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GATE 2020: Electrical Machines Rapid Revision Quiz 1
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Question 1
A 4–pole induction motor, supplied by a slightly unbalanced three–phase 50 Hz source, is rotating at 1440 rpm. The electrical frequency in Hz of the induced negative sequence current in the rotor is
Question 2
The resistance and reactance of a 100 kVA, 11000/400V, Δ - Y distribution transformer are 0.02 p.u. and 0.07 p.u. respectively. The phase impedance of the transformer referred to the primary is
Question 3
A 4-pole, separately excited, wave wound DC machine with negligible armature resistance is rated for 230 V and 5 kW at a speed of 1200 rpm. If the same armature coils are reconnected to form a lap winding, what is the rated voltage (in volts) and power (in kW) respectively at 1200 rpm of the reconnected machine if the field circuit is left unchanged?
Question 4
To conduct load test on a dc shunt motor, it is coupled to a generator which is identical to the motor. The field of the generator is also connected to the same supply source as the motor. The armature of the generator is connected to a load resistance. The armature resistance id 0.02 p.u. armature reaction and mechanical losses can be neglected. With rated voltage across the motor, the load resistance across the generator is adjusted to obtain rated armature current in both motor and generator. The p.u. value of this load resistance is
Question 5
A 400V, 50 kVA, 0.8 pf leading Δ-connected, 50 Hz synchronous machine has a synchronous reactance of 2Ω and negligible armature resistance. The friction and wind age losses are 2 kW and the core loss is 0.8 kW. The shaft is supplying 9kW load at a power factor of 0.8 leading. The line current drawn is
Question 6
The armature resistance of a permanent magnet dc motor is 0.8Ω. At no load, the motor draws 1.5 A from a supply voltage of 25 V and runs at 1500 rpm. The efficiency of the motor while it is operating on load at 1500 rpm drawing a current of 3.5 A from the same source will be
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Dec 20ESE & GATE EE
