Need help understanding brushed DC motor torque loss.

[JonDick13926]

I pulled a window motor out of a minivan about a month ago because under a load it wouldn't spin (it basically wouldn't move the window up and down) but would spin fine unloaded.

I replaced it and the new one worked fine, and I sent the vehicle out the door. But I kept the motor so I could mess around with it later and try to figure out why it failed. I took it apart and didn't see anything obvious (obvious to me anyway). The brushes and commutators looked good. The coils looked good (though I'll admit I wasn't 100% sure what to look for). Finally, I scoped the motor and graphed the amperage, and the graph was rough. Each ripple for each commutator looked different, and none of them looked as good as you would expect from a healthy motor like this. Obviously this motor is bad, but I'd like to understand specifically why this happens, as in where in the motor does this fault originate and why does the motor seem to lose the torque it once had?

 

[matt_i]

Just my experience with 7.5hp DC drives which are obviously larger but torque loss or inability to control torque, is almost always a contact issue between brushes and commutator.

In simplest terms, to build the magnetic field needed to repulse the permanent magnet and spin the rotor, electricity must be conducted with minimal resistance. Otherwise field strength is low, rotor has low torque, etc.

 

[JonDick13926]

Just my experience with 7.5hp DC drives which are obviously larger but torque loss or inability to control torque, is almost always a contact issue between brushes and commutator.

In simplest terms, to build the magnetic field needed to repulse the permanent magnet and spin the rotor, electricity must be conducted with minimal resistance. Otherwise field strength is low, rotor has low torque, etc.

Thanks for the information.

So in the case of the motor I'm dealing with, one possible culprit would be the springs holding the brushes against the commutator? I did notice that they weren't exactly pressed up tightly.

 

[MikeF2316]

You didn't mention if the motor current is high or low.

I've seen the magnets come unbonded from the case. This generally leads to noise and high current draw.

When the varnish that insulates the rotor wire starts to fail, windings short together, or to ground. This results in higher current draw as well.

The buildup of oxide on the commutator or worn brushes that are too short to slide smoothly without jamming, or broken springs result in lower current draw.

Some motors designed with a low duty cycle in mind have a kind of thermal fuse that increase resistance as it warms up. I've seen these fail in the high resistance state. But this wouldn't cause the jerky current readings you're seeing.

 

[matt_i]

So in the case of the motor I'm dealing with, one possible culprit would be the springs holding the brushes against the commutator? I did notice that they weren't exactly pressed up tightly.

In my opinion yes, if the contact pressure isn't correct then that's a problem. Its not a huge spring pressure as the idea isn't to build a grinding wheel that wears out the brushes in short order, but it has to be enough to keep the resistance as low as possible.

Window motors are pretty low price-point motors, they seem to be better than the past when power windows were a new thing, but still not a place where a lot of money is spent.