COMPUTATIONS AND CIRCLE DIAGRAMS

Short notes

Objective questions (Answers are given below)

1.In the circle diagram for a 3-φ induction motor,
the diameter of the circle is determined by
(a) rotor current
(b) exciting current
(c) total stator current
(d) rotor current referred to stator.

Answer : C

2.Point out the WRONG statement.
Blocked rotor test on a 3-φ induction motor
helps to find
(a) short-circuit current with normal voltage
(b) short-circuit power factor
(c) fixed losses
(d) motor resistance as referred to stator.

2. c

3.In the circle diagram of an induction motor,
point of maximum input lies on the tangent
drawn parallel to
(a) output line
(b) torque line
(c) vertical axis
(d) horizontal axis

3. d

4.An induction motor has a short-circuit current
7 times the full-load current and a full-load slip
of 4 per cent. Its line-starting torque is …….
times the full-load torque.
(a) 7 (b) 1.96
(c) 4 (d) 49

4. b

5.In a SCIM, torque with autostarter is ……. times
the torque with direct-switching.
(a) K2 (b) K
(c) 1/K2 (d) 1/K
where K is the transformation ratio of the autostarter.

5. a

6.If stator voltage of a SCIM is reduced to 50 per
cent of its rated value, torque developed is
reduced by ……. per cent of its full-load value.
(a) 50 (b) 25
(c) 75 (d) 57.7

6. c

7.For the purpose of starting an induction motor,
a Y-Δ switch is equivalent to an auto-starter of
ratio…….per cent.
(a) 33.3 (b) 57.7
(c) 73.2 (d) 60.

7. b

8.A double squirrel-cage motor (DSCM) scores
over SCIM in the matter of
(a) starting torque
(b) high efficiency under running conditions
(c) speed regulation under normal operating
conditions
(d) all of the above.

8. d

9.In a DSCM, outer cage is made of high
resistance metal bars primarily for the purpose
of increasing its
(a) speed regulation
(b) starting torque
(c) efficiency
(d) starting current.

9. b

10.A SCIM with 36-slot stator has two separate
windings : one with 3 coil groups/ phase/pole
and the other with 2 coil groups/phase/pole.
The obtainable two motor speeds would be in
the ratio of
(a) 3 : 2 (b) 2 : 3
(c) 2 : 1 (d) 1 : 2

10. a

11.A 6-pole 3-φ induction motor taking 25 kW
from a 50-Hz supply is cumulatively-cascaded
to a 4-pole motor. Neglecting all losses, speed
of the 4-pole motor would be ……. r.p.m.
(a) 1500 (b) 1000
(c) 600 (d) 3000.

and its output would be ……. kW.
(e) 15 (f) 10
(g) 50/3 (h) 2.5.

11. c, f

12.Which class of induction motor will be well
suited for large refrigerators?
(a) Class E (b) Class B
(c) Class F (d) Class C

12. d

13.In a Schrage motor operating at supersynchronous
speed, the injected emf and the
standstill secondary induced emf
(a) are in phase with each other
(b) are at 90º in time phase with each other
(c) are in phase opposition
(d) none of the above.
(Power App.-III, Delhi Univ. July 1987)

13. a

14.For starting a Schrage motor, 3-φ supply is
connected to
(a) stator
(b) rotor via slip-rings
(c) regulating winding
(d) secondary winding via brushes.

14. b

15.Two separate induction motors, having 6 poles
and 5 poles respectively and their cascade
combination from 60 Hz, 3-phase supply can
give the following synchronous speeds in rpm
(a) 720, 1200, 1500 and 3600
(b) 720, 1200 1800
(c) 600, 1000, 15000
(d) 720 and 3000
(Power App.-II, Delhi Univ.Jan 1987)

15. a

16.Mark the WRONG statement.
A Schrage motor is capable of behaving as a/ an
(a) inverted induction motor
(b) slip-ring induction motor
(c) shunt motor
(d) series motor
(e) synchronous motor.

16.d

17.When a stationary 3-phase induction motor is
switched on with one phase disconnected
(a) it is likely to burn out quickly unless
immediately disconnected
(b) it will start but very slowly
(c) it will make jerky start with loud growing
noise
(d) remaining intact fuses will be blown out
due to heavy inrush of current

17. a

18.If single-phasing of a 3-phase induction motor
occurs under running conditions, it
(a) will stall immediately
(b) will keep running though with slightly increased slip
(c) may either stall or keep running
depending on the load carried by it
(d) will become noisy while it still keeps
running.

18. C

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