Fixing a DC motor with a worn commutator

My latest project has been to fix a faulty motor from a Wurlitzer CD jukebox. The motor powers the CD grabber arm and had been only working intermittently. Jukeboxes of this sort are quite expensive things to get repaired and I've been told there are only a handful of specialists in the UK who charge a lot of money for their expertise. I figured it wouldn't do any harm to have a look.

Tools/Equipment used (not an exhaustive list, but all of these things came in handy)
    Multimeter
    Bench top vice
    Flat screwdrivers of various sizes
    Cotton buds (or Qtips for those of a US persuasion)
    Soldering iron and solder
    Solder sucker
    Needlenose pliers
    Fine sand paper
    Araldite two part epoxy
    Fine fishing line
    Acetone (or some other solvent for removing dirt and grease)
    Contact grease (maybe not essential but thought it might increase life)
    Power supply for testing (I used a drill battery)
    


When I measured the resistance accross the poles of the motor with a meter if found that in certain places the resistance would be in the order of a few kΩ. I would expect something in the order of tens of Ohms for a coil, there was obviously something wrong so I removed the motor assembly and had a look. The motor casing was quite thick and took a lot of effort to unfold the metal tabs with a flat screwdriver and pointy pliers.

A view of the motor with the end cap removed.
As can be seen from the photo the motor has had plenty of use. When new the commutator would have been perfectly cylindrical. Over many years the brushes have worn a track in the copper deeper and deeper until the point of failure when the brushes reach the insulated core. The missing copper shown by the arrow explains why at some points the resistance is very high, there is no longer a good contact with the coils. This also serves to explain why the motor worked intermittently, it is only when the motor happens to stop at a bad part of the commutator that it wont start again and why I was able to get it to work by manually turning the shaft and re-applying power.
The motor was filled with the dirt from the worn commutator. I used cotton buds, acetone and a small brush to remove as much dirt as possible from the motor. Unfortunately I was not able to completely remove the rotor as the gear was seized onto the shaft. Do take care to remove any stray bits of copper that may have broken off as they could end up causing unwanted shorts.

A view of the end cap of the motor, the terminal inputs can be seen at approximately 1 o clock and 7 o clock on the image. Noise suppression capacitors ground to the case (seen in yellow). The brushes themselves are the curved black blocks of carbon in the very centre. I'm surprised that there is so much of the brushes left and so little of the commutator. The brushes are pushed towards the commutator by the sprung copper strips. There is also what appears to be a resistor in series with each contact, I am not sure what purpose this serves as testing with a multimeter showed it to have negligible resistance.

I thought it would be a shame to have to replace the motor on the grounds that part of the commutator was worn, a look at the top picture shows that there is still a good deal of usable copper contact left on the commutator if the brushes could be moved to the right place. I set about moving the brushes further towards the coil end of the commutator.

I started by desoldering the leads that connect the input to the brush arms. Note where the arms are placed before you remove them. I figured that if I raised the arms about 3mm I would get a good contact with the commutator without hitting the bumps where the coil wires join to the commutator. When the leads are desoldered, clamp the end cap in a vice and pull the brush arm upwards. The brush arms had spades at the bottom which slid into a slot in the end cap.

A view of the brushes after removal.

The motor end cap with the brushes removed. I gave it a good clean and roughed up the surface with some fine sand paper.
The object was to get the brushes to sit at a higher level than they previously were. In order to keep them in place I mixed up some araldite and inserted it in and around the slots in the end cap (using a piece of bare wire or a paper clip should make it easy to get it into the nooks and crannies). I then pushed the brush arms back into the slots but stopping about 3mm short of the full way. I then added an extra bit of araldite to make it more secure. Avoid getting any on:
    the spring part of the arm (this needs to stay flexible)
    the leads (they will need to be resoldered later)
    anywhere that would interfere with fitting the end cap back onto the casing
    anywhere that would interfere with the turning of the rotor

Once the epoxy has set, resolder the leads as they were. You should now have something that looked just like before but the brushes are now a bit higher.

In order to get the end cap back on, the brushes must be pulled out of the way. In order to do this I drilled two small holes on the end cap and looped some fine fishing line through each brush arm and out of the hole (it is important that you use a loop of fishing line and not just tie it onto the brush arm as it would be impossible to remove without disassembling the motor and going back to square one). This will require some patience and a steady hand. In the end you should have two pieces of fishing line coming out of each side of the motor (so four in total) pulling on these lines will enable you to pull the brushes clear of the commutator in order to put the end cap back on the motor.
The newly raised motor brushes held in with epoxy, and the loops of fishing line inserted ready for reassembly.

If this is not clear, here's a short video:

Fit the end cap back onto the motor while gently pulling on the fishing line. I found that a tiny screwdriver poked through the centre of the end cap was useful for aligning the rotor, it always tends to fall to one side because it is attracted to the stator magnets "always with the magnets".

Once the end cap is back on connect the motor to a power supply such as a cordless drill battery and with a bit of luck it should be alive! My motor was a bit noisy for a few seconds and then worked fine. This might've just been the brushes hitting the rough parts of the commutator and soon settling into position.

If you are satisfied that the motor is working then its time to cut one side of each loop of fishing line, they should then pull free.

The very last thing to do is to fold the metal tags back in to secure the end cap on to the case. I would avoid doing this too many times as they will eventually break, but then if you've got araldite...

The brushes are now in contact with the unused part of the commutator, because of previous wear it is only about half of the width of the brush that is in contact, so this will cause it to about wear about twice as fast. I think that this process will significantly extend the life of the motor and only it cost a few hours work and some epoxy. The jukebox is now back in action and swapping CDs just fine.

If you are interested in trying this, tried it before or have any questions please send me an email
mesoiam [at] gmail.com


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