MEGGERING A MOTOR PHASE TO PHASE

[WB82]

I always megger a 3 phase 480V motor phase to ground, but I am wondering if meggering phase to phase can tell me something about the state of the winding insulation as well, and what kind of a reading I would be looking for considering the windings are "shorted" together. On say an ABB motor that can be set up for a Wye or delta connection, would you want to remove the little bus bars, and meg each phase to one another separately, or just keep them in what ever configuration while doing this. Also we use a fluke digital megger, and generally look for a reading of >500M on a good motor, but is there a general rule of thumb to use as far as how low the reading can go before a motor is actually considered bad???? Thank you for any knowledge you can share!

 

[Besoeker]

I always megger a 3 phase 480V motor phase to ground, but I am wondering if meggering phase to phase can tell me something about the state of the winding insulation as well

Unless you can get to all six ends of the windings and isolate the phases from each other, it won't tell you a lot.

 

[RESI]

WB82- I work in a power plant of a large utility company. Typically we megger motor just one phase of the motor to ground as the phases are all connected inside the motor. Most, if not all our motors only have 3 leads coming out of the connection box. The megger test is checking the condition of the ground wall insulation of the motor. We also check the phase to phase resistance reading and are looking for no more than a 10% difference max. If some of the windings are shorted or you have bad connections, then the resistance readings will be unacceptable. Sometimes you have to dis-connect the motor and eliminate the leads from the MCC to the motor. The only test that will find shorts between individual coils of wire in a phase is the surge test. At our plant, we consider a motor startable if it megs 1 megohm per each 1KV of running voltage plus 1KV. So, if you have 480 volt motor, we look for a minimum of 1.5 megohm or we won't start it. If a motor has to have heat applied to get the megger reading up, then it will need a rewind eventually as the insulation is either tracking to ground or absorbing moisture. On a good motor, a megger reading between phases will show a short. Another test we might do is called a Polarization Index test. It is like taking a 10 minute megger reading. You then divide the ten minute reading by the one minute reading to get a number between 1 and 10. Depending on what you get, it tells you the condition of your ground wall insulation. For a explanation of the PI test, see:

http://electromotores.com/PDF/InfoT?cnica/EASA/Using Polarization Index.pdf

 

[Besoeker]

If some of the windings are shorted or you have bad connections, then the resistance readings will be unacceptable. Sometimes you have to dis-connect the motor and eliminate the leads from the MCC to the motor. The only test that will find shorts between individual coils of wire in a phase is the surge test.

Appreciate your experience. And your point about shorted turns.
:thumbsup:

 

[Cow]

I test between T leads as well as to ground. To date, I've only found one motor that showed full scale readings between one T lead and the other two(open winding) and I've tested A LOT of motors. But I still do it. Everytime.

I would of never known why the motor was tripping the breaker if I hadn't tested between windings, because the phase-ground readings were fine.

 

[weressl]

WB82- I work in a power plant of a large utility company. Typically we megger motor just one phase of the motor to ground as the phases are all connected inside the motor. Most, if not all our motors only have 3 leads coming out of the connection box. The megger test is checking the condition of the ground wall insulation of the motor. We also check the phase to phase resistance reading and are looking for no more than a 10% difference max. If some of the windings are shorted or you have bad connections, then the resistance readings will be unacceptable. Sometimes you have to dis-connect the motor and eliminate the leads from the MCC to the motor. The only test that will find shorts between individual coils of wire in a phase is the surge test. At our plant, we consider a motor startable if it megs 1 megohm per each 1KV of running voltage plus 1KV. So, if you have 480 volt motor, we look for a minimum of 1.5 megohm or we won't start it. If a motor has to have heat applied to get the megger reading up, then it will need a rewind eventually as the insulation is either tracking to ground or absorbing moisture. On a good motor, a megger reading between phases will show a short. Another test we might do is called a Polarization Index test. It is like taking a 10 minute megger reading. You then divide the ten minute reading by the one minute reading to get a number between 1 and 10. Depending on what you get, it tells you the condition of your ground wall insulation. For a explanation of the PI test, see:

http://electromotores.com/PDF/InfoT?cnica/EASA/Using%20Polarization%20Index.pdf

All well and done, but lets make it clear that you are talking about several different tests that are performed with different instruments. You can't perform a winding resistance test with a Megger, nor can you use a low powered resistance meter for PI or insulation integrity test.

 

[ATSman]

All well and done, but lets make it clear that you are talking about several different tests that are performed with different instruments. You can't perform a winding resistance test with a Megger, nor can you use a low powered resistance meter for PI or insulation integrity test.

So....? With that said, I am wondering what your point is?

 

[ATSman]

I always megger a 3 phase 480V motor phase to ground, but I am wondering if meggering phase to phase can tell me something about the state of the winding insulation as well, and what kind of a reading I would be looking for considering the windings are "shorted" together. On say an ABB motor that can be set up for a Wye or delta connection, would you want to remove the little bus bars, and meg each phase to one another separately, or just keep them in what ever configuration while doing this. Also we use a fluke digital megger, and generally look for a reading of >500M on a good motor, but is there a general rule of thumb to use as far as how low the reading can go before a motor is actually considered bad???? Thank you for any knowledge you can share!

Just by the tone of your question about the phase to phase test it would appear you do not understand the purpose of a megger test: to verify the integrity of the insulation system whether it be a motor, transformer, circuit breaker, etc. The rule is to separate all conductors as much as you can and perform the test between each conductor and ground and from conductor to conductor; all possible insulating paths. Connecting the megger phase to phase will show as a 0 or shorted reading because it is not a test of an insulating path. I hope this explains the point I am trying to make.

 

[kwired]

You can meg from each lead to ground - this is well known.

You can meg from one winding to another as long as there is no connection between the two. The internal connection of the wye point on a wye connected motor will prevent testing between the windings internally connected but you could test from these three to the other windings.

You can not meg across a coil and detect shorts from one turn to another in the same coil.

 

[richxtlc]

I test between T leads as well as to ground. To date, I've only found one motor that showed full scale readings between one T lead and the other two(open winding) and I've tested A LOT of motors. But I still do it. Everytime.

I would of never known why the motor was tripping the breaker if I hadn't tested between windings, because the phase-ground readings were fine.

Testing with a megger winding lead to winding lead on a motor for continuity may give you a false reading, as the voltage produced by the megger can jump a small gap in the winding indicating a complete winding, when if fact, it is opened. To measure the resistance of the windings to determine if there is a short circuit or shorted turns, you need a bridge or at least a DLRO.

 

[richxtlc]

Using a PI test to determine the condition of the motor can be misleading. If you are testing a newer motor, one whose megger readings are in the gigaohms range, then the ratio between the 1 min and 10 min reading may end up being 1. This is a false bad reading. On older motors, this test works well.

 

[Cow]

Testing with a megger winding lead to winding lead on a motor for continuity may give you a false reading, as the voltage produced by the megger can jump a small gap in the winding indicating a complete winding, when if fact, it is opened. To measure the resistance of the windings to determine if there is a short circuit or shorted turns, you need a bridge or at least a DLRO.

You brought up a good point, you're absolutely right. Thanks.

 

[ATSman]

More PI Stuff

More PI Stuff

http://electromotores.com/PDF/InfoTécnica/EASA/Using Polarization Index.pdf