why 3 phase motor no load pf is high?

I measured a small induction motor in the electrical lab for pf, and the pf was found to be 0.95. The motor has high resistance to the winding, but still the pf is supposed to be less at no load, as the motor draws only reactive power at no load right?


rated pf is 0.7

5 Answers

  • Steven
    Lv 5
    7 years ago
    Favorite Answer

    Yes. pf should be less at no load. Compare the current drawn by the motor at no load and full load. It should draw less current at no load. If not so, then there is some problem with motor winding or the winding is getting earthed. Use Megger.

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  • Anonymous
    7 years ago

    0.95 pf is extremely high for an induction motor, even at full load.

    Firstly I'd say re-check your calculations.

    Secondly how did you calculate, or measure the pf?

    IE. if you used a variable speed drive,(VSD), if so where did you take measurements, before or after the VSD? (A VSD will have a power factor at near unity).

    Smaller motors do have a higher DC resistance in the windings than larger motors.

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  • 7 years ago

    Yes it should be less than the rated pf. Check if the rated voltage is applied to the motor input terminals. If it is already applied then check for the measuring devices. If nothing changes them most likely there's a fault in the motor winding.

    Source(s): Elec Eng
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  • amadi
    Lv 4
    4 years ago

    Now you are speakme in small print what used to be your concern. You will have two screw ups.... [1] No grounding on desktop case and motor case. If a good floor was there, a excessive voltage leakage can be shorten away and blew the fuse. [2] possible motor winding leaking or pitched wire somewhere inside the motor. This failure can't be found with 1.5V kind ohm-meter. You desire a 250V to 500V mega to seek out this fault. I bit it shall learn just about zero ohm between any winding and motor case with 500V atmosphere. You might make a easy mega this way...... Rectify instantly from AC energy source with a bridge to obtain about +200V or +400V utilizing this DC power source in sequence with a 500V environment DC voltmeter. Now the meter terrible and the DC vigour negative are the test leads of a residence made mega. Any leakage motor shall show a reading about 200V or 400V.

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  • 7 years ago

    since the motor is induction which is more inductive and not a synchronous it will behave the same manner it behaved.If you want to improve the power factor introduce a capacitor in your circuit

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