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Motor - Low insulation resistance
Posted: Fri Apr 29, 2022 5:42 pm
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?
Re: Motor - Low insulation resistance
Posted: Sat Apr 30, 2022 11:23 am
by AlecK
"3760" specifies the min IR for a Class I appliance as 1 megohm
Re: Motor - Low insulation resistance
Posted: Sat Apr 30, 2022 3:16 pm
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
Re: Motor - Low insulation resistance
Posted: Sat Apr 30, 2022 4:56 pm
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.
Re: Motor - Low insulation resistance
Posted: Mon May 30, 2022 8:37 pm
by MikeNZ
JamieP wrote: ↑Fri Apr 29, 2022 5:42 pm
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?
Hi Jamie,
What sort of motor was this?
As in size (kW), voltage?
I'm sort of picking that this was some sort of a 3 phase cage motor?
If so, what were your winding resistances?
These can be a really good indication of the condition of a motor, if the IR is a bit out of wack.
Two things can come into play, either the winding insulation is really damp and low to Earth or the motor has been sitting doing nothing because it threw a winding.
The windings should be within 5% of each other resistance-wise, as long as that is the case, there is no real reason why that motor should not spin up.
Like yourself, I would be quite hesitant to connect something like that, however once that built up to temperature, the readings you get could be totally different to what you got before-hand.
Re: Motor - Low insulation resistance
Posted: Tue May 31, 2022 8:53 am
by JamieP
Couldn't tell you sorry, I declined to connect and made them swap it out. I felt happiest this way but it just got me thinking where others draw the line with these things because as you say, I have heard of low readings disappeared once run and dried out or similar
Thanks for the input mate
Re: Motor - Low insulation resistance
Posted: Tue May 31, 2022 11:58 pm
by MikeNZ
Nah,
That's all good.
No-one like to be anywhere near anything that goes BANG when it's first turned on.
And my god that has happened a LOT of times in my years in the trade!