OBJECTIVE TEST On Armature reaction and commutation

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OBJECTIVE TEST

1. In d.c. generators, armature reaction is produced actually by

(a) its field current

(b) armature conductors

(c) field pole winding

(d) load current in armature

2. In a d.c. generator, the effect of armature reac- tion on the main pole flux is to

(a) reduce it

(b) distort it

(c) reverse it

(d ) both (a) and (b)

3. In a clockwise-rotating loaded d.c. generator, brushes have to be shifted

(a) clockwise

(b) counterclockwise

(c) either (a) or (b)

(d) neither (a) nor (b).

4. The primary reason for providing compensat- ing windings in a d.c. generator is to

(a) compensate for decrease in main flux

(b) neutralize armature mmf

(c) neutralize cross-magnetising flux

(d ) maintain uniform flux distribution.

5. The main function of interpoles is to minimize ............ between the brushes and the commu- tator when the d.c. machine is loaded.

(a) friction

(b) sparking

(c) current

(d ) wear and tear

6. In a 6-pole d.c. machine, 90 mechanical degrees correspond to ............ electrical degrees.

(a) 30

(b) 180

(c) 45

(d) 270

7. The most likely cause(s) of sparking at the brushes in a d.c. machine is /are

(a) open coil in the armature

(b) defective interpoles

(c) incorrect brush spring pressure

(d) all of the above

8. In a 10-pole, lap-wound d.c. generator, the num- ber of active armature conductors per pole is

50. The number of compensating conductors

per pole required is

(a) 5

(b) 50

(c) 500

(d) 10

9. The commutation process in a d.c. generator basically involves

(a) passage of current from moving armature to a stationary load

(b) reversal of current in an armature coil as it crosses MNA

(c) conversion of a.c. to d.c.

(d) suppression of reactance voltage

10. Point out the WRONG statement. In d.c. gen- erators, commutation can be improved by

(a) using interpoles

(b) using carbon brushes in place of Cu brushes

(c) shifting brush axis in the direction of armature rotation

(d) none of the above

11. Each of the following statements regarding interpoles is true except

(a) they are small yoke-fixed poles spaced in between the main poles

(b) they are connected in parallel with the armature so that they carry part of the armature current

(c) their polarity, in the case of generators is the same as that of the main pole ahead

(d) they automatically neutralize not only reactance voltage but cross-magnetisation as well

12. Shunt generators are most suited for stable parallel operation because of their voltage characteristics.

(a) identical

(b) dropping

(c) linear

(d) rising

13. Two parallel shunt generators will divide the total load equally in proportion to their kilo- watt output ratings only when they have the same

(a) rated voltage

(b) voltage regulation

(c) internal IaRa drops

(d) boths (a) and (b)

14. The main function of an equalizer bar is to make the parallel operation of two over-compounded

d.c. generators

(a) stable

(b) possible

(c) regular

(d) smooth

15. The essential condition for stable parallel op- eration A Two d.c. generators having similar characteristics is that they should have

(a) same kilowatt ouput ratings

(b) droping voltage characterisitcs

(c) same percentage regulation

(d) same no-load and full-load speed

16. The main factor which loads to unstable paral- lel operation of flat-and over-compound d.c. generators is

(a) unequal number of turns in their series field windings

(b) unequal series field resistances

(c) their rising voltage characteristics

(d) unequal speed regulation of their prime movers

17. The simplest way to shift load from one d.c. shunt generator running in parallel with another is to

(a) adjust their field rheostats

(b) insert resistance in their armature circuits

(c) adjust speeds of their prime movers

(d) use equalizer connections

18. Which one of the following types of generators does NOT need equalizers for satisfactory par- allel operation ?

(a) series

(b) over-compound

(c) flat-compound

(d) under-compound.

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