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Electric Current and Potential

A 4 pole, 440...

Question

A 4 pole, 440 V, 3-phase star connected, alternator is connected to bus bar delivering a real power of 500 MW at pf of 0.9 lag. Neglect all losses at constant excitation. Choose the correct statement(s) regarding alternator.

A

If prime mover fails, machine will act as motor and working on zero power factor leading.

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B

Current delivered is zero if prime mover fails.

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C

Terminal voltage will be less than excitation emf in case of prime mover fails.

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D

Terminal voltage will be equal to excitation emf in case of prime mover fails

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Solution

The correct option is C Terminal voltage will be less than excitation emf in case of prime mover fails.

(a, b)
When prime mover fails real power delivered by alternator is zero as there is no losses, the real power required by machine is zero but current direction will change and machine will act as motor at leading pf and at no losses the pf will be zero. So terminal voltage will be less than excitation emf in case of leading pf.

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