Electrical machines miscellaneous Easy Questions and Answers | Page - 16

Electrical machines miscellaneous

Damping winding in a synchronous motor is generally used to

1. prevent hunting and provide the starting torque
1. reduce eddy currents
1. minimise vibrations
1. reduce noise level

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NA

Correct Option: A

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Voltage across the open-circuited field terminals of a synchronous machine under slip test is

1. d.c.
1. d.c. of slip frequency
1. a modulated supply frequency ac voltage with slip frequency envelope
1. a.c. of supply frequency

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When then driving power from the prime mover driving the alternator is lost, but alternator remains connected to the supply network and the field supply also remains, the alternator will behave as a synchronous motor and will also continue to rotate the same direction.

Correct Option: B

When then driving power from the prime mover driving the alternator is lost, but alternator remains connected to the supply network and the field supply also remains, the alternator will behave as a synchronous motor and will also continue to rotate the same direction.

Power angle characteristic of a salient - pole synchronous machine working with a constant field excitation on an infinite bus system is shown in the given figure. If load on the machine is suddenly changed from P₁ to P₂ (P₂ > P₁), then machine will fail to maintain stability if

1. shaded area A₁ is greater than shade area A₂
1. shaded area A₁ is equal to shaded area A₂
1. shaded area A₁ is less than shaded area A₂
1. δ_m > δ₀

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The answer is based on 'equal area criterion' of stability. If area A₁ is greater than the area A₂, then machine will fail to maintain stability.

Correct Option: A

The answer is based on 'equal area criterion' of stability. If area A₁ is greater than the area A₂, then machine will fail to maintain stability.

A synchronous machine working in the motor made, is fed from an infinite bus and is delivering half full-load. If an increase in field current causes an increase in the armature current, then the motor will

1. deliver reactive power and active power to the bus
1. absorb reactive power and active power from the bus
1. absorb reactive power from the bus and deliver active power to the bus
1. deliver reactive power to the bus and absorb active power from the bus.

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The condition stated corresponds to overexcitation and it is known that an overexcited synchronous motor runs under leading p.f. condition. Therefore, motor (loaded) delivers reactive power and absorbs active power from the bus.

Correct Option: D

The condition stated corresponds to overexcitation and it is known that an overexcited synchronous motor runs under leading p.f. condition. Therefore, motor (loaded) delivers reactive power and absorbs active power from the bus.

For a p-pole machine, the relation between electrical (θ_e) and mechanical (θ_m) degree is given by

1. θ_e = 2
  p.θ_m
1. θ_e = p.θ_m
1. θ_e = θ_m
1. θ_e = p
  2θ_m

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NA

Correct Option: D

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