of 0.1 Ω and a full-load armature current of 20 A.
Determine
a. the value of the armature current on starting.
b. the value of the counter EMF with full-load applied.
15. a. What is motor armature reaction?
b. State three effects that armature reaction has on the operation of a DC motor.
16. Explain how interpoles minimize the effects of armature reaction.
17. a. A motor rated for 1750 rpm at no load has a 4 percent speed regulation. Calculate the speed of the motor with full load applied.
b. In what way does a DC motor’s armature resis- tance affect its speed regulation?
18. a. How is the base speed of a DC motor defined?
b. How is the speed of a DC motor controlled below base speed?
c. How is the speed of a DC motor controlled above base speed?
19. With armature voltage control of a DC shunt motor, what is the effect on the rated torque and horse- power when the armature voltage is increased?
20. With field current control of a shunt DC motor, what is the effect on the rated torque and horse- power when the armature voltage is increased?
21. Field loss protection must be provided for DC motors. Why?
22. List several control functions found on a DC motor drive that would not normally be provided by a traditional DC magnetic motor starter.
1. Give two reasons why DC motors are seldom the first motor of choice for most applications.
2. What special types of processes may warrant the use of a DC motor?
3. Explain the function of the commutator in the oper- ation of a DC motor.
4. a. How is the direction of rotation of a permanent- magnet motor changed?
b. How is the speed of a permanent-magnet motor controlled?
5. Summarize the torque and speed characteristics of a DC series motor.
6. Why should a DC series motor not be operated without some sort of a load coupled to it?
7. In what way is the shunt field winding of a shunt motor different from that of the series field winding of a series motor?
8. Compare the starting torque and load versus speed characteristics of the series motor to those of shunt wound motor.
9. How are the series and shunt field windings of the compound wound DC motor connected relative to the armature?
10. In what way is a cumulative-compound motor connected?
11. Compare the torque and speed characteristics of a compound motor with those of the series and shunt motors.
12. How can the direction of rotation of a wound DC motor be changed?
13. Explain how counter EMF is produced in a DC motor.
PART 2 Review Questions
PART 3 Three-Phase Alternating Current Motors
Rotating Magnetic Field
The main difference between AC and DC motors is that the magnetic field generated by the stator rotates in the case of AC motors. A rotating magnetic fi eld is key to the operation of all AC motors. The principle is simple. A magnetic field in the stator is made to rotate electrically around and around in a circle. Another magnetic field in the rotor is made to follow the rotation of this field pattern by being attracted
and repelled by the stator field. Because the rotor is free to turn, it follows the rotating magnetic field in the stator.
Figure 5-31 illustrates the concept of a rotating mag- netic field as it applies to the stator of a three-phase AC motor. The operation can be summarized as follows:
• Three sets of windings are placed 120 electrical degrees apart with each set connected to one phase of the three-phase power supply.
• When three-phase current passes through the stator windings, a rotating magnetic field effect is produced that travels around the inside of the stator core.
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102 Chapter 5 Electric Motors
This means the higher the frequency, the greater the speed and the greater the number of poles the slower the speed.
Motors designed for 60 Hz use have synchronous speeds of 3,600, 1,800, 1,200, 900, 720, 600, 514, and 450 rpm.
The synchronous speed of an AC motor can be calculated by the formula:
S = 120 f
_____
P where
S = synchronous speed in rpm
f = frequency, Hz, of the power supply P = number of poles wound in each of the
single-phase windings
• Polarity of the rotating magnetic field is shown at six selected positions marked off at 60 degree inter- vals on the sine waves representing the current flow- ing in the three phases, A, B, and C.
• In the example shown, the magnetic field will rotate around the stator in a clockwise direction.
• Simply interchanging any two of the three-phase power input leads to the stator windings reverses direction of rotation of the magnetic field.
• The number of poles is determined by how many times a phase winding appears. In this example, each winding appears twice, so this is a two-pole stator.
There are two ways to define AC motor speed. First is synchronous speed. The synchronous speed of an AC motor is the speed of the stator’s magnetic field rota- tion. This is the motor’s ideal theoretical, or mathemati- cal, speed, since the rotor will always turn at a slightly slower rate. The other way motor speed is measured is called actual speed . This is the speed at which the shaft rotates. The nameplate of most AC motors lists the actual motor speed rather than the synchronous speed (Figure 5-32).
The speed of the rotating magnetic field varies directly with the frequency of the power supply and inversely with the number of poles constructed on the stator winding.
RPM Synchronous
(1800-rpm) 1725
RPM
Figure 5-32 Synchronous and actual speed.
