220VDC motor slow and jerking

Question

I'm troubleshooting issue of a motor from a washing machine I bought from a garage sales. Based on the service manual, I'm suppose to replace the motor assembly. So I decided to take a closer look at the motor assembly. I notice that the 2 carbon brushes are no equal in length. One is significantly shorter and only 3mm of extension left when secured.

After about a week, one night it tripped the RCD. I didn't plug in to confirm the source of the trip, but measuring Live vs Earth give me 4MOhm of resistance, which I think might not be the cause. So next day I open the motor again and clean it with WD40. After drying and putting it back, the motor jerks and turn slowly when I tested it. I'm not sure if this is caused by the cleaning, shorten carbon brush, burn coil (physically I don't see any color change on the coil) or control board (The driving source rectified 220VAC going through 3 relay controlling the rotor and stator for speed and direction). I'll try replace the carbon brush later evening.

At the mean time any input on this? It's not easy to get a motor assembly in my country, so I would like to save the washing machine as much as possible.

EDIT: Since this motor uses brush, so no issue with capacitor or such. The bridge rectifier and relay is the only component that drive the motor from direct AC. This motor is commonly used in Whirlpool and Samsung as well.

EDIT: The exact model of the motor is here and a similar one as shown at the video here. The schematic for a similar model which uses a similar motor is here at page 14.

The jerk happens at a constant frequency, which caused me to worried that it's a burn amateur, probably only 1 winding. But I can't see any obvious color change on the winding.

This is how the carbon brush looks like. I've only manage to get a bigger sized on, so I'll shape it and try it out in a few days time.

EDIT: I've replaced the carbon brush. Still the same. This is the video of how it looks like.

EDIT: madrivereric was right. The 5 winding on the armature is shorted. After running the motor using 12VDC, I found 5 continuous section on the commutator that doesn't produce torque, but has current. So I isolate the 5 section, and now the motor runs smoothly. But there's arc when the brush enters and exit the 5 section. Can I short the 5 section +1 before and after so the current can connect continuously? I know I'll loose some torque, at least I can try it out while waiting for the motor to arrive, which might take months. Btw, I think the armature is wave wounded.

Answer 1

Using a straight dc motor for a washing machine drive is new to me. I'd expect a straight induction motor (likely cap start and/or cap run) or perhaps a universal motor. The fact that it has brushes means it is either a universal or a dc motor. In both cases, good brush contact is needed in order to excite the armature coils on the rotor. Intermittent contact of the brushes will result in intermittent torque.

The uneven wear of the brushes indicates that they may need replacement. Without knowing the the original dimensions of the brushes, I can't say if 3mm is enough or not, but since you're having problems with the motor, I'll guess its probably not.

You might also ohmmeter check the armature windings for continuity since there could be a broken connection between the brush contact and winding for example.