OBJECTIVE QUESTIONS ( INDUCTION MOTOR)

1.The induction motors have .........power factor.

(a) zero                                                      (b) unity

(c) lagging                                                 (d) leading.                                                                        

2.An eight-pole wound rotor induction motor operating on 60 Hz supply is driven at 1800 r.p.m.

by a prime mover in the opposite direction of revolving magnetic field. The frequency of rotor

current is

(a) 60 Hz                                                    (b) 120 Hz

(c) 180 Hz                                                  (d) none of the above.

3. A 3-phase, 4-pole, 50-Hz induction motor runs at a speed of 1440 r.p.m. The rotating field

produced by the rotor rotates at a speed of .......r.p.m. with respect to the rotor.

(a) 1500                                                       (b) 1440

(c) 60                                                           (d) 0

4. In a 3-φ induction motor, the rotor field rotates at synchronous speed with respect to

(a) stator                                                       (b) rotor

(c) stator flux                                                (d) none of the above.

5.The power factor of a squirrel-cage induction motor is

(a) low at light loads only                             (b) low at heavy loads only

(c) low at light and heavy loads both            (d) low at rated load only.

6. The torque developed by a 3-phase induction motor depends on the following three factors:

(a) speed, frequency, number of poles                            (b) voltage, current and stator impedance

(c) synchronous speed, rotor speed and frequency      (d) rotor emf, rotor current and rotor p.f.


7.The speed of the motor can be varied by ___.
a) Changing supply frequency                                    c) Using multi speed windings
b) Changing no. of poles                                              d) All the above

8. In a capacitor start single-phase induction motor, the capacitor is connected

a)  in series with main winding.                        b) in series with auxiliary winding.

c)  in series with both the windings.                d)  in parallel with auxiliary winding.

ANSWERS

1. c
2. c ( Ns = 120f / p ) = 120 * 60 /8 = 900 rpm,    slip S = [900 - (-1200)] / 900 = 3
        rotor current frequency = S * f = 3* 60 =180 Hz)
3. c (speed of rotor field = 120 f /p =120*50/4 =1500 r.p.m
        speed of rotor =1440 rpm
        rotor field speed w.r.t rotor =1500-1440 = 60 rpm)
4. a
5. a
6. d
7. d
8. b
   
   
   Reference

1. Electrical technology by B.L THAREJA