HV motor running with low insulation 1

[riqsoares]

I have in my plant a 2500 kW, 13200 V, 20 poles brushless synchronous motor. The machine was running and when it stopped i did the insulation test with 5 kV. The insulation test result was 3 MΩ. The machine is installed in sheltered place and it does not have any signs of oil, dirty or moisture. When we do the insulation test with the guard probe connected together with the ground probe, the result was 7 GΩ.
The machine is now running normally. Could someone explain me this phenomenon?
Regards

 

[RRaghunath]

Apparently, it is the high capacitance of the winding that is playing mischief.
By earthing the guard probe, I guess you are some how draining the capacitive charge current away from the IR instrument.
Did you try measuring PI value of the motor (ratio of 10min IR to 1min IR)??

 

[riqsoares]

The PI value was 1,3 without the guard probe. We did not measure it with the guard probe.

 

[electricpete]

Can you describe your test setup better for both tests guarded and unguarded.

 

[edison123]

Purpose of guard terminal in a megger.

http://blog.protecequip.com/blog/protec/protec/what-is-the-purpose-of-a-guard-terminal

If you meggered with the motor supply cables still connected to the motor, the use of guard terminal will drastically change the IR value. If you are meggering just the motor winding without the supply cables, use of guard terminal will not affect the winding IR much. In our shop, we never use guard terminal for measuring winding IR.

Muthu

http://www.edison.co.in

 

[riqsoares]

The supply cables were desconnected from the motor.
I am attaching the test setup.

 

[panter]

I think that 5kV is small test voltage for your motor. Can you separate conductors in star and test insulation resistance of all three coils interphase .
Good luck !

 

[krisys]

Referring to the link shared by Muthu, cleaning the terminal box insulators might improve the IR value. This is just my guess. Motor terminals are the only portion where the conductors are exposed.

 

[flexoprinting]

I agree with Panter and think the test voltage is too low, Here is a link I found with recommendations on page 6.

http://www.techni-tool.com/site/ART...te Guide to Electrical Insulation Testing.pdf

Chuck

 

[riqsoares]

My main question is how a HV motor can work such low insulation.
I can not do the insulation test now because the motor is running.

 

[panter]

Riqsoares ,
insulation resistance between phases should definitely be measured .13200 / 1,73 is 7,500 V to the ground and if you're lucky a few megaomhs may be enough for a while .

 

[edison123]

Low or good IR value is not a direct indication of winding dielectric strength. I have seen machines with poor IR running fine while those with high IR failing. IR is more of a trending measurement than a single determinant value about winding condition since it changes with temperature, contamination etc.

Muthu

http://www.edison.co.in

 

[krisys]

This motor certainly requires tan δ test or some other tests which has a capability to split the resistive and capacitive components, leakage resistance current components etc. After analysing the contribution from each component only one can take a rational decision and troubleshoot.