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[Guest]

A certain application features a VFD driving a 50hp, 2-pole, 460-VAC motor. The VFD is refuses to start the motor, and trips the over-current protection. A technician measured the motor windings phase to phase; theyare 0.3, 0.3,0 .4 ohms and open to ground.

The technician updates the issue by reporting the drive was incorrectly setup for a 4 pole (default) motor. The drive had been running for over a month before problem started. She thinks someone may have started changing parameters, then decided to restore factory defaults.

What's wrong?

[daqmaster]

Troubleshooting:

These appear bit low since normal test-lead resistance usually is in that range. Is the motor delta connected?
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The motor winding readings look fine for a motor that size (about 37kW) -- one should expect a very low resistance. Is the operator trying to start the motor at full speed immediately -- this would cause a massive inrush current and would probably trip the drive. If this is the case you have a few options: either ramp the drive up to speed over a finite period of time, possibly by the analog signal from a PLC (is this how the system is controlled?) or also by looking at the parameters for the drive's ramp(they all normally have one, it just depends on how much one wants to vary the speed).
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Further Questions:

Is this a new installation, or has this problem just begun on an existing installation?
What is the motor driving?
Is the load free and easily able to rotate?
Is it possible to uncouple the motor from the load and see if the VSD will spin the motor on it's own?
It is possible that there is just insufficient torque to start the motor from zero speed. This can be due to a jammed load, or a drive that is undersized.
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Assuming roughly one percent winding loss in the motor, one arrives at less than 0.1 ohms winding resistance. So the measured value is definitely test-lead resistance. The large variation, 0.3 - 0.4 ohms, is probably a result of how hard the test probes were pressed against the bolts. A winding showing such a large spread (about 30 percent) in resistance would have indicated a failure.