GATE 2015 EE – Question 44

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 · 2 marks · Numerical answer

A 3-phase 50 Hz square wave (6-step) VSI feeds a 3-phase, 4 pole induction motor. The VSI line voltage has a dominant 5th harmonic component. If the operating slip of the motor with respect to fundamental component voltage is 0.04, the slip of the motor with respect to 5th harmonic component of voltage is ________.

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Correct answer: 1.16 to 1.22

Explanation

The fifth harmonic voltage sets up a field that rotates backwards at 5 times the synchronous speed, while the rotor turns forward at $0.96$ times synchronous speed. The slip is $s_5 = \frac{5 + 0.96}{5} = 1.192$.