polyphase Motor Troubleshooting and Repair 32

AC MOTOR RELAY CONTROLS 36 The Manual Control Circuit 36 The Basic Electric Control Circuit 38 A Reversing Circuit 40 Two-speed Magnetic Starters 41

SELF-CHECK ANSWERS 47

APPENDIX 49

EXAMINATION 53

v Industrial Alternating Current Motors

BASIC AC MOTOR THEORY

Alternating Current and Coils

Alternating current (AC) motors are one of the most popular forms of motors used in industry. Visit any industrial plant and you’ll most likely find AC motors in sizes from a fraction of a horsepower to 1,000 horsepower or more. AC motors are very popular because the normal current supplied to industry is in the form of alternating current. Where direct current (DC) motors require special rectifiers or controllers, AC motors can be connected directly to the AC line power or controlled by simple contactors. Up until a few years ago, when industry needed adjustable or variable-speed motors, a DC motor was most likely used. However, with the use of a special control called an inverter, AC motors can now easily be used for these purposes. When direct current is applied to a coil of wire, it creates a magnetic field around each wire in the coil. The magnetic fields from the coils can run together and get quite strong in magnetic force or flux. A metal core or pole that’s surrounded by the magnetic field will concentrate that field and exhibit a north and south magnetic pole. When alternating current is applied to that same coil of wire, a magnetic field is also formed. This field will be different from the magnetic field that’s created by direct current in that the field will alternate in two ways.

1 Figure 1A displays a single cycle of AC. Note that the voltage and current rise in a positive direction until a positive maximum (A) voltage is reached. The voltage and current then reverse and head towards zero. After crossing zero, the voltage and current head for a negative peak where the voltage and current reverse once again and head towards zero. This positive and negative (B) cycle occurs sixty times per second (60 Hertz or 60 Hz). FIGURE 1—At each peak in the AC cycle, the magnetic flux in the motor’s coil or pole also Figure 1B displays the magnetic flux peaks. density, or the strength of the magnetic force produced by the coil as the applied AC goes through one cycle. The field strength isn’t constant but rather follows the applied voltage and current through the cycle. Figure 2 displays the other property of a coil that’s supplied by AC. In Figure 2A, the AC signal has created a north pole (N) on the left of the core and a south pole (S) on the right of the core. In Figure 2B, the AC signal is now going through its negative transition and the poles have reversed with the south to the left and the north to the right.

FIGURE 2—As this AC cycle changes from a positive half cycle to a negative half cycle, the N S poles will switch from north to south and back (A) to north in a continuous cycle.

S N

(B)

2 Industrial Alternating Current Motors So as you can see, AC voltage and current applied to a coil create a magnetic field with constantly changing properties: 1. The magnetic field strength constantly varies. 2. The magnetic pole polarities constantly alternate from the north to the south pole and back.

Magnetic Induction There’s one other property of AC and coils that should be discussed. That property is magnetic induction. Figure 3 displays two DC-powered circuits. In Figure 3A, the switch has just been closed. The change in voltage from zero to some value of DC voltage creates a magnetic field around the primary coil P. If the primary and secondary coils are very close together, this change of magnetic field from zero to a higher level will magnetically induce a field on the secondary coil S. For a brief instant, the meter would measure a pulse of voltage. After the pulse arrived, there would be no further voltage generated in the secondary coil or viewed at the meter. When the switch is opened as in Figure 3B, the collapsing field will also induce a pulse of voltage in the secondary coil that can be viewed on the meter. Again, after this pulse arrives, there will be no further voltage changes in the secondary.

FIGURE 3—If DC is SWITCH applied to a coil, a mag- CLOSED netic field is created around that coil. The DC field is induced on the P SOURCE S RL secondary coil S when the V switch is closed (A) or opened (B).

V (A) COM

SWITCH OPENED

DC SOURCE P S RL V

V (B) COM

Industrial Alternating Current Motors 3 Figure 4 displays the same circuit as Figure 3, except that the voltage source is an AC source. The primary coil P will, of course, develop a changing magnetic field that follows the two properties listed previously. Its strength and magnetic polarity will change in step with the changing AC cycle. Also, this coil will induce a magnetic field in the secondary coil S. This coil’s magnetic field will follow the same properties of the primary field. At the secondary coil, both a magnetic field and an AC voltage will be created.

FIGURE 4—When AC is supplied to the same circuit as in Figure 3, the AC constantly changing pri- POWER mary field at P is induced P SOURCE S RL on the secondary coil S. V An AC voltage will be measured across RL by the meter. V COM

Most rotors of AC motors will operate on magnetic induction. The magnetic fields created by the field windings or stators will induce a voltage on the conductors of the rotor. These induced voltages will create magnetic fields around the motor conductors that are attracted to or repelled by the stator fields to rotate the rotor.

A Basic AC Motor

It might appear that we have everything needed to build a motor. We have a changing magnetic field in the stator or field windings and an induced current and voltage in the rotor that creates the second magnetic field. There’s one problem, however. Figure 5 displays a simplified view of a motor using two stator poles and a permanent-magnet rotor. The coils are supplied with a source of AC voltage. Assume that on the first AC half-cycle, the left pole near the permanent magnet will become a north pole and the right pole will become a south pole. The permanent magnet will quickly try to turn until it’s horizontally aligned with the field

4 Industrial Alternating Current Motors 1/120 poles. However, of a second later, the STATOR STATOR field poles will switch magnetic polarity and POLE 1 ROTOR POLE 2 attempt to repel the permanent magnet. At N 1/120th of a second later, the magnetic poles again reverse and again attempt to attract S the permanent magnet. What’s happening here is that the perma- AC POWER nent magnet rotor will move a certain small SOURCE amount at 60 cycles per second creating a hum rather than rotating motion. This motor would only work if the rotor was FIGURE 5—This illustration shows a basic spun at high speed and then the AC power two-pole motor with a permanent magnet rotor. source was applied to the motor. Adding two additional stator poles and coils wouldn’t help. The permanent magnet would vibrate or hum between two poles or at some other point in its rotation.

Getting the Motor to Turn

Something must be done to create a true rotating magnetic field within the motor, one that the magnetic field in the rotor will follow. This rotating magnetic field can be set up in several ways. For example, multiple windings sets can be placed in offset positions in the stator. This is done in shaded-pole and split- phase induction motors. A capacitor can also be used to place a set of windings out of phase with the main windings as in a capacitor-start motor. Finally, there’s a motor that starts like a universal motor and runs like an induction motor that’s called a repulsion-induction motor. More will be seen on these types of motors in the next section.

Industrial Alternating Current Motors 5 Self-Check 1

At the end of each section of Industrial Alternating Current Motors, you’ll be asked to pause and check your understanding of what you’ve just read by completing a “Self-Check” exercise. Answering these questions will help you review what you’ve studied so far. Please complete Self-Check 1 now.

1. AC motors are popular in industry because they’re a. easy to fix. b. easily connected to line power. c. quieter than DC motors.

2. When AC is applied to a coil of wire, the magnetic poles will (reverse polarity/get stronger).

3. A coil that is supplied by AC can influence a second nearby coil by means of magnetic (induction/reluctance).

Check your answers with those on page 47.

