Turn, Turn, Turn

Back in 1998, when I was spending my first summer living aboard Eolian, the 12V electric motor which powers the refrigeration system failed.  Back then, 23 years ago, I pulled the motor and walked up into the South Lake Union area to a motor rebuild shop.  A day later, I had the rebuilt motor back in hand and reinstalled it.

Fast forward those 23 years, and the motor has again failed - the brushes have worn out.  But now we are berthed in Anacortes where there are no motor rebuild shops, and the South Lake Union area is now all expensive high-rise apartments and businesses.

So this time, I decided to do the work myself.   First, I found that the bearings are in great shape - it is only the brushes need replacing.   Well, that and the commutator needs to be cleaned up.

(Quick lesson here for those not familiar with the internals of a DC electric motor.  The armature - the spinny part - has on one end a device called the commutator.  This is a series of pairs of copper bars, with the members of each pair on opposite sides of the armature.  Each pair member bar is the termination of one end of a coil of wire which, when powered, makes an electromagnet.  Things are arranged such that when the brushes contact the pair of bars and the magnet is powered up, it is just outside the field magnet and is strongly attracted to it.  But just as it gets there, the brushes break contact with that coil and make contact with the next one, via the next pair of bars on the commutator.  Thus the armature spins.)

Inspecting the commutator, it was obvious that it had seen arcing once the brushes had worn so short that they were not making good contact anymore.  But even more importantly, that motor repair shop which did the previous repair had done a very sloppy job of turning down the commutator.  First, they cut a trench that was just wide enough to accommodate the brushes, thus ensuring that if there was any play in the brush holder, the brushes would contact the edges of the trench and wear prematurely.  Second, they left a series of deep grooves in the commutator.  Now one might say, "Well, the brush will just wear to fit the contour of the commutator, grooves and all."  But again, if there is any play in the brush holder, then movement of the brush will defeat that reasoning, resulting in rapid brush wear.

So, OK - on to the repair.

I constructed a jig that would support the armature on its bearings, and connected an electric drill to the armature shaft.  This way, I could spin the armature while holding a file against the commutator, smoothing it out.  A lathe would have been much faster, but I don't have one.

 

It worked well, albeit slowly.  First I cut off the outboard wall on the commutator, making it all the same diameter.  (I couldn't do this at the inboard end, because this is where the wires are connected.)  Then I filed the rest of the surface smooth, and followed with 100, 220 and finally 440 grit sandpaper.

Before and After

I think it came out great!  (In the before picture I had already done some cleanup.)

I couldn't find brushes of the correct size, so I bought some that were too big and ground them down to fit.  This was an easy job, but messy since they are made out of graphite, which ends up going everywhere.

I checked the bearings (they are sealed bearings); they are in perfect condition.

The motor is now re-installed, and we have refrigeration once again.  And all that refrigeration downtime gave me a perfect opportunity to clean the interior of the refrigerator and freezer compartments - so bonus!