MCQ ON ALTERNATORS

Objective Questions

Short notes

Objective questions (Answers are given below)

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.
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.
The main disadvantage of using short-pitch winding
in alterators 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.
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.
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.
If an alternator winding has a fractional pitch of
5/6, the coil span is ……. degrees.
(a) 300 (b) 150
(c) 30 (d) 60.
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.
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.
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.
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.
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.
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.

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.
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.
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.
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.
At lagging loads, armature reaction in an alternator
is
(a) cross-magnetising (b) demagnetising
(c) non-effective (d) magnetising.
At leading p.f., the armature flux in an alternator
……. the rotor flux.
(a) opposes (b) aids
(c) distorts (d) does not affect.
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
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
The power factor of an alternator is determined
by its
(a) speed
(b) load
(c) excitation
(d) prime mover.
For proper parallel operation, a.c. polyphase alternators
must have the same
(a) speed (b) voltage rating
(c) kVA rating (d) excitation.
Of the following conditions, the one which does
not have to be met by alternators working in
parallel is
(a) terminal voltage of each machine must be
the same
(b) the machines must have the same phase
rotation
(c) the machines must operate at the same
frequency
(d) the machines must have equal ratings.
After wiring up two 3-φ alternators, you checked
their frequency and voltage and found them to
be equal. Before connecting them in parallel,
you would
(a) check turbine speed
(b) check phase rotation
(c) lubricate everything
(d) check steam pressure.
Zero power factor method of an alternator is
used to find its
(a) efficiency
(b) voltage regulation
(c) armature resistance
(d) synchronous impedance.
Some engineers prefer `lamps bright’ synchronization
to ‘lamps dark’ synchronization because
(a) brightness of lamps can be judged easily
(b) it gives sharper and more accurate synchronization
(c) flicker is more pronounced
(d) it can be performed quickly.
It is never advisable to connect a stationary
alternator to live bus-bars because it
(a) is likely to run as synchronous motor
(b) will get short-circuited
(c) will decrease bus-bar voltage though momentarily
(d) will disturb generated e.m.fs. of other alternators
connected in parallel.

Two identical alternators are running in parallel
and carry equal loads. If excitation of one alternator
is increased without changing its steam
supply, then
(a) it will keep supplying almost the same load
(b) kVAR supplied by it would decrease
(c) its p.f. will increase
(d) kVA supplied by it would decrease.
Keeping its excitation constant, if steam supply
of an alternator running in parallel with another
identical alternator is increased, then
(a) it would over-run the other alternator
(b) its rotor will fall back in phase with respect
to the other machine
(c) it will supply greater portion of the load
(d) its power factor would be decreased.
The load sharing between two steam-driven
alternators operating in parallel may be adjusted
by varying the
(a) field strengths of the alternators
(b) power factors of the alternators
(c) steam supply to their prime movers
(d) speed of the alternators.
Squirrel-cage bars placed in the rotor pole faces
of an alternator help reduce hunting
(a) above synchronous speed only
(b) below synchronous speed only
(c) above and below synchronous speeds both
(d) none of the above.
(Elect. Machines, A.M.I.E. Sec. B, 1993)
For a machine on infinite bus active power can
be varied by
(a) changing field excitation
(b) changing of prime cover speed
(c) both (a) and (b) above
(d) none of the above .
(Elect. Machines, A.M.I.E. Sec. B, 1993)

a 2. d 3. b 4. d 5. d 6. b 7. c 8. d 9. a 10. b 11. a 12.a 13. d 14. b 15. c 16. a 17. d 18. b 19. b 20. c 21. b 22. b 23.d 24. b 25. b 26. b 27. b 28. a 29. c 30. c. 31. c 32. b

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