101 BASICS SERIES
LEARNING MODULE 16:
BASICS OF MOTORS AND MOTOR CONTROL
Cutler-Hammer
BASICS OF MOTORS AND MOTOR CONTROL
WELCOME
Welcome to Module 16, which is about the basics of motors and motor control.
An electric motor is a machine that converts electrical energy to mechanical energy.
There are two main groups of electrical motors: DC and AC motors. This module will
discuss both types of motors, and how to control them.
FIGURE 1. TYPICAL ELECTRIC MOTOR
Like the other modules in this series, this one presents small, manageable sections
of new material followed by a series of questions about that material. Study the
material carefully then answer the questions without referring back to what you’ve
just read. You are the best judge of how well you grasp the material. Review the
material as often as you think necessary. The most important thing is establishing a
solid foundation to build on as you move from topic to topic and module to module.
A Note on Font
Styles
Key points are in bold.
Viewing the
Glossary
You may view definitions of glossary items by clicking on terms and words that are
underlined and italicized in the text. You may also browse the Glossary by clicking
on the Glossary bookmark in the left-hand margin.
Glossary items are italicized and underlined the first time they appear.
1
BASICS OF MOTORS AND MOTOR CONTROL
WHAT YOU
WILL LEARN
We’ll step through each of these topics in detail:
Section Title
•
•
4
•
Magnetic Fields
4
•
Current Flow
4
•
Induced Motion
5
•
Commutator
8
DC Motors
9
•
Simple DC Motor
•
Practical DC Motor
10
•
Electromagnets
11
•
Motor Components
12
•
Reversing a DC Motor
12
•
DC Motor Types
13
9
•
Review 1
14
•
AC Motors
15
•
•
2
Motor Theory
Page Number
•
What Makes an AC Motor Different From a DC Motor?
15
•
Single-Phase
15
•
Three-Phase
16
The Squirrel Cage Induction Motor
17
•
Induction Principle
17
•
Applying the Induction Principle to the AC Motor
17
•
Three-Phase Motor
19
•
Construction of Three-Phase Motors
21
•
Wye and Delta
21
•
Dual Voltage
22
Review 2
23
BASICS OF MOTORS AND MOTOR CONTROL
WHAT YOU
WILL LEARN
(CONTINUED)
•
•
•
•
Speed Control
24
•
Force, Work and Torque
24
•
Power and Horsepower
25
•
Putting it All Together
26
•
Application Types
27
•
Speed Control for a DC Motor
28
•
Speed Control for an AC Motor
29
Starting the Motor
32
•
Across the Line
32
•
Minimizing Inrush Current
32
Reversing the Motor
33
•
Manual Reversing Starter
33
•
Magnetic Reversing Starter
33
Braking the Motor
34
•
DC Injection Braking
34
•
Dynamic Braking
35
•
Review 3
36
•
Glossary
37
•
Review Answers
40
•
Appendix A: Typical Multispeed Motor Connections
41
3
BASICS OF MOTORS AND MOTOR CONTROL
MOTOR
THEORY
To understand motor theory, we need to cover the underlying principles of magnetic
fields, current flow, and induced motion.
NOTE: There are two theories regarding the flow of current. Electron Flow Theory
states that current flows from negative to positive. Conventional Flow
Theory states that current flows from positive to negative.
This module uses Electron Flow Theory. For more information on these
theories, see Module 2, Fundamentals of Electricity.
Magnetic Fields
Between the poles of a magnet, there exists a magnetic field. The direction of
the magnetic field is called magnetic flux. Magnetic flux moves from the north pole
to the south pole, as shown in Figure 2.
FIGURE 2. LINES OF MAGNETIC FLUX FLOW FROM NORTH POLE TO SOUTH POLE
Current Flow
Now, let’s consider a wire (conductor) with an electric current flowing through it. A
magnetic field surrounds the wire, as shown in Figure 3.
⊕ = CURRENT
FLOWING INTO
THE PAGE
FIGURE 3. LEFT HAND FLUX RULE: LINES OF MAGNETIC FLUX SURROUND A CONDUCTOR
Understanding the direction of the magnetic flux around the conductor is
critical to understanding motor motion. The direction of the magnetic flux can be
determined using the left hand flux rule.
Imagine grasping the wire with your left hand, making sure your thumb points in the
direction of the current flow. Your fingers will curl around the wire in the direction of
the magnetic flux.
In Figure 3, the current is flowing into the page, so the lines of flux rotate
counterclockwise around the wire.
4
BASICS OF MOTORS AND MOTOR CONTROL
Induced Motion
When this current-carrying conductor is placed between the poles of a magnet,
both magnetic fields distort. In Figure 4, the conductor will tend to move upward,
since the current is flowing into the page.
The force exerted upward depends on the strength of the magnetic field between
the poles of the magnet, and the strength of the current through the conductor.
A simple method for determining the direction of motion is the right hand motor rule.
In Figure 4, the index finger points in the direction of the magnetic flux (N to S), the
middle finger points in the direction of current flow through the conductor, and the
thumb points in the direction of the conductor movement.
DIRECTION OF
MAGNETIC
FLUX
DIRECTION OF
CONDUCTOR
MOVEMENT
DIRECTION
OF CURRENT
FLOW
TO
ELECTRICAL
DC SOURCE
+ -
FIGURE 4. RIGHT HAND RULE: WIRE IS MOVED UPWARD
This means that if you know the direction the current is flowing, and the
orientation the poles, you can determine which way the conductor will move
through the magnetic field.
Applying the right hand motor rule to Figure 4, the conductor will move upward
through the magnetic field. If the current through the conductor were to be reversed,
the conductor would move downward.
Note that the conductor current is at a right angle to the magnetic field. This is
required to bring about motion, since no force is felt by a conductor if the current
and the field direction are parallel.
5
BASICS OF MOTORS AND MOTOR CONTROL
Induced Motion
(continued)
Now, suppose we change the single conductor into a simple coil or loop of wire.
This coil is called an armature, and is shown in Figure 5.
DIRECTION OF
ROTATION
B
C
A
D
ARMATURE
COMMUTATOR
FIGURE 5. ARMATURE ROTATING
Both sections of the armature AB and CD have a force exerted on them. Why does
the coil want to move in a counterclockwise motion?
Recall that the magnetic flux rotates around the conductors. Armature sections AB
and CD have the current flowing in opposite directions. This means the magnetic
flux flows around them in opposite directions, as shown in Figure 6.
