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c) 3 d) 1/3*sqrt(3) View Answer Answer: a Explanation: The no load current of the transformer is directly proportional to the dimension of the transformer. 3. For a single phase, 50 Hz transformer, if the open circuit test is conducted at 40 Hz. What is the variation in the no-load power factor of the transformer? a) Decreases b) Increases c) Remains constant d) None of the mentioned View Answer Answer: a Explanation: If the frequency reduces then flux density increases, so the flux and then the magnetizing current. When magnetizing current increases power factor angle increases so the power factor decreases. 4. The desired voltage regulation for a single phase, 50 Hz transformer is zero. Its pu ohmic drop is 4% and pu reactive drop is 4%. The required power factor angle for the operation in degrees a) 45 b) 90 c) 30 d) 60 View Answer Answer: a Explanation: The power factor angle is, tan(phi)=pu resistive drop/pu reactive drop = 4/4= 1 5. While conducting lab experiments on DC machine loading, the students are given a machine with its rating as 100 kW maximum rating. Then a) machine can’t be overloaded after 100 kW b) machine can be overloaded after 100 kW c) machine can be overloaded by 5% for a small time duration d) machine can be overloaded by 10% View Answer Answer: a Explanation: As the maximum rating is specified and operation condition are also same, it cannot be overloaded beyond the maximum. 6. A 3-phase, five limbed shell type transformer, star connected, 200V, 50 Hz, 100 kVA will have a) no third harmonic current in its phase but has sinusoidal flux b) third harmonic current in its phase but has flat-topped flux
