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GATE 2017 EE – Question 18

Electrical Machines · Three-phase induction machines: principle of operation, types, performance, torque-speed characteristics, no-load and blocked-rotor tests, equivalent circuit, starting and speed control · 1 mark · Multiple choice

A 4 pole induction machine is working as an induction generator. The generator supply frequency is 60 Hz. The rotor current frequency is 5 Hz. The mechanical speed of the rotor in RPM is

  1. 1350
  2. 1650
  3. 1950
  4. 2250

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Correct answer: (C) 1950

Explanation

The synchronous speed is $\frac{120 \times 60}{4} = 1800$ rpm. The rotor current frequency is $|s| f = 5$ Hz, so $|s| = \frac{5}{60} = 0.0833$. A generator runs above synchronous speed, so the rotor speed is $1800 \times (1 + 0.0833) = 1950$ rpm.