AC Machines Final Review Flashcards
Does a three-phase SCIM operate at its higher power factor when it is operating at no load or full load? Explain
Power factor is highest when motor is operating at full load. VARs remain the same from no load to full load, but as power out increases, power in also increases.
A fully-loaded synchronous motor has its excitation adjusted so that its power factor is unity. If the excitation is decreased slightly without making any other change, which of the following would occur?
a. the motor may run away
b. the torque angle would decrease
c. the power factor would become lagging
d. the motor would definitely pull out of synchronism
c. the power factor would become lagging
Would decreasing the frequency of the voltage applied to the stator of a three-phase SCIM cause the rotor speed to increase?
No, speed would decrease
State two advantages of choosing a three-phase SCIM having a high reactance rotor design.
Lower inrush current, high breakdown torque
The rotor of a three-phase, double squirrel cage induction motor:
a. has high resistance bars near the rotor surface
b. has high reactance bars near the rotor surface
c. has two different synchronous speed ratings
d. has a much longer rotor shaft
a. has high resistance bars near the rotor surface
State two advantages of choosing a three-phase SCIM having a high resistance rotor design.
High starting torque, lower inrush current
State two disadvantages of choosing a three-phase SCIM having a high resistance rotor design.
Poor speed regulation, poorer efficiency
If you reversed two of the rotor circuit leads for a 3-phase wound rotor motor, the motor would:
a. overheat very quickly
b. run at a greatly reduced speed
c. operate as previous
d. rotate in the opposite direction
c. operate as previous
Why does a single phase capacitor start motor have greater starting torque than a standard split phase motor?
The capacitor start motor produces a 90° phase displacement between the current in the run winding and the current in the start winding. This in turn produces a more uniform rotating magnetic field resulting in a better starting torque.
If the frequency of the voltage applied to the stator of a 3-phase induction motor was increased;
a. the rotor speed would decrease
b. the rotor speed would increase
c. the rotor speed would remain the same, but the motor would draw less current
d. the rotor speed would remain the same, but the motor would draw more current
b. the rotor speed would increase
How many leads are brought out and how are they identified for a three-phase wound rotor motor?
six - T1, T2, T3; M1, M2, M3
Should a three-phase wound rotor induction motor be started with maximum or minimum resistance placed in the rotor circuit?
Maximum resistance
If the centrifugal switch fails to open as a split phase motor accelerates to its rated speed, what will be the likely result?
Less torque, slower speed, poorer speed regulation, burn out of the start winding
If a 8.3HP, 480V, 3-phase squirrel cage induction motor was started with 277V, 3 phase, then the;
a. inrush current would be 173% of normal
b. initial starting torque would be 33% of normal
c. full-load speed would be 57% lower than normal
d. rotor reactance would be 173% higher than normal
b. initial starting torque would be 33% of normal
The maximum permissible setting of the overcurrent devices used to protect a motor branch circuit when using time delay fuses is ____% of the FLA
175%
What purpose does the amortisseur winding serve on a synchronous motor?
It is often used as a start winding but also serves as a “dampening” winding
If a squirrel-cage induction motor were started with only 90% of its rated voltage, the starting current would likely _______ (increase/decrease) to ____%
a. decrease to 81%
b. decrease to 90%
c. increase to 110%
d. increase to 121%
b. decrease to 90%
What is the advantage of the double squirrel cage motor when compared to other types of induction motors?
excellent starting torque and good speed regulation
If a 3-phase induction motor operates at a higher percent slip, then the;
a. rotor frequency increases
b. stator frequency decreases
c. induced rotor voltage decreases
d. induced stator voltage increases
a. rotor frequency increases
Single phasing occurs when one line of a three phase motor opens. How would you recognize this problem?
Motor runs rough, clamp-on ammeter indicates zero amps in one line and more than rated current in the other two lines
Will the inrush current to a 3-phase wound rotor induction motor be as high as that of a typical SCIM when all the resistance is in the rotor circuit at starting?
No
The centrifugal switch of a split-phase AC motor fails to open after starting. The most likely result is;
a. tripping of the OLR or burn-out of the starting winding
b. a great increase of the initial starting torque
c. a higher than normal running speed
d. continuous sparking at the brushes
e. a lower than normal line current
a. tripping of the OLR or burn-out of the starting winding
The angular displacement (torque angle) between the centres of the stator field and the rotor field of a synchronous motor is determined by?
load (primarily)
If a three-phase SCIM rated 5HP, 240 volt was started with 208 volts applied to its terminals, what percentage of starting torque would be developed as compared to if it were started at its rated 240 volts?
75.1%
Why is the starting torque of a capacitor-start single phase motor better than that of a resistance-start motor?