A
A B
C
C
B
A
A B
C
C
B
A
A B
C
C
B
A
A B
C
C
B
A
A B
C
C
B A
Phase C 1
1 2 3 4 5 6 7
2 4 6
3 5 7
Phase A
Three- phase input Phase B
C A
B A
B
C
C
B
A
A B
C
C
B
O
O S N
S N
N
S S O O N
S
N O N S O
O
O S N
S N
S
N N O O S N
S O S N O
O
O S N
N S
A
A
C B
C B
Phase A Two poles wound in each single-phase winding
Phase B Phase C
Stator connection to three-phase supply
Rotor
L1 L2 L3
Figure 5-31 Rotating magnetic field.
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PART 3 Three-Phase Alternating Current Motors 103 Squirrel Cage Induction Motor
An induction motor rotor can be either wound rotor or a squirrel cage rotor. The majority of commercial and indus- trial applications usually involve the use of a three-phase squirrel-cage induction motor. A typical squirrel-cage induc- tion motor is shown in Figure 5-35. The rotor is constructed using a number of single bars short-circuited by end rings and arranged in a hamster-wheel or squirrel-cage configura- tion. When voltage is applied to the stator winding, a rotat- ing magnetic field is established. This rotating magnetic field causes a voltage to be induced in the rotor, which, because the rotor bars are essentially single-turn coils, causes currents to flow in the rotor bars. These rotor currents establish their own magnetic field, which interacts with the stator magnetic field to produce a torque. The resultant production of torque spins the rotor in the same direction as the rotation of the magnetic field produced by the stator. In modern induction motors, the most common type of rotor has cast-aluminum conductors and short-circuiting end rings.
The resistance of the squirrel-cage rotor has an impor- tant effect on the operation of the motor. A high-resistance Induction Motor
The AC induction motor is by far the most commonly used motor because it is relatively simple and can be built at less cost than other types. Induction motors are made in both three-phase and single-phase types. The induction motor is so named because no external voltage is applied to its rotor. There are no slip rings or any DC excitation supplied to the rotor. Instead, the AC current in the sta- tor induces a voltage across an air gap and into the rotor winding to produce rotor current and associated magnetic field (Figure 5-33). The stator and rotor magnetic fields then interact and cause the rotor to turn.
A three-phase motor stator winding consists of three separate groups of coils, called phases, and designated A, B, and C. The phases are displaced from each other by 120 electrical degrees and contain the same number of coils, connected for the same number of poles. Poles refer to a coil or group of coils wound to produce a unit of magnetic polarity. The number of poles a stator is wound for will always be an even number and refers to the total number of north and south poles per phase. Figure 5-34 shows a typical connection of coils for a four-pole, three- phase Y-connected induction motor.
E X A M P L E 5 - 3
Problem: Determine the synchronous speed of a four- pole AC motor connected to a 60-Hz electrical supply.
Solution:
S = 120 f_____
P
= 120 ________ × 60 4
= 1,800 rpm
Current
Magnetic fields
Figure 5-33 Induced rotor current.
Figure 5-34 Stator coils for a Y-connected four-pole, three-phase inductor motor.
Photo courtesy Swiger Coil LLC., www.swigercoil.com.
T1
T3 T2
Phase A
Phase B Phase C
Stator coil groupings
Stator poles N
S S
A B
C A B
C A B A C
B C
N
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104 Chapter 5 Electric Motors
torque. This type is suitable for equipment with very high inertia starts such as cranes and hoists.
Operating characteristics of the squirrel-cage motor include the following:
• The motor normally operates at essentially constant speed, close to the synchronous speed.
• Large starting currents required by this motor can result in line voltage fluctuations.
• Interchanging any two of the three main power lines to the motor reverses the direction of rotation.
Figure 5-37 shows the power circuit for reversing a three-phase motor. The forward contacts F, when closed, connect L1, L2, and L3 to motor terminals T1, T2, and T3, respectively. The reverse contacts R, when closed, connect L1, L2, and L3 to motor rotor develops a high starting torque at low starting cur-
rent. A low-resistance rotor develops low slip and high efficiency at full load. Figure 5-36 shows how motor torque varies with rotor speed for three NEMA-type squirrel-cage induction motors:
NEMA Design B —Considered a standard type with normal starting torque, low starting current, and low slip at full load. Suitable for a broad variety of appli- cations, such as fans and blowers, that require normal starting torque.
NEMA Design C —This type has higher than stan- dard rotor resistance, which improves the rotor power factor at start, providing more starting torque. When loaded, however, this extra resistance causes a greater amount of slip. Used for equipment, such as a pump, that requires a high starting torque.
NEMA Design D —The even higher rotor resistance of this type produces a maximum amount of starting
0 50 100 150 200 300
Type D Type C
Type B 250
25 50 75 100
% synchronous speed
% full-load torque
Figure 5-36 Typical squirrel-cage motor speed–torque characteristics.