AB
CD
⊕ = CURRENT
FLOWING INTO
THE PAGE
¤ = CURRENT
FLOWING OUT
OF THE PAGE
FIGURE 6. MAGNETIC FLUX AROUND THE ARMATURE SECTIONS
6
BASICS OF MOTORS AND MOTOR CONTROL
Induced Motion
(continued)
When the magnetic field of the magnets are put in the picture, the two magnetic
fields distort. A turning force, or torque, acts on the coil. The lines of force act like
stretched rubber bands that tend to contract. The result is that the armature rotates
in a counterclockwise direction.
Figure 7 illustrates a cross-sectional view of the induced motion.
⊕ = CURRENT
FLOWING INTO
THE PAGE
¤ = CURRENT
FLOWING OUT
OF THE PAGE
CD
AB
FIGURE 7. CREATING TORQUE: A CROSS SECTION
The interaction between the two magnetic fields causes a bending of the lines of
force. Where the lines straighten out, they cause the armature to rotate. The left
conductor (AB) is forced downward, and the right conductor (CD) is forced upward,
causing a counterclockwise rotation.
7
BASICS OF MOTORS AND MOTOR CONTROL
Commutator
As we mentioned earlier, when the armature is positioned so that the loop sides are
at right angles to the magnetic field, a turning force is exerted. But what happens
when the coil rotates 180°°?
A problem arises here. The magnetic field in the conductor is now opposite that of
the field, and this will tend to push the armature back the way it came, stopping the
rotating motion.
To solve this problem, some method must be employed to reverse the current in the
armature every one-half rotation so that the magnetic fields will work together to
maintain a positive rotation.
A device called a commutator performs this task. Two stationary brushes, one
supplied with positive DC current, the other with negative DC current, supply current
to the two rotating commutator segments.
As the armature and commutator rotate together, the commutator reverses the
direction of the current through the armature. In this way, magnetic fields are
always running in the direction needed to contribute to a continuing turning effort.
ARMATURE
ROTATION
+
+
TO DC
POWER
TO DC
POWER
BRUSH
CURRENT
FLOW
COMMUTATOR
POSITION “A” - TORQUE
POSITION “B” - NEUTRAL
+
+
TO DC
POWER
TO DC
POWER
POSITION “C” - TORQUE
POSITION “D” - NEUTRAL
FIGURE 8. THE COMMUTATOR REVERSES THE CURRENT THROUGH THE ARMATURE
Now we are getting somewhere. With the armature continuously rotating through
the magnetic field, mechanical energy is created from electrical energy.
8
BASICS OF MOTORS AND MOTOR CONTROL
DC MOTORS
Simple DC Motor
What we have just described is a DC motor. Direct current is fed to the commutator.
The commutator is connected to the armature in such a way that the current
direction (called polarity) is switched every half-turn of the armature. This allows the
armature to continue turning in the magnetic field, creating mechanical energy from
electrical energy.
However, this simple DC motor has a few shortcomings. Each time the armature is
parallel to the magnetic field (called a neutral position), no torque is produced.
(Refer back to Figure 8.)
Recall that when the armature is positioned so that the loop sides are at right
angles to the magnetic field, torque is exerted. But, as the armature turns in a circle,
there are two points at which it is parallel to the magnetic field – at ¼ and ¾ of a
turn – and no torque is generated. (Refer back to Figure 8.)
The change in the amount of torque is shown graphically in Figure 9. The speed of
the motor varies because of the changes in torque. Most devices require a motor
to turn at a uniform speed, so the simple DC motor just described would not be
suitable.
MAX
----- TORQUE
- - - SPEED
MIN
0
¼
½
¾
REVOLUTIONS
1
FIGURE 9. SIMPLE DC MOTOR TORQUE AND SPEED GRAPH
Another problem with a simple DC motor is that it does not start easily. This is
particularly true if the armature is in or near a neutral position. The armature must
be moved out of the neutral position to start the motor.
9
BASICS OF MOTORS AND MOTOR CONTROL
Practical DC
Motor
In a practical DC motor, the armature is never in a neutral position, and the
torque is always at its maximum. This is accomplished by using an armature with
more than one loop. A four loop armature is shown in Figure 10. As you can see,
each loop of the armature is connected to a pair of commutator segments.
BRUSH
COMMUTATOR
(FOUR SEGMENTS)
+
LOOPS
-
TO DC
SOURCE
BRUSH
FIGURE 10. FOUR-LOOP ARMATURE
When current flows through the brushes, all four loops act together, producing full
torque at all times. There is no neutral armature position where torque is absent.
Also, notice that the brushes are larger than the gaps between the commutator
segments. This means that contact with the commutator is maintained at every
instant of rotation of the armature.
A DC motor of this type has uniform torque, both for running and for starting. It is
a definite improvement over the simple DC motor.
IN THE WORKPLACE
This is a common cordless drill
that might be used by a building
maintenance person. It is run on
a battery and uses a DC motor.
The small size of the DC motor
makes the drill very light,
portable and convenient to use.
CORDLESS DRILL USING A DC MOTOR
10
BASICS OF MOTORS AND MOTOR CONTROL
Electromagnets
In the previous drawings, we have shown the armature rotating between a pair of
magnetic poles. Practical DC motors do not use permanent magnets; they use
electromagnets instead.
Electromagnets work very similarly to permanent magnets. To make one, simply
wrap an iron rod with insulated wire and run current through the wire, as shown in
Figure 11. The iron rod develops a magnetic field, and North and South magnetic
poles.
DIRECTION OF
CURRENT FLOW
FIGURE 11. ELECTROMAGNET
The electromagnet has two advantages over the permanent magnet.
•
By adjusting the amount of current flowing through the wire, the strength of the
electromagnet can be controlled.
•
By changing the direction of current flow, the poles of the electromagnetic can
be reversed. In the diagram above, switching the leads on the battery terminals
would change the direction current flow.
(Connecting the leads to an AC source would switch the direction of current flow
automatically. We will consider AC later in this module.)
11
BASICS OF MOTORS AND MOTOR CONTROL
Motor
Components
We have already discussed three of the four major components that make up a DC
motor: the armature, the brushes, and the commutator. The fourth is the field coils
(also called field poles or stationary windings).
Figure 12 shows a disassembled view of a typical four-pole DC motor.
SHAFT
FIELD POLES
COMMUTATOR
WINDINGS
FAN
FIGURE 12. A TYPICAL FOUR-POLE DC MOTOR, ASSEMBLED AND DISASSEMBLED
Note that many turns (or windings) are used to make up the field poles. The larger
the poles, the stronger the field.
The larger the number of coils used in a DC motor, the smoother the motor will
run. However, the number of field coils used must always be even. Each set of coils
consists of a North and a South pole.
