1. What should be done to keep the rotor voltage to an acceptable level?
a) rotor to stator turns must be properly adjusted
b) stator to rotor turns must be properly adjusted
c) stator turns must be adjusted
d) rotor turns must be adjusted
Explanation: The effective ratio of stator to rotor turns should be adjusted to keep the rotor voltage to an acceptable level. The choice of this turns ratio is arbitrary and is controllable by the designer.
2. The rotor voltage on open circuit between slip rings should not exceed 500 V for small machines.
a) true
b) false
Explanation: The rotor voltage on open circuit between slip rings should not exceed 500 V for the small machines. The voltage is limited to a small value in order to protect persons working the motor if the brush gear is not perfectly protected.
3. How should the rotor voltage be with respect to the high voltage and large machines?
a) low
b) moderate
c) high
d) very high
Explanation: The high voltage and large machines should have high rotor voltage. If the rotor voltage is kept low, the rotor current becomes large, involving use of large conductor sections
4. What is the range of the rotor voltage for the large machines?
a) 1000-1500 V
b) 1000-1750 V
c) 500-1500 V
d) 1000-2000 V
Explanation: The minimum value of the rotor voltage should be 1000 V. The maximum value of the rotor voltage should not exceed 2000 V.
5. What is the formula for rotor turns per phase?
a) rotor turns per phase = (winding factor for stator/winding factor for rotor) * (Rotor voltage per phase/Stator voltage per phase) * Number of turns per phase for stator
b) rotor turns per phase = (winding factor for stator/winding factor for rotor) / (Rotor voltage per phase/Stator voltage per phase) * Number of turns per phase for stator
c) rotor turns per phase = (winding factor for stator/winding factor for rotor) * (Rotor voltage per phase/Stator voltage per phase) / Number of turns per phase for stator
d) rotor turns per phase = (winding factor for stator/winding factor for rotor) / (Rotor voltage per phase/Stator voltage per phase) / Number of turns per phase for stator
Explanation: Firstly, the winding factor for stator is obtained along with the winding factor for stator. Next the ratio of the rotor voltage per phase to the stator voltage per phase. Finally, the number of turns per phase for stator is also calculated to obtain the rotor turns per phase.
6. What is the formula for the full load rotor mmf?
a) 65% of stator mmf
b) 75% of stator mmf
c) 85% of stator mmf
d) 90% of stator mmf
Explanation: The full load rotor mmf is taken as 0.85 of stator mmf. Full load rotor mmf = 0.85 *
(stator current * no of stator turns) / no of rotor turns.
7. The value of the current density of rotor is chosen almost equal to that in the stator.
a) true
b) false
Explanation: There occurs a lot of excessive copper loss in the rotors. The value of current density of rotor is almost equal to that in the stator.
8. What type of conductor is chosen for the small motors?
a) round
b) bar
c) skewed
d) rectangular
Explanation: Round conductors are used for the small motors. Bar conductors are being used for the large motors.
9. What type of winding is made use of for the small motors?
a) mush windings
b) cross windings
c) interconnected windings
d) rounded windings
Explanation: For the small motors, it is a normal practice to use mush windings. The mush windings should be housed in the semi-closed slots.
10. What type of winding is made use of for the large motors?
a) mush windings
b) bar type windings
c) cross windings
d) rounded windings
Explanation: For the small motors mush windings are made use of. For the large motors, a double layer bar type winding is made use of.