Measureing Insulation Resistance of AC Induction Motors 1

[cranetrainer]

I am new to this site, but I have found a lot of valuable information. I have a question that I hope someone can help answer. I am a trainer and I am in the process of developing training material on using a Megger to measure the insulation resistance of AC Induction Motors. When I research this topic it seems everyone has a different opinion on how to test the motor. I was always taught to measure each leg of the motor to ground (independently), yet others seem to say you only need to measure from one leg of the Motor to ground. Can anyone tell me what is the accepted method, should we go with one leg or all three to ground?

 

[oldfieldguy]

Cranetrainer--

The most common AC three-phase motors have all three legs connected internally, so a DC test like using a Megger stresses a all three legs at the same time. A quick round of point-to-point testing with an ohmmeter will verify this.

Some motors have more leads brought out for various reasons, and in this case, you use the nameplate and/or your trusty ohmmeter to see how the windings are (or aren't) connected, then you ground everything except the winding you're testing.

Here's a link:

http://www.biddlemegger.com/biddle/Stitch-new.pdf

You will find more then most folks will need in that document.

old field guy

 

[fangas]

In the field, I've always lifted the leads off the final control (Drive, contactor, SS) and megged from there.

Ed

 

[JHanson717]

Ditto to fangas, that also checks the integrity of the wiring in between. if you fail there, d/c it in the pecker-head to further isolate it...

 

[edison123]

The idea behind measuring the IR of each lead in a 3 lead motor/generator is to ensure that one of the leads (or part of the winding to which it is connected) was not isolated due to internal winding failure, terminal loose connection etc. In the event of such a failure, the IR value may differ from lead to lead. In a healthy machine, the three IR values should be the same.

Of course, you could also check for the leads to winding continuity by meggering between the leads (which should read zero) if the winding is healthy.

 

[electricpete]

There are advantages to testing each leg to ground beyond detection of an open circuit.
1 – let’s say you have temperature corrected readings of 200 megaohms, 1000megaoms, 1000 megaohms on a 4kv motor. We are alerted to become suspicious of localized influence (test lead, defect, etc) on the one phase that measured 200 megaohms. In contrast if we measured all three together we might see 180 megaohms and not think twice because we have passed the IEEE43 limit 100 megaohms.
2 – the individual phase test stresses the phase to phase insulation particularly in the endwinding area. Contamination or defects in the endwinding area might be completely missed by megger since there is no ground plane anywhere near, but can show up when voltage is applied to a single phase with other phases grounded.


Btw - star to oldfieldguy for a great link.

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[cranetrainer]

Thank you Edison123, this gives me one answer to tell my class if this question pops up.

I have been meggering for years and have always meggered all three legs. I know motors are connected in either wye or delta so internally the windings are connected so it only makes sense to megger just one leg, but that's not the way I was taught in school.

If the motor is working fine and your measurements are part of a PM, the only difference you see would be the resistance of the winding itself. So would you still recommend going to all three legs as part of a PM measurement.

I researched this online, and found many engineering school course materials, and instructions from megger OEM's that say go to all three legs, even simple sites such as EHow.com say go to all three legs.

Do any of you have other reasons like Edison123 for using all three legs.

thanks for the help

 

[electricpete]

There are advantages to testing each leg to ground beyond detection of an open circuit.

I should have mentioned of course that it takes longer. Whether the additional benefit to be gained by separating phases is worthwhile is a matter of judgement. Going by memory, I think IEEE43 tells you to separate the phases.

Also a very few large motors have embedded neutral - not feasible to separate.

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[electricpete]

a matter of judgement

And there is no one size fits all. If it is a large critical motor in the shop you can bet I want them to separate the phases. If it is a small non-critical motor during routine check and it passes, it might not be worthwhile.

Also if you do test all together and get a bad result, that would be a time when it may be worthwhile to separate phases for investigation

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[rbulsara]

If the motor has only three leads brought out, it is internally connected (wye or delta) and you can only check one phase to ground and would suffice.

If you the motor had all six (or more) leads brought out, you can check each winding individually to ground.

Whether one phase is shorted or all , it will not make a difference as the whole motor needs to be re-wound.

Rafiq Bulsara

http://www.srengineersct.com

 

[rbulsara]

By the way as electricpete stated, you must check that there is continuity among all three phases (check for open) in three lead motors.

Rafiq Bulsara

http://www.srengineersct.com

 

[Zogzog]

[q] When I research this topic it seems everyone has a different opinion on how to test the motor.[/q]

Opinions are a dime a dozen, but there are only a few standards. NETA/ANSI is the most widely accepted standard for motor testing and any training program you develop should be based on that standard, after all it is an ANSI standard and also meets the requirements of IEEE 43 and NEMA MG1.