Electrical Machines - Induction Motors

1. A 3-phase, 4-pole, 400 V, 50 Hz squirrel-cadge induction motor is operating at a slip of 0.02. The speed of the rotor flux in mechanical rad/sec, sensed by a stationary observer is closest to
(A) 1500 
(B) 1470
(C) 157
(D) 154 (GATE 2017)
Hint: 
\omega  = \frac{2}{P}(2\pi f)

2. In a constant V/f induction motor drive, the slip at the maximum torque:
(A) is directly proportional to the synchronous speed.
(B) remains constant with respect to the synchronous speed.
(C) has an inverse relation with the synchronous speed.
(D) has no relation with the synchronous speed. (GATE 2016)

3. The direction of rotation of a single-phase capacitor run induction motor is reversed by:
(A) interchanging the terminals of the AC supply
(B) interchanging the terminals of the capacitor
(C) interchanging the terminals of the auxiliary winding
(D) interchanging the terminals of both the windings. (GATE 2016)

4. In a constant V/f control of induction motor, the ratio V/f is maintained constant from 0 to base frequency, where V is the voltage applied to the motor at fundamental frequency f. Which of the following statements relating to low frequency operation of the motor is TRUE?
(a) At low frequency, the stator flux increases from its rated value
(b) At low frequency, the stator flux decreases from its rated value
(c) At low frequency, the motor saturates
(d) At low frequency, the stator flux remains unchanged at its rated value (GATE 2014)

5. Leakage flux in an induction motor is 
(a) Flux that leaks through the machine
(b) Flux that links both stator and rotor windings
(c) Flux that links none of the windings
(d) Flux that links the stator winding or the rotor windings but not both (GATE 2013)

6. The slip of an induction motor normally does not depend on
(a) Rotor speed 
(b) Synchronous speed
(c) Shaft torque 
(d) Core-loss component (GATE 2012)
Hint: S = ({N_s} - {N_r})/{N_s}

7. A balanced 3-phase voltage is applied to a starconnected induction motor, the phase to neutral
voltage being V. The stator resistance, rotor resistance referred to the stator, stator leakage reactance, rotor leakage reactance referred to the stator, and the magnetizing reactance are denoted by {r_s},{r_r},{x_s},{x_r}\,and\,{X_m} respectively. The magnitude of the starting current of the motor is given by:
(a) \frac{V}{{\sqrt {{{({r_s} + {r_r})}^2} + {{({x_s} + {x_r})}^2}} }}
(b) \frac{V}{{\sqrt {{r_s}^2 + {{({r_s} + {x_m})}^2}} }}
(c) \frac{V}{{\sqrt {{{({r_s} + {r_r})}^2} + {{({X_m} + {x_r})}^2}} }}
(d) \frac{V}{{\sqrt {{r_s}^2 + {{({X_m} + {x_r})}^2}} }} (GATE 2010)

Hint: {I_{st}} = \frac{V}{{({r_s} + {r_r}) + j({X_s} + {X_r})}}

8. A 3-phase squirrel cage induction motor supplied from a balanced 3-phase source drives a mechanical load. The torque–speed characteristics of the motor (solid curve) and of the load (dotted curve) are shown. Of the two equilibrium points A and B, which of the following options correctly describes the stability of A and B?

(a) A is stable B is unstable
(b) A is unstable B is stable
(c) Both are stable
(d) Both are unstable (GATE 2009)


9. For a 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. (GATE 2006)

10. For an induction motor, operating at a slip s, the ratio of gross power output to air gap power is equal to
(a) {(1 - s)^2}
(b) (1 - s)
(c) \sqrt {(1 - s)}
(d) 1 - \sqrt s (GATE 2005)
Hint: {P_{gross}} = air\,gap\,power - rotor\,copper\,loss = {P_g} - s{P_g}

Answers

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

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