I'm not a VFD expert but the possibility of having the wires worn so that only a couple of strands are attached does not seem likely to me. To greatly reduce the speed a lot of voltage would have to be dropped in those few strands of wire. That voltage drop and the amount of current flowing into the motor would generate a lot of heat and probably melt the wires. Then the motor wouldn't run at all.
I think most VFD's anticipate the speed/torque curve and make modifications to the voltage/current when it detects that more of a load is placed on the motor. When a load is placed on the motor, the motor has to generate more power to maintain the speed (or close to the speed). To generate more power, the motor has to take more power from the line. The VFD should detect the increased current flow or the voltage drop (depending on how the VFD does its control) and make adjustments to supply more power to the motor. The motor will slow down because there will be a greater slip but the slow-down should be fairly small.
So - to give you a short answer, my guess is that you have a problem in the VFD. But I'm not a VFD expert by a long shot.
[and as Ken suggest below, certainly check the wires just in case. It's quick, easy, and cheap to do.]
Last edited by Mike Henderson; 01-11-2008 at 04:21 AM.
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