IR test on a 5kV wound rotor motor

[mull982]

We need to perform an acceptance test on a 5kV 6500hp wound rotor motor. I plan on using a 5000V megger and performing a PI test and comparing the 10min and 1min values.

What is an acceptabalbe or minimum PI value I should see for this size/type motor?

Mhat is the minimum IR value in Megaohms I sould see for this motor?

Aside from testing the stator windings do I need to perfrom a seperate IR, or PI test on the rotor windings of this wound rotor motor?

I notice that our engineering spec for the project calls for Hi-Potting of all the 5kV motors. I believe it calls for 11kV DC for 1 min with a minimum IR value of 25Mohms. What are thoughts or standards on this Hi-Pot test for these 5kV motors? Good idea / bad idea? Are these standards correct?

 

[zog]

We need to perform an acceptance test on a 5kV 6500hp wound rotor motor. I plan on using a 5000V megger and performing a PI test and comparing the 10min and 1min values.

What is an acceptabalbe or minimum PI value I should see for this size/type motor?

PI should be >1.0 for sure, that is the only "real" spec. But NEMA has some other criteria as outlined in "A stich in time", if you don't have it, get it.

Mhat is the minimum IR value in Megaohms I sould see for this motor?

100M at 40 degrees C is the min spec for this motor per ANSI/NETA ATS.

Aside from testing the stator windings do I need to perfrom a seperate IR, or PI test on the rotor windings of this wound rotor motor?

Yep

I notice that our engineering spec for the project calls for Hi-Potting of all the 5kV motors. I believe it calls for 11kV DC for 1 min with a minimum IR value of 25Mohms. What are thoughts or standards on this Hi-Pot test for these 5kV motors? Good idea / bad idea? Are these standards correct?

Thoughts? Whoever wrote the standard knows squat about hipotting, there is no spec and they dont read in Megohms. It is a withstand test and should be done per ANSI/IEEE standard 95.

ANSI/NETA also recommends a PF test on motors >2300V.

 

[mull982]

PI should be >1.0 for sure, that is the only "real" spec. But NEMA has some other criteria as outlined in "A stich in time", if you don't have it, get it.

We performed the PI test this morning at 2500V. The results for the stator all had about a 1.5 PI ratio and for the rotor we had a 1.1, 1.5, and 2.0 for the phases.

If you are saying that greater than 1 is acceptable then it looks like everything here is o.k. I have seen other posts of yours where you said that 1 or greater for a DAR test is acceptable. So it looks like in summary that anything greater than 1 for theset two test is acceptable.

Is 2500V good for testing these motors or should we have used 5000V?

100M at 40 degrees C is the min spec for this motor per ANSI/NETA ATS.

All reading leveled out to above 100M during 10min interval. It looks like we are good here. I saw in other posts where it was stated that for 480V motors 5M was minimum. So as a rule of thumb can it be said that 5M is minimum for 480V motors, and 100M is minimum for 5kV motors as far as acceptance?

Thoughts? Whoever wrote the standard knows squat about hipotting, there is no spec and they dont read in Megohms. It is a withstand test and should be done per ANSI/IEEE standard 95.

ANSI/NETA also recommends a PF test on motors >2300V.

I agree, this spec did look a bit strange. For the rest of the project which has about 6 5kV motors I am going to use a PI test at either 2500V or 5000V as opposed to any Hi-Potting.

 

[mull982]

In case your curious these were the results of the test:

Stator:

A Phase - 1min -500M 10min - 750M PI- 1.5

B Phase - 1min - 500M 10min - 750M PI - 1.5

C Phase - 1min - 500M 10min - 750M PI - 1.5


Rotor:

A Phase - 1min - 67.5M 10min - 100M PI - 1.49

B Phase - 1min - 105M 10min - 115M PI - 1.10

C Phase - 1min - 112.5 10min - 225M PI - 2

 

[zog]

We performed the PI test this morning at 2500V. The results for the stator all had about a 1.5 PI ratio and for the rotor we had a 1.1, 1.5, and 2.0 for the phases.

If you are saying that greater than 1 is acceptable then it looks like everything here is o.k. I have seen other posts of yours where you said that 1 or greater for a DAR test is acceptable. So it looks like in summary that anything greater than 1 for theset two test is acceptable.

1.0 is the minimum spec for NETA/ANSi testing on both tests, but NEMA goes more in depth here and says the follwoing:

Condition DAR PI
Dangerous ------ <1.0
Questionable 1.0-1.25 1.0-2.0
Good 1.4-1.6 2-4
Excellent >1.6 >4

Is 2500V good for testing these motors or should we have used 5000V?

2500VDC is fine, that is the minimum test voltage

All reading leveled out to above 100M during 10min interval. It looks like we are good here.

Should be >100M after 1 minute. Did you temp correct the readings?

I saw in other posts where it was stated that for 480V motors 5M was minimum. So as a rule of thumb can it be said that 5M is minimum for 480V motors, and 100M is minimum for 5kV motors as far as acceptance?

No, depends on the motor type. I just gave you the specs for the motor you described. I don't believe in rules of thumb, they lead to misconceptions, I believe in real specs.

I agree, this spec did look a bit strange. For the rest of the project which has about 6 5kV motors I am going to use a PI test at either 2500V or 5000V as opposed to any Hi-Potting.

ANSI/NETA still requires a (hipot) withstand test, just needs to be done right and the tech needs to know what they are looking for. But, the spec they gave you makes little sense.

 

[mull982]

Should be >100M after 1 minute. Did you temp correct the readings?

No I did not temp correct readings. The 100M seems to be based on 40degC? It was about 80deg F which corrosponds to only 26degC so it looks like my readings should actually be higher than reported. (adjustment factor)

When performing a megger test on 5kV motors which have surge arrestors and surge capacitors, do you need to disconnect the arrestors and capacitors if you megging from upstream of the motor.

I know with the capacitors they will block the DC current, so there should be no leakage current to ground to worry about? What about arrestors? Should either of these be a concern when testing as far as results?

 

[zog]

No I did not temp correct readings. The 100M seems to be based on 40degC? It was about 80deg F which corrosponds to only 26degC so it looks like my readings should actually be higher than reported. (adjustment factor)

Yep, they look much better now

When performing a megger test on 5kV motors which have surge arrestors and surge capacitors, do you need to disconnect the arrestors and capacitors if you megging from upstream of the motor.

Usually, yes.

I know with the capacitors they will block the DC current, so there should be no leakage current to ground to worry about? What about arrestors? Should either of these be a concern when testing as far as results?

They should be disconnected also. They have thier own tests to be performed on them and will effect your readings of your motor tests.

 

[ELA]

Thoughts? Whoever wrote the standard knows squat about hipotting, there is no spec and they dont read in Megohms. It is a withstand test and should be done per ANSI/IEEE standard 95.

ANSI/NETA also recommends a PF test on motors >2300V.

I do not work much with motors but I have written hi-pot test specifications for other equipment and UL864 which pertains to that equipment states the test level as:"

c) For circuits rated more than 150 volts AC rms (212 volts DC) ? 1000 volts AC plus twice the
rated voltage (1414 volts plus 2.828 times the rated AC rms voltage, when a DC potential is
used)."


This could be where the person who wrote the spec got the 11K from? I am not saying their spec is correct but-
we test a unit rated at 240 VAC at a high pot setting of 2.09KVdc

 

[bob]

The results of a HiPot test are not in megohms. The results are in micro-amps. The voltage is usually twice the operating voltage + 1000 volts.
Below is more information that applys to your installation. Check the hi-pot information in the middle of the atricle.

http://www.reliabilityweb.com/art04/motor_pdm.htm