Thursday, September 13, 2012

Electrical Machines - Alternators : Part 8

[1] 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 blow synchronous speeds both
(d) None of the above


[2] The stationary alternator should not be connected 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 emf's of other alternators connected in parallel.


[3] With a unity load p.f, the effect of armature reaction on the main field flux of an alternator is

(a) Distortional
(b) Magnetising
(c) Demagnetising
(d) Nominal


[4] At lagging loads, armature reaction in an alternator is

(a)  Cross-magnetising
(b)  Demagnetising
(c)  Non-effective
(d)  Magnetising


[5] The frequency of voltage generated by an alternator having 4 poles and rotating at 1800 rpm is

(a) 60 Hz
(b) 7200 Hz
(c) 120 Hz
(d) 450 Hz


[6] The main disadvantages 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.


[7] Zero power factor method of an alternator is used to find its

(a) Efficiency
(b) Voltage regulation
(c) Armature resistance
(d) Synchronous impedance


[8] Armature reaction in an alternator mainly affects

(a) Rotor speed
(b) Terminal voltage per phase
(c) Frequency of armature current
(d) Generated voltage per phase


[9] The effect of increasing air gap length in the induction motor will increase the

(a) Power factor
(b) Speed
(c) Magnetising current
(d) Air gap flux


[10] The principle of operation of a 3 phase induction motor is most similar to that of a

(a) Synchronous motor
(b) Repulsion start induction motor
(c) Transformer with a shorted secondary
(d) Capacitor start, induction run motor


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5:27 PM

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