Electrical Machines - Induction Motors

1. The purpose of starting winding in a single-phase induction motor is to?
(a) Produce rotating flux in conjunction with main winding.
(b) Limit temperature rise of the machine.
(c) Reduce losses
(d) Increase losses (SSC JE, 2015)

2. If static voltage of a squirrel cage induction motor is reduced to 50 percent of its rated value, torque developed is reduced by how many percentage of its full load value?
(a) 50% 
(b) 25%
(c) 75% 
(d) 57.7% (SSC JE, 2015)
Hint: T \propto {V^2}

3. Which of the following motor is non-self starting?
(a) DC series motor
(b) Slip ring induction motor
(c) Synchronous motor
(d) Squirrel cage induction motor (SSC JE, 2015)

4. An eight pole wound rotor induction motor operating on 60 Hz supply is driven at 1800 rpm by a prime mover in the opposite direction of revolving magnetic field. The frequency of rotor current is 
(a) 120 Hz 
(b) 180 Hz
(c) 60 Hz 
(d) 200 Hz (SSC JE, 2015)
Hint: N = 120 x 60/8 = 900;
s = (900 + 1800)/900; f = N x s (SSC JE, 2015)

5. In a 3-phase induction motor crawling happens at
(a) any speed
(b) no-load speed
(c) odd multiples of fundamental
(d) even multiples of fundamental (SSC JE, 2014)

6. A 4-pole, 3-phase induction motor runs at 1440 rpm on a 50 Hz supply. Find the slip speed.
(a) 2940 rpm 
(b) 1500 rpm
(c) 1440 rpm 
(d) 60 rpm (SSC JE, 2014)

7. In capacitor start single phase induction motor, the current in the
(a) supply lines leads the voltage
(b) starting winding lags the voltage
(c) main winding leads the voltage
(d) starting winding leads the voltage (SSC JE, 2014)

8. In a single phase induction motor, speed sensitive centrifugal switch is connected in ⸺ winding.
(a) parallel with main
(b) series with main
(c) parallel with starting
(d) series with starting (SSC JE, 2014)

9. At starting, the current through the starting winding (Is) of single phase induction motor
(a) lags ‘V’ by 90⁰
(b) leads ‘V’ by 90⁰
(c) is nearly in phase with ‘V’
(d) leads ‘V’ by 75⁰ (SSC JE, 2014)

10. In a single phase induction motor at start, the two revolving fields produce
(a) unequal torques in the rotor conductors
(b) no torque in the rotor conductor
(c) equal and opposite torques in the rotor conductors
(d) equal torques in same direction in the rotor conductors (SSC JE, 2014)

Answers

    1. (a)    2. (d)    3. (c)    4. (b)    5. (c)
    6. (b)    7. (d)    8. (d)    9. (c)    10. (c)

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