# DC Motor Megger Readings



## MDB1988 (Nov 13, 2017)

I work at a ski resort and was recently asked to help the lift maintenance department with their annual lift motor inspections and maintenance. I have plenty of experience with AC motors but this was my first foray into DC motors.

So far I have concerns about 2 motors. These tests were done at 500V and cleaning the motors did not change them. Both probably run 1000-1500 hours a year. Frequent stops/starts/speed changes.

#1
Bull Electric
1999, reconditioned at least once around 2010.
500HP
Shunt Wound
Armature to ground measures 0.4 MOhms. This is with the brushes removed and the probe directly connected to the commutator.
All other readings are 550 MOhms, the max on this meter.
Some commutator scoring from a brush that was worn down to the copper.

#2
GE
Unknown age/reconditioning
75HP
Stab Shunt Wound
Armature to ground measures 0.1 MOhms. This is with the brushes removed and the probe directly connected to the commutator.
Field to ground measures 9 MOhm.
Some commutator scoring from a brush that was worn down to the copper.


All the research I've done indicates these 2 fail miserably due to their low armature to ground readings. The problem arises when I look at the inspections from previous years and see that these have had similar low readings since at least 2012. If these are truly failing readings then I can't fathom why the previous managers, one of whom was a master electrician/electrical engineer, did nothing about them. Unfortunately no one involved in them works here anymore.

We're currently looking for paperwork from before 2012, hopefully some from right after the motors were reconditioned.


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## Zog (Apr 15, 2009)

What were the 30 sec, 1 min, and 10 min readings? What was the winding temp at the time of the tests?


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## MDB1988 (Nov 13, 2017)

Instantaneous readings and at 1min were the same. Didn't do any longer than that. Temps were probably 25-30c.


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## MDB1988 (Nov 13, 2017)

Correction on the 75HP one, I worked on it some more today and the armature itself actually measures 550 MOhms to ground. The 0.1 reading is coming from the lead that goes from the peckerhead to the first brush holder. I also noticed that the S1/S2 leads are just connected to each other instead of in series with A1/A2.


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## paulengr (Oct 8, 2017)

OK I'm an old DC guy. Still not really fond of them even after doing it for years. I'd suggest leaving these to the few specialists that haven't retired or died yet if you don't know what you are looking at. DC motors are electrically much simpler than AC but mechanically much more complicated.



1. You need to do temperature correction on insulation resistance or you can't compare it to anything at all when you get intermediate values. It's one thing when it's 500 megaohms plus, or dead shorted, but quite another when it's somewhere in between.

2. 10 minute reading is required to calculate PI. It's the ratio of 10 minute to 1 minute value and should be about 2-5.

3. Generally ROTOR (armature) readings are crap. The last edition of IEEE 43 recommended down to about 1 megaohm per kilovolt. You didn't say what your armature voltages are but 150, 300, and 500 are all typical ratings. The typical minimums you see like 5 megaohms for <1000 V are based on AC stator coils or DC field coils, not AC or DC rotor values which are much lower. The same goes for synchronous and wound rotors, too.

4. DC motors NEED tender loving care at least once a year if not more often. You MUST, I repeat MUST at a bare minimum check for brush wear, commutator patterns, sparking, and above all else MUST clean it with dry cloths, NO SOLVENT under any circumstances, to get the carbon dust out that accumulates. If you don't, you will have a flashover sooner or later. Also check everything at the end of the season, not the beginning. The film that develops on the commutator is a mix of carbon, copper, and MOISTURE. During periods of nonuse like through the summer, the film disintegrates and comes off leaving a clean commutator which is actually not good that leads to high brush wear, hot spots, poor connectivity, and other problems. Also solvents including most cleaners and especially RTV if they are even in the same room with a DC motor wipe out the film within hours.


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## MDShunk (Jan 7, 2007)

I've never been able to get decent megger measurements from a DC motor without completely disassembling and almost surgical cleaning of everything- especially the brush holder mechanicals. The carbon brush dust is diabolical. I agree very much that the end-of-season (and more frequent) attention to brush wear and cleaning is most important. I've seen moisture in the brush dust reconstitute it into hard little marbles that wreak havoc on everything.


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