Reversing a
DC Motor
The direction of rotation of a DC motor may be reversed using one of these
methods:
•
Reversing the direction of the current through the field
•
Reversing the direction of the current through the armature
The industrial standard is to reverse the current through the armature. This is
accomplished by reversing the armature connections only.
12
BASICS OF MOTORS AND MOTOR CONTROL
DC Motor Types
There are basically three DC motor types: The series motor, the shunt motor, and
the compound motor. Internally and externally they are practically the same. The
difference between them is the way the field coil and armature coil circuits are
wired.
The series motor (Figure 13) has the field coil wired in series with the armature.
It is also called a universal motor because it can be used in DC or AC applications.
It has a high starting torque and a variable speed characteristic. The motor can start
heavy loads, but the speed will increase as the load is decreased.
SERIES FIELD
S2
ARMATURE
S2
A1
A2
A2
S1
A1
ARM
S1
DC
VOLTAGE
FIGURE 13. DC SERIES MOTOR: SCHEMATIC AND WIRING DIAGRAM
The shunt motor (Figure 14) has the armature and field circuits wired in
parallel, giving essentially constant field strength and motor speed.
SHUNT FIELD
F2
ARMATURE
F2
A2
F1
ARM
A1
F1
A2
A1
DC
VOLTAGE
FIGURE 14. DC SHUNT MOTOR: SCHEMATIC AND WIRING DIAGRAM
The compound motor (Figure 15) combines the characteristics of both the
series and the shunt motors. A compound motor has high starting torque and
fairly good speed torque characteristics at rated load. Since complicated circuits are
needed to control the compound motors, this wiring arrangement is usually only
used on large bi-directional motors.
SHUNT FIELD
F2
SERIES FIELD
ARMATURE
F2
S2
A1
S2
A2
S1
S1
F1
A2
A
A1
F1
DC
VOLTAGE
FIGURE 15. DC COMPOUND MOTOR: SCHEMATIC AND WIRING DIAGRAM
13
BASICS OF MOTORS AND MOTOR CONTROL
REVIEW 1
Answer the following questions without referring to the material just presented.
Begin the next section when you are confident that you understand what you’ve
already read.
1. The right hand rule is illustrated here. What does each finger indicate?
Thumb _____________________
Index ______________________
Middle _____________________
2. The 2 main problems with the simple DC motor are:
______________________________________________________
______________________________________________________
3. Label the simple DC motor’s speed/torque graph below:
MAX
----- ________
- - - ________
MIN
0
__
__
__
REVOLUTIONS
__
4. The 2 methods for reversing a DC motor are:
______________________________________________________
______________________________________________________
5. The 3 DC motor types are:
___________________________
___________________________
___________________________
14
BASICS OF MOTORS AND MOTOR CONTROL
AC MOTORS
While there are only three general types of DC motors, there are many different AC
motor types. This is because each type is confined to a narrow band of operating
characteristics. These characteristics include torque, speed, and electrical service
(single-phase or polyphase). These operating characteristics are used to determine
a given motor’s suitability for a given application.
What Makes an
AC Motor
Different From a
DC Motor?
In a DC motor, electrical power is conducted directly to the armature through
brushes and a commutator. An AC motor does not need a commutator to
reverse the polarity of the current, as AC changes polarity “naturally.”
Also, where the DC motor works by changing the polarity of the current running
through the armature (the rotating part of the motor), the AC motor works by
changing the polarity of the current running through the stator (the stationary part of
the motor).
The many types of AC motor may be split into two main groups: single-phase and
polyphase.
Single-Phase
A single-phase power system has one coil in the generator. Therefore, one
alternating voltage is generated. The voltage curve of a single-phase AC generator
is shown in Figure 16.
FIGURE 16. VOLTAGE CURVE OF A SINGLE-PHASE AC GENERATOR
Single-phase motors are generally motors with horsepower ratings of one or
below. (These are generally called fractional horsepower motors.) They are
generally used to operate mechanical devices and machines requiring a relatively
small amount of power.
Types of single-phase motors include: shaded-pole, capacitor, split-phase,
repulsion, series (AC or universal) and synchronous.
However, the single-phase motor is generally not used because it is inefficient,
expensive to operate, and is not self starting.
We will not go into detail here regarding how each single-phase motor type
functions.
15
BASICS OF MOTORS AND MOTOR CONTROL
Three-Phase
Three-phase or polyphase motors run on three-phase power.
A three-phase power system has three coils in the generator. Therefore, three
separate and distinct voltages will be generated. The voltage curve is shown in
Figure 17.
FIGURE 17. VOLTAGE CURVE OF A THREE-PHASE AC GENERATOR
We will discuss how three-phase power works in more detail shortly.
Types of three-phase motors include: induction (squirrel-cage or wound), rotor
types, commutator, and synchronous.
In an AC environment, the squirrel cage induction motor is the most widely
used. We will focus only on this type of motor.
16
BASICS OF MOTORS AND MOTOR CONTROL
THE
SQUIRREL
CAGE
INDUCTION
MOTOR
Induction
Principle
Before we discuss the squirrel cage motor further, let’s consider the term induction.
Induction refers to electrically charging a conductor by putting it near a
charged body.
The principle of the induction motor was first discovered by Arago in 1824. He
observed that if a non-magnetic metal disk and a compass are pivoted with their
axes parallel, so that one (or both) of the compass poles are located near the edge
of the disk, spinning the disk will cause the compass needle to rotate. The direction
of the induced rotation in the compass is always the same as that imparted to the
disk.
You can prove it to yourself if you like. Mount a simple copper or aluminum disk and
a large compass on a vertical stem, so that each may be rotated on its own bearing,
independently of the other. Spin the disk, and watch the compass needle. There is
no more effective way to demonstrate the principle of induction.
FIGURE 18. DEMONSTRATING THE PRINCIPLE OF INDUCTION
Applying the
Induction
Principle to the
AC Motor
So, how do we apply the concept of induction to a motor?
Recall that the AC motor works by changing the polarity of the current running
through the stator (the stationary part of the motor). The stator plays the role of the
metallic disk described above. A rotating magnetic field is established in the stator.
The conductor, called the rotor, “follows” the rotating magnetic field by beginning to
rotate, just like the compass needle described above.
17
BASICS OF MOTORS AND MOTOR CONTROL
Applying the
Induction
Principle to the
AC Motor
(Continued)
The induction motor uses a rotor of a special design. It resembles a cage used for
exercising squirrels. This is why it is called a squirrel cage rotor.
