Electrical Machines (Alternators)

1. When speed of an alternator is changed from 3600 r.p.m. to 1800 r.p.m., the generated
e.m.f./phases will become
(a) one-half
(b) twice
(c) four times
(d) one-fourth.

2. The magnitude of the three voltage drops in an alternator due to armature resistance, leakage reactance and armature reaction is solely determined by
(a) load current, Ia
(b) p.f. of the load
(c) whether it is a lagging or leading p.f. load
(d) field construction of the alternator.

3. Armature reaction in an alternator primarily affects
(a) rotor speed
(b) terminal voltage per phase
(c) frequency of armature current
(d) generated voltage per phase.

4. Under no-load condition, power drawn by the prime mover of an alternator goes to
(a) produce induced e.m.f. in armature winding
(b) meet no-load losses
(c) produce power in the armature
(d) meet Cu losses both in armature and rotor windings.

5. As load p.f. of an alternator becomes more leading, the value of generated voltage required to give rated terminal voltage
(a) increases
(b) remains unchanged
(c) decreases
(d) varies with rotor speed.

6. With a load p.f. of unity, the effect of armature reaction on the main-field flux of an alternator is
(a) distortional
(b) magnetising
(c) demagnetising
(d) nominal.

7. At lagging loads, armature reaction in an alternator is
(a) cross-magnetising
(b) demagnetising
(c) non-effective
(d) magnetising.

8. At leading p.f., the armature flux in an alternator ⸻ the rotor flux.
(a) opposes
(b) aids
(c) distorts
(d) does not affect.

9. The voltage regulation of an alternator having 0.75 leading p.f. load, no-load induced e.m.f. of 2400V and rated terminal voltage of 3000V is ⸻ percent.
(a) 20
(b) − 20
(c) 150
(d) − 26.7

20. If, in a 3-ϕ alternator, a field current of 50A produces a full-load armature current of 200 A on short-circuit and 1730 V on open circuit, then its synchronous impedance is ⸻ ohm.
(a) 8.66
(b) 4
(c) 5
(d) 34.6

Answers

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

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