LV Motor insulation resistance

[dogzzz]

Motor Insulation resistance yes in general when measured comes in the order of hundreds of meg-ohms.

But there can be a condition that due to mositure or motor not being in use for few days the insulation resistance goes down.

As per IEC 60364-6 for low voltage motors , @ 500/1000 V if the insulation resistance comes out to be 1 Mega Ohms, then the motor is safe to be operated.

Further when the motor will run , it will end up gaining more insulation mega ohms due to heat kicking out moisture.

Opinion needed.

 

[itsmoked]

Yes that generally works - the 1Mohm rule. However if you really care about the motor you'll dry it out first. Anything that warms it up 20F above ambient will generally work wonders. Clip-on lamps with rare incandescent bulbs work well.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

What can possibly happen is if the motor temperature rises too fast, the moisture deeper in the insulation will vaporize and not escape, making a "bubble" that can separate it from the magnet wire. Failure becomes accelerated then. It's best to warm it up before using it.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[edison123]

IEC is right on all points.

Though, it's a good idea to keep these motors, LV/MV doesn't matter, warm via inbuilt space heaters/external external heaters to keep the IR values high. All it takes is a few watts.

Muthu

http://www.edison.co.in

 

[panter]

Some standards and experiences for minimum insulation imply result of 1 kilo ohm per volt but at the same time somebody says it must be at operating temperature, which may be 100 degrees C.
If you measured insulation resistance at ambient temperature, then it should recalculated with correction factor and minimum insulation resistance of the windings for 400 V motor should be over 20 mega Ohm.

Good luck

 

[electricpete]

All good input.

IEEE 43-2000 (r2006) is a little different. Reference temperature for instulation resistance temperature correction is 40C (104F). Minimum corrected resistance for machines rated below 1kv is given as 5 mega-ohms although there is an allowance for windings made below 1970 and for field windings the minimum given is 1 megaohms.

The scenario of potential insulation damage due to rapid vaporization of moisture would not apply to random wound windings to my thinking since the insulating materials are primarily varnish and phase separator paper and groundwall paper (in contrast a form wound machine can have many layers of tape and the scenario seems more plausible). But out of an abundance of caution I sometimes have concern for starting these motors in this condition. For one thing the moisture in conjunction with conductive contaminants make it more likely to bridge between inevitable magnet wire film insulation imperfections (by NEMA magnet wire specs these imperfections are expected/allowed, just not so many of them that they would likely end up next to each other) resulting in turn to turn short. (If you get through the first few minutes you're fine, but will the motor make it that far... plenty fail soon after start for various reasons) For another thing although we assume it's moisture based on most experience and circumstances, there's always the possibility that it's telling us about something else we should be attending to (drying the machine first allows us to confirm it was moisture and not something else).

imo it's not an exact science and there's plenty of room for situational judgement (what is normal, how critical is the application etc).

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(2B)+(2B)' ?

 

[dogzzz]

Thanks good info .
Better to follow IEEE 43-2000 then
Thanks