6 Industrial Alternating Current Motors SINGLE-PHASE AC MOTORS

Uses of Single-phase Motors

A typical industrial plant can use many different types of single-phase AC motors. These motors power small fans, pumps, chillers, compressors, and other equipment. Homes contain many single-phase motors such as in the air conditioner, refrigerator, washer, dryer, and dehumidifier’s compressor. Single-phase AC motors are small motors rated from the fractional horsepower sizes to about 10 horsepower (HP). On single phase AC systems the input to the motor circuit will normally be 120 VAC. This voltage can be supplied by a simple line cord, switch, and fuse, or may be part of a complex computer control system. In either case the supplied voltage is often termed AC High, or Hi, and AC Low, or Lo. The AC High is the line that carriers the voltage and the AC Low side is the ground or neutral potential line. Figure 6 displays these two types of AC power sources.

FIGURE 6—This figure LINE shows a standard 120 VAC supply in (A) and a 120 VAC MOTOR 120 VAC 240 VAC supply in (B).

NEUTRAL (A)

0 A 120 VAC 240 VAC NEUTRAL MOTOR 240 VAC

0 B 120 VAC

(B)

Industrial Alternating Current Motors 7 In Figure 6A, the voltage and current are the common 120 VAC. This is the same AC supply that’s delivered to each convenience outlet. Figure 6B shows 240 VAC single-phase power. This form of energy is developed by using two 120 VAC power sources that are 180 electrical degrees apart. The total peak voltage is then 240 VAC (120 VAC plus 120 VAC). This form of energy shouldn’t be confused with 240 VAC three-phase or polyphase power. Three-phase power has three individual phases or sources of electricity where split-phase power has two. Split-phase power is commonly used to lower the current draw of the motor. For example, in a home workshop, the split-phase-powered motor can be used to power an air compressor or exhaust fan. At 120 VAC, a three-quarter horsepower motor (3/4 HP) can draw as much as 15 amps. At 240 VAC, this same motor will draw only 7.5 amps. This allows a smaller wire size to be used in 240 VAC-powered equipment.

The Rotor

The key component of most AC motors is the rotor. The rotor is made up of laminations that are made of high-quality steel that’s notched at equal distances around its outside diameter. These laminations are riveted together and a steel shaft is pressed through this assembly. This is shown in Figure 7A. Copper or aluminum bars are placed, or cast, into the notches. Two end rings short out these bars at each end of the rotor, as shown in Figure 7B. The end rings often have short tabs that are used to move the air inside the motor to help cool the bars. In some cases, the lamination slots and, therefore the rotor bars, are skewed instead of placed horizontally on the rotor. These skewed rotor bars are used to develop a greater starting torque and less magnetic hum than horizontal bars. If you were to strip a rotor of its rotor bar and end rings, it would look like a kind of pet exercise wheel. This type of rotor has, therefore, been given the name squirrel-cage rotor.

8 Industrial Alternating Current Motors FIGURE 7—The squirrel- cage rotor shown here in ROTOR (A), (B), and (C) is used BARS in almost all types of AC motors.

LAMINATIONS

(A)

TABS

(B) END END RING RING

(C)

This type of rotor acts through magnetic induction, giving the entire motor the name AC induction motor. When this rotor is placed in the magnetic fields produced by the stator, tremendous currents are created in the rotor bars of the motor. These currents create magnetic fields that are concen- trated in the poles of the laminations. These magnetic fields then interact with the changing magnetic fields in the stator to create the rotation of the rotor.

The Shaded-pole Motor

One of the simplest forms of AC motors is the shaded-pole motor. Shaded-pole motors are widely available in sizes from 1/100 to 1/20 HP. The fans on computers, small industrial blow- ers, humidifiers, etc., are all examples of shaded-pole motors.

Industrial Alternating Current Motors 9 The shaded-pole motor, like most AC MAIN WINDINGS motors, uses a small squirrel-cage rotor. (4) The key to the operation of this type of motor is the field or stator windings. Figure 8 displays the windings used in a typical shaded-pole motor. The windings SHADING WINDINGS are basically very similar to those used for (4) a four-pole motor of any type, AC or DC. Notice, however, the notch at one side of each pole. One turn of fairly heavy wire is wound in this notch. On some motors, you’ll see this winding as one turn of a copper or aluminum band. Earlier, the motor with the permanent mag- FIGURE 8—Shaded-pole motors use small net rotor simply vibrated when electricity shading coils in the stator winding of the was supplied to the windings. The shading motor. These shading coils offset the magnetic fields to help start the motor. coils in this example are present in order to shift the magnetic field enough to start the motor. Once the motor is turning at near field speed, the shading coils and their magnetic field aren’t required to continue rotation. When AC is first supplied to the motor, magnetic fields are formed on each of the motor’s four poles. Current is induced into the squirrel-cage rotor bars and into the shading coil on each pole. The induced magnetic field on the shading coil is out of phase with the magnetic fields of the poles. This out- of-phase magnetic field helps pull the rotor in the direction of the shading poles. This action gives the motor sufficient starting torque to start turning the rotor and its load—a fan blade for example. When the motor reaches near full speed, the voltage and current that’s induced into the shading winding becomes very small. As this occurs, the magnetic field that surrounds the shading pole gets weaker and weaker until it becomes very small. This causes the magnetic center of the pole to shift on each of the four poles towards the center of each pole. This is

10 Industrial Alternating Current Motors shown in Figure 9. In Figure 9A, the center CSP of the magnetic field for the pole is shown CP as CP and that of the shading pole as CSP. In Figure 9B, where the shading pole’s magnetic field has dropped out, the center of the physical pole has become the center of the magnetic pole. Often the main field windings of a shaded- (A) pole motor are protected by a thermal fuse. If the stator windings get too hot, this fuse will melt and open the motor circuit. This C fuse is often inside a glass capsule and, P once opened, can’t be repaired, but can be replaced. Usually a shaded-pole motor will fail due to an overload condition. This condition can be caused by dirty or damaged fan blades (B) that cause excessive drag on the motor’s rotor. Also, the bearings, usually an oil- FIGURE 9—As the shaded-pole motor comes up to speed, the magnetic centers change impregnated copper-sleeve bearing, will from their composit in (A) to a single center wear and allow the rotor to strike or rub on each pole as shown in (B). against the stator poles, stalling the rotor. Shaded-pole motors aren’t normally reversible. The motor will start turning and will continue to turn in the direction from the main pole toward the shaded pole. To reverse such a motor, it’s required that you disassemble the motor, press the stator from the case or housing, turn the stator end- for-end, and press the stator back into the motor. A few reversible shaded-pole motors have been manufactured. These motors use a shading pole on each side of the main pole and use an external switch to select which poles are connected and, therefore, which direction the motor will rotate.

Industrial Alternating Current Motors 11 The Split-phase Motor

The limitation of the shaded-pole motor is the amount of horsepower that can be economically obtained from the design. At 1/20 horsepower, it’s more economical to build and use a split-phase motor. The split-phase motor also uses a squirrel-cage rotor. The split-phase motor also uses a wound stator designed with two sets of windings. These are shown in Figure 10. As shown in Figure 10, there are four run windings labeled R and four start windings labeled S. The run windings, like the main windings, will be energized at all times the motor is turning and will maintain the rotation when the motor is at full speed.

