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A 3-phase, 400 V, 4 pole, 50 Hz star connected induction motor has the following parameters referred to the stator:
R'2 = 1Ω, Xs = X'r = 2Ω
Stator resistance, magnetizing reactance and core loss of the motor are neglected.
The motor is run with constant V/f control from a drive. For maximum starting torque, the voltage and frequency output, respectively, from the drive, is closest to,
- 400 V and 50 Hz
- 200 V and 25 Hz
- 100 V and 12.5 Hz
- 300 V and 37.5 Hz
Correct answer: 100 V and 12.5 Hz
Solution
At standstill (s=1) maximum torque occurs when total leakage reactance equals R2'=1 ohm. Since Xs+Xr'=4 ohm at 50 Hz, the frequency must drop to 50*(1/4)=12.5 Hz; keeping V/f=400/50=8 gives V=8*12.5=100 V. So 100 V and 12.5 Hz.
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