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%