The rotor consists of circular end rings joined together with metal bars. Note
that the metal bars are placed directly opposite each other and provide a complete
circuit within the rotor, regardless of the rotor's position. Rotors normally have
several bars, but only a few are shown here for clarity.
FIGURE 19. THE ROTOR OF A SQUIRREL CAGE INDUCTION MOTOR
Squirrel cage motors are usually chosen over other types of motors because
of their simplicity, ruggedness and reliability. Because of these features,
squirrel-cage motors have practically become the accepted standard for AC, allpurpose, constant speed motor applications. Without a doubt, the squirrel-cage
motor is the workhorse of the industry.
The squirrel cage induction motor has certain advantages over the DC motor.
18
•
There are only two points of mechanical wear on the squirrel cage motor: the
two bearings.
•
Since it has no commutator, there are no brushes to wear. This keeps
maintenance minimal.
•
No sparks are generated to create a possible fire hazard.
BASICS OF MOTORS AND MOTOR CONTROL
Three-Phase
Motor
An induction motor depends upon an electrically rotating magnetic field, not a
mechanically rotating one. (A mechanically rotating field would work, but an
electrically rotating magnetic field has significant advantages.) How is an electrically
rotating field obtained? It all starts with the phase displacement of a three-phase
power system.
Three-phase power can be thought of as three different single-phase power
supplies. They are called A, B, and C. In the three-phase motor, each phase of the
power supply is provided with its own set of poles, located directly across from each
other on the stator, and offset equally from each of the other two phases’ poles.
PHASE C
PHASE B
PHASE A
FIGURE 20. THREE PAIRS OF FIELD COILS ON THE STATOR, SET 120°° APART
The three currents start at different times. Phase B starts 120° later than phase A
and phase C starts 120° later than phase B. This is shown on the sine wave graph
in Figure 21, which indicates the way the magnetic field will point at various times in
the cycle.
PHASE A
PHASE B
PHASE C
FIGURE 21. MAGNETIC FIELD ROTATION PROVIDING TORQUE TO TURN THE MOTOR
Introducing these different phase currents into three field coils 120° apart on the
stator produces a rotating magnetic field, and the magnetic poles are in constant
rotation.
19
BASICS OF MOTORS AND MOTOR CONTROL
Three-Phase
Motor
(Continued)
The magnetic poles chase each other, simultaneously inducing electric currents in
the rotor (generally, bars of copper imbedded in a laminated iron core). The induced
currents set up their own magnetic fields, in opposition to the magnetic field that
caused the currents. The resulting attractions and repulsions provide the torque to
turn the motor, and keep it turning.
If each magnetic pole were to "light up" whenever it was energized, the effect would
appear as though the lights were "running" around the stator, much as the lights on
some electric signs simulate a running border.
Let’s walk through one revolution of the motor to see how it works.
First, the A poles of the stator are magnetized by phase A. Then, the B poles are
magnetized by phase B. The rotor turns, due to the induced current. Then, the C
poles are magnetized by phase C. The rotor turns, due to the induced current. The
rotor has completed one-half turn at this point.
FIGURE 22. ROTATING MAGNETIC FIELD TURNS THE MOTOR
Now, the A poles of the stator are magnetized again, but the current flow is in the
opposite direction. This causes the magnetic field to continue to rotate, and the
rotor follows. Then, the B poles are magnetized by phase B. The rotor turns, due to
the induced current. Then, the C poles are magnetized by phase C. The rotor turns,
due to the induced current.
FIGURE 23. ROTATING MAGNETIC FIELD TURNS THE MOTOR
The rotor has completed one full revolution at this point, and the process repeats
itself.
20
BASICS OF MOTORS AND MOTOR CONTROL
Construction of
Three-Phase
Motors
The three-phase motor is probably the simplest and most rugged of all electric
motors. To get a perspective on how important the three-phase motor is, all you
need to know is that this motor is used in nine out of ten industrial applications.
All three-phase motors are constructed with a number of individually wound
electrical coils. Regardless of how many individual coils there are in a threephase motor, the individual coils will always be wired together (series or
parallel) to produce three distinct windings, which are called phases. Each
phase will always contain one-third of the total number of individual coils. As we
mentioned, these phases are referred to as phase A, phase B and phase C.
Three-phase motors vary from fractional horsepower size to several thousand
horsepower. These motors have a fairly constant speed characteristic but a wide
variety of torque characteristics. They are made for practically every standard
voltage and frequency and are very often dual voltage motors. (We will look briefly
at dual voltage motors later.)
Wye and Delta
All three-phase motors are wired so that the phases are connected in either a Wye
(Y) or Delta (∆) configuration.
In a Wye (Y) configuration (Figure 24), one end of each of the three-phases is
connected to the other phases internally. The remaining end of each phase is
then brought out externally and connected to the power line. The external leads are
labeled T1, T2 and T3, and are connected to the three-phase power lines labeled
L1, L2 and L3, respectively.
L1
L2
L3
PHASE
C
INTERNAL
CONNECTION
OF ONE END OF
EACH PHASE
T3
T2
PHASE
B
PHASE
A
T1
MOTOR STARTER
FIGURE 24. WYE CONFIGURATION
21
BASICS OF MOTORS AND MOTOR CONTROL
In a Delta (∆
∆ ) configuration (Figure 25), each winding is wired end to end to
form a completely closed loop circuit. At each of the three points where the
phases are connected, a lead is brought out externally. They are labeled T1, T2
and T3, and are connected to the three-phase power lines labeled L1, L2 and L3,
respectively.
L1
L2
L3
PHASE
A
T3
PHASE
B
T2
T1
PHASE
C
MOTOR STARTER
FIGURE 25. DELTA CONFIGURATION
In either case, for the motor to operate properly, the three-phase line supplying
power to the motor must have the same voltage and frequency ratings as the
motor.
Dual Voltage
Many three-phase motors are made so that they can be connected to either of two
voltages. The purpose in making motors for two voltages is to enable the same
motor to be used with two different power line voltages. Usually, the dual
voltage rating of industrial motors is 230/460V. However, the nameplate must
always be checked for proper voltage ratings.
When the electrician has the choice of deciding which voltage to use, the
higher voltage is preferred. The motor will use the same amount of power, giving
the same HP output for either high or low voltage, but as the voltage is doubled
(230 to 460), the current will be cut in half. With half the current, wire size can be
reduced and savings can be realized on installation.
22
BASICS OF MOTORS AND MOTOR CONTROL
REVIEW 2
Answer the following questions without referring to the material just presented.
Begin the next section when you are confident that you understand what you’ve
already read.
