Magnified view: brush and commutator – PMDC motor (Bodine Electric Company)ġ. Since an ohmmeter can’t be used to directly measure the armature circuit resistance, the resistance has to be calculated.
#Motor winding resistance values free#
The same oxide film and free particles don’t represent a significant resistance to the higher voltage and higher current of a typical DC motor power supply. The additional resistance comes from the oxide film formed on the surface of the commutator and the free particles in the gap, as shown in the image below (the graphitic film is conductive). This is because armature circuit resistance cannot be measured accurately with an ohmmeter.īecause the connection between the brushes and the commutator is not a solid one, the gap between the two components represents additional resistance to the low voltage and low current of a typical ohmmeter power source. But turning the armature provided no better resistance reading. You might have thought that the armature was in a position where the brushes were bridging two commutator bars on each side. If you have tried to measure armature circuit resistance with an ohmmeter (via the motor leads), you have found that the result is an unusually high reading. Since an ohmmeter can’t be used to directly measure the armature circuit resistance in a PMDC motor or gearmotor, the resistance has to be calculated.
This is a fairly simple procedure using an ohmmeter on an AC motor or a brushless DC motor, but it is not so simple with a brush type (PM) DC motor or gearmotor.
When a permanent magnet DC (PMDC) motor fails to operate properly, or fails to run at all, one sensible troubleshooting step is to measure the winding resistance to determine if there’s an open circuit or a short circuit, either between turns of the same winding, between two windings in the same motor, or between a winding and ground. Permanent Magnet DC (PMDC) Motor and Gearmotors
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