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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.
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.
2 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.
3 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 permanentmagnet motor. Control of the ECM™ output speed is accomplished with solid-state switches. The solidstate 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 threephase 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.
4 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 variablefrequency drive maintains a preset volts/hertz ratio power to the motor that it is controlling.
5 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
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
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.
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
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.
8 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.
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.
10 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.
Copyright © 1997, 2009, By Refrigeration Service Engineers Society.
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