GATE EE 2015 Set 1 — Question 44
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Electrical Machines → Inverters → Harmonics in Inverters
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Question
A 3-phase square wave (6-step) VSI feeds a 3-phase, 4 pole induction motor. The VSI line voltage has a dominant harmonic component. If the operating slip of the motor with respect to fundamental component voltage is , the slip of the motor with respect to harmonic component of voltage is ________.
Correct answer
1.16 to 1.22
Solution
Step 1: Fundamental synchronous speedStep 2: Rotor speed from the fundamental slipStep 3: Field produced by the 5th harmonicIn a three-phase six-step VSI the harmonic voltages form a negative-sequence set, so the harmonic field rotates backwards at five times the fundamental synchronous speed:Step 4: Slip with respect to the 5th harmonicEquivalently . The slip exceeds unity because the rotor turns against the harmonic field.Answer:
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