Generator with low IR

[Emad Shaaban]

We are currently assessing the condition of a three-phase 2.5 kVA, 400V emergency generator, which is used for black start only. During our routine checks, we observed the following insulation resistance values:
• Initial readings (before cleaning):
• Phase to ground (IR): as low as 404 kΩ
• Phase to phase: approximately 1.2 MΩ

We then dismantled the rotor and performed the following maintenance steps:
• Cleaned the windings with isopropyl alcohol
• Coated the sleeve with electrical varnish
• Dried the unit using an air dryer

Post-maintenance insulation readings:
• Phase to ground: improved significantly to 1 GΩ
• Phase to phase: now reads 15 MΩ

My question is:
Are these 15 MΩ phase-to-phase readings acceptable for such a small generator used only for emergency/black start purposes? Or should further action be considered to improve these values

 

[waross]

When faced with a low insulation resistance (often below 1 Meg Ohm) I calculate the losses and make a judgement call as to the severity of the losses and the associated heating.

Phase to phase = 15 MΩ.
15 MΩ at 400 Volts = 0.027 mA
0.027 mA at 400 Volts = 0.0107 Watts per phase.
Total heating = 0.0107 x 3 = 0.032 Watts losses.
I would not be concerned.

 

[edison123]

Phase to phase IR? Is it 6 leads generator? If not, you should be getting zero megohms between phases.

 

[wcaseyharman]

For low voltage equipment I generally see the pass/fail at 1 megohm, with I think comes from a NETA (inter-National electrical testing association) testing guideline. I don’t remember the section, if you’re interested I can go back and find it and get you the reference.

Casey

 

[waross]

Is it 6 leads generator?

In North America, quite possibly a 12 lead generator.

For low voltage equipment I generally see the pass/fail at 1 megohm,

I never worried much unless the resistance was below 1 megOhm.

 

[Emad Shaaban]

For low voltage equipment I generally see the pass/fail at 1 megohm, with I think comes from a NETA (inter-National electrical testing association) testing guideline. I don’t remember the section, if you’re interested I can go back and find it and get you the reference.

Casey

Would you please do that for me please