Asymmetry Fault 11kv Motor

[ljessop]

Good morning/ evening all.

I have just recieved a call in regards to an asymmetry fault on an 11kV motor.

We have low resistance tested the rotor {wound rotor with liquid resistance starter with power factor correction banks}. Checked all brush connections, inspected the 11kV circuit breaker and even tested the relay for the starter. The only thing that has not been done, is the star point on the HV motor lifted and tested the individual windings. My suspicion is that we have a shorted turn/s in the windings. We do not have the gear to turns/ratio test the windings available, but we have a spare motor. The power factor bank has been checked as well and is okay.

 

Background: Suspected direct lightening strike during storm.....

 

The assymetry fault has not been seen before, but I would suspect that the current is pulling back the voltage in one phase giving an imbalance.

 

Need some input.............rapidly !

[marke]

Hello ljessop

 

Welcome to the forum.

 

If you suspect that you may have sorted turns in the stator, then one thing that you can easily do, is to put a low voltage across each winding in turn and measure the current draw. The motor will not try to rotate.

I would suggest that if the motor is star connected, you try around 230VAC from the star point to each phase input and you should get a measurable current flow. If the currents are all even, then the windings are probably OK or are breaking down under voltage. If you have an imbalance in current, then you have a winding issue. (I typically use 24VAC for a 400V motor)

 

Best regards,

Mark.

[ljessop]

Hello ljessop

 

Welcome to the forum.

 

If you suspect that you may have sorted turns in the stator, then one thing that you can easily do, is to put a low voltage across each winding in turn and measure the current draw. The motor will not try to rotate.

I would suggest that if the motor is star connected, you try around 230VAC from the star point to each phase input and you should get a measurable current flow. If the currents are all even, then the windings are probably OK or are breaking down under voltage. If you have an imbalance in current, then you have a winding issue. (I typically use 24VAC for a 400V motor)

 

Best regards,

Mark.

 

 

Thanks mark. We split the star point and resistance tested the windings. No real problems. Went back to the LRS and found a high resistance across the contactor on one phase that feeds the slip rings.

Cheers,

 

Luke

[marke]

Hi Luke

 

Thanks for the feedback. So often we are left in a vacuum as to whether the advise has helped or not.

 

Sounds like you got it sorted.

Best regards,

Mark.