FIGURE 10—The stator of RUN WINDINGS (R) a split-phase motor has (4) two sets of windings, the run windings R and the start or auxiliary windings S.

START WINDINGS (S) (4)

STATOR

BASE

12 Industrial Alternating Current Motors The start windings are similar to the shaded-pole windings. However, these windings are made of many turns of lighter- gauge wire than shaded-pole windings. The start windings will be wound across the same stator poles used for the run windings and will be 90 electrical degrees from the run windings. Also, the start windings are electrically disconnected from the power source after the motor has obtained a certain speed. When a split-phase motor starts, the shaft begins to turn and then, when it’s almost up to speed, a definite click can be heard coming from inside the motor. As the motor de- energizes, a second click signals the motor slowing. This clicking noise is caused by a centrifugal switch assembly that connects or disconnects the AC power source to or from the start windings. The centrifugal switch assembly is almost always mounted in the rear section of the motor opposite the output shaft. It consists of two parts, the centrifugal mechanism and the switch assembly. The centrifugal mechanism (Figure 11) is mounted to the rotor shaft. The weights are close to the shaft, pulled together by a soft spring. When the shaft reaches full speed, the weights will move out under centrifugal force. This motion causes a free-moving disc on the rotor’s shaft to move back- wards toward the rear of the motor. This disc, in turn, makes contact with the centrifugal switch assembly. When enough

FIGURE 11—Most split- phase or capacitor-start motors use a centrifugal mechanism on the rear end of the squirrel-cage rotor.

SWITCH MOTOR SIDE SIDE

SPRING(2)

MOVABLE DISC WEIGHT

Industrial Alternating Current Motors 13 pressure is placed on the centrifugal switch, as shown in Figure 12, the contacts of the switch will open, disconnecting the MOTOR start windings from the AC power source. TERMINAL This action occurs at about 70 percent of SWITCH the motor’s full speed. The opening and closing of the switch can be noted by the definite click heard as the motor reaches speed or slows down when de-energized. If the switch were to fail, the motor wouldn’t start, causing instead a loud hum. With safety in mind, you can add power to see if the motor rotates. If it does rotate, the run windings are in good FIGURE 12—A centrifugal switch mechanism is triggered by a free-moving disc on the rotor condition, but the centrifugal switch or the that connects or disconnects the AC power start windings have failed. Often you’ll see source to the start windings. the start windings called the auxiliary windings by some motor manufacturers. Two methods of showing a split-phase motor are given in Figure 13. The centrifugal switch is the device marked C.S. in the illustration. Split-phase motors are easily reversed. Simply open the motor’s junction box and reverse the run or start windings in reference to the other winding. The example in Figure 13 is drawn from looking into the motor’s shaft. The motor in Figure 13A will turn clockwise. The motor in Figure 13B will turn counterclockwise using the same reference point. Some split-phase motors are set up to run on either 120 or 240 VAC. This is performed by having two run windings and a single start winding. If the run windings are connected in series, the motor will operate at the higher voltage. If the run windings are connected in parallel, the motor will operate at the lower voltage. These connections are shown in Figure 14. Follow the connections in the illustration for each connection diagram and see how these series and parallel arrangements of the run windings are formed. Note that the start windings are always connected to a source of 120 VAC. To reverse this

motor, simply switch the locations of T5 and T8 in either connection diagram.

14 Industrial Alternating Current Motors FIGURE 13—Shown here DIRECTION are two methods of denoting a split-phase motor. AC HIGH C.S.

RUN A

START AC LOW B

(A) DIRECTION

RUN AC AC HIGH LOW A C.S. START B

(B)

Split-phase motors normally fail due to either centrifugal switch or start winding failure, bearing failure, and run winding failure. If the motor is cycled ON and OFF at regular intervals, the centrifugal switch, followed by a start winding failure, are the two most common types of failures. Due to the motor’s small size, a centrifugal switch may be replaced but the start winding is rarely rewound. Instead the motor is simply replaced. Centrifugal switch and start run winding failures are usually noted as a motor humming loudly instead of rotating. In some rare cases, the centrifugal switch’s contacts can stick closed due to contact welding. This problem is identified by a motor that quickly heats up, trips the overloads, or blows the fuse shortly after it starts.

Industrial Alternating Current Motors 15 FIGURE 14—The leads of an industrial split-phase motor are terminated T8 with wires labeled with T1 T numbers such as those shown here. Also shown are the connections to T2 operate the motor on T3 120 or 240 VAC.

T T T T T T 2 4 5 1 8 3 T4 T5 T2 T3 T8 T1

AC AC AC AC HIGH LOW HIGH HIGH

LOW VOLTAGE HIGH VOLTAGE (120 VAC) (240 VAC)

Bearing problems are usually noted by excessive shaft rotation resistance or excessive noise. Bearing noise changes audio pitch as the bearing degrades. At first, a faulty bearing will have a high-pitched click or whine. Then the pitch of the sound will lower until the bearing completely decays. Run winding problems can show up as excessive motor heat or as a lack of motor torque. Usually, run winding problems can be found with a meter set to measure resistance, which is then compared to a known good motor. You can also disassemble the motor, supply the run windings with a source of DC current, and use a metal bar or compass near each winding to check magnetic field strength. If a run winding is shorted to the motor’s case, the motor will normally open the fuse or circuit breaker in its power circuit.

16 Industrial Alternating Current Motors The Capacitor Motor

A version of a split-phase motor is the capacitor motor. If you’ve studied motors for any length of time, you’ve probably seen a small motor that has a small half-tubular-shaped metal enclosure on its case. The case houses a capacitor. Capacitors serve many different purposes. C1 Capacitors can store electric energy and AB can pass AC while blocking DC. However, there’s another property of capacitors that R1 R2 can be used to great advantage when start- D ing a motor. (A) Figure 15A displays a simple circuit with an

AC current source, a load resistor R1, and a ES ER2 series circuit of C1 and R2. In Figure 15B, you see two sine waves. The solid sine wave is taken between points A and D or across the load resistor R1. This waveform is in phase with the source. The dashed line in Figure 15B reflects the waveform across points B and D. Here the capacitor has delayed the applied voltage by 90 degrees. (B) Delayed applied voltage can easily create a rotating magnetic field within a split- FIGURE 15—This illustration displays how a capacitor can cause the voltage to lag the cur- phase-style motor. The advantages in using rent in an AC circuit. a capacitor to start a motor are that the motor will draw less current on starting and that the motor will provide more starting torque. Figure 16 displays the connection of a typical capacitor-start motor. Note that this is the same connection system as the split-phase motor, with the exception that there’s a capacitor in series with the centrifugal switch and start or auxiliary winding. As with the split-phase motor, the motor’s direction can be reversed by reversing the start or the running winding’s connections with respect to the opposite winding. Also, the capacitor-start motor comes in dual-voltage models to operate on 120 or 240 VAC. On most capacitor motors, the capacitor is used to start the motor and is then disconnected by means of a centrifugal switch or relay contact. On some small motors (under 1/4 HP),

