Testing three phase motors

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Waxrichard

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Hi so I been doing electrical maintenance for years. I have not went to school but all on hands. In the foundry I worked in they showed me how to test a three phase motor with a ohm meter well I now work for a company and they say you can not properly test a three phase motor with an ohm meter that a motor megger must be used to properly test a three phase motor and they used it by going from the ground wire to each leg. Basically I want to see if it's true that you should not test a three phase motor with a ohm meter? That a megger should always be used.
 
It all depends on what you want to test. An ohm meter will tell you whether the windings are continuous and whether there are any direct grounds but they won't tell you if the insulation is failing (but a Megger should). Neither of those will tell you how the motor operates or whether there are shorted windings.
 
What level maintenance ?
Sounds like in the foundry if a motor stopped running they sent you out to replace it, first checking if it was shorted or open to determine if other system elements were impacted by the failure.
If so, then simple continuity or short to ground test with ohmmeter might be sufficient.

If OTOH the test is for preventative maintenance, the megger required. AND OTHER testing also.
 
You can use a VOM to test a motor--BUT--when looking for grounded windings,,if the ohm meter shows good then you need to use a megger as described--IF the ohm meter shows grounded there is no need for the megger
 
As others mentioned depends on what you want to test. simple ohm meter can tell you if you have an open circuit condition or a fairly severe ground fault condition, it won't tell you how close you might be to insulation failure and in cases where there is a turn to turn fault you may not ever know about it either.

Certainly helpful for some basic go/no go tests though.
 
Thank you for replying as you can tell I'm totally new to a megger and person who said use the megger is pretty new to it. Now say you did the test with the ohm meter each leg to ground and you did leg to leg and it all checks good is there a need to use the megger and will a megger tell if the motor is getting weak close to fail?
 
drcampbell said:
A two-wire ohmmeter won't be accurate enough to detect shorted turns, but a four-wire ohmmeter might.
Click to expand...
If you are going behind a two wire ohmmeter, an impedance meter that applies an AC test voltage will be even better at finding a shorter turn.

Sent from my XT1585 using Tapatalk
 
A DMM can only apply 9VDC to the circuit as it tests resistance, so any "leaky" (failing) motor insulation is not going to have a problem with that and will give you a false sense of it being OK. The only thing it really tells you is if the windings are broken (open circuit) or shorted hard to the frame. That sort of damage is generally the result of something burning, so it's visibly obvious anyway. That's why they told you that using a DMM / VOM is not really that useful of a test. The Megger puts 500V or 1000V DC onto the windings and will find the little tiny flaws in the winding insulation that affect the motor at the voltage it actually operates at.
 
Case in point: a handful of years ago we upgraded a machine to give it vfd control of the motor instead of a soft starter so they could control speed. The motor ran just fine from the soft starter. No obvious issues from what anyone could tell and it didn’t look like the motor was all that old. However, when we took out the soft starter and replaced it with a drive, as soon as we tried to run the motor from the drive the first time the drive kept tripping. Something about a faulty motor connection or ground fault if I remember correctly from the fault code.

So we grabbed our DMMs and did some basic checks. Everything seemed just fine. Winding resistances seemed reasonable, and no detectable shorts to ground. So we connected it back up to the drive. Same thing. Kept tripping and refusing to run.

So then we brought in our megger, and it ends up showing us that the winding insulation in the motor was starting to fail. Only a hundred or so mega-ohm to ground I believe. We showed this to the customer, they bought a new motor, and then everything ran great on the new vfd.

If it wasn’t for what the megger could show us that a DMM couldn’t, I might have had a hard time explaining why this high end vfd wasn’t able to start a motor that ran just fine from the soft starter.
 
bwat said:
Case in point: .... Same thing. Kept tripping and refusing to run.

So then we brought in our megger, and it ends up showing us that the winding insulation in the motor was starting to fail. Only a hundred or so mega-ohm to ground I believe. We showed this to the customer, they bought a new motor, and then everything ran great on the new vfd.
Click to expand...

Older motor likely had high capacitance and ground currents thru the bearings causing the trip. Nothing to do with IR, high frequency capacitive currents - which also cause bearing failures on older motors hooked to VFD.
 
Waxrichard said:
Thank you for replying as you can tell I'm totally new to a megger and person who said use the megger is pretty new to it. Now say you did the test with the ohm meter each leg to ground and you did leg to leg and it all checks good is there a need to use the megger and will a megger tell if the motor is getting weak close to fail?
Click to expand...
Yes--If it all checks good with a DVM then you need to test with a megger--A DVM will not breakdown failing insulation--You need the 500 to 1000V level to do that
 
regarding using a megger to test the windings, its motor ground to A, Gound to B Ground to C then A-B, A-C, B-C right? i keep getting 660 on my fluke 1577
 
This is an excellent publication by Biddle, mfg of the megger insulation tester.
A megger is very useful and should be part of your maintenance kit. With a megger, you measure insulation testing on a regular basis and compare the readings, they can tell you if the motor windings are about to fail. A one time reading does not tell you much, you are looking for trends.
Along with the megger, take amp readings, open circuit voltage, running voltage, and IR temp readings.
The traditional megger uses a hand crank, but I had a nice Fluke combo DVM and megger, I didn't trust the megger part at first but it worked great.
 
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