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SYNCHRONOUS MACHINES

 Construction & types of synchronous machines

 Armature reaction

 Phasor diagram

 Two axis theory

 Voltage regulation

 Parallel operation of alternators

 Synchronous motors

 Hunting

 

 

Internet website: Simulator for synchronous generator

 Objective questions

HOME takes you to the start page after you have read these Topics. Start page has links to other topics.

 

http://powerlearn.ee.iastate.edu  -- Simulator for synchronous generator

 

Construction & types of synchronous machines

 

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

 

Armature reaction

 

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

 

Phasor diagram

 

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

TOP

Voltage regulation

 

Voltage regulation by different methods

TOP

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

TOP

Hunting

 

Hunting in synchronous machines, damper winding                   

TOP

 

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

load power factor  is

 

a)       unity

b)       zero lagging

c)       zero leading

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