Industrial Alternating Current Motors 17 the capacitor can remain in the motor’s

CAP. electric circuit. This type of motor is often C.S. AC termed a permanent split-capacitor motor. IN The capacitor itself is normally encased in a RUN START metal or plastic shell and will have either AC IN push-on or screw-type terminals. This is a non-polarized type of capacitor with a value ranging from a few microfarads to about 250 microfarads (mFd, Fd, or F). The rating of capacitance is important in replacing a failed unit, although you can test a FIGURE 16—The internal connections for a capacitor-start motor are shown in this motor’s capacitor by replacing it with a diagram. known good unit of a value close to the original rated value (within 20%). There are two conditions, however, that are very important when working on a motor’s capacitor. 1. A capacitor holds a potentially lethal charge of electricity. This charge should be bled off twice with a jumper wire or screwdriver across the capacitor’s terminals. Normally the start or auxiliary winding bleeds off the capacitor when the motor stops. However, if the centrifugal switch or winding has failed, the capacitor can and will remain charged for a few days. 2. Capacitors have a second rating called working voltage. This working voltage is the AC voltage across the capacitor that’s safe for the capacitor, often a value of 250 WVAC. This means that the maximum voltage that can appear across the capacitor is 250 VAC. The same types of problems found with the split-phase motor are common in the capacitor motor. The centrifugal switch and start winding are the most common causes of motor failure, followed by the bearings and the stator’s run windings. The capacitor can also fail. Capacitors and centrifugal switches can be changed in small motors, but stators are seldom rewound. Two to 10 HP motors will often be rewound for economical reasons.

18 Industrial Alternating Current Motors The Repulsion-type Motor

Repulsion-type motors are a form of motor that’s a cross between a wound-rotor motor and a standard induction motor. The repulsion motor has a stator with the typical single-phase run windings. However, instead of start or auxiliary windings, the rotor contains a wound set of coils and a commutator that’s very similar to those on a DC motor. This type of repulsion motor will typically short-circuit the brushes that ride in the commutator. The magnetic axis created by the rotor’s windings is offset by the position of the brushes, allowing the motor to start and run as a form of induction motor. The speed of the motor can be varied by repositioning the brushes. One different kind of repulsion motor is the repulsion- induction motor. This motor has an armature that looks much like a DC motor’s armature. In addition, another section of the same armature has squirrel-cage rotor bars or windings. This motor will start as a repulsion motor and then, at a specified speed, the brushes are disconnected and the squirrel-cage windings operate as in a typical split-phase motor.

Industrial Alternating Current Motors 19 Self-Check 2

1. The special switch that’s used to start a split-phase motor is the (centrifugal/pressure) switch.

2. The simplest of the small AC motor types uses (shaded/wire wound) coils.

3. The type of motor that has both a wound and a squirrel-cage rotor is the (split-phase/ repulsion) motor.

Check your answers with those on page 47.

20 Industrial Alternating Current Motors POLYPHASE MOTORS

Polyphase Motor Basics

So far, the motors we’ve looked at use single-phase or split- phase AC power. These motors typically range from small fractional HP to about 10 HP. These motors run fans, pumps, air compressors, and other forms of small equipment. Three-phase motors are available in fractional HP to thousands of HP, at voltages from 120 VAC to thousands of volts. Most of the motors seen on the floor of industrial plants will be three-phase motors from 1/2 to 25 HP. Larger motors power exhaust fans, circulating pumps, conveyors, and compressors. These motors can be rated at 1,000 HP or more and operate at a voltage of 4,400 VAC or higher. First, let’s look at the power that’s supplied to a three-phase motor. Figure 17 displays A BC three-phase power. Phase A power is shown as a solid line, phase B is shown as a long dashed line, and phase C is shown as short dashed lines. These phases are separated from each other by 120 degrees, making 120° 120° 120° them an ideal power source for creating the rotating magnetic fields needed inside a motor. Figure 18 displays the arrangement of coils FIGURE 17—Three-phase or polyphase AC in the stator of a four-pole three-phase voltage waveforms are shown here. Notice motor. It might seem that these coils would the phase relationships of the voltage peaks. be connected to a phase and then to a ground to get the rotating magnetic field developed inside the motor. However, these stator windings are connected across the phases in one of two methods. The electrical neutral is not used on a typical three-phase motor, and the ground is used to ground the motor’s metal parts for safety purposes.

Industrial Alternating Current Motors 21 A C B

C B

A A

B A

FIGURE 18—The three-phase coils are normally connected phase-to-phase and not phase-to-neutral or ground.

The Delta-connected Motor

The delta connection is very common in electrical work. Many transformer systems are connected in a delta configuration. The coils of a three-phase motor can also be connected in this configuration. Figure 19 displays a typical delta connection for a three-phase motor. Note how the L1 OR A phases are connected so that each phase is a source of power for the other two phases. The actual wiring for a delta-connected dual-voltage three-phase motor is shown in L OR B 2 Figure 20. In this high-voltage connection,

L3 OR C the coils are connected in series. The winding terminal markings are shown below the internal coil diagram. Note that the three

input AC phases are listed as L1, L2, and L3 instead of A, B, and C. FIGURE 19—A basic symbol for a delta connection is shown here.

22 Industrial Alternating Current Motors L1

T9 T4

T6 T7

T3 T8 T2 L 3 L2 T5

L1 L2 L3

T1 T2 T3

T7 T8 T9

T4 T5 T6

FIGURE 20—A high-voltage, or 440 VAC, delta connection is shown here. The terminal or wire connections are shown below the wiring diagram.

Figure 21 displays the low-voltage connections used for a delta-connected motor. The coil connections look very compli- cated. However, this is simply placing two coils of the motor in parallel across each set of phases. The terminal board or wire connections for this motor are shown below the coil connection. Often these connections are made in the motor’s junction box on terminals with studs, jumpers, and nuts. The motor’s wires can be connected together with wire nuts, eyelet terminals and screws or bolts and nuts, or other suit- able electrical connections. The most common shop floor motors will operate at 220 or 440 VAC. Larger motors will often have a single higher-voltage range.

Industrial Alternating Current Motors 23 L 1 L1

T 1 T1

T6 T7

T4 T9 T T7 6 T9 T4

T8 T5 L3 L L 2 3 L2 T3 T T 2 3 T5 T8 T2

L1 L2 L3

T1 T2 T3

T7 T8 T9

T4 T5 T6

FIGURE 21—Two low-voltage delta, or 220 VAC, connection diagrams and the terminal block or wiring diagrams are given here. The two upper connection diagrams are identical but are drawn differently to help you identify the connections.

The Wye-connected Motor

A very common type of three-phase motor is the wye- connected motor. The basic symbol for such a motor is shown in Figure 22. As with the delta-connected motor, all nine L1 OR A coil leads are brought out to the motor junction box. These leads are labeled T1 through T9. The high-voltage connections for a wye- connected motor are shown in Figure 23. In L2 OR B this illustration, notice that the coils are L3 OR C placed in series. This causes part of the incoming AC voltage to be dropped across each coil.

FIGURE 22—An elementary diagram for a wye- or star-connected motor is given here.

24 Industrial Alternating Current Motors FIGURE 23—This illustra- L1 T1 tion displays how to connect a wye-connection motor for 440 VAC.

