Motor Current Varying Without Load
Posted 20 November 2009 - 05:35 AM
I have been facing a problem with a 10KW, 22A, 66HZ, 380V VFD compatible motor. The motor rotates a shaft having slitting knifes. First time it happened that motor current increased up to 34A. During checking I found that at 0-20HZ, its current was near about 34A (without load) and as speed increased current decreased at 13A. There was no any jamming in rotor moment found. We re-winded the motor, replaced the bearings, connected the motor at direct line, checked it current without load and found the same behavior as written above.
Any bode have any idea? For me, this is a rotor problem but it looked like ok.
Posted 20 November 2009 - 05:45 PM
I support your suspicion of the rotor. It happens - laminations insulation (there should be some although all looks so monolithic) degrades, some bars get distorted, rotor may be slightly shifthed along the axe, and the motor would draw more current and heat up. With current differences 34 and 13 - it must be very hot.
The rewinders, at a good repairing shop should have the appliance that measures rotor losses - so it was some time ago, am not sure what they use now. It's like a half-transformer, into which you put the rotor (thus forming its "second winding"). You may want to bring the rotor to them.
Posted 20 November 2009 - 06:52 PM
It sounds to me as though the V/Hz ratio is wrong at the low speeds.
Typically this is due to a voltage or torque boost setting, or a 60Hz motor programmed in as a 50 Hz motor.
You say that the motor is a 66Hz motor. I am not sure if this is a typo or actually a 66Hz motor.
In this case, the motor data must be programmed in as a 66Hz motor. If you set it as a 60Hz motor, you will overflux the motor and the current will be too high. At higher speeds, the VFD may be voltage limiting and this will reduce the flux.
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Posted 24 November 2009 - 06:10 AM
Thanks to friends for replying.
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