This set of Electrical Machines Multiple Choice Questions & Answers (MCQs) focuses on “Flux and MMF Phasors in Synchronous Machines”.
1. A 3 phase synchronous machine is _________ excited machine.
c) two stage
d) feedback based
Explanation: The 3 phase synchronous machine has two excitations. Ac excitation at armature while dc excitation at field winding.
2. Field winding of a dc machine
a) always absorbs power
b) absorbs real power at lagging load
c) delivers power while behaving as generator
d) always delivers power
Explanation: Field is always given supply from dc system so it always absorbs power as it is necessary to produce the flux.
3. In large synchronous machine, field winding is placed on ______ and ac supply on ______
a) rotor, stator
b) stator, rotor
c) armature, slots
d) pole shoes, stator
Explanation: The field winding is placed on the stator as it will carry less current while the ac supply is fed at stator as it will have to carry large amount of current.
4. The pilot exciter in dc exciters is ________
a) dc shunt generator feeding field winding of mains
b) universal motor
c) stepper motor feeding the field winding of mains
d) any of the mentioned
Explanation: Pilot exciter is dc shunt generator which supplies the field of main winding.
5. The main problems faced by conventional DC exciters are/is _______
a) cooling and maintainance
b) wear and tear
c) additional parasitic losses
d) all of the mentioned
Explanation: The conventional dc exciters have all the above mentioned losses which make them out dated and least efficient.
6. The static excitations mainly comprises of _______
a) brushless excitation system
b) thyrister based excitation system
c) synchronous motor excitation system
d) brushless and thyristor based excitation system
Explanation: SCRs are used in the rectifier configuration as a static system for excitation for dc field winding.
7. The excitation system and speed of alternator has _______
a) linear response
b) exponential response
c) rectangular hyperbola response
d) circular response
Explanation: As the excitation energy is directly taken from alternator terminals, excitation voltage is directly proportional to speed.
8. The brushless excitation and conventional DC excitation systems are comparable but one significant change is _______
a) pilot exciter is brushless permanent magnet motor
b) three phase rectifier feeds directly to main exciter
c) a three phase rectifier is fed by main exciter
d) no direct rectification is provided to the pilot exciter
Explanation: The brushless excitation involves a brushless permanent motor which acts as an auxiliary exciter.
9. The main exciter used in DC excitation is ________
a) field on stator
b) armature on stator
c) field on rotor
d) field on poles
Explanation: The main exciter used in DC excitation is field on stator.
10. If the DC excitation is suddenly dropped to 0 ,the three phase alternator _________
a) runs as motor
b) stops to zero speed in few seconds
c) continues to run as motor but at lower speed
d) no change in the operating conditions
Explanation: When the dc excitation is disconnected from the supply, it will start acting as synchronous motor as the prime mover will still be mechanically coupled.
Sanfoundry Global Education & Learning Series – Electrical Machines.
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