Motor - Low insulation resistance

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JamieP
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Motor - Low insulation resistance

Post by JamieP »

Asked to connect a motor. This was a used motor so I checked the winding resistances, insulation resistance between windings and IR from windings to frame. My IRs were around 0.5M through to around 2.5M at the highest. I decided I wasn't happy to connect.

My question is was this a fair judgement? I understand we have IR values in 3000 but do they apply to things like motors our would there possibly be different requirements in product standards.

Maybe I'm over thinking this but as always best to ask others and see other opinion

Basically take the used part out of the equation because I suppose it makes no difference if used or new, except you'd probably expect higher values for new. Would you connect a motor that IRs less than 1MOhm? Would this change if the motor was disconnected as is for a certain purpose and simple asked to be reconnected as is?
AlecK
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Re: Motor - Low insulation resistance

Post by AlecK »

"3760" specifies the min IR for a Class I appliance as 1 megohm
JamieP
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Re: Motor - Low insulation resistance

Post by JamieP »

This wouldn't be within the scope of 3760 though, would it?

Or are you just applying that as a closest available guide to this situation
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gregmcc
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Re: Motor - Low insulation resistance

Post by gregmcc »

Done lots of motors in the past, the only reasons for a low test would be moisture ingress or it been on the way to burnt out. 1/2 meg is too low right off the mark, 2.5 meg would be marginal at best.

Even if used at 2.5meg I would expect a total failure in the near future if used.

So yes a fair judgment call.

depending on the if it's a moisture ingress, you could get it dried out in a oven and see how it tests, if it's a burnt out problem then that's a rewind or replace.

Also remember that a IR test isn't 100% perfect, had a 100kw motor that IR tested perfect, hooked up to a softstarter and the starter tripped, unhooked the softstarter IR tested - ok, hooked up and ran again, tripped again, this time disconnected really quickly and got a poor test and watched the IR go up to a really good test.
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