The capacitor start motor produces a 90° phase displacement between the current in the run winding and the current in the start winding. This in turn produces a more uniform rotating magnetic field resulting in better starting torque
The direction of flux across the main pole face of the stator of a single phase, shaded pole motor is from ________ to _______
main pole face to shaded pole portion
If a squirrel cage induction motor were started with 110% of its rated voltage, the starting current would likely _______ (increase/decrease) to ____%
increase to 110%
If a squirrel cage induction motor were started with 110% of its rated voltage, the starting torque would likely ________ (increase/decrease) to ____%
increase to 121%
If a squirrel cage induction motor were started with only 90% of its rated voltage, the full load current would likely __________ (increase/decrease) to _____%
increase to 111%
What would happen to the rotor speed of a three-phase SCIM if the frequency of the applied voltage was increased?
It would increase
What is meant by the term percent slip?
(Synchronous speed - rotor speed/synchronous speed) x 100
The single phase AC motor which exhibits the best percent speed regulation is the:
a. capacitor-start motor
b. repulsion-induction motor
c. standard split-phase motor
d. permanent capacitor motor
e. shaded pole synchronous motor
e. shaded pole synchronous motor
What is the advantage and disadvantage of having a high reactance rotor design for a 3-phase SCIM?
Advantage - low inrush current, good speed regulation, good breakdown torque
Disadvantage - poorer starting torque than standard
What is another name commonly applied to the AC series motor?
Universal motor
A SCIM has a service factor of 1.15. What is this term used to determine?
Service factor is a multiple that when applied to the rated horsepower of a squirrel cage induction motor, indicates a permissible loading that may be carried continuously without damaging the motor. This value is used when determining the maximum permissible setting of the overload devices
To reverse rotation for a capacitor-start split-phase motor, it is necessary to reverse the connections to the:
a. line leads only
b. leads of the capacitor
c. leads of the running winding
d. leads to the centrifugal switch
e. none of the above; the rotor must be reversed end-for-end
c. leads of the running winding
When the field rheostat of a synchronous motor is adjusted to produce the lowest stator line current, then the PF of the motor is:
a. at unity
b. leading
c. lagging
d. not affected
a. at unity
If a stethoscope is not available to you, what instrument could you use to detect a faulty bearing in a SCIM?
Screwdriver of vibration displacement indicator
How would you reverse the direction of rotation of a wound rotor motor?
Interchange any two line leads
Is it possible to vary the torque of a wound rotor motor? If so, how?
Yes - insert or remove resistance in the rotor circuit
Is it possible to vary the speed of a wound rotor motor? If so, how?
Yes - insert of remove resistance in the rotor circuit or vary the applied frequency
What would happen to the speed of a wound rotor motor if the frequency of the applied voltage were increased?
It would speed up
What is the difference between the stator of a SCIM and the stator of a wound rotor induction motor?
There is no difference
What is the difference between the rotor of a SCIM and that of the wound rotor motor?
The rotor of a wound rotor motor consists of windings brought out to slip rings. External resistance can be added to the rotor circuit through brushes
When comparing the starting winding to the running winding of a standard split phase motor, one would expect to find:
a. the starting winding has fewer turns of larger wire
b. the is no physical difference between the two; both are identical in all respects
c. the starting winding is placed over the top of the running winding in the stator slots
d. the starting winding has twice the number of coil groups compared to the running winding
c. the starting winding is placed over the top of the running winding in the stator slots
What would be the main reason one would go to the expense of purchasing a three-phase wound rotor induction motor rather than simply purchasing a SCIM?
Higher starting torque, smoother acceleration of heavy loads, limited speed control
What apparatus enables the connection of the rotor to be made to an externally wye connected rheostat in a three-phase wound rotor induction motor?
Slip rings and brushes
Will a three-phase wound rotor induction motor start if the rotor circuit is opened? If not, why?
No - there will be no current in the rotor circuit
A three-phase wound rotor induction motor fails to initially start but will start rotation when the 3 slip rings are short-circuited. What problem would this likely indicate?
Open in the secondary resistor circuit
The division of load between diesel powered alternators operating in parallel is accomplished by:
a. the use of an equalizer bus
b. adjusting the field rheostat
c. adjusting the applied power
d. adjusting the speed of the alternators
c. adjusting the applied power
List the essential parts of a split phase induction motor.
Start winding, run winding, stator core, end bells, squirrel cage rotor
What happens if the centrifugal switch contacts fail to close when the motor stops?
Motor will not start next time it is turned on
Explain how the direction of rotation of a split phase induction motor is reversed.
Interchange the start winding connection
The sharing of VARs between alternators operating in parallel is accomplished by:
a. the use of an equalizer bus
b. adjusting the applied power
c. adjusting the field excitation
d. adjusting the speed of the alternators
c. adjusting the field excitation
What is one limitation of a capacitor start induction run motor?