T6 T8

L3 T3

T2 L2

L1 L2 L3

T1 T2 T3 HIGH VOLTAGE CONNECTION T4 T5 T6

T7 T8 T9

The low-voltage arrangement for a wye-connected motor is shown in Figure 24. Here, the coil sets are placed in parallel across a lower incoming line voltage. The terminal connections are also shown below the coil sets.

Multiple-speed Operation

Many types of three-phase motors are wound in a special arrangement that allows the motor to operate at two different speeds with the motor performing at constant torque at each speed.

Industrial Alternating Current Motors 25 FIGURE 24—This illustration shows how to L1 T1 connect a wye-connected motor to a source of 220 VAC. T4

T9 T8 T6

T 3 T5 L3

L2 T2

L1 L2 L3

T1 T2 T3 LOW VOLTAGE CONNECTION T7 T8 T9

T4 T5 T6

The AC three-phase or polyphase motor is a synchronous- speed motor. This means that the squirrel-cage-wound rotor will attempt to lock in exactly at a speed that’s a function of line frequency and the number of poles inside the motor. The operating speed of a motor is found by using the following equation:

Where S = speed in rpm F = frequency of the applied AC signal (in Hz) P = number of poles (always an even number) Table 1 displays these values for a 60 Hz input line frequency.

26 Industrial Alternating Current Motors Table 1

Motor Poles Speed RPM

2 3600

4 1800

6 1200

8 900

10 720

12 600

14 514.3

16 450

18 400

20 360

As mentioned, the motor will attempt to lock into these speeds. A tachometer placed on the motor’s shaft would measure these speeds minus a small deviation called slip. For example, a motor may measure 1,782 RPM instead of 1,800 RPM because of slip. Slip will increase as motor load is increased until it reaches a condition where the motor stops turning. At this time, the motor is drawing maximum current and this current value is often identified as locked rotor current. In normal operation, the motor will have a small amount of slip and will be rotating at a value similar to one of those seen in the tables. A two-speed motor must, therefore, change its pole number to change its speed. This motor will, however, be locked into only one voltage for which its internal coils are wound. A typical variable-speed wye-connected motor is shown in Figure 25. The connection diagram displays how the motor should be connected for low- and high-speed operations. For low speed, the three input phases L1, L2, and L3 connect to

T1, T2, and T3 respectively. Terminals T4, T5, and T6 are left separated from each other and insulated from the incoming power lines.

Industrial Alternating Current Motors 27 FIGURE 25—This type of motor offers two speeds T4 when it’s connected as illustrated in the lower chart.

T1 T3

T5 T6

SPEED L1 L2 L3 OPEN CONNECT

LOW T1 T2 T3 T4 , T5 , T6 HIGH T T T 6 4 5 T1 , T2 , T3

For high speed, T1, T2, and T3 are shorted together. Now L1,

L2, and L3 are connected to T4, T5, and T6. If you were to draw this diagram as a grouping of internal motor coils at low speed, you would have a delta. At high speed, the coils would be connected as a parallel wye or double-wye connection. These two arrangements will give a constant torque two-speed motor. A second type of two-speed motor uses the opposite arrangement of a two-parallel wye connection for low speed and a series delta for high speed. Such a motor is shown in Figure 26. This type of arrangement will offer a constant horsepower (HP) at either the lower or higher speeds.

28 Industrial Alternating Current Motors FIGURE 26—This motor T 4 also offers two speeds when connected as shown in the chart.

T3 T1

T5 T6

SPEED L1 L2 L3 OPEN CONNECT

LOW T1 T2 T3 T4 , T5 , T6

HIGH T6 T4 T5 T1 , T2 , T3

Actually, there are various types of multispeed motors, from wye- to delta-connected, to one-winding two-speed through two-winding three-speed motors. The label on the motor’s case will identify the type of motor and display its connection diagram for various speeds. AC single-phase motors are typically controlled with a rather simple switch assembly. Three-phase or polyphase motors use more complex relays called motor starters or contactors. Multispeed motors will also often use motor starters or contactors to control the motor’s speeds. More will be seen on these devices in the next section.

Motor Nameplate Information

A nameplate is attached to every motor to identify the motor and the motor manufacturer. A typical motor nameplate is shown in Figure 27.

Industrial Alternating Current Motors 29 This nameplate is useful in identifying the motor for replacement purposes. Look at this label closely. THREE = PHASE INDUCTION MOTOR The frame type is listed in the upper left FRAME 215 MODEL 1718 - 1A5 corner. This number or number/letter HP 5.0 RPM 1750 combination describes the physical PHASE 3 CYCLES 60 characteristics of the motor, such as the VOLTS 220 A 12.2 RATING CONT. dimensions of the mounting holes, height VOLTS 440 A 6.1 of the shaft above the baseplate, and so INSULATION CODE H forth. A list of frame styles is given in the 55C SERVICE FACTOR 1.15 Appendix section of this study unit.

L1 L2 L3 L1 L2 L3 The model number is the identification number given the motor by the manufac- HIGH LOW turer. This number is helpful when VOLTAGE replacing the motor if an exact replacement is desired. FIGURE 27—A nameplate for a three-phase AC motor is shown here. The next two columns define the motor’s HP and RPM with the next lower two values signifying that this is a three-phase motor that operates at 60 Hz. The next line identifies the voltages and the currents at the applied voltages. This motor operates at 220 VAC at 12.2 amps or 440 VAC at 6.1 amps. Notice how this motor draws one-half of the current at the higher voltage than it does at the lower voltage. The rating CONT. stands for continuous. This means that at the rated voltage and frequency, the motor will develop and maintain at least the horsepower listed on the nameplate. You may also see the terms inching, plugging, or jog duty for those motors that operate for only short periods of time. Celsius rise and insulation code ratings go hand-in-hand. The celsius rise rating denotes the motor’s temperature rise above ambient (surrounding) temperature. The insulation code ratings reveal the temperature rating of the insulation used on the windings of the motor. Table 2 displays a list of the more common temperature ratings.

30 Industrial Alternating Current Motors Table 2

Class Temperature C

A 105°

B 130°

F 155°

H 180°

These values listed are the maximum values of temperatures that can appear on the windings before the insulation will begin to break down and fail. Returning to Figure 27, the service factor of the motor serves as a numerical multiplier. The rated horsepower of the motor can be multiplied by the service factor number to give the true maximum horsepower of the motor for the rated voltage and frequency. Another group of numbers commonly found on the nameplate signifies the bearings used in the motor. For example, Front Bearing 62062RS means that the bearing at the shaft end of the motor is a 6206 bearing with two rubber seals. You may also see an AFBMA number. This abbreviation stands for the Antifriction Bearing Manufacturer’s Association numbering system, which requires that you decode the number to find the bearing inside and outside diameters. You may also see a CODE listing followed by a letter. This letter signifies the locked rotor current in kilovolt amperes or KVA. This value of current is used to provide the proper circuit protection values. One final number sometimes seen on a motor nameplate is a Power Factor or PF number. A motor is an inductive device. This means the current will lag the voltage in the motor’s circuit. The power factor is simply a number from 0 to 1 that reflects the cosine of the angle of the phase difference between the applied current and the motor’s circuit current. A PF of 1.0 would be ideal; however, a PF of 0.85 to 0.95 is

Industrial Alternating Current Motors 31 more common. The problem with low PF motors is that they may require the use of power-factor-correction capacitors to bring the PF closer to 1. All of the ratings on a nameplate are important. To replace a motor, you should use an exact replacement or a motor with higher ratings. However, in most cases, you can’t increase the horsepower rating of the new motor without increasing the size of the circuit protection devices, components, and conductors.

