ADJUSTABLE SPEED DRIVES By: Richard D

Service Application Manual SAM Chapter 620-130 Section 6A

ADJUSTABLE SPEED DRIVES By: Richard D. Beard P.E. Consultant, RSES Manufacturers’ Service Advisory Council

INTRODUCTION

Many commercial and industrial machines and processes require adjustable speed. Adjustable speed usually makes a machine more universally compatible and increases its versatility. Adjustable-speed drives also are being used in residential equipment, including air conditioners, refrigerators, heat pumps, furnaces, and other devices driven by motors. These drives optimize speed and torque, making them generally more efficient than non-adjustable-speed drives.

An adjustable-speed motor is one in which the speed can be varied gradually over a wide range—but, once adjusted, it remains nearly unaffected by the load. A variable-speed motor is one in which the speed varies with the load, usually decreasing when the load increases.

The term "adjustable speed" implies that some external adjustment, which is independent of load, will cause the speed to change. A variable-frequency inverter drive is an example. The term "variable speed" describes a drive in which load changes inherently cause significant changes in speed. A direct current series motor, for example, exhibits this characteristic.

An adjustable variable-speed motor is one in which the speed can be adjusted gradually. However, once adjusted for a given load, the speed will vary with changes in the load. A multispeed motor is one that can be operated at any one of two or more definite speeds, each being practically independent of the load. The multispeed motor is neither an adjustable-speed nor a variable-speed drive. Multispeed motors usually have two, three, or four definite operating speeds.

DIRECT CURRENT MOTORS

In general, direct current (DC) motors are classified by how the field windings are connected to the armature. The field windings, sometimes referred to simply as "fields," are the stationary coils attached to the frame of a DC motor. The armature is the rotating part. There are two basic types of DC motors, called shunt motors and series motors. A compound motor combines characteristics of both types. Other types of DC motors include universal motors and permanent-magnet motors.

SHUNT MOTORS

The shunt-wound motor is the most widely used type of DC motor built with armature and field windings. The name originates from the fact that the field windings are connected in parallel (shunt) across the armature, as shown in Figure 1.

Service Application Manual SAM Chapter 620-130 Section 6A

ADJUSTABLE SPEED DRIVES By: Richard D. Beard P.E. Consultant, RSES Manufacturers’ Service Advisory Council

DC shunt-wound motor.

In the separately excited shunt motor, the field circuit is energized from a separate source of DC power, as shown in Figure 2.

Separately excited DC shunt-wound motor.

This field circuit power supply is independent from the armature circuit power supply. Once the desired speed has been obtained through variations of the voltage applied to the armature or field, a shunt motor provides relatively small changes in speed under changing load conditions. Shunt motors are frequently used on adjustable-speed DC drives because of this characteristic of excellent speed regulation.

Service Application Manual SAM Chapter 620-130 Section 6A

ADJUSTABLE SPEED DRIVES By: Richard D. Beard P.E. Consultant, RSES Manufacturers’ Service Advisory Council

SERIES MOTORS

As the name implies, the field windings in a series-wound motor are connected in series with the armature, as shown in Figure 3.

Series DC motor.

Both the field and the armature carry full motor current. In series motors, motor speed is a function of load, once the speed has been adjusted by the voltage applied from the DC power supply. Series motors are commonly used as traction motors for transportation equipment drives, cranes, and hoists.

COMPOUND MOTORS

A compound motor is one in which there are two field windings, as shown in Figure 4.

Compound DC motor.

One is a shunt field connected in parallel with the armature, and the other is a series field connected in series with the armature. By properly selecting the shunt and series field windings, the designer can make the motor more nearly like a shunt or a series motor.

Service Application Manual SAM Chapter 620-130 Section 6A

ADJUSTABLE SPEED DRIVES By: Richard D. Beard P.E. Consultant, RSES Manufacturers’ Service Advisory Council

OTHER TYPES OF DC MOTORS

Universal motors are series-wound motors that can be operated on either DC or AC power. Performance is the same, regardless of which power supply is used. The operating characteristics of universal motors are similar to those of DC series motors. However, universal motors are small in size, with ratings usually less than one (1) hp. Typical applications of universal motors include power hand tools.