1. Name the two groups of AC motors.
___________________________
___________________________
2. Explain why an AC motor does not need a commutator:
_______________________________________________________________
_
3. Three-phase power can be thought of as three different ____________
___________ _________ ____________.
4. Fill in the blanks on the diagram below.
L __
L __
L __
PHASE
___
T3
PHASE
___
T2
T1
PHASE
___
MOTOR STARTER
5. Does the diagram above show a WYE or DELTA configuration?
Circle the correct answer.
23
BASICS OF MOTORS AND MOTOR CONTROL
SPEED
CONTROL
Speed control is essential in many applications. Mining machines, printing
presses, cranes and hosts, elevators, and conveyors, among others, all depend on
speed control.
In choosing the speed control method for an application, there are three main
factors to consider:
•
Type of equipment (load) the motor drives
•
Application type
•
Motor type
We will discuss each of these factors in turn.
Loads and application types are as varied as the types of motors available.
However there are two fundamental motor types: AC and DC. Each type has its
own ability to control different loads at different speeds.
In order to select the correct motor type for a given application, it is necessary
to understand the load requirements first. To understand these requirements,
you need to be familiar with the concepts of force, work, torque, power and
horsepower, and how they relate to speed.
Force, Work
and Torque
Work is done when a force overcomes a resistance. Work is measured with the
formula:
Work = Distance x Force
If you carry a 10-pound bag of groceries 50 feet, 500 foot-pounds (ft-lb.) of work is
done.
In the case of an electric motor, force is not exerted in a line, but in a circle, about a
cylindrical shaft. As you recall, turning force is called torque.
Torque = Radial Distance x Force
If you apply 100 pounds of force to a motor shaft at a radial distance of 5 feet, 500
foot-pounds (ft-lb.) of torque is applied to the shaft.
FORCE
RADIAL
DISTANCE
FIGURE 26. TORQUE = RADIAL DISTANCE X FORCE
24
BASICS OF MOTORS AND MOTOR CONTROL
Power and
Horsepower
Power takes into consideration how fast work is accomplished. Power is the
rate of doing work. The formula to determine power is:
Power = Work/Time
If the 10-pound bag of groceries was connected to a very small motor, it might take
the motor several minutes to move the load 50 feet. If a larger motor was used, it
might move the load in only a few seconds.
The reason for this difference is the amount of work that can be delivered in a given
amount of time. Obviously, a larger motor should be able to deliver more work in a
given time than one that is considerably smaller. It is this difference that determines
the power rating of the motor.
Motors are rated in horsepower (HP). One horsepower is equal to 33,000 ft-lbs.
per minute. (Electrical power can also be measured in watts. One horsepower is
equal to 746 watts of electrical power.) Let’s figure horsepower for a motor to move
those groceries. Recall that:
Work = Distance x Force
If you carry a 10-pound bag of groceries 50 feet, 500 foot-pounds of work is done. If
you connect the bag to a motor that can move it 50 feet in 15 seconds, what is the
horsepower of the motor?
Power = Work/Time
Power = 500 ft-lb/.25 minutes
Power = 2000 ft-lb. per minute
And since 33,000 ft-lb. per min equals 1 HP, (2000 / 33,000) the motor has about
0.06 horsepower.
25
BASICS OF MOTORS AND MOTOR CONTROL
Putting it All
Together
Torque, horsepower, and speed are all interrelated when turning a load.
Horsepower is proportional to torque and speed. The following formula ties them
all together:
HP = (T x N)/5252
Where:
HP = the horsepower provided by the motor
T=
the torque of the motor in foot-pounds
N=
the synchronous speed of the motor in rpm
This means that if either speed or torque remains constant while the other
increases, horsepower increases. Conversely, if either torque or speed decreases
while the other remains constant, horsepower will decrease.
Below is a chart that shows the relationship of horsepower, torque and speed.
SPEED
INCREASES
TORQUE
CONSTANT
é
SPEED
DECREASES
TORQUE
CONSTANT
ê
SPEED
CONSTANT
TORQUE
INCREASES
é
HORSEPOWER
INCREASES
é
HORSEPOWER
DECREASES
ê
HORSEPOWER
INCREASES
é
HORSEPOWER
DECREASES
ê
SPEED
CONSTANT
TORQUE
DECREASES
ê
SPEED
INCREASES
TORQUE
DECREASES
é
ê
HORSEPOWER
REMAINS CONSTANT
SPEED
DECREASES
TORQUE
INCREASES
ê
é
HORSEPOWER
REMAINS CONSTANT
FIGURE 27. HORSEPOWER, TORQUE AND SPEED RELATIONSHIP
26
BASICS OF MOTORS AND MOTOR CONTROL
Application
Types
When a motor is driving a load, it will be called upon to deliver either a constant or a
variable torque, and either a constant or variable horsepower. The amount of torque
and horsepower required, will depend upon the speed and size of the load.
There are three main application types. Let’s consider each briefly.
•
Constant Torque/Variable Horsepower
This type of load is often found on machines that have friction-type loads, such
as conveyors, gear-type pumps, and load lifting equipment.
The horsepower required increases when the speed increases. The torque
requirement does not vary throughout the speed range except for the extra
starting torque needed to overcome the breakaway friction. The torque remains
constant because the force of the load does not change.
•
Constant Horsepower/Variable Torque
This type of load is used for loads that demand high torque at low speeds and
low torque at high speeds. Examples of these loads are machines that roll and
unroll paper or metal.
Since the linear speed of the material is constant, the horsepower must also be
constant. While the speed of the material is kept constant, the motor speed is
not. At start, the motor must run at high speed to maintain the correct material
speed while torque is kept at a minimum. As material is added to the roll, the
motor must deliver more torque at a slower speed. In this application, both
torque and speed are constantly changing while motor horsepower remains the
same.
•
Variable Torque/Variable Horsepower
This type of load is used for loads that have a varying torque and horsepower at
different speeds. Typical applications are fans, blowers, centrifugal pumps,
mixers and agitators.
As the motor speed is increased, so is the load output. Since the motor must
work harder to deliver more output at faster speeds, both torque and
horsepower are increased.
27
BASICS OF MOTORS AND MOTOR CONTROL
Speed Control
for a DC Motor
Now that you understand what factors are important in choosing a motor for an
application, we are ready to look at how to actually control the speed of the motor.
Let’s start with the DC motor.
The base speed of a motor is the speed at which the motor will run with full line
voltage applied to the armature and the field.
