SYNCHRONOUS MACHINES

 Construction & types of synchronous machines Armature reaction Phasor diagram Synchronous motors Internet website: Simulator for synchronous generator

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Types of synchronous machines, simple working of synchronous machine as a generator and motor, expression for induced emf. Generator & motor operation

Resultant mmf vector, armature reaction reactance, synchronous impedance, simple equivalent circuit

Phasor diagram for cylindrical rotor machine, synchronous impedance from open circuit & short-circuit tests, short circuit ratio, generator load characteristics, armature reaction ampere turns and leakage reactance, Potier method

Two axis theory

Two-reaction theory of salient pole machine, phasor diagram & power angle relations, reluctance machine

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Voltage regulation

Voltage regulation by different methods

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Parallel operation of alternators

Synchronous machine on infinite bus (generator & motor action) and synchronization, synchronous machine operation on infinite bus at

1.        Constant excitation with variable load

2.        Constant load under variable excitation (O-curves & V-curves)                                                                                                                            TOP

Synchronous motors

Starting of synchronous motors, V-curves, Torque & power angle curves

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Hunting

Hunting in synchronous machines, damper winding

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# Objective questions

1. The damper winding in the synchronous moor is often used to

a)       prevent hunting only

b)       prevent hunting & provide the starting toque

c)       maintain  synchronism

d)       provide the starting torque only.

Ans. b

1. Which of the following statements is true?

Short-pitched coils in the alternators are used to

a)       reduce the size of the machine

b)       reduce the harmonics or to eliminate the harmonics from the generated emf

c)       provide accurate phase difference of 120 degrees  between phases

d)       reduce copper losses.

Ans. b

1. The armature flux of an alternator  at unity power factor load  will be

a)       demagnetizing

b)       of square waveform

c)       cross-magnetizing

Ans. c

1. The armature reaction of an alternator  will be completely magnetizing  when

a)       unity

b)       zero lagging

d)       0.8 lagging

Ans. c

1. If two alternators are running in parallel and he voltage of one machine is suddenly increased

a)       the machine winding will burn

b)       both machines will stop

c)       synchronizing torque will be produced to restore synchronism

Ans. c

1. If the  input  to the prime mover of an alternator ( in a system of three alternators) is kept constant, and the excitation is changed, then

(1)     the reactive component of the output is changed

(2)     the active component of the output is changed

(3)     the power factor of the load remains  the same

(4)     (a) and (b)  occur simultaneously.

Ans. a

1. The coupling angle or load angle of a synchronous motor is defined as the angle between the

a)       rotor and the stator poles  of same polarity

b)       rotor and the stator poles  of opposite  polarity

c)       rotor and stator teeth

Ans. b