Motor current unbalance and met results

[EEENGRX]

Check out these results for megging a motor after an over amping issues was noticed. phase 2 GND
A- 168.5
B - 186.2
C- 131.7
And wire only phase to GND results are
A-B - 12.03
A-C - 9.49
B-C - 14.83

Every other tests like phase to phase and motor leads looks goodWhat are your thoughts. And btw this is a 3 phase 200HP 460V SF- 1.15 SFA-265A motor. Let me know yours thoughts

 

[che12345]

Please clarify:
1. Motor winding is in Star or in Delta ?
2. Phase 2 (to?) GND A- 168.5, B - 186.2 ...values are in kohm or Mohm ?
3. wire only phase to GND results are : A-B - 12.03, A-C - 9.49...

a) why A-B, A-C, ... should be A-GND, B-GND and C-GND?
b) the values are in kohm or Mohm ?
Che Kuan Yau (Singapore)

 

[EEENGRX]

Yes it’s phase 2(to) GND
And it’s in
GΩ. Not sure about the connection if it’s start or delta

 

[EEENGRX]

Both values are in Ω
GΩ and test voltage is 1KV

 

[electricpete]

What does any of this have to do with the title of current unbalance. I'm going to assume that was a typo and there is no current unbalance under discussion.

Both values are in Ω
GΩ and test voltage is 1KV

I don't know why you wrote both Ω and GΩ. It's one or the other.

I'm going to assume you mean GΩ and none of your readings represent Ω

In that case, no it's not a problem at all to have minor unbalances in GΩ readings. Very small differences in surface leakage currents near the test lead, or grounded time / residual voltage can easily cause these differences.

 

[wcaseyharman]

If those are in Gohms then I would move on. If I recall correctly the IEEE 43 insulation resistance test standard suggests above 5 Gohm the accuracy of an insulation resistance test is questionable anyway because the currents are so small (that is in referencing a polarization index test, but I think the idea applies here regarding variation between phases.)

 

[che12345]

" ...Check out these results for megging a motor after an over amping issues was..."
1. What do you mean "over amping" ?
a) heavy short-circuit current, breaker trips on instantaneous trip ?,
b) the starter thermal over-load relay trips on over-load.
2. Some deviation in the IR (in Gohm ?) is normal.
3. check the following
a) voltage at the motor terminal A-B, B-C, C-A ; A-E, B-E, C-E.
b) each line, full load current.
c) motor and load bearings, the rotor and the load should be able to be turned by hand freely.
Che Kuan Yau (Singapore)