Polyphase Motor Troubleshooting and Repair

A polyphase motor, like a transformer, is nearly an ideal machine. Almost all of the applied electrical energy is con- verted to mechanical energy. The lost energy is mostly in the form of heat. Some motors are open motors where the end bells have open slots. A fan at the end of the motor moves air through the motor to cool the rotor and the stator. A typical maintenance task is to clean these slots to provide for proper air flow. If the motor is operated in a dusty location, the motor will often fill up with dust until it begins to bind. Obviously, a good cleaning with compressed air or disassembly and cleaning is in order. Some motors are enclosed to prevent dirt from entering the motor. These motors are termed totally enclosed. These motors may or may not have a fan on the rear shaft of the motor to help cool the case. There are also drip-proof motors that are specially sealed to help prevent moisture from entering the motor. Finally, there are hermetically sealed or explosion proof motors for use in areas where there are flammable gases, dusts, or liquids. Any type of motor will fail when the service life of the bearings is exceeded or when the bearings are overloaded. Bearing failures account for most of the problems with polyphase motors.

32 Industrial Alternating Current Motors Most bearing failures will result in an audible high-pitched whine or a rapid ringing noise at first and then degrade to a low-pitched rumble as the bearing progressively fails. Often, if there are grease fittings, you can apply grease with a grease gun to the bearing to see if it gets quieter. Bearing failures require that the motor be disassembled and the bearings replaced. Winding problems normally are revealed as an unbalanced load. A good clamp-type ammeter can be used to measure the current in each phase (L1, L2, and L3). If one phase is about 10 percent higher or lower than the other two phases, that phase is suspect. Shorted windings will usually result in a hot-running, vibrating motor, with a loss of torque or horsepower. If the winding shorts to the motor’s case, the motor’s overload protection device will normally open the circuit. Rotor bar conductor failures are more common in large motors (100 HP and up). These rotor bar failures usually show up as a lack of horsepower or torque and especially as excessive vibration. However, a condition known as rotor eccentricity can mimic rotor bar failures. This condition is normally caused by someone bolting the motor down to an uneven plate or mount. As the motor’s bolts are torqued down, the motor’s case deforms until the rotor is no longer centered within the stator. This is also called a soft foot condition. Two rather new technologies are being used to detect these and other motor problems. These technologies are vibration analysis and electrical signature analysis. Vibration analysis electronically measures motor vibration and identifies the cause of each frequency within a computer-stored spectrum. By careful analysis of these spectrums, the causes of excessive vibration (such as soft foot, faulty rotor or machine balance, misalignment, and so forth) can be identified and corrected. Electrical signature analysis also stores a spectrum, but this kind is an electrical frequency spectrum delivered by a transducer or clamp-type ammeter. Problems such as stator winding failures or rotor bar failures are easily identified using this technology.

Industrial Alternating Current Motors 33 The key pieces of motor test equipment are simply your senses of smell and touch, a good meter for measuring voltage and resistance, and an accessory clamp-type ammeter for measuring current. You can hear a motor’s bearings and feel excessive temperature, even inches or feet away from a motor. You can also take voltage measurements to make sure the proper voltages are reaching the motor, and take resistance readings phase-to-phase and phase-to-ground to check for shorted or open stator windings. You can also use the clamp-type ammeter to check for phase-to-phase balance in current draw. However, don’t always condemn the motor as faulty at the first sign of trouble. Remember that the motor is moving or driving something and that a jammed machine or a failed bearing in the load device won’t be corrected by changing the motor. Insufficiently low supply voltage is a common cause of hot-running AC motors. This condition, which often results in the motor’s thermal cutout switch tripping, is often caused by improperly sized power-supply-circuit wiring. Small wires increase the voltage drop in the supply circuit and, therefore, reduce the voltage at the motor. Whenever in doubt and whenever possible, uncouple the motor from the load and check each component individually. One final test conducted on motors is often part of a preven- tative maintenance (PM) program. A clamp-type ammeter is used to measure motor-starting current or inrush current on each phase as the motor is started from a standstill. These values can be four or more times the normal running current values. The values should also be within 10 percent of each other and can be recorded for trending purposes. If the inrush current has suddenly increased, the motor or the load device is beginning to have a problem that should be identified and repaired.

34 Industrial Alternating Current Motors Self-Check 3

1. The number that can be multiplied by the horsepower to get a maximum horsepower rating for the motor is the (service/power) factor rating.

2. The dimensions of the mounting holes of the motor are identified by the (case/frame) type.

3. If you are connecting a constant-torque delta-wound motor for low speed, you should short

out (T1, T2, and T3/T4, T5, and T6 )

Check your answers with those on page 47.

Industrial Alternating Current Motors 35 AC MOTOR RELAY CONTROLS

AC motor controls can be divided into many categories. For our purposes, we will look at two sections of AC controls, the control circuit and the power circuit. The control circuit contains the manual operators, switches, or sensors used to energize and close the contacts or energize the coil of the motor starter or contactor. The power circuit contains the large contacts and thermal overload protection devices that connect the motor to the source of AC electricity.

The Manual Control Circuit

The simplest of all control circuits is the manual control circuit. This circuit is shown in Figure 28. This illustration shows how a manually operated motor controller operates. In Figure 28A, a single-phase AC motor is powered by a single AC line feed. This feed is controlled by a toggle switch-type device that, from the outside, looks like a typical wall light switch. However, the body of the switch is much heavier, as are its internal contacts. Also incorporated into the switch’s housing is a location for mounting one or two thermal overload elements. These elements are used to monitor the current flow to the motor.

FIGURE 28—A simple AC manual starter circuit for HIGH AC a 120 or 220 VAC motor HIGH is shown here. SWITCH AC HIGH

ENCLOSURE

OVERLOAD

MOTOR MOTOR

AC LOW

(A) (B)

36 Industrial Alternating Current Motors The typical thermal overload element is mounted on a ratchet-type assembly that protrudes from the switch. If too much current flows though the overload element, the element heats up to a point where the ratchet is released. This opens the circuit to the motor in a way similar to that used with a circuit breaker. In this situation, the switch lever will move away from the ON position to a neutral position between ON and OFF. To reset the switch/overload protection device, simply move the switch to the OFF position and then turn the switch back to ON. Figure 28B displays how a manual switch should be connected for a 240 VAC single-phase or split-phase system. Note that both of the phases of the source are placed through the switch’s contacts and through their own thermal overload elements. If either or both phases experience an overcurrent, then both circuits will be opened to the motor. It’s neither wise nor legal, according to the National

Electrical Code, to disconnect only a single L1 L2 L3 phase of a 240 VAC system. The system must also be grounded in areas such as the metal switch enclosure and the motor’s case.

A manual three-phase starter is shown MANUAL in Figure 29. All three phases of the incom- STARTER ing AC lines are controlled by the push- button-type switch. A square ON button is pressed to close these contacts and a square OFF button is pressed to open these contacts. All three phases are shown with 30 overload protection. Often in older manual MOTOR starters, only two phases are protected by overload protection, while the third phase is allowed to pass from the switch’s load side contact directly to the motor. As with the FIGURE 29—A manual three-phase starter will single-phase system, if an overcurrent con- disconnect or connect all three motor wires to the three incoming phases. dition exists in any phase, all phases will be opened.

