Megging Motors?

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jerjwillelec

jerjwillelec

Senior Member
Location
Nevada, IA
Occupation
Master Electrician
I am troubleshooting some vent fans at a grain elevator. 480 volt 2 HP. Some are tripping breakers and some are tripping overloads. I'm new to motor megging so I want to make sure I'm doing it correctly. I have a Fluke 1507. Any tips for me?
 
jerjwillelec said:
I am troubleshooting some vent fans at a grain elevator. 480 volt 2 HP. Some are tripping breakers and some are tripping overloads. I'm new to motor megging so I want to make sure I'm doing it correctly. I have a Fluke 1507. Any tips for me?
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I would start by doing basic voltage and current measurements.

Overload tripping often means some kind of mechanical overload.

Cb tripping is more likely to be a problem with the motor like a shorted winding.
 
Thank you so far...I'm just wondering what readings I should be getting and what leads I should meg. I've done 1 & 2 & 3 all to ground. Some read > 11 G ohm and some considerably less like 1900 M ohm. CB trips instantly on start up on some. OL trips within seconds on others
 
jerjwillelec said:
Thank you so far...I'm just wondering what readings I should be getting and what leads I should meg. I've done 1 & 2 & 3 all to ground. Some read > 11 G ohm and some considerably less like 1900 M ohm. CB trips instantly on start up on some. OL trips within seconds on others
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Plenty of good threads here, also download "A stitch in time" from the megger website.

We can't answer your questions because you didnt say what the motor voltage was, what type of motor it is, what your test voltage was, was the time of the test was, what your DAR/PI readings were, or most important of all when testing motors, if your readings are tempature corrected or not. Temp correction is key for motors and transformers and readings are worthless info without it.
 
zog said:
Plenty of good threads here, also download "A stitch in time" from the megger website.

We can't answer your questions because you didnt say what the motor voltage was, what type of motor it is, what your test voltage was, was the time of the test was, what your DAR/PI readings were, or most important of all when testing motors, if your readings are tempature corrected or not. Temp correction is key for motors and transformers and readings are worthless info without it.
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I would not say they are quite worthless.

Every time I have had someone megger something and it was not what it should be it was a very low reading and pretty obvious there was a problem.
 
petersonra said:
I would not say they are quite worthless.

Every time I have had someone megger something and it was not what it should be it was a very low reading and pretty obvious there was a problem.
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That's been my experience too.

But...
First place I would start is, were they working fine and now they no longer work, or is this a new installation (or new replacement equipment) and this is part of the commissioning process? The point is, I don't typically start off the troubleshooting process by megging the motors, that's for the end of the process when you have eliminated all the easier issues. It sucks to read a bad motor circuit and replace the motor, only to discover it never WAS the motor. The guy footing the bill will NEVER call you again...

For example, are the breaker instantaneous trip settings correct for those motors? If the motors were recently changed out with new Energy Efficient motors, nuisance SC tripping is a really common problem because the new motor designs have less resistance, so settings for older motors no longer work.

Other possible causes of SC tripping: moisture in the conduits, moisture in the motor windings, dust and moisture in the controllers. etc. etc.

As to the OLs tripping in 3 seconds, is there a change in the duct work or vane controls? Sometimes people fail to realize that a broken duct or a WIDE OPEN vane control system will OVERLOAD a blower motor, they are typically designed to start with low or NO FLOW, meaning the vanes are CLOSED and/or the ducts are intact. Some people mistakenly think that if the vanes are closed, the blower will overload, when it is EXACTLY THE OPPOSITE.
 
ActionDave said:
I would put the megger away for now and start checking voltage, amps, connections, looking for dry bearings, stuff line that.
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I took this advice and found multiple motors with bad branch circuit conductors. I would still like to know more about megging but will do that another time. Two of my motors are still instantly tripping the breaker on start up so I will start a new post for that. Thank you!
 
Jraef said:
First place I would start is, were they working fine and now they no longer work, or is this a new installation (or new replacement equipment) and this is part of the commissioning process? The point is, I don't typically start off the troubleshooting process by megging the motors, that's for the end of the process when you have eliminated all the easier issues. It sucks to read a bad motor circuit and replace the motor, only to discover it never WAS the motor. The guy footing the bill will NEVER call you again...

For example, are the breaker instantaneous trip settings correct for those motors? If the motors were recently changed out with new Energy Efficient motors, nuisance SC tripping is a really common problem because the new motor designs have less resistance, so settings for older motors no longer work.

Other possible causes of SC tripping: moisture in the conduits, moisture in the motor windings, dust and moisture in the controllers. etc. etc.
Click to expand...

This is an existing system that has been in place for many years. There are 6 silos with 2 vent (exhaust) fans each. I think they have slowly been failing over the years to the point that 80% weren't working and moisture was building up and ruining grain. The motors, starters, and breakers haven't been touched until now so to my knowledge none of the settings have been changed. I did find multiple issues with the branch circuit conductors and we will be re pulling them next fiscal year...I am close to getting one fan per silo running which will get them by. Still have two motors tripping breakers so I'm not quite done.
 
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