Three-phase stator winding
Squirrel cage rotor Enclosure
Stator
Rotor
Figure 5-35 Squirrel-cage induction motor.
Figure 5-37 Power circuit for reversing a three-phase motor.
Photo courtesy Eaton Corporation, www.eaton.com.
OL
L1 L2 L3
F F F R R R
T2
T1 T3
Motor
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PART 3 Three-Phase Alternating Current Motors 105 current to flow in the stator windings, just as an increase in the current in the secondary of a transformer results in a corresponding increase in the primary current.
You may recall that power factor (PF) is defined as the ratio of the actual (or true) power (watts) to the apparent power (volt-amperes) and is a measure of how effectively the current drawn by a motor is converted into useful work. The motor exciting current and reactive power under load remain about the same as at no load. For this reason, whenever a motor is operating with no load, the power factor is very low in comparison to when it is oper- ating at full load. At full load the PF ranges from 70 per- cent for small motors to 90 percent for larger motors.
Induction motors operate at their peak efficiency if they are sized correctly for the load that they will drive. Over- sized motors not only operate inefficiently, but they also carry a higher first cost than right-sized units.
The moment a motor is started, during the acceleration period, the motor draws a high inrush current. This inrush current is also called the locked-rotor current . Common induction motors, started at rated voltage, have locked- rotor starting currents of up to 6 times their nameplate full- load current. The locked-rotor current depends largely on the type of rotor bar design and can be determined from the NEMA design code letters listed on the nameplate.
High locked-rotor motor current can create voltage sags or dips in the power lines, which may cause objectionable light flicker and problems with other operating equip- ment. Also, a motor that draws excessive current under locked-rotor conditions is more likely to cause nuisance tripping of protection devices during motor start-ups.
A single-speed motor has one rated speed at which it runs when supplied with the nameplate voltage and frequency. A multispeed motor will run at more than one speed, depending on how the windings are connected to form a different number of magnetic poles. Two- speed, single-winding motors are called consequent pole motors. The low speed on a single-winding consequent pole motor is always one-half of the higher speed. If requirements dictate speeds of any other ratio, a two- winding motor must be used. With separate winding motors a separate winding is installed in the motor for each desired speed.
Consequent pole single-winding motors have their stator windings arranged so that the number of poles can be changed by reversing some of the coil currents.
Figure 5-38 shows a dual-speed three-phase squirrel-cage single-winding motor with six stator leads brought out.
By making the designated connections to these leads, the windings can be connected in series delta or parallel wye.
The series delta connection results in low speed and the parallel wye in high speed. The torque rating would be terminals T3, T2, and T1, respectively, and the
motor will now run in the opposite direction.
• Once started, the motor will continue to run, with a phase loss, as a single-phase motor. The current drawn from the remaining two lines will almost double, and the motor will overheat. The motor will not start from standstill if it has lost a phase.
The rotor does not revolve at synchronous speed, but tends to slip behind. Slip is what allows a motor to turn.
If the rotor turned at the same speed at which the field rotates, there would be no relative motion between the rotor and the field and no voltage induced. Because the rotor slips with respect to the rotating magnetic field of the stator, voltage and current are induced in the rotor.
The difference between the speed of the rotating magnetic field and the rotor in an induction motor is known as slip and is expressed as a percentage of the synchronous speed as follows:
Percent slip = _____________________________ Synchronous speed – Actual speed Synchronous speed × 100 The slip increases with load and is necessary to produce useful torque. The usual amount of slip in a 60-Hz, three-phase motor is 2 or 3 percent.
E X A M P L E 5 - 4
Problem: Determine the percent slip of an induction motor having a synchronous speed of 1,800 rpm and a rated actual speed of 1,750 rpm.
Solution:
Percent slip
= ____________________________ Synchronous speed – Actual speed Synchronous speed × 100
= ____________ 1,800 – 1,750 1,800 × 100
= 2.78%
Loading of an induction motor is similar to that of a transformer in that the operation of both involves chang- ing flux linkages with respect to a primary (stator) wind- ing and secondary (rotor) winding. The no-load current is low and similar to the exciting current in a transformer.
Thus, it is composed of a magnetizing component that cre- ates the revolving flux and a small active component that supplies the windage and friction losses in the rotor plus the iron losses in the stator. When the induction motor is under load, the rotor current develops a flux that opposes and, therefore, weakens the stator flux. This allows more
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106 Chapter 5 Electric Motors
ratings in fixed-frequency applications. The multiple leads may be designed to allow either series to parallel reconnec- tions, wye to delta reconnections, or combinations of these.
