DC Motor Problem

AIman

I have completely rebuilt a DC motor and am having problems troubleshooting a current issue that happens once every rotation.

The armature was tested and checks out ok as do the tach and encoder. New bearings are in place and all wiring has been inspected along with the magnets

However, when the voltage supply is hooked up there is up to a 20% change in current during each rotating. The oscilloscope and meter both show that the current drain happens and you can hear, but there isn't anything, to indicate a drag of some kind on the motor during the same point of rotation.

All the math verifies what the oscillascope is showing but there is litterally nothing found to indicate why a current drain happens.

Any ideas?

Chris Savage

Alman,

Did you mean to post this in the projects forum?

Hal Albach

Some things to check for would be commutator segments shorted together or possibly armature winding insulation breakdown. A close inspection of the commutator with an ohm meter will locate adjacent shorted segments. Also measure each segment against all other segments for insulation breakdown. Only opposite segments should show a reading. An inductance meter should be able to find an armature loop that has shorted to itself.

Mark_T

One quick check, remove the brushes and check the rotation is smooth all the way round. Replace brushes, check
as best you can its still smooth.

And as mentioned above check segment-to-segment resistance (probably need a kelvin measurement)
all the way round the commutator. Its pretty common for one or two windings to burn out in an armature,
certainly seen it in a couple of universal electric drill motors.