Electrical Machines - Alternators
1. The power factor of an alternator is determined by its
(d) prime mover
2. For proper parallel operation, a.c. polyphase alternators must have the same
(b) voltage rating
(c) kVA rating
3. 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.
4. After wiring up two 3-phase 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.
5. Zero power factor method of an alternator is used to find its
(b) voltage regulation
(c) armature resistance
(d) synchronous impedance.
6. 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.
7. 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.
8. 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
9. 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.
10. 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.
1. b 2. b 3. d 4. b 5. b
6. b 7. b 8. a 9. c 10. c.