In some smaller DC motors, permanent magnets are used in place of field windings. Permanent-magnet DC motors provide shunt motor characteristics with speed adjustment obtained by changing the power supply voltage to the armature.

A relatively new type of permanent-magnet DC motor is the electronically commutated motor, or ECM™ , from General Electric. This motor operates on AC power—however, it is a brushless DC permanent- magnet motor. Control of the ECM™ output speed is accomplished with solid-state switches. The solid- state switches eliminate the need for the traditional mechanical commutator and brushes. The permanent magnets are in the rotor, while the windings in the stator create electronically controlled rotating electromagnets.

POLYPHASE ALTERNATING CURRENT MOTORS

The speed of an alternating current (AC) induction motor is related to the frequency of the power supply. The equation that shows this relationship is:

The speed determined by this equation is actually the synchronous speed, which is the speed of the rotating magnetic flux in the stator of the motor. The stator is the stationary part of an induction motor. The rotor is the rotating part. The rotating magnetic flux is created as current flows through the three- phase stator windings. This rotating magnetic flux induces voltage into the rotor bars, which causes current to flow in the rotor. As a result, electromagnets are created in the rotor. These electromagnets follow the electromagnetic field of the stator and cause the rotor to rotate.

The amount by which the rotor speed lags behind the speed of the rotating magnetic flux in the stator is called the slip. This determines the rotor voltage. Without slip, no voltage would be induced into the rotor. Because of the slip, there is a difference between the speed of the rotating magnetic flux in the stator and the speed of the rotor.

Thus, there are actually three speeds to consider:

1. the synchronous speed (of the rotating stator flux)

2. the speed of the rotor

3. the slip speed, which is the difference between the synchronous speed and the rotor speed, and is usually expressed as a percentage.

Service Application Manual SAM Chapter 620-130 Section 6A

ADJUSTABLE SPEED DRIVES By: Richard D. Beard P.E. Consultant, RSES Manufacturers’ Service Advisory Council

SYNCHRONOUS MOTORS

A brief description of synchronous motor operation is included here, even though synchronous motors are not adjustable-speed motors. Polyphase synchronous motors have stators and stator windings very similar to those of induction motors. The primary difference between synchronous motors and induction motors is in the rotor construction. The rotor of the synchronous motor has distinct (salient) poles wound with insulated magnet wire and connected in series. In addition, bar windings are placed in the upper part of each pole. These bar windings are similar to the windings in the rotor of a squirrel-cage induction motor. The synchronous motor starts and accelerates as an induction motor. Because of the slip, the rotor reaches a speed that is somewhat slower than the speed of the rotating magnetic flux in the stator. Direct current then is applied to the rotor circuit, which includes each of the poles. This DC power is connected to the rotor circuit through two insulated slip rings mounted on the rotor shaft. Carbon brushes make contact with the slip rings. When the rotor circuit is energized, each pole becomes an electromagnet. These magnets lock into step with the stator flux electromagnets. The rotor then turns at exactly the same speed as the rotating magnetic flux in the stator. When this happens, there is no slip and the motor runs at synchronous speed.

WOUND-ROTOR MOTORS

Wound-rotor motor construction differs from that of the induction motor only in the rotor. Rather than having a rotor with bar windings whose ends are connected together, as in a squirrel-cage induction motor, the wound-rotor motor has insulated coils of magnet wire inserted in the rotor core iron. These coils are similar to the stator winding coils. One end of each phase winding is connected to one of the three slip rings mounted on the shaft and insulated from it. Connections to the slip rings are made through carbon brushes, so that any value of secondary resistance may be added to the rotor circuit. With the three slip rings connected together, the wound-rotor motor runs exactly like the squirrel-cage induction motor. Reduced speed is obtained by connecting resistance into the rotor circuit. Thus, the wound-rotor motor is an adjustable secondary resistance motor that provides adjustable and variable speed, by means of changing the value of the external resistance in the rotor circuit.