The capacitor takes time to heal after each start, therefore this motor should not be started too frequently in a short period of time
A 3 phase AC motor is said to be “single-phasing” when:
a. one of the overloads has tripped
b. two of the overloads have tripped
c. no current is flowing in one of the lines
d. no current is flowing in two of the lines
c. no current is flowing in one of the lines
A NEMA design ‘B’ squirrel cage induction motor will usually develop its maximum torque (i.e.: breakdown torque) at approximately _____% slip.
24%
Each phase winding of a 3-phase alternator has a resistance of 4.8 ohms when measured with an ohmmeter. The generator is delta-connected. When the ohmmeter is placed across the terminals, line-to-line, what will the resistance measure?
3.2 ohms
How can the speed of a single phase universal motor be controlled?
Insert resistance in series with the armature. Decrease the average voltage across the armature.
How can the direction of rotation of a single phase universal motor be reversed?
Interchange the armature leads
A 3-phase wound rotor motor should be started with;
a. no load on the motor
b. all resistance cut out of the rotor circuit
c. a reduced-voltage applied to the stator circuit
d. maximum resistance placed in the rotor circuit
d. maximum resistance placed in the rotor circuit
What would be the result of changing the direction of rotation of the single phase universal motor without also shifting the brushes to the neutral plane?
No problem
Why is it that a series DC motor will not operate satisfactorily on AC voltages?
High heat losses due to hysteresis losses and eddy current losses. The rotor (armature) and stator are not laminated to prevent these losses.
What is the main advantage of a single phase universal motor?
High torque development at a slow speed
Will a universal motor operate faster on a DC or single phase AC supply?
Approximately the same speeds
What precaution must we observe when utilizing a single phase universal motor?
Motor will overspeed with loss of load
Of the single phase motors discussed in class, what motor exhibits the best speed regulation from no load to full load?
Synchronous shaded pole motor
Of the single phase motors discussed in class, what motor exhibits the poorest speed regulation from no load to full load?
AC series universal motor
A 9-lead, 3-phase, dual-voltage motor is connected with 3 leads connected to each line wire. This would typically represent a:
a. wye connection for high voltage operation
b. wye connection for low voltage operation
c. delta connection for high voltage operation
d. delta connection for low voltage operation
d. delta connection for low voltage operation
How is the rotating field developed in a single phase shaded pole motor?
The development of flux in the shaded pole portion is delayed due to the inductive effect created by the short circuited single turn loop around this portion of the stator iron.
If a squirrel cage induction motor was operated at 110% of its rated voltage, the full load current would likely __________ (increase/decrease) to ____%
Decrease to about 91%
The laminated sheet steel used in the construction of a SCIM has a __________ resistance and a __________ permeability.
high resistance and a high permeability
What is one of the main disadvantages of the single phase shaded pole motor?
poor torque development. Both starting torque and full load torque are small.
What is one of the main advantages of the single phase shaded pole motor?
Cost is cheap and they are difficult to overload
Name two possible applications of the single phase shaded pole motor
Bathroom fan, fan for an overhead projector, can opener
The excitation is adjusted so that the power factor of a fully loaded synchronous motor is at unity. What would happen if the excitation was decreased?
Current would increase. Power factor would decrease and go lagging.
How could the speed of a synchronous motor be increased?
Increase the applied frequency of the supply voltage
If a fully-loaded synchronous motor is operating at unity PF and only the mechanical load is reduced, what would happen?
The torque angle decreases, power factor goes leading, power out decreases.
When the line current of a synchronous motor is at its minimum value, what would be the PF of the motor?
Unity
How could the direction of rotation of a 3-phase synchronous motor be reversed?
Interchange two line leads to the stator
When should the DC excitation be applied to the rotor of a synchronous motor in order to bring the motor into synchronism?
At about 95% to 97% of rated speed
What is meant by the term “pull out torque”?
Too much load causes the torque angle to increase to the point where the magnetic bond cannot hold the rotor at synchronous speed. At this point the motor will pull out of synchronism and decelerate to a stand still
When a synchronous motor is operated with little or no load, and its field excitation is adjusted so as to provide a leading power factor, it can be used for power factor correction. When operated in this manner it is sometimes called a ___________
Synchronous condenser
A fully-loaded synchronous motor is operating at unity PF. If only the mechanical load is reduced, you would expect:
a. the motor PF to go leading
b. the motor PF to go lagging
c. the motor speed to increase
d. the AC line current to increase
a. the motor PF to go leading
When operating an alternator at no load, a decrease in the speed of the prime mover will cause a(n):
a. decrease in frequency and an increase in voltage
b. increase in frequency and an increase in voltage
c. decrease in frequency and a decrease in voltage
d. increase in frequency and a decrease in voltage
c. decrease in frequency and a decrease in voltage
