Electrical Machines (Alternators)

1. The frequency of voltage generated by an alternator having 4-poles and rotating at 1800 r.p.m. is .......hertz.
(a) 60
(b) 7200
(c) 120
(d) 450.

2. A 50-Hz alternator will run at the greatest possible speed if it is wound for ....... poles.
(a) 8
(b) 6
(c) 4
(d) 2.

3. The main disadvantage of using short-pitch winding in alternators is that it
(a) reduces harmonics in the generated voltage
(b) reduces the total voltage around the armature coils
(c) produces asymmetry in the three phase windings
(d) increases Cu of end connections.

4. Three-phase alternators are invariably Y-connected because
(a) magnetic losses are minimised
(b) less turns of wire are required
(c) smaller conductors can be used
(d) higher terminal voltage is obtained.

5. The winding of a 4-pole alternator having 36 slots and a coil span of 1 to 8 is short-pitched
by ....... degrees.
(a) 140
(b) 80
(c) 20
(d) 40.

6. If an alternator winding has a fractional pitch of 5/6, the coil span is ....... degrees.
(a) 300
(b) 150
(c) 30
(d) 60.

7. The harmonic which would be totally eliminated from the alternator e.m.f. using a fractional pitch of 4/5 is
(a) 3rd
(b) 7th
(c) 5th
(d) 9th.

8. For eliminating 7th harmonic from the e.m.f. wave of an alternator, the fractional-pitch must be
(a) 2/3
(b) 5/6
(c) 7/8
(d) 6/7.

9. If, in an alternator, chording angle for fundamental flux wave is α, its value for 5th harmonic is
(a) 5α
(b) α/5
(c) 25α
(d) α/25.

10. Regarding distribution factor of an armature winding of an alternator which statement is
false?
(a) it decreases as the distribution of coils (slots/pole) increases
(b) higher its value, higher the induced e.m.f. per phase
(c) it is not affected by the type of winding either lap, or wave
(d) it is not affected by the number of turns per coil.


Answers

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

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