SQUIRREL-CAGE INDUCTION MOTORS

Since the speed of a polyphase induction motor is equal to the synchronous speed minus the slip, electrical speed control must adjust one of these two speeds. Slip speed control is utilized in wound-rotor adjustable-speed motors. The universal availability of AC power, together with the simplicity and relatively low cost of squirrel-cage induction motors, resulted in the development of economical and practical adjustable-frequency drives. By providing adjustable-frequency power to the induction motor, adjustable output speed over the desired range can be obtained.

Motor speed can be increased from standstill by using a variable-frequency control. By varying both frequency and voltage, full torque can be maintained up to breakdown value at relatively high power factor and with current proportional to the torque. Automatic control of both voltage and frequency also permits a higher torque to be obtained for starting when required.

The speed of an induction motor can be determined by using the previous equation. The synchronous speed of an AC induction motor depends on the number of poles in the winding and the power supply frequency. When variable-frequency power is supplied, the motor will run at a speed determined by the frequency, since the number of poles is fixed. The voltage supplied must be proportional to the frequency to be sure that both the volts and the hertz are correct for the particular motor design. A variable- frequency drive maintains a preset volts/hertz ratio power to the motor that it is controlling.

Service Application Manual SAM Chapter 620-130 Section 6A

ADJUSTABLE SPEED DRIVES By: Richard D. Beard P.E. Consultant, RSES Manufacturers’ Service Advisory Council

All motors are designed with specific torque characteristics that are classified by the National Electrical Manufacturers Association (NEMA). These NEMA designs include classes A, B, C, D, and F. Even though two induction motors may have the same horsepower rating, their torque characteristics, including breakaway or starting torque, pull-up torque, maximum torque and full load torque, may be different depending on their NEMA design. Refer to NEMA MG-1 for additional information on motor torque.

Variable-frequency controls utilize input power with fixed voltage and frequency, as is available from the electric utility company. These controls provide variable voltage and frequency to the motor and maintain the correct volts/hertz ratio required by the motor. This is usually accomplished by using a DC converter and an AC inverter.

The DC converter part of the variable-frequency control rectifies the AC power to DC power. This is done because it is easier to generate a variable frequency from DC power than from AC power. Some drives operate with a variable DC voltage, while others use a fixed voltage. Many drives use a pulse width modulated design and operate from a fixed DC voltage. There are several other types of designs in use on variable-frequency drives. Solid-state devices, such as power diodes for fixed DC voltage and silicon controlled rectifiers (SCRs) for variable DC voltage, are used for the conversion process.

The AC inverter is used to provide the required output frequency. Solid-state switching devices such as SCRs and transistors are connected to the DC power from the converter. Positive and negative switching of the DC voltage produces AC power. A microprocessor or logic board determines the frequency of switching in the inverter. This results in a range of output frequencies. A typical output frequency range is 1 to 400 Hz.

The key to successful performance of variable-frequency controls is for the motor to provide the required torque at all speeds. In order to accomplish this, the control must maintain sufficient current to the motor so that it will develop the required NEMA torque.

Always refer to the manufacturer's manual that applies to the adjustable-speed drive. This will include valuable technical information, including:

• safety precautions

• component description, identification, and specifications

• installation guidelines

• wiring diagrams and procedures

• drive operation instructions

• function code descriptions

• troubleshooting

• parts and service availability.

SINGLE-PHASE AC MOTORS

Although universal motors are DC devices, they are frequently used in applications with single-phase AC power supplies. Speed adjustment of these universal motors used with AC power supplies is easily obtained by phase modulation. Solid-state devices such as diodes and SCRs are used to provide constant speed characteristics with varying torque requirements. The control causes the gate of the SCR

Service Application Manual SAM Chapter 620-130 Section 6A

ADJUSTABLE SPEED DRIVES By: Richard D. Beard P.E. Consultant, RSES Manufacturers’ Service Advisory Council to trigger into conduction, so that part of the half-cycle supply voltage is applied to the motor. The point on the sine wave where the voltage is applied to the motor (see Figure 5) is determined by the setting of the speed control potentiometer. The control needs to be matched to the motor application so that each speed selected is stable under varying load conditions. This usually can be achieved over a speed range of one to three.

