Checking motor windings
- Thread starter JdoubleU
- Start date
- Status
To ground, but not between turns.
zog
Senior Member
- Location
- Charlotte, NC
Yes, but the motor could have other issues, an insulation resistance test is only one of many that should be done.
Your IR test should be a 10 minute test with readings taken at 30 sec, 1 minute, and 10 minutes, then calulate your DAR and PI. Winding megger readings need to be tempature corrected to 40C to have any real meaning.
Cow
Senior Member
- Location
- Eastern Oregon
- Occupation
- Electrician
480 volt motors I test from T lead to ground, then between T leads. Testing at 1000v with my Fluke 1507. I shoot for at least 50 megs from T-Gr and 0 megs between T leads. I have found one motor that showed open(infinite resistance) between T leads, that's why I check.
My method isn't scientific at all, I don't do any timed tests or any DAR/PI functions. I simply start testing, if the resistance starts climbing I'll watch it for 10-15 secs or so. If it starts dropping, I'll usually watch it until it stops.
Usually when I'm called out it's because a breaker/overload is tripping. Almost all of the motors I've tested have tested pretty good to complete junk. There hasn't been a whole lot of in between, so it's usually been pretty straight forward to troubleshoot without having to make any real judgement calls.
On the other hand I've tested a lot of wire that's read 0.1-0.3 megs, etc and yet the motor is still running. Moisture in conduits seems to play a big part in your megger readings. I've pulled some wire out that's megged low and couldn't ever find a nick or scrape in it. I think it must develop very small cracks in the insulation you just can't see with the naked eye.
I've had no formal training and haven't read that Biddle megger book some guys on the forum recommend, so you can take my advice for what it's worth. Nothing.:grin:
My method isn't scientific at all, I don't do any timed tests or any DAR/PI functions. I simply start testing, if the resistance starts climbing I'll watch it for 10-15 secs or so. If it starts dropping, I'll usually watch it until it stops.
Usually when I'm called out it's because a breaker/overload is tripping. Almost all of the motors I've tested have tested pretty good to complete junk. There hasn't been a whole lot of in between, so it's usually been pretty straight forward to troubleshoot without having to make any real judgement calls.
On the other hand I've tested a lot of wire that's read 0.1-0.3 megs, etc and yet the motor is still running. Moisture in conduits seems to play a big part in your megger readings. I've pulled some wire out that's megged low and couldn't ever find a nick or scrape in it. I think it must develop very small cracks in the insulation you just can't see with the naked eye.
I've had no formal training and haven't read that Biddle megger book some guys on the forum recommend, so you can take my advice for what it's worth. Nothing.:grin:
ptonsparky
Tom
- Occupation
- EC - retired
I'm. With Cow, most of my Meg checks are to confirm the DOAs. Rarely do we get to do PM checks and without history of the motor it is usually just a shot in the dark. I have found the occasional bad wire connector and a wire or two. Guess we did a diagnose a bad jumper or link on a submersible gas pump the other day via my megger.
ptonsparky
Tom
- Occupation
- EC - retired
Found it first by checking phase to phase resistance at controller while motor was still connected. Then confirmed with Meg check.
zog
Senior Member
- Location
- Charlotte, NC
Depends on the motor age, insulation type, motor type, voltage, HP and winding type.
IR 1 minute = kV+1 for most windings made before 1970, all field windings, and others not lited below.
IR 1min=100 Meg for most DC armature and AC windings built after 1970
IR 1min= 5 Meg for most machines and random wound stator coils and form wound coils rated below 1kV.
All values temp corrected to 40 degrees C.
winding resistance can also be very useful
winding resistance can also be very useful
'Would a phase to ground meg test to the motor windings be a sufficient test for the motors winding insulation.'
---the winding resistance can be very useful e.g. usind a digital multimeter like for a 3 phase motor the resistance between phases shoul be almost the same if not there is damage a motor may megger good to ground but, the windings between phases could be open, a general rule is to look for 1,000 ohms of resistance for every volt e.g. a 480v motor should not be under 480,000 ohms usually rounded to 0.5 meg ohms
a single phase motor is different the start and run winiding need to be isolated and checked the start should have a higher than the running
winding resistance can also be very useful
'Would a phase to ground meg test to the motor windings be a sufficient test for the motors winding insulation.'
---the winding resistance can be very useful e.g. usind a digital multimeter like for a 3 phase motor the resistance between phases shoul be almost the same if not there is damage a motor may megger good to ground but, the windings between phases could be open, a general rule is to look for 1,000 ohms of resistance for every volt e.g. a 480v motor should not be under 480,000 ohms usually rounded to 0.5 meg ohms
a single phase motor is different the start and run winiding need to be isolated and checked the start should have a higher than the running
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