The speed of a DC motor is controlled by varying the applied voltage across the
armature, the field, or both. When armature voltage is controlled, the motor will
deliver a constant torque characteristic. When field voltage is controlled, the motor
will deliver a constant horsepower characteristic.
MOTOR SPEED IN RPM
REDUCING FIELD VOLTAGE
INCREASES SPEED ABOVE
BASE SPEED
BASE SPEED OF THE MOTOR
(FULL FIELD AND ARMATURE
VOLTAGE APPLIED)
REDUCING ARMATURE VOLTAGE
DECREASES SPEED BELOW
BASE SPEED
DC APPLIED VOLTAGE
FIGURE 28. FIELD VOLTAGE VS. ARMATURE VOLTAGE IN CONTROLLING A DC MOTOR’S SPEED
DC motors are used in industrial applications that require either variable speed
control, high torque, or both. Since the speed of most DC motors can be
controlled smoothly and easily from zero to full speed, DC motors are used in many
acceleration and deceleration applications.
The DC motor is ideal in applications where momentarily higher torque output is
needed. The DC motor can deliver three to five times its rated torque for short
periods of time. (Most AC motors will stall with a load that requires twice the rated
torque.)
For these reasons, DC motors are used to run large machine tools, cranes and
hoists, printing presses, cranes, elevators, shuttle cars and automobile starters.
28
BASICS OF MOTORS AND MOTOR CONTROL
Speed Control
for an AC Motor
Since each motor type has its own characteristics of horsepower, torque and speed,
different motor types are more suited for different applications.
The basic characteristics of each AC motor type are determined by the design of
the motor and the supply voltage used. These design types are classified and
given a letter designation, which can be found on the nameplate of motor
types listed as “NEMA Design.”
NEMA
Design
A
Starting
Torque
Normal
Starting
Current
Normal
Breakdown
Torque
High
Full Load
Slip
Low
Typical
Applications
Machine Tool
Fan
Centrifugal Pump
B
Normal
Low
High
Low
Machine Tool
Fan
Centrifugal Pump
C
High
Low
Normal
Low
Loaded
Compressor
Loaded Conveyor
D
Very High
Low
---
High
Punch Press
The most commonly used AC NEMA Design motor is the NEMA B.
IN THE WORKPLACE
The conveyor on this beer
bottling line is powered by a
NEMA Design B motor.
The NEMA Design B motor is a
general purpose AC induction
motor. It is the most commonly
used NEMA Design motor,
because it offers a good balance
of function against price.
MOTOR
NEMA DESIGN B MOTOR AT WORK
29
BASICS OF MOTORS AND MOTOR CONTROL
Speed Control
for an AC Motor
(continued)
The induction motor is basically a constant speed device. The speed at which
an induction stator field rotates is called its synchronous speed. This is because it is
synchronized to the frequency of the AC power at all times. The speed of the
rotating field is always independent of load changes on the motor, provided the line
frequency is constant.
Synchronous speed is determined by the number of poles that the motor has,
and the frequency being supplied to it. The equation for determining the
synchronous speed of a motor is:
N = 120f/P
Where:
N=
the synchronous speed of the motor in revolutions per minute (RPM)
f=
the frequency supplied to the motor in Hertz (Hz)
P=
the number of poles the motor has
Motors designed for 60 Hertz use (standard in the US) have synchronous speeds
as follows:
Poles
2
4
6
8
10
12
14
16
RPM
3600
1800
1200
900
720
600
514
450
Induction motors do not run at synchronous speed; they run at full load speed,
which is the rotational speed of the rotor. Full load speed is always slower. The
percent reduction in speed is called percent slip. The slip is required to develop
rotational torque. The higher the torque, the greater the slip.
The motor speed, under normal load conditions, is rarely more than 10% below
synchronous speed. If the motor is not driving a load, it will accelerate to nearly
synchronous speed. As the load increases, the percent slip increases.
For example, a motor with a 2.8% slip and 1800 rpm synchronous speed would
have a slip of 50 rpm, and a full load speed of 1750 rpm (1800 - 50 = 1750 rpm). It
is this full load speed that will be found on the motor's nameplate.
From the formula, it is evident that the supply frequency and number of poles are
the only variables that determine the speed of the motor.
Varying the voltage is not a good way to change the speed of the motor. In fact, if
the voltage is changed by more than 10%, the motor may be damaged. This is
because the starting torque varies as the square of the applied voltage.
30
BASICS OF MOTORS AND MOTOR CONTROL
Speed Control
for an AC Motor
(continued)
Since the frequency or number of poles must be changed to change the speed of
an AC motor, two methods of speed control are available. These are:
•
Changing the frequency applied to the motor
Changing the frequency requires a device called an adjustable frequency drive
to be inserted upstream from the motor. This device converts the incoming 60
Hz into any desired frequency, allowing the motor to run at virtually any speed.
For example, by adjusting the frequency to 30 Hz, the motor can be made to run
only half as fast.
We will look at adjustable frequency drives in much more detail in Module 20,
Adjustable Frequency Drives.
•
Using a multispeed motor
Multispeed AC motors are designed with windings that may be reconnected
to form different numbers of poles. They are operated at a constant
frequency.
Two-speed motors usually have one winding that may be connected to provide
two speeds, one of which is half the other.
Motors with more than two speeds usually include many windings. These can be
connected many ways to provide different speeds. Refer to APPENDIX A:
Typical Multispeed Motor Connections.
IN THE WORKPLACE
Everyone is familiar with this
piece of equipment. The portable
three-speed oscillating fan can
be found in most homes.
The fan’s multispeed motor
contains many windings that can
be connected three different
ways. This allows the user to set
the fan to run at any of the three
preset speeds.
THREE-SPEED OSCILLATING FAN
31
BASICS OF MOTORS AND MOTOR CONTROL
STARTING
THE MOTOR
Across the Line
A starter is a device that is used to start a motor from a stop. The across-the-line
starter is by far the most common. This type of starter places the motor directly
across the full voltage of the supply lines, hence the name: "across-the-line.” When
an induction motor is placed across-the-line, it will accelerate to full speed in a
matter of seconds.
What applications are suitable for this type of rapid acceleration? Pumps of all
types, fans and blowers, and most machines such as drill presses, lathes and
grinders are suitable.
We will discuss starters in much more detail in Module 19, Starter Basics.
Small DC motors are generally started by simply closing the line switch. No auxiliary
starting equipment is necessary to limit the initial rush of current. The same practice
applies to most small (and some large) polyphase motors.
Minimizing
Inrush Current
During an AC motor’s start-up accelerating period, a large amount of current is
required to start the motor rotating and bring it up to speed. This is called inrush
current. Currents 6 to 8 times the full load rating of the motor are not
uncommon when the motor is started across-the-line.
