Jameell
New User
- Location
- Philadelphia
I have 50hp, 3 phase motor I believe to be bad. What readings should I be getting from the Megger?
460 motor
- Thread starter Jameell
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- Status
- Occupation
- Licensed Electrician
Welcome to the forum.
The simple answer to your question is, It depends. Temperature, moisture, length of conductor can all make a difference. Are you meggering just the motor?
You did not much info in your OP so I'll just throw some general troubleshooting experience I've had.
In all my years of motor troubleshoots I'd say almost eighty percent of the time if someone thinks the motor is bad then the problem is not the motor. Motors, for the most part don't go kinda bad, they either run or they burn up. Some other things to check are the bearings, all the connections up and down the circuit, and problems with the equipment the motor is driving are other things to check. I would check things out carefully with a volt meter and check amps with the motor uncoupled form the load before I started meggering anything.
Write back and let us know. There are some smart members here that can be of help.
The simple answer to your question is, It depends. Temperature, moisture, length of conductor can all make a difference. Are you meggering just the motor?
You did not much info in your OP so I'll just throw some general troubleshooting experience I've had.
In all my years of motor troubleshoots I'd say almost eighty percent of the time if someone thinks the motor is bad then the problem is not the motor. Motors, for the most part don't go kinda bad, they either run or they burn up. Some other things to check are the bearings, all the connections up and down the circuit, and problems with the equipment the motor is driving are other things to check. I would check things out carefully with a volt meter and check amps with the motor uncoupled form the load before I started meggering anything.
Write back and let us know. There are some smart members here that can be of help.
ptonsparky
Tom
- Occupation
- EC - retired
Occasionally a motor will develop an internal winding to winding short that will not show up with a Megger test. It may even pass the smell test. You will need to check each winding resistance with a good ohm meter. Fluke has one on their Megger.
Russs57
Senior Member
- Location
- Miami, Florida, USA
- Occupation
- Maintenance Engineer
IMHO, minimum of 1 meg. More important is all winding's have close to the same readings.
That motor in a clean conditioned air space could read 500+ megs. Put it outside in a moist humid place, like a cooling tower, and it could read less than a meg and still be okay (but I'd be inclined to dry it out with a belly band heater and see if readings come up).
That motor in a clean conditioned air space could read 500+ megs. Put it outside in a moist humid place, like a cooling tower, and it could read less than a meg and still be okay (but I'd be inclined to dry it out with a belly band heater and see if readings come up).
IEEE 43 and EASA both state 5 megaohms temperature corrected as a minimum on stators under 1000 V, tested at 500 V for 1 minute. Temperature correction can often reduce the raw reading by 70% so a “passing” value might be over 15 megaohms for a healthy motor. Most of the time it either never comes close or is much better than this. 1 megaohm is definitely a failed motor. Even cooling tower motors are awful but never that bad or at least they will just burn up if they are.
On rotors the standard is 1 megaohm plus 1 megaohm per kilovolt. This is pretty much a hard limit. Some EASA members argue for a simple 1 megaohm per kilovolt standard but I’ve never seen a working rotor that low except a green rotor not yet dipped and baked.
Now in reality sometimes you need to decide if you can safely start regardless of the book number. A safe value to attempt starting if you suspect it’s just moisture is using the rotor minimum for the stator. If it fails the next megger reading will be lower as the fault develops into a bigger one.
Lots of other bad or wrong advice. I do this test and others all the tine diagnosing motors. See this article,
https://www.linkedin.com/pulse/motor-testing-doing-right-paul-campbell
On rotors the standard is 1 megaohm plus 1 megaohm per kilovolt. This is pretty much a hard limit. Some EASA members argue for a simple 1 megaohm per kilovolt standard but I’ve never seen a working rotor that low except a green rotor not yet dipped and baked.
Now in reality sometimes you need to decide if you can safely start regardless of the book number. A safe value to attempt starting if you suspect it’s just moisture is using the rotor minimum for the stator. If it fails the next megger reading will be lower as the fault develops into a bigger one.
Lots of other bad or wrong advice. I do this test and others all the tine diagnosing motors. See this article,
https://www.linkedin.com/pulse/motor-testing-doing-right-paul-campbell
MTW
Senior Member
- Location
- SE Michigan
Paul, your link is dead. I'd be interested in seeing it, Thanks
MTW Ω
MTW Ω
Here it is again. I just looked. LinkedIn does not delete.
https://www.linkedin.com/pulse/motor-testing-doing-right-paul-campbell
https://www.linkedin.com/pulse/motor-testing-doing-right-paul-campbell
Jameell
New User
- Location
- Philadelphia
Thanks to everyone for the feed back. Turns out the motor was bad. Each leg was going directly to ground which threw me for a loop, never seen all 3 do that.
ptonsparky
Tom
- Occupation
- EC - retired
You would typically find them all going to ground. It’s not worth the effort to isolate which winding. The motor is still bad.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
And even if a motor shop finds it economical to rewind the motor, I doubt that they will choose to rewind just the bad winding!
You can’t. The coils are varnished/epoxied after installation. Coil removal is done in a burnout oven except in very high pole count large mill motors (thousands of HP). It’s all or nothing.
Highly doubt you had a short to ground on “all three lines”. The resistance in the coils is milliohms but megaohms or gigaohms in the insulation. So short all leads and test. Grounded is grounded.
kwired
Electron manager
- Location
- NE Nebraska
Unless you can completely isolate each winding you will read ground fault through interconnections and it will show up on all input leads. A wye connected motor with inaccessible wye point - you can not isolate the windings for individual testing. Typical dual voltage 9 lead motor - one of the two sections of windings usually has such wye point that is inaccessible, a fault in one phase will still be detectable via the other two input leads. If fault is near the wye point it might even be close to same reading no matter which lead you test.
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