Inverter duty motor terminals insulation

[thinker]

We received a special design inverter duty motor built to MG1, Part 31 with max voltage 460V (RMS). It has three phase terminals (bus adapters) with clearance through air and creepage distance between phases 0.5". It looks like the motor manufacturer has used RMS voltage rating for clearance selection. The motor insulation per MG1 Part 31 must be capable to withstand repeatable voltage spikes with the peak value up to 3 times line-to-line voltage, and the motor insulation should comply with that. My question is: should the terminal spacing be determined based on higher than RMS values, and is the existing clerance 0.5" adequate? Thanks in advance for your inputs.

 

[DickDV]

I'm not an expert on the testing techniques but an MG1 P31 endorsed motor has to withstand 1600V high frequency pulses, not three times the supply voltage. Clearly, that would have to include the motor terminals which would have to be spaced accordingly.

 

[edison123]

Post a picture of the terminals here.

 

[thinker]

DickDV,
MG1, 31.4.4.2 states:"... Motors with base rating voltage not exceeding 600V: Vpeak=3.1*Vrated".
Still, the question is: if the motor sees 1600V pulses, why spacing between terminals is selected for 460V RMS?

 

[DickDV]

Hmmm! Clearly I've not read the actual spec. But everything I've ever read referencing MG1 P31 states 1600V peak.

Now I would like an explanation since 3.1 times peak would be 3.1 x 660 = 2046.

Anybody out there that can clarify this?

 

[thinker]

3.1 multiplier used with RMS, not peak value. The actual formula is: Vpeak<1.1x2xSqrt2xVrated=3.1xVrated

 

[jraef]

Don't know for sure, but it could be that standing wave pulses, to which the insulation material is designed to withstand, is not considered to be the same as continuous voltage as far as terminal spacing goes. So the dielectric capabilities of the air gap may be more akin to BIL ratings. For example if you look at MV switchgear, the insulation level and bus spacing may be designed around 5kV, but the BIL rating will often end up being 60kV.


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