Objective tests on computations and circle diagrams.

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OBJECTIVE TESTS – 35

1. In the circle diagram for a 3-f 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.

2. Point out the WRONG statement.

Blocked rotor test on a 3-f 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.

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.

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

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.

6. If stator voltage of a SCIM is reduced to 50 percent 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

7. For the purpose of starting an induction motor, a Y-D switch is equivalent to an auto-starter of ratio.......per cent.

(a) 33.3

(b) 57.7

(c) 73.2

(d) 60.

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.

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.

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

11. A 6-pole 3-f 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.

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

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)

14. For starting a Schrage motor, 3-f supply is connected to

(a) stator

(b) rotor via slip-rings

(c) regulating winding

(d) secondary winding via brushes.

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)

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.

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

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.

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