From this, we can see that the power company will be rather concerned, since they
have to supply the actual current necessary to start (and also to run) the motor. So,
it is desirable (if not necessary) to limit the initial rush of current to a reasonable
value, about 1.25 to 5 times the full load rating. There are several ways of doing
this:
32
•
(AC/DC) Inserting resistance in the line, and then cutting the resistance
gradually as the motor comes up to speed.
•
(AC) Using a reduced voltage starter, which we will discuss in much more detail
in Module 21, Reduced Voltage Starters.
•
(AC) Using a wound rotor type of motor, which employs a resistor controller for
the starting function and which may also serve as a speed control device.
•
(AC) Using the Wye-Delta method, in which the stator is connected in a Wye at
the instant of starting, and in Delta after the motor has reached normal speed.
•
(AC) Using an adjustable frequency drive, which we will discuss in much more
detail in Module 20, Adjustable Frequency Drives.
BASICS OF MOTORS AND MOTOR CONTROL
REVERSING
THE MOTOR
In applications where it is desirable to run a motor in both forward and reverse,
there are a few options for providing a reversing capability.
Manual
Reversing
Starter
A manual reversing starter can be used to change the direction of rotation of a
three-phase, a single-phase or a DC motor. It is made by simply connecting two
manual starters together. The electrical diagram is shown in Figure 29.
POWER TERMINAL CONNECTIONS
START
START
MECHANICAL
INTERLOCK
FORWARD
CONTACTS
F
F
F
R
R
REVERSE
R CONTACTS
STOP
STOP
MOTOR TERMINAL CONNECTIONS
FIGURE 29. MANUAL REVERSING STARTER
This type of device is generally used to run lower horsepower motors, such as
those found on fans, small machines, pumps and blowers.
Magnetic
Reversing
Starter
A magnetic reversing starter performs the same function as a manual reversing
starter. Electrically, the only difference between manual and magnetic starters
is the addition of forward and reversing coils and the use of auxiliary contacts.
The forward and reversing coils replace the pushbuttons of a manual starter. The
auxiliary contacts provide additional electrical protection and circuit flexibility.
33
BASICS OF MOTORS AND MOTOR CONTROL
BRAKING THE Two common methods used for braking a motor are DC injection braking and
dynamic braking. We will look at both in detail, starting with electric braking.
MOTOR
DC Injection
Braking
DC injection braking is a method of braking in which direct current (DC) is applied
to the stationary windings of an AC motor after the AC voltage is removed. This
is an efficient and effective method of braking most AC motors. DC injection braking
provides a quick and smooth braking action on all types of loads, including highspeed and high-inertia loads.
Recall that opposite magnetic poles attract and like magnetic poles repel. This
principle, when applied to both AC and DC motors, is the reason why the motor
shaft rotates.
In an AC induction motor, when the AC voltage is removed, the motor will coast to a
standstill over a period of time, since there is no induced field to keep it rotating.
Since the coasting time may be unacceptable, particularly in an emergency
situation, electric braking can be used to provide a more immediate stop.
By applying a DC voltage to the stationary windings once the AC is removed, a
magnetic field is created in the stator that will not change polarity.
In turn, this constant magnetic field in the stator creates a magnetic field in the
rotor. Since the magnetic field of the stator is not changing in polarity, it will attempt
to stop the rotor when the magnetic fields are aligned (N to S and S to N).
STATOR
ROTOR
FIGURE 30. DC INJECTION BRAKING
The only thing that can keep the rotor from stopping with the first alignment is the
rotational inertia of the load connected to the motor shaft. However, since the
braking action of the stator is present at all times, the motor is braked quickly and
smoothly to a standstill.
Since there are no parts that come in physical contact during braking, maintenance
is kept to a minimum.
34
BASICS OF MOTORS AND MOTOR CONTROL
Dynamic Braking
Dynamic braking is another method for braking a motor. It is achieved by
reconnecting a running motor to act as a generator immediately after it is
turned off, rapidly stopping the motor. The generator action converts the
mechanical energy of rotation to electrical energy that can be dissipated as heat in
a resistor.
Dynamic braking of a DC motor may be needed because DC motors are often used
for lifting and moving heavy loads that may be difficult to stop.
There must be access to the rotor windings in order to reconnect the motor to act as
a generator. On a DC motor, access is accomplished through the brushes on the
commutator.
In this circuit, the armature terminals of the DC motor are disconnected from the
power supply and immediately connected across a resistor, which acts as a
load. The smaller the resistance of the resistor, the greater the rate of energy
dissipation and the faster the motor slows down.
The field windings of the DC motor are left connected to the power supply. The
armature generates a voltage referred to as “counter electromotive force” (CEMF).
This CEMF causes current to flow through the resistor and armature. The current
causes heat to be dissipated in the resistor, removing energy from the system
and slowing the motor rotation.
The generated CEMF decreases as the speed of the motor decreases. As the
motor speed approaches zero, the generated voltage also approaches zero. This
means that the braking action lessens as the speed of the motor decreases. As a
result, a motor cannot be braked to a complete stop using dynamic braking.
Dynamic braking also cannot hold a load once it is stopped, because there is no
more braking action.
For this reason, electromechanical friction brakes are sometimes used along with
dynamic braking in applications that require the load to be held, or in applications
where a large heavy load is to be stopped. This is similar to using a parachute to
slow a race car before applying the brakes.
FIGURE 31. DYNAMIC BRAKING IS OFTEN USED WITH ELECTROMECHANICAL FRICTION BRAKING
Dynamic braking for AC motors can be handled with an adjustable frequency drive.
We will discuss adjustable frequency drive in much more detail in Module 20,
Adjustable Frequency Drives.
35
BASICS OF MOTORS AND MOTOR CONTROL
REVIEW 3
Answer the following questions without referring to the material just presented.
1. Fill in the blanks for the following formulas:
Work = _________ x_________
Power = _________ / __________
2. Work out the horsepower rating of a motor that moves a load of 1000 pounds a
distance of 330 feet in one minute.
Answer: _________ HP
3. A conveyor is an example of a ________ Torque / _________ Horsepower
application.
4. Name the two devices that can be used to reverse the direction of a motor.
________________________________
________________________________
5. Reducing the voltage supplied to the field of a DC motor will cause the motor
speed to INCREASE or DECREASE. Circle the correct answer.
6. Using the synchronous speed formula, calculate the full load speed of a motor
with 8 poles running on 60 Hz with a slip of 2.2%.