Industrial Alternating Current Motors 37 The Basic Electric Control Circuit

The most basic form of three-phase AC motor control is shown in Figure 30. The circuit in Figure 30A is the control circuit for the across-the-line motor starter in Figure 30B. In Figure 30A, the control circuit is shown in ladder-logic for- mat. The two side sections or rails denote the power supply to the circuit. This power supply is normally a low voltage, such as 24 or 48 volts. If the low voltage source is an AC voltage, this voltage is supplied by a step-down or control transformer. The input to the transformer may be 120, 240, or 480 VAC while the output of the transformer is 12, 24, or

FIGURE 30—This circuit is often called a seal cir- 24 V COM START STARTER cuit since the relay’s COIL auxiliary contacts seal STOP the circuit around the M START push button. M OVERLOAD CONTACTS AUXILIARY CONTACTS

(A)

L1 L2 L3

COIL AUXILIARY M CONTACTS

OVERLOAD CONTACTS

30 MOTOR

(B)

38 Industrial Alternating Current Motors 48 VAC. If the source of the control voltage is a DC voltage, then the source is typically a stand-alone power supply that has an output voltage of 12 or 24 VDC. Sometimes a computer control system’s internal power supply is also used to power the field devices. However, electrical noise from the wires, relays, and solenoids can play havoc with sensitive computer circuits making the stand-alone power supply the better choice. The lower voltages in the control circuits are used to prevent an electric shock hazard where operator pushbuttons and selector switches are used. The horizontal sections or rungs contain the circuit elements. In this circuit, the rung contains a STOP and START push button. The STOP push button is normally closed, allowing for a closed circuit to this point. The START push button is normally open and must be pressed to close the circuit to the coil of the motor starter labeled M. To the right of coil M, there’s a set of contacts off the thermal overload section of the motor starter. As long as the thermal overload section hasn’t tripped due to overcurrent, these contacts will be closed. When the START button is pressed, the circuit will be com- pleted, energizing the coil of the motor starter M. An auxiliary contact of its motor starter parallels the START push button. When the coil is energized, the mechanical motion of the motor starter will close this contact along with the three large contacts for the motor. The auxiliary contact will then close across the START push button, keeping the coil of the motor starter energized. This allows the START push button to be released and the motor to stay energized until either the STOP push button is pressed or the overloads trip. Figure 30B shows the coil and the three contacts that connect the motor to the three-phase AC lines. Below these contacts are the thermal overloads with their auxiliary contact that’s placed in the right side of the coil’s circuit in this example.

Industrial Alternating Current Motors 39 A Reversing Circuit

Often a motor must be reversed during normal machine operations, such as in loading or unloading the machine or in clearing its jaws. Figure 31 gives us an example of such a circuit.

FIGURE 31—This is the control circuit for a 24 V MOTOR COM FORWARD reversing motor starter COIL for a three-phase motor. STOP MR OL MF MF

MOTOR REVERSE COIL MF MR MR

The top rung of this figure is very similar to our STOP/START circuit given earlier. The STOP push button is normally closed, allowing power to flow through to the two control circuits. When the MOTOR FORWARD push button is pressed, the motor forward coil will be energized. However, if the motor’s reverse coil is already energized, its contacts MR on the top rung will be open, keeping the forward motor coil from energizing. This is a safety interlock contact that allows only one coil to be energized at one time. The motor reverse section operates in a similar manner. Figure 32 displays the power control section of the reversing circuit. To reverse the direction of any AC three-phase motor, you simply swap any two phases to the motor. As seen in this figure, when the forward starter has its coil energized,

the three left contacts will close. This places L1 on T1, L2 on

T2, and L3 on T3. Let’s assume the motor now turns clockwise or to the right. If the forward coil MF is de-energized and the reverse coil is energized, a different connection path is taken. Now, the three contacts to the right of the illustration are

closed and those on the left are open. Now L1 connects to T3,

40 Industrial Alternating Current Motors L2 to T2, and L3 to T1. Because two phases L1 L2 L3 are swapped, this motor will turn in an opposite direction or counterclockwise in this example. A careful study of this illustration shows MF MR the importance of interlocking the coils of the contactors to prevent both from turning on at the same time. If this were to occur, it would be like shorting phases L1 and L3 together. This action will almost always draw such a great current that the main fuses will blow or the main circuit breaker T1 T2 T3 will trip. The current draw is in fact so great that fuses can literally explode! 30 Some reversing motor starters have MOTOR mechanical interlocks, a kind of teeter-tot- ter bar between the contact carriers of each section of the motor starter. This bar physi- FIGURE 32—In order to reverse an AC motor, cally prevents both carriers from rising into two of the three incoming phases are contact with the fixed contacts at the same swapped before they reach the motor. time.

Two-speed Magnetic Starters

The control circuit for a LOW/HIGH speed AC motor control system is very similar to the one used for FORWARD/ REVERSE control. This circuit is shown in Figure 33. As with the reversing starter, the SLOW/FAST starter is interlocked to prevent both coils from energizing simultane- ously. For example, if the motor’s SLOW push button is pressed, the MS coil will be energized. This coil will seal itself on with its contacts in parallel with the MOTOR SLOW push button. A second set of contacts from the MS starter will open the circuit to the FAST, or MF, coil, preventing it from being energized until the motor SLOW coil is de-energized.

Industrial Alternating Current Motors 41 FIGURE 33—This is a typical control circuit for a + V MOTOR MOTOR COM two-speed motor. In this SLOW SLOW COIL circuit, the STOP push STOP MF button must be pressed MS to change speeds. MS

MOTOR MOTOR FAST FAST COIL MS MF MF

A second version of the SLOW/FAST control circuit is shown in Figure 34. This circuit is modified to eliminate the need to press the STOP push button when you desire to change motor speeds.

FIGURE 34—This is also a circuit for a reversing + V system. In this circuit, MOTOR MOTOR the STOP push button STOP FAST SLOW doesn’t need to be pressed to change MOTOR COM SLOW speeds. MF COIL MS

MF MF MOTOR FAST COIL MS MF

This circuit starts out with the typical STOP push button in its normally closed state. Next, there’s a normally closed contact of the FAST push button and a jumper wire to the normally closed contact of the SLOW push button. Beneath each of these contacts are normally open contacts that will close when the associated push button is pressed.

42 Industrial Alternating Current Motors Let’s assume that you want slow speed. By pressing the SLOW push button, you open the top set of contacts on that push button and close the lower set. This supplies power to the motor SLOW coil MS through the normally closed auxiliary interlocking contacts of the MF coil. The MS coil will seal itself around the normally open contacts of the motor SLOW push button. If high speed is desired, simply push the FAST push button. The normally closed contact set will open, drop- ping out or de-energizing the MS coil. At the same time, the MF coil will energize as soon as the interlocking contacts of the MS coil close as the coil de-energizes. The FAST coil will be energized and that starter will be enabled. The power circuit for a two-speed motor is shown in Figure 35. In slow speed, the contactor acts like a simple across-the-line motor starter. The three-phase lines L1, L2, and L3 are connected to T1, T2, and T3 through the three contacts on the right side of the diagram.

