# Megger motor windings



## ElectricMetalTuba (Mar 20, 2017)

Is there a limit to how many times you can run insulation tests? Is it uncommon for people to test the motor from the switch. I don't want to open the motor every time.


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## sparkiez (Aug 1, 2015)

Of course there is a limit, but what you really need to look at is the voltage rating on the motor and the (likely quite a bit higher) voltage rating on the wire. Do not exceed either voltage rating on either, but you will typically meg at the voltage rating on the motor.

There is no problem with megging the wire on the outgoing side of the local disconnect or at the outgoing side of the overloads on the contactor, but if you get bad results you will need to open the motor and check at the motor.

Typically I don't have a megger on hand so I will check with a meter. If the meter gives me bad readings, then I know there is a problem. If I get good readings I will go get the megger and check again.

DO NOT EVER meg a VFD.


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## Flyingsod (Jul 11, 2013)

With a megger your not going to damage winding insulation. There are other insulation tests that assuredly will damage the insulation. I don't expect you are doing then though since your title mentioned megger.

If your are megging on a routine basis for preventive maintenance purposes you should also be taking humidity readings at the same time. There's a formula to use that will give clearer results when you graph your readings.

If it's just for troubleshooting a megger is like the last things I use. It's pretty rare that a motor megs bad but reads good with a dmm. 

Sent from my C6725 using Tapatalk


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## MechanicalDVR (Dec 29, 2007)

Flyingsod said:


> With a megger your not going to damage winding insulation. There are other insulation tests that assuredly will damage the insulation. I don't expect you are doing then though since your title mentioned megger.
> 
> If your are megging on a routine basis for preventive maintenance purposes you should also be taking humidity readings at the same time. There's a formula to use that will give clearer results when you graph your readings.
> 
> ...


I've seen many compressors that would look fine with an ohm meter that don't meg well. 

A 9vdc circuit is nowhere near the test of say a 500v spike.


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## Flyingsod (Jul 11, 2013)

MechanicalDVR said:


> I've seen many compressors that would look fine with an ohm meter that don't meg well.
> 
> A 9vdc circuit is nowhere near the test of say a 500v spike.


I'm guilty of forgetting about compressors. I remember you told me about them before. I'll have to try and do better remembering so I can pass the info along when I'm taking about megging motors. Thanks


Sent from my C6725 using Tapatalk


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## MechanicalDVR (Dec 29, 2007)

Flyingsod said:


> I'm guilty of forgetting about compressors. I remember you told me about them before. I'll have to try and do better remembering so I can pass the info along when I'm taking about megging motors. Thanks
> 
> 
> Sent from my C6725 using Tapatalk


While compressors are the primary example don't disregard air over motors in a process system that are in corrosive environments when they operating and you are testing when they are down. 

When the windings are hot and sucking in say salt air is far different when they have been off since yesterday and you are there first thing today testing.


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## Big John (May 23, 2010)

sparkiez said:


> DO NOT EVER meg a VFD.


 Ugh. That's gotta be my least favorite mistake.

We had one where a guy screwed up his decimal places. It was supposed to be a 100 volt test. 10,000 volts later.... 

FUUUUUUDDDDDDGGGGGEEEE....


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## JRaef (Mar 23, 2009)

sparkiez said:


> There is no problem with megging the wire on the outgoing side of the local disconnect or at the outgoing side of the overloads on the contactor, but if you get bad results you will need to open the motor and check at the motor.


To this point; if you megger the leads at the starter, and you get a reading saying something is bad, you will not know whether it's in the motor or in the leads. But if you get an all clear, then no need to take the motor connections apart. So I'd say that most people do it this way because it's easier and 99% of the time on PM checks, you don't actually find a problem exists. But if you ALREADY know there is a problem, assume you will need to do both, so you may as well start at the motor, because then you can do the check at that connection point going both ways.

And to re-re-re emphasize, _*DISCONNECT THE VFD *_if there is one. Soft Starters too.


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## sparkiez (Aug 1, 2015)

Flyingsod said:


> With a megger your not going to damage winding insulation. There are other insulation tests that assuredly will damage the insulation. I don't expect you are doing then though since your title mentioned megger.
> 
> If your are megging on a routine basis for preventive maintenance purposes you should also be taking humidity readings at the same time. There's a formula to use that will give clearer results when you graph your readings.
> 
> ...


You have not spent much time in a spray-down environment. I saw it about once a month in a food processing facility. On a funny note, we had a guy that would insist the motor be megged even if a meter showed it to be bad. :laughing:


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## MechanicalDVR (Dec 29, 2007)

sparkiez said:


> You have not spent much time in a spray-down environment. I saw it about once a month in a food processing facility. On a funny note, we had a guy that would insist the motor be megged even if a meter showed it to be bad. :laughing:


Did you straighten him out?


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## sparkiez (Aug 1, 2015)

MechanicalDVR said:


> Did you straighten him out?


No chance of him listening to me. He is the one that taught me to use the megger in the first place.


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## MechanicalDVR (Dec 29, 2007)

sparkiez said:


> No chance of him listening to me. He is the one that taught me to use the megger in the first place.


Did he have you put the leads to your nose to smell the burn?










And cranker up?


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## sparkiez (Aug 1, 2015)

MechanicalDVR said:


> Did he have you put the leads to your nose to smell the burn?
> and cranker up?


Hehe, no but I did have a newbie hold the leads one time, I just couldn't bring myself to push the button. Poor guy was clueless.

It went something like, "Here, check this out. Clamp these onto your fingers."


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## MechanicalDVR (Dec 29, 2007)

sparkiez said:


> Hehe, no but I did have a newbie hold the leads one time, I just couldn't bring myself to push the button. Poor guy was clueless.
> 
> It went something like, "Here, check this out. Clamp these onto your fingers."


Yeah that could be bad but the nose thing goes way back and is funny as hell.


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## Big John (May 23, 2010)

sparkiez said:


> Hehe, no but I did have a newbie hold the leads one time, I just couldn't bring myself to push the button. Poor guy was clueless...


 We had a greenhorn meg himself: Hooked up the leads and then learned on the conductor for balance while bent over to push start....

I didn't even feel bad about laughing.


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## sparkiez (Aug 1, 2015)

Big John said:


> We had a greenhorn meg himself: Hooked up the leads and then learned on the conductor for balance while bent over to push start....
> 
> I didn't even feel bad about laughing.


Fortunately for me, I can say I have never done this. We went over meggers in school and what they did and their purpose for testing insulation. We didn't have one specifically because of the whole "students shocking each other bit."

Only been doing this a few years, but I made a decision a long time ago to "be a sponge forever." Heck yes, put me with that grump old 65 year old man. He can cuss and yell and belittle me all he wants as long as I learn those old school tricks to add to my troubleshooting arsenal. I'll carry his tools + mine and do all of the hard labor to save him from exerting himself with a smile on my face. And it has just made me that much better, that much faster.


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