Motor Phase to Phase Resistance

[Barry1961]

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

 

[Skogsgurra]

Sounds a little bit low since normal test lead resistance usually is in that range. How did he measure? Is the motor delta connected?

Gunnar Englund

http://www.gke.org

 

[Kevinprior]

Hi All,
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.

 

[Marke]

Hello Barry1961

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

 

[Barry1961]

Thanks for quick responses.

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

 

[Skogsgurra]

Assuming about 1 percent winding losses in the motor, you arrive 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.

Gunnar Englund

http://www.gke.org

 

[stardelta]

The phase resistances for a motor of that size will be very low but you really need to use a Ductor type ohmeter to get an accurate figure, an ordinary ohmeter is simply not accurate enough at such low resistances. If possible remove any shorting straps from the block and measure the resistance of each phase circuit as this will give the highest and most accurate reading. Winding resistances can vary slightly between phases as a result of the winding design. Its a common practise to slightly decrease coil size over each progressive layer particularly on 2 pole machines that use a full slot winding design. A good repair shop will have suitable test equipment and be able to carry out a surge test possibly even on site which will give you a simple yes/no answer on the winding condition. Another thing to consider is the condition of the insulation resistance "between phases" and not just to earth, it should be of a similar value for a servicable winding and at least 1MOhm. No mention of this test was made in the original posting and a failure of this nature would also cause the faults you describe.

 

[electricpete]

On meausuring winding resistance, I agree with stardelta you need to select test equipment carefully and an ordinary ohmmeter is not accurate.

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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[itsmoked]

electricpete; so you connect the 4 leads to two points? Same mentality as a 4 wire RTD?