FIGURE 35—This is the L1 L2 L3 power circuit for the two- speed motor starter.

MF MS

T5 T1 T4 T2 T6 T3

Industrial Alternating Current Motors 43 However, when the FAST starter is chosen, its coil is energized, and all six contacts close. The left three of these supply power

to T4, T5, and T6. The right three short out

T1, T2, and T3. AC motor starters are often controlled by a single output from a programmable controller PC or PLC. In regular hard-wired circuits, however, the AC motor starter coils can have quite a few input devices, such as selector switches, pressure switches, light outputs, and so forth. These devices are in series with the coil. Before condemning a motor starter as faulty, make sure the coil is receiving the proper amount of voltage by measuring across the coil’s terminals with a meter. These terminals are where the wires attach to the small terminals in the motor starters, as shown in Figure 36. FIGURE 36—A magnetic across-the-line three- This starter can also be tested for faulty phase motor starter has the AC line inputs at the top and the motor output at the bottom. contacts by measuring with a meter set to (Photo courtesy of Allen-Bradely, a Rockwell AC volts and placing the leads of the meter International Company) as follows:

L1 to T1

L2 to T2

L3 to T3 In each of these readings, little, if any, voltage should be measured. A reading of about 2.0 VAC or more indicates a faulty contact. A reading of 10 V or more indicates a burnt contact that will soon fail. Full line voltage across a measuring point means that the contact is open. In the same starter, there are no thermal overload elements in place. They would be mounted on the lower screws where the square white overload reset push button is located.

44 Industrial Alternating Current Motors Self-Check 4

1. In a 240 VAC single-phase manual motor starter, how many AC power lines should be disconnected by switches or contacts?

a. 1 b. 2 c. 3

2. On some older three-phase manual starters, how many phases of the starter have overload protection? a. 1 b. 2 c. 3

3. When an auxiliary contact of one motor starter is placed in series with the coil of a second motor starter, the contact is called an (interchangeable/interlocking) contact.

Check your answers with those on page 47.

Industrial Alternating Current Motors 45 NOTES

46 Industrial Alternating Current Motors Self-Check 1

1. b s r e w s n A

2. Reverse polarity s r e w s n A 3. induction

Self-Check 2

1. centrifugal 2. shaded 3. repulsion

Self-Check 3

1. service 2. frame

3. T4, T5, and T6

Self-Check 4

1. b 2. b 3. interlocking

47 NOTES

48 Self-Check Answers APPENDIX A x i d n e p p A x i d n e p p A

FULL LOAD CURRENT OF SINGLE-PHASE AC MOTORS AT 120 AND 240 VAC

HP 120 V 240 V

1/6 4.4 2.2 1/4 5.8 2.9 1/3 7.2 3.6 1/2 9.8 4.9 3/4 13.8 6.9

1 16 8 11/2 20 10 2 24 12 3 34 17

5 56 28 7 80 40 10 100 50

49 APPENDIX B

FULL LOAD CURRENT FOR TYPICAL 240 V AND 480 V THREE-PHASE MOTORS

HP 240 V 480 V

1/2 2 1 3/4 2.8 1.4 1 3.6 1.8

11/2 5.2 2.6 2 6.8 3.4 3 9.6 4.8

5 15.2 7.6 71/2 22 11 10 28 14

15 42 21 20 54 27 25 68 34

30 80 40 40 104 52 50 130 65

60 154 77 75 192 96 100 248 124

125 312 156 150 260 180 200 480 240

50 Appendix APPENDIX C

NEMA FRAME DIMENSIONS FOR AC MOTORS

F F

M + N

E E

Appendix 51 Motor NEMA Frame Dimension — Inches Frame D E F U V M+N

5 3 7 1 42 2 /8 1 /4 2 /32 3/8 – 4 /32 1 3 3 48 3 2 /8 1 /8 1/2 – 5 /8 1 7 1 1 56 3 /2 2 /16 1 /2 5/8 – 6 /8 1 15 1 7 66 4 /8 2 /16 2 /2 3/4 – 7 /8 7 143T 31/2 23/4 2 /8 2 61/2

145T 31/2 23/4 21/2 7/8 2 7 7 182 41/2 33/4 21/4 /8 2 71/4 182T 41/2 33/4 21/4 11/8 21/2 73/4 7 184 41/2 33/4 23/4 /8 2 73/4 184T 41/2 33/4 21/4 11/8 21/2 81/4

213 51/4 41/4 23/4 11/8 23/4 91/4 213T 51/4 41/4 23/4 13/8 31/8 95/8 215 51/4 41/4 31/2 1 23/4 10 215T 51/4 41/4 31/2 13/8 31/8 103/8 254T 61/4 5 41/8 15/8 33/4 123/8

1 1 3 1 1 254U 6 /4 5 4 /8 1 /8 3 /2 12 /8 256T 61/4 5 5 15/8 33/4 131/4 256U 61/4 5 5 13/8 31/2 13 1 3 7 3 1 284T 7 5 /2 4 /4 1 /8 4 /8 14 /8 284Ts 7 51/2 41/4 15/8 3 131/2

1 3 5 5 1 284U 7 5 /2 4 /4 1 /8 4 /8 14 /8 286T 7 51/2 51/2 17/8 43/8 147/8 286U 7 51/2 51/2 15/8 45/8 151/8 1 1 1 3 324T 8 6 /4 5 /4 2 /8 5 15 /4 324U 8 61/4 51/4 17/8 53/8 161/8

326T 8 61/4 6 21/8 5 161/2 1 7 1 326TS 8 6 /4 6 1 /8 3 /2 15 326U 8 61/4 6 17/8 53/8 167/8 5 3 5 3 364T 9 7 5 /8 2 /8 5 /8 17 /8 5 1 1 7 364U 9 7 5 /8 2 /8 6 /8 17 /8

365T 9 7 61/8 23/8 55/8 177/8 1 1 1 3 365U 9 7 6 /8 2 /8 6 /8 1 /8 1 7 404T 10 8 6 /8 2 /8 7 20 404U 10 8 61/8 23/8 67/8 197/8 7 7 3 405T 10 8 6 /8 2 /8 7 20 /4

405U 11 8 67/8 23/8 67/8 205/8 444T 11 9 71/4 33/8 81/4 231/4 444U 11 9 71/4 27/8 83/8 233/8 445T 11 9 81/4 33/8 81/4 241/4 445U 11 9 81/4 27/8 83/8 243/8

52 Appendix 22. Which one of the following types of AC motors uses brushes and a commutator? A. Shaded-pole C. Split-phase B. Repulsion-induction D. Capacitor start

23. The deviation in a motor’s speed between the nameplate value and the actual value is called A. stagger. C. lag. B. slip. D. decline.

24. The most common failure item with a three-phase induction motor system is the A. bearings. C. rotor bars. B. stator windings. D. controller or starter.

25. Three current readings are taken on a motor and are 7.7A, 8.1A, and 8.4A. What’s the condition of this motor? A. The motor is in good working condition. B. Phase A is too low. C. Phase C is too high. D. Phase B should be 8.3A.

56 Examination