AC MACHINES

1. A single phase induction motor with only the main winding excited would exhibit the following response at synchronous speed
a) rotor current is zero
b) rotor current is non-zero and is at slip frequency
c) forward and backward totaling fields are equal
d) forward rotating field is more than the backward rotating field.
ans. d

2. A 230 V, 50 Hz, 4 pole, single-phase induction motor is rotating in the clockwise direction at a speed of 1425 rpm. If the rotor resistance at standstill is 7.8 Ohm, then the effective rotor resistance in the backward branch of the equivalent circuit will be
a) 2 ohm
b) 4 ohm
c) 78 ohm
d) 156 ohm
ans. b

3. For single phase capacitor start induction motor which of the following statements is valid?
a) The capacitor is used for power factor improvement
b) The direction of rotation can be changed by reversing the main winding terminals.
c) The direction of rotation cannot be changed.
d) The direction of rotation can be changed by interchanging the supply terminals.
ans. b

4. In a single-phase induction motor driving a fan load, the reason for having a high resistance rotor is to achieve
a) low starting torque
b) quick acceleration
c) high efficiency
d) a reduced size
ans. b

5. The type of single-phase induction motor having the highest power factor at full load is
a) shaded pole type
b) split-phase type
c) capacitor -start type
d) capacitor-run type.
ans. d

6. The torque-speed characteristics of a repulsion motor resembles which of the following dc motor characteristic?
a) separately excited
b) shunt
c) series
d) compound
ans. c

7. Ratio of the rotor reactance X to the rotor resistance R for a 2-phase servo motor
a) is equal to that of normal induction motor
b) is less than that of a normal induction motor
c) is greater than that of a normal induction motor
d) may be less or greater than that of a normal induction motor.
ans. b

8. In a stepper motor, the detent torque means
a) minimum of the static torque with the phase winding excited
b) maximum of the static torque with the phase winding excited
c) minimum of the static torque with the phase winding unexcited
d) may be less or greater than that of a normal induction motor.
ans. b

9. For a given stepper motor, the following torque has the highest numerical value
a) detent torque
b) pull-in torque
c) pull-out torque
d) Holding torque
ans. c

10. The following motor definitely has a permanent magnet rotor.
a) DC commutator motor
b) Brushless dc motor
c) Stepper motor
d) Reluctance motor
ans. c