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Anonymous Poster #1

Electrical Motor 13, 8 KV Low Insulation Resistance Test Result

11/27/2018 8:56 AM

I have a problem with my motor 13, 8 KV, When we start, there's ground fault 105 Ampere, and we found there's differential current on phase C 147 Ampere. After we perform Insulation Resistance Test with 5 KV (3 minutes) we found result:

Phase A to ground: 115 Kohm (voltage inject 350 VDC)

Phase B to ground: 1, 5 Mohm (voltage inject 2500 VDC)

Phase C to ground: 2, 1 Mohm (voltage inject 2500 VDC)

Phase A to Phase B: 1, 5 MOhm (voltage inject 2500 VDC)

Phase A to phase C: 1, 8 Mohm (voltage inject 2500 VDC)

Phase B to Phase C: 4, 5 Mohm (voltage inject 2500 VDC)

1. What happens with the motor? Is the motor short to the ground on phase A?

2. Why is the Megger Test just 2500 KV while we are setting 5000 V?

3. When we perform Insulation Resistance Test with analog Megger Test, 5 KV, the result shows 0 Ohm. Why?

I hope I can hear a good answer soon, thank you

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#1

Re: Electrical motor 13,8 KV low insulation resistance test result

11/27/2018 9:03 AM

A1) It looks that way, though the fault is not linear with respect to applied voltage.

A2) The question is best directed towards the individual actually carrying out the test and the test protocol, rather than a global, international, anonymous internet forum: what <...we...> are doing depends also upon the boundary of the <...we...>, which hasn't been shared with the forum.

A3) The measurement seems to reinforce A1)

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#2

Re: Electrical Motor 13, 8 KV Low Insulation Resistance Test Result

11/27/2018 8:09 PM

Maybe you could use CR4 to organize a prayer circle to make this pesky ground fault go away.

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#3

Re: Electrical Motor 13, 8 KV Low Insulation Resistance Test Result

11/28/2018 6:16 AM

1. The motor has a poor resistance between all phases and ground so that a complete coil was damaged .

2. Digital instrument has a current limit , 350V/115 Kohms is 3mA so that when it reaches it, it will not increase the output voltage.

3. Analog instrument has no current limit and it shows zero

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#4

Re: Electrical Motor 13, 8 KV Low Insulation Resistance Test Result

11/28/2018 8:56 AM

ANSEWRS

1) Phase A: is groundwall insulation shorted (kilo-scale); 41 m-A current.

Recommendation: clean the stator and perform a new Megger test. If results will be similar, then phase-A has a groundwall failure and phases B-C have a poor insulation.

Other two questions already answered by Panta.

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#5

Re: Electrical Motor 13, 8 KV Low Insulation Resistance Test Result

11/28/2018 11:00 AM

I am assuming that your motor is star connected?

If so yes, A phase is showing signs of insulation fatigue maybe due to heat and/or harmonics.

You have two options

1. run it until it blows up
2. rewind it now

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#6

Re: Electrical Motor 13, 8 KV Low Insulation Resistance Test Result

11/28/2018 11:15 AM

INSPECT stator lead(s): cracks.....

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