Figure 5-39 shows typical connections for dual-voltage wye and delta series and parallel reconnections. These types of reconnections should not be confused with the reconnection the same at both speeds. If the winding is such that the
series delta connection gives the high speed and the paral- lel wye connection the low speed, the horsepower rating is the same at both speeds.
Single-speed AC induction motors are frequently supplied with multiple external leads for various voltage
T1
T5
T2 T3
L2 L1 L3 T4
T6
Series delta - low speed Parallel wye - high speed
L2 L1 L3
T1 T3 T2
T5 T4
T6
NEMA Nomenclature—6 Leads
High Low
6 1 Speed
4 5
2 3
L2 L3
1&2&3 join 4-5-6 open
2 wye Typical connection
1 delta L1
Figure 5-38 Dual-speed, three-phase squirrel-cage single-winding motor.
© Baldor Electric Company. Reprinted with their permission. Photo Baldor, www.baldor.com.
T1
T4 T7
T8 T5
T2 T3
T9 T6
T1
T4 T7 T9
T6
T8 T5
T2 T3
9-lead dual-voltage wye- connected motor
6 5 4
Low voltage (parallel)
L3
9 8 7
3 2 1
L2 L1
High voltage (series)
6 5 4
9 8 7
3 2 1
L3 L2 L1
9-lead dual-voltage delta- connected motor
9 4 7
5 8 6
3 2 1
Low voltage (parallel)
L3 L2 L1
High voltage (series)
L3 L2 L1
9 4 7
5 8 6
3 2 1
Figure 5-39 Typical connections for dual-voltage wye and delta series and parallel reconnections.
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PART 3 Three-Phase Alternating Current Motors 107 high-inertia load with a standard cage motor would require between 400 and 550 percent start current for up to 60 sec- onds. Starting the same machine with a wound-rotor motor (slip-ring motor) would require around 200 percent current for around 20 seconds. For this reason, wound rotor types are frequently used instead of the squirrel-cage types in larger sizes.
Wound-rotor motors are also used for variable-speed service. To use a wound-rotor motor as an adjustable-speed drive, the rotor control resistors must be rated for continuous current. If the motor is used only for a slow acceleration or high starting torque but then operates at its maximum speed for the duration of the work cycle, then the resistors will be removed from the circuit when the motor is at rated speed.
In that case they will have been duty cycle–rated for starting duty only. Speed varies with this load, so that they should not be used where constant speed at each control setting is required, as for machine tools.
Three-Phase Synchronous Motor
The three-phase synchronous motor is a unique and spe- cialized motor. As the name suggests, this motor runs at a constant speed from no load to full load in synchronism with line frequency. As in squirrel-cage induction motors, the speed of a synchronous motor is determined by the number of pairs of poles and the line frequency.
A typical three-phase synchronous motor is shown in Figure 5-41.The operation of the motor can be summa- rized as follows.
• Three-phase AC voltage is applied to the stator windings and a rotating magnetic field is produced.
of multispeed polyphase induction motors. In the case of multispeed motors, the reconnection results in a motor with a different number of magnetic poles and therefore a differ- ent synchronous speed at a given frequency.
Wound-Rotor Induction Motor
The wound-rotor induction motor (sometimes called a slip-ring motor) is a variation on the standard cage induc- tion motors. Wound-rotor motors have a three-phase winding wound on the rotor, which is terminated to slip rings as illustrated in Figure 5-40. The operation of the motor can be summarized as follows.
• The rotor slip rings connect to start-up resistors in order to provide current and speed control on start-up.
• When the motor is started, the frequency of current flowing through the rotor windings is nearly 60 Hz.
• Once up to full speed, the rotor current frequency drops down below 10 Hz to nearly a DC signal.
• The motor is normally started with full external resistance in the rotor circuit that is gradually reduced to zero, either manually or automatically.
• This results in a very high starting torque from zero speed to full speed at a relatively low starting current.
• With zero external resistance, the wound-rotor motor characteristics approach those of the squirrel cage motor.
• Interchanging any two stator voltage supply leads reverses the direction of rotation.
A wound-rotor motor is used for constant-speed appli- cations requiring a heavier starting torque than is obtain- able with the squirrel-cage type. With a high-inertia load a standard cage induction motor may suffer rotor damage on starting due to the power dissipated by the rotor. With the wound rotor motor, the secondary resistors can be selected to provide the optimum torque curves and they can be sized to withstand the load energy without failure. Starting a
T1 T2 T3
External variable resistors
Figure 5-40 Wound-rotor induction motor. Figure 5-41 Three-phase synchronous motor.
Photo courtesy ABB, www.abb.com.
Stator field windings L1
L2
L3 Rotor field
winding F1
F2
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