Electronics > Beginners

Understanding series wound DC motor waveform ?

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nurbster:
I used an insulation tester on this motor measuring approximately 50 megaohms @ 250V  between leads and shaft as well as between leads and enclosure.  I'm not experienced enough with such a tester, but I believe this is indicative of a fault ?  I then opened up motor to reveal windings that looked darker than normal ( perhaps heat damaged ) and continued with insulation testing showing the same 50 megaohms readings between leads and lamination stack.  Resistance measurements between commutator bars ( 180 degrees and adjacent ) seemed normal.  Can anybody comment on insulation testing measurements ?  What is the methodology experienced people would use to troubleshoot this motor in the field ?  The insulation tester can reveal interesting things because you don't even have to open up the motor to get to individual commutator bars and so it seems valuable to try when first diagnosing, but I would appreciate knowing how others would approach this situation.

amyk:
50MR seems OK for a 12V motor.

If that timescale is in seconds, that is really slow for this type of motor... I count ~55 commutations in 1.2 seconds, which even if assuming you have a tiny 6-bar commutator is only around 500 RPM.

We need more information about the motor, but based on what you've given so far it doesn't sound right for a series DC motor to be turning at that speed with no load. I'd expect well over 1000 RPM.

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