GATE 2014 EE (EE1) – Question 21

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 · Numerical answer

An 8-pole, three-phase, 50 Hz induction motor runs at 700 rpm. Find rotor-current frequency (Hz).

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Correct answer: 3.2 to 3.5

Explanation

Synchronous speed is120f/P=120×50/8=750rpm. At700rpm, slip is(750−700)/750=1/15. Rotor-current frequency is sf=(1/15)×50=**3.33333Hz**. It is the slip frequency, not the mechanical rotation frequency.