Brushed DC motor diagnostics - Page 1
Forum section: electronics / repair

author: Manul
author_rank: Super Contributor
timestamp: February 17, 2024, 11:16:27 pm
content: I'm not a motor guy, so I would appreciate a comment from those who know. I have a brushed DC motor with separate (isolated, not series, not parallel) field winding. Question: how would youproove, that the motor is goodifthere is no spec sheet and no other motor to compare with?Some data:Field winding: 71 Ohms, 1.6H (@100Hz)Armature: ~1.5 Ohms, 21mH (@100Hz)I tried to measure armature circuit more precisely using CC and measuring voltage drops (rotor stationary).@ 1A DCTotal resistance: 1.45 - 1.65 OhmsCumulative brush resistance: 0.26 - 0.5 Ohms (17 - 30% of total)@ 3A DCTotal resistance: 1.33 - 1.57 OhmsCumulative brush resistance: 0.18 - 0.34 Ohms (14 - 22% of total)Field winding seems ok, I don't think there is anything wrong, I suppose it is what it is? Now brush resistance seems a bit highish. I assume, brush resistance should be compared to armature resistance and should be quite small in proportion. It drops slightly with higher current. I'm not sure if I measured it absolutely correctly, because brush is probably contacting more than one contact, and I was measuring voltage drop touching just one. Anyway, motor rotates and produces torque. Brushes have enough length and pressure (visually).Does it look like a good motor? Are such electrical measurements enough toproovethat it is good? I could measure rpm and max torque, but with no spec sheet it would be meaningless (in a sense that it prooves nothing).

author: amyk
author_rank: Super Contributor
timestamp: February 18, 2024, 03:01:51 am
content: If all the armature coils test equally and the field coils are also all the same, and there's no sign of overheating or other physical damage, it's good. Based on the given data I suspect it was designed to run as a shunt field motor.

author: Manul
author_rank: Super Contributor
timestamp: February 18, 2024, 05:11:01 pm
content: As far as I know, it was used bidirectionally by changing polarity of field winding. I understand that such electric motor is not a rocket science, but it's literally the first ever DC motor I try to diagnose. I work with electronics normally. So I know, that it is possible, that I don't know something.What concerns me is brush resistance, which seems highish and not very stable. So for instance, maybe that may indicate oxidation of comutator, or heat damage (drying) of graphite material used for brushes, so their composition is not anymore up to spec. Maybe when motor rotates, it's even worse and motor does not develop nominal torque anymore. Again, I really don't know. Overall, nothing seems burned or smells bad. I need to say either it meets the specs or it does not - without having the actual specs. And put my signature. That's the problem.

author: Jwillis
author_rank: Super Contributor
timestamp: February 19, 2024, 09:12:43 pm
content: Brushes may look Ok, but if the their is excessive arcing on the commutator that would indicate damaged brushes. They can develop micro cracks and pits that don't seem apparent upon visual inspection. The damage to brushes that causes micro cracks and pitting is due to overloading which creates excessive heat. These micro cracks can also increase the resistance across the graphite further reducing the efficiency causing more heat and further damage to the brush and commutator.Sometimes electric motors will have a arc suppression ring installed above the armature contacts. This is a ring of TVS diodes that is pressed on the shaft snug but not super tight. Often the arc suppression ring is mistaken as a thrust washer and gets re-installed upside down.