Discussion
A 30 kW induction motor is found to be driving a load of only 4 kW. Its power factor as measured at the terminals will be
Answer: A
Correct answer: poor, the magnetising current dominating
The magnetising current of an induction motor is set by the applied voltage and is nearly constant from no load to full load, typically 30 to 40 per cent of the rated current. At light load the in-phase, work-producing component is small while that magnetising component is undiminished, so the total current is largely reactive and the power factor may fall to 0.2 or 0.3. The machine can never draw a leading current from the supply, and matching the motor to its load is the standard remedy for a poor plant power factor.
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