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BARC 2018 EE: Electrical Machines Nuclear Quiz 2

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Question 1

A 400/100 V, 5 kVA, single phase two-winding transformer is to be used as an autotransformer to supply 400 V from a 500 V source. When tested as a two winding transformer at rated load and 0.8 p.f. lagging, its efficiency was found to be 0.95. Find its efficiency as an autotransformer at rated load and 0.8 pf lagging.

Question 2

A turbo-generator, connected to an infinite bus is operating at 0.8 power factor lag. If its excitation is increased, with steam input kept constant then it would
1- Feed more leading KVAR to the bus.
2- Feed more lagging KVAR to the bus.
3- Feed more real power to the bus.
4- Start operating at power factor < 0.8.
From these, the correct answer is

Question 3

What is the speed of the slip ring induction motor when 3-phase balanced voltage source at 50 Hz is applied. If a 3-phase slip ring induction motor is wound for 4 poles on stator and 6 poles on the rotor?

Question 4

Figure shows an ideal three–winding transformer. The three windings 1, 2, 3 of the transformer are wound on the same core as shown. The turn’s ratio N1:N2:N3 is 4:2:1. A resistor of 10W is connected across winding–2. A capacitor of reactance 2.5W is connected across winding–3. Winding–1 is connected across a 400 V, ac supply. If the supply voltage phasor V1 = 400∠0 °, the supply current phasor I1 is given by

Question 5

A three-phase, salient pole synchronous motor is connected to an infinite bus. It is operated at no load at normal excitation.
The field excitation of the motor is first reduced to zero and then increased in the reverse direction gradually. Then the armature current
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Jun 24ESE & GATE EE