Electrical Machines - Induction Motors

1. The rotor slots, in an induction motor are usually not quite parallel to the shaft because it
(a) improves the power factor
(b) improves the efficiency
(c) helps the rotor teeth to remain under the stator
(d) helps in reducing the tendency of the rotor teeth to remain under the stator teeth (SSC JE, 2014)

2. The synchronous speed of a three phase induction motor having 20 polar and connected to a 50 Hz source is
(a) 1200 rpm 
(b) 300 rpm
(c) 600 rpm 
(d) 1000 rpm (SSC JE 2014)
Hint: {N_s} = 120f/P

3. When the rotor of a three phase induction motor is blocked, the slip is
(a) 1
(b) 0
(c) 0.1 
(d) 0.5 (SSC JE, 2014)

4. A 3-phase 4 pole induction motor works on 3-phase 50 c/s supply. If the slip of the motor is
4%. The actual speed will be
(a) 720 rpm 
(b) 1550 rpm
(c) 1460 rpm 
(d) 1440 rpm (SSC JE, 2014)
{N_s} = 120f/P\\
\% S = \frac{{{N_s} - {N_r}}}{{{N_s}}}x100

5. The single phase Induction Motor (IM) which does not have centrifugal switch is
(a) capacitor start single phase IM
(b) resistance split single phase IM
(c) capacitor start capacitor run single phase IM
(d) permanent capacitor run single phase IM (SSC JE, 2013)

6. In squirrel-cage induction motor, the rotor conductors are
(a) open circuited
(b) short circuited via end rings
(c) short circuited via external reactance
(d) short circuited via external resistance (SSC JE, 2013)

7. if the centrifugal switch of a single-phase resistance split induction motor does not open after starting of motor, the motor
(a) will run above normal speed
(b) will run below normal speed
(c) will draw very small current
(d)will draw high current and get over- heated (SSC JE, 2013)

8. Which one of the following is a speed control method of three-phase squirrel cage induction motor?
(a) Plugging method
(b) Star-delta switch method
(c) Pole-changing method
(d)Centrifugal clutch method (SSC JE, 2013)

9. In star-delta starting of three-phase induction motor the starting voltage is reduced to
(a) √3 times of normal voltage
(b) 3 time of normal voltage
(c) 1/3 times of normal voltage
(d) 1/√3 times of normal voltage (SSC JE, 2013)

10. If the starting torque of a 3 phase induction motor is {T_{st}} for DOL starting, that for star-delta starting of the same motor is
(a) {T_{st}}/3
(b) {T_{st}}/\sqrt 3
(c) \sqrt 3 {T_{st}} 
(d) 3{T_{st}} (SSC JE, 2013)


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