Answer: _________ RPM
36
BASICS OF MOTORS AND MOTOR CONTROL
GLOSSARY
Adjustable
Frequency Drive
This device converts the incoming 60 Hz power into any
desired frequency, allowing an AC motor to run at virtually
any speed.
Armature
The turning conductor in a DC motor.
Base Speed
The speed at which a DC motor will run with full voltage
applied to the armature and the field
Brushes
The stationary components of the commutator, providing
current to the rotating commutator segments.
Coils
The stationary windings of the DC motor that generate an
electromagnetic field.
Commutator
A device used in a DC motor to reverse the current in the
armature every one-half rotation so that the magnetic fields
will work together to maintain rotation.
Compound Motor
A DC motor that combines the characteristics of both the
series and the shunt motors.
Conventional Flow A theory regarding the flow of current. It states that current
flows from positive to negative.
Theory
DC Injection
Braking
A method of braking an AC motor in which direct current
(DC) is applied to the stationary windings of an AC motor
after the AC voltage is removed.
Delta
A motor connection arrangement where each winding is
wired end to end to form a completely closed loop circuit.
Dual Voltage
Motor
A motor made for two voltages. It enables the same motor
to be used with two different power line voltages.
Dynamic Braking
A method of braking a DC motor by reconnecting a running
motor to act as a generator immediately after it is turned
off. Reconnecting the motor in this way makes the motor
act as a loaded generator that develops a retarding torque,
rapidly slowing the motor.
Electron Flow
Theory
A theory regarding the flow of current which states that
current flows from negative to positive.
Full Load Speed
The true speed at which a motor turns, found on the
nameplate. To calculate, take Synchronous Speed minus
Percent Slip. It is the speed of the rotor.
Horsepower
A unit of power measurement, used for rating the amount of
Work a motor can do. One horsepower equals 33,000 footpounds per minute of Work.
37
BASICS OF MOTORS AND MOTOR CONTROL
38
Induction
The process of producing a current by the relative motion of
a magnetic field across a conductor.
Left Hand
Flux Rule
The relationship of the factors used to determine is which
direction the magnetic flux moves around a conductor.
Imagine grasping the wire with your left hand, making sure
your thumb points in the direction of the current flow. Your
fingers will curl around the wire in the direction of the
magnetic flux.
Magnetic Flux
The direction of a magnetic field.
Magnetic
Reversing Starter
A device that performs the same function as a manual
reversing starter. Electrically, the only difference between
manual and magnetic starters is the addition of forward and
reversing coils and the use of auxiliary contacts.
Manual Reversing
Starter
A device used to change the direction of rotation of a threephase, a single-phase or a DC motor. It is made by simply
connecting two manual starters together.
Neutral Position
The position at which the armature in a DC motor is parallel
to the magnetic field, where no torque is produced.
Percent Slip
The percentage difference between a motor’s Synchronous
Speed and its Full Load Speed.
Polarity
Direction of current flow through a conductor.
Poles
The stationary windings of the DC motor that generate an
electromagnetic field.
Power
A measure of work done per unit of time.
Reduced Voltage
Starter
A type of starter that ramps up the power to a motor
gradually to cut down on current draw at start-up.
Right Hand
Motor Rule
The relationship between the factors involved in
determining the movement of a conductor in a magnetic
filed. The index finger points in the direction of the magnetic
field (N to S), the middle finger points in the direction of
electron current flow in the conductor, and the thumb points
in the direction of the force on the conductor.
Rotor
The rotating part of an AC motor.
Series Motor
A DC motor with the field coil wired in series with the
armature coil. It is also called a universal motor.
Shunt Motor
A DC motor with the field coil wired in parallel with the
armature coil.
BASICS OF MOTORS AND MOTOR CONTROL
Starter
A device that is used to start a motor from a stop
Stationary
Windings
The stationary windings of the DC motor that generate an
electromagnetic field.
Stator
The stationary part of an AC motor.
Squirrel Cage
Induction Motor
The most common AC motor type, named for the rotor’s
resemblance to a cage used for exercising squirrels.
Synchronous
Speed
The rotational speed of the stator, defined by the formula:
N = 120f/P
Where:
N=
the synchronous speed of the motor in revolutions
per minute (RPM)
f=
the frequency supplied to the motor in Hertz (Hz)
P=
the number of poles the motor has
REFERENCE
Torque
Turning or rotational force.
Work
Applying a force over a distance.
Wye
A motor connection arrangement where one end of each of
the three-phases is connected to the other phases
internally. The remaining end of each phase is then brought
out externally.
In preparing this training module, some material was taken from the publication
listed below:
Gary Rockis and Glenn A. Mazur, Electrical Motor Controls. (Homewood, IL:
American Technical Publishers, Inc., 1997).
39
BASICS OF MOTORS AND MOTOR CONTROL
REVIEW 1
ANSWERS
1. Thumb:
Index:
Middle:
Direction of the conductor movement
Direction of the magnetic flux
Direction of current flow through the conductor
2. When the armature is parallel to the magnetic field, no torque is produced.
They are hard to start.
3. Blanks on the bottom of the graph, from left to right: “1/4”, “1/2”, “3/4”.
Blanks on the side of the graph, from top to bottom: “Torque”, “Speed”.
4. Reversing the direction of the current through the field.
Reversing the direction of the current through the armature.
5. Series, shunt and compound
REVIEW 2
ANSWERS
1. Single phase and polyphase
2. AC changes polarity “naturally.”
3. single-phase power supplies
4. Blanks from left to right: “L1”, “L2”, “L3”, “B”, “C”, “A”.
5. Delta
REVIEW 3
ANSWERS
1. Work = Distance x Force
Power = Work/Time
2. 10
3. Constant Torque / Variable Horsepower
4. Manual reversing starter; Magnetic reversing starter
5. Increase
5. About 800 RPM
40
BASICS OF MOTORS AND MOTOR CONTROL
Common motor connection arrangements, conforming to NEMA standards, are
APPENDIX A:
used when connecting motors. The diagrams on these two pages are typical
TYPICAL
arrangements, but do not depict all possible arrangements.
MULTISPEED
MOTOR
CONNECTIONS
41
BASICS OF MOTORS AND MOTOR CONTROL
42
Cutler-Hammer
Milwaukee, Wisconsin U.S.A.
Publication No. TR.90.06.T.E
February 1999
Printed in U.S.A. (GSP)
101 Basics Series and 201 Advanced Series are trademarks of Cutler-Hammer University, Cutler-Hammer and Eaton Corp.
©1999, Eaton Corp.