Motor Phase to Phase Resistance
Motor Phase to Phase Resistance
(OP)
I have a customer with VFD driving a 50hp, 2 pole, 460v motor. The VFD is does not want to start the motor at all and is tripping on over current. He measured the motor winding from phase to phase and got .3, .3, .4 ohms and open to ground.
This sounds low to me but I would think there would be some short to ground if the windings were burnt.
Do the readings sound like they are in the ballpark?
Barry1961
This sounds low to me but I would think there would be some short to ground if the windings were burnt.
Do the readings sound like they are in the ballpark?
Barry1961





RE: Motor Phase to Phase Resistance
Gunnar Englund
www.gke.org
RE: Motor Phase to Phase Resistance
To me the motor winding readings look o.k. for a motor that size (about 37kW) I would expect a very low resistance.
Is he trying to start the motor at full speed straight away, 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 (I don't know if this is how the system is controlled) or also by looking in the parameters for the drives ramp, they all normally have one, it just depends uopn how much you want to vary the speed.
RE: Motor Phase to Phase Resistance
Is this a new installation, or has this problem just begun on an existing instalation?
What is the motor driving?
Is the load free and easily able to rotate?
Is if possible to uncouple the motor from the load and see if the VSD will spin the motor on it's own?
It is posible 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.
Best regards,
Mark Empson
http://www.lmphotonics.com
RE: Motor Phase to Phase Resistance
I found out he had the drive setup for a 4 pole (default) motor.
The drive had been running for over a month before problem started. I think someone may have started changing parameters then decided to restore factory defaults. I am getting some real fuzzy answers to questions I am asking.
I am surprised that any windings that big would measure less than a ohm. But I have not worked on many motors that size.
Barry1961
RE: Motor Phase to Phase Resistance
Gunnar Englund
www.gke.org
RE: Motor Phase to Phase Resistance
RE: Motor Phase to Phase Resistance
A ductor if I'm not mistaken is a high current device. I don't think you need that.
You need an instrument with Kelvin type probes (4 lead resistance method). Or else a bridge type resistance measurement.
The idea of the 4-lead measurement is that one set of leads is the current leads which provide the power to maintain the voltage, the other set of leads in parallel is a voltage sensing lead. The voltage sensing lead is connected to a high-impedance voltage sense circuit so the lead resistance and connection resistance are not critical.
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RE: Motor Phase to Phase Resistance