One cycle of AC voltage sine wave.

Single-phase motors include the shaded-pole motor, the permanent split-capacitor motor, the split-phase motor, the capacitor-start motor, and the capacitor-start, capacitor-run motor. The last three types utilize a switching device for connecting and disconnecting the starting winding. A relay or switch operated by a centrifugal mechanism normally is used to energize and de-energize the starting winding. Since the switching usually takes place at approximately 65% of the rated speed, these three types of motors generally are not used with electronic adjustable-speed controls. If adjustable-speed controls were applied, the starting windings could be switched in and out of the circuit as the speed was adjusted to near the switching speed. Overheating of the starting windings would result, causing motor failures. These three types of motors can be of the multispeed design by using two or more windings, each with a different number of poles. Multispeed motors can be used in place of adjustable-speed drive motors for certain applications.

A shaded-pole motor has distinct (salient) poles in the stator, each wound with coils of magnetic wire that are connected in series. A single-turn, closed-loop shading coil is located in a section of each pole face, as shown in Figure 6.

Service Application Manual SAM Chapter 620-130 Section 6A

ADJUSTABLE SPEED DRIVES By: Richard D. Beard P.E. Consultant, RSES Manufacturers’ Service Advisory Council

Shading Coils in Shaded-Pole Motor On shaded-pole motors, the shading coil is in a copper insert on each pole that determines which way the rotor and shaft will turn when the motor is energized. The rotor and shaft will rotate from the side of the pole without a shading coil to the side that contains a shading coil (from A to B).

This short-circuited coil causes the flux in the area of the shading coil to be phase-delayed. As a result, starting torque is developed. This type of motor has a greater slip than other types of induction motors. A typical four-pole, 60-Hz shaded-pole motor has a rated speed of 1,550 rpm and a slip speed of 250 rpm. Other four-pole, 60-Hz induction motors typically have a rated speed of 1,725 rpm, with a 75-rpm slip speed. Shaded-pole motors of the multispeed design are equipped with extended windings that can be connected to a control for switching to lower speeds, as shown in Figure 7.

Service Application Manual SAM Chapter 620-130 Section 6A

ADJUSTABLE SPEED DRIVES By: Richard D. Beard P.E. Consultant, RSES Manufacturers’ Service Advisory Council

Shaded-pole motor.

The permanent split-capacitor motor has a distributed main winding and a distributed start winding with a capacitor connected in series with it, as shown in Figures 8 and 9. The start winding provides starting torque and is designed to remain energized continuously. The multispeed permanent split-capacitor motor is equipped with extended main windings, which can be connected to a control for switching to lower speeds, as shown in Figure 9.

Service Application Manual SAM Chapter 620-130 Section 6A

ADJUSTABLE SPEED DRIVES By: Richard D. Beard P.E. Consultant, RSES Manufacturers’ Service Advisory Council

Windings in permanent split-capacitor motor.

Service Application Manual SAM Chapter 620-130 Section 6A

ADJUSTABLE SPEED DRIVES By: Richard D. Beard P.E. Consultant, RSES Manufacturers’ Service Advisory Council

Permanent split-capacitor motor.

Electronic circuits, which include solid-state components, are used to apply voltage to the motor for the precise part of each half cycle required to maintain the output speed that has been selected. The phase control varies the motor speed of these single-phase motors by changing the voltage that is applied to the motor winding. Detailed information on each adjustable-speed drive control should be available from the manufacturer. With the rapid technological advancement that has taken place in adjustable-speed drives, it is extremely important to use the appropriate instruction manual when servicing this equipment.