GATE 2021 EE – Question 28

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

The power input to a 500 V, 50 Hz, 6-pole, 3-phase induction motor running at 975 RPM is 40 kW. The total stator losses are 1 kW. If the total friction and windage losses are 2.025 kW, then the efficiency is ________%.

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Correct answer: 90

Explanation

The synchronous speed is $\frac{120\times50}{6}=1000$ rpm, so the slip is $\frac{1000-975}{1000}=0.025$. The air-gap power is $40-1=39$ kW and the mechanical power developed is $(1-s)\times39=38.025$ kW. The output is $38.025-2.025=36$ kW, so $\eta=\frac{36}{40}=90\%$.