12
Induction Motors
OBJECTIVE
OBJECTIVE--TYPE QUESTIONS
1. The speed of an induction motor depends upon
(a) number of stator poles
(b) supply frequency
(c) input voltage to the stator (d) all the above
2. The no load current of an induction motor in terms of full load current is
around
(a) 30%
(b) 50%
(c) 5%
(d) 10%
3. One of the following is the most commonly used motor in power system
(a) synchronous motor
(b) DC shunt motor
(c) 3B induction motor
(d) relectance motor
4. 3B-induction motor works on the principle of
(a) Faraday’s laws of electromagnetic induction
(b) mutual inductance
(c) Lenz’s law
(d) Ampere’s law
5. Synchronous speed of 6-pole induction motor working on 60 Hz supply is
(a) 1000 rpm
(b) 1500 rpm
(c) 1200 rpm
(d) 3000 rpm
6. The magnetising current in a 3B induction motor is larger than in a transformer of the same KVA rating because of
(a) large air gap between stator and rotor
(b) moving parts
(c) rotor
(d) all of the above
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12.2
Basic Electrical Engineering
7. The full load speed of a 3B, induction motor working on 50 Hz supply is
720, then its synchronous speed in rpm will be
(a) 750
(b) 1000
(c) 1200
(d) 700
8. The number of poles of the induction motor in the above example is
(a) 8
(b) 6
(c) 4
(d) 16
9. If the phase sequence of the supply to the rotor fed induction motor is
RYB, then the rotor rotes
(a) in the same sense as RYB (b) in the opposite sense on RYB
(c) at double the speed
(d) none of the above
10. A 3B induction motor operating at a slip ‘s’ when working with supply
frequency of ‘f ’, then the frequency of rotor induced currents will be
(a) f
(b) f/s
(c) sf
(d) zero
11. The slip of a large induction motor varies around
(a) 3%
(b) 8%
(c) 6%
(d) 5%
12. An induction motor is operating at a slip ‘S’ and its synchronous speed is
NS, then its operating speed is
(b) (1 + 3)NS
(a) NS
(c) ZNS
(d) (1 – S)NS
13. A centre-zero dc galvanometer suitably shunted is the rotor circuit of
3-phase, 6 pole, 50 Hz induction motor makes 90 complete oscillation per
minute. Then the frequency of rotor current is
(a) 1.5 Hz
(b) 2 Hz
(c) 1 Hz
(d) 3 Hz
14. The operating slip of the motor in the above problem is
(a) 3%
(b) 2%
(c) 2.5
(c) 4%
15. The direction of rotation of a 3-phase induction motor can be reversed by
(a) reversing the supply terminals
(b) reversing the rotor terminals
(c) cross-connecting any two supply terminals
(d) reversing the supply to the star
16. If Pg, Pc and Pm are the input to the rotor, rotor copper loss and gross
mechanical power developed in a 3-phase induction motor, then the ratio
Pg : Pc : Pm is
(a) 1 : (s – 1) : s
(b) 1 : s : (s + 1)
(c) 1 : s : (1 – s)
(d) 1 : s : (s – 2)
17. If the rotor input to a 3 phase inductor operating at a slip 4% is 100 W,
then the corresponding rotor copper loss is
(a) 400 W
(b) 40 W
(c) 4 W
(d) 800 W
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Induction Motors
12.3
18. If r2 and x2 are the stand still rotor resistance and reactance/ph, of an
induction motor and s is the operating slip, then the condition for maximum torque is
(b) r2 = x2
(a) r2 = sx2
x
x
(c) r2 = 2
(d) r2 = 2
2
s
19. A 3B induction motor has stand still rotor reactance/ph of 4 9. If the
starting torque is equal to maximum torque then the value resistance in the
rotor circuit is
(a) 3 W
(b) 2 W
(c) 1 W
(d) 4 W
20. If ‘Pg’ is the rotor input/Ph and ‘Ws’ is synchronous angular velocity of
the rotor, then the gross torque developed/Ph in N-m is
(b) Pg/Ws
(a) Pg ´ Ws
(c)
21.
22.
23.
24.
25.
26.
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Pg
(d) Pg(1 – Ws)
(1 − Ws )
The torque in an 3-phase induction motor is proportional to supply voltage
as
(b) V
(a) V2
(c) 1/V
(d) V3
The economical method of starting of 3 phase induction motor is by
(a) direct starting
(b) auto-transformer
(c) rotor-resistance starting
(d) star delta starter
The running performance is better in the following type 3-phase induction
motor
(a) squirrel cage type
(b) slipring type
(c) capacitor-run type-cage induction motor
(d) capacitor start type
The speed of the revolving magnetic field w.r.t rotor conductors of a
3 phase induction motor is
(b) 0
(a) (NS – N)
(d) N
(c) NS + N
If NS is the synchronous speed and N is the speed of the rotor of 3-phase
induction motor, then the speed of R.M. field w.r.t stator conductor is
(b) N
(a) NS
(d) (NS + N)
(c) (NS – N)
If NS is the synchronous speed, N is the speed of the rotor, then the speed
of R.M. field in space in a rotorfed induction motor is
(b) N
(a) NS
(c) (NS – N)
(d) (NS + N)
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