It’s Never The Motor Brushes

Posted on Posted in Repair Stories, Tech Tips

So, in “Hard Luck Cases“, I mentioned learning a valuable lesson. When I got the mixer on the bench, I didn’t even plug it in to see if it ran. Once I saw its overall condition, with the corroded power cord, crud, and all the rest, I just started right in on disassembling it. As a result, I had no idea whether it actually ran.

The first thing I did was remove the motor brushes. There was no particular reason to do this — at the time I wasn’t planning on disassembling the motor at all, and the brushes can stay in unless they’re worn out.

This was a mistake.

After many hours of disassembly, cleaning, and reassembly, I plugged in the brand-new power cord and moved the speed control to “Stir”.

Nothing.

Checked everything. Measured voltages across the speed control plate. Fine. Swapped in a new phase control board. No change. Completely disassembled the motor (which involved emptying the gear case so that I could remove the armature, which has to be driven out through the accessory shaft hole). Measured resistance across the field coil and armature. Both OK. Since I had everything out I also replaced the brush holders, which were in rough shape.

Somewhere in the middle of all this, my brilliant wife asked: “Did you switch it on before you took it apart, to see if it ran at all? I always do that when I put a new sewing machine on the bench.” Reader, I had not. And she’s absolutely right: I should have turned it on to make sure it ran. Otherwise, I’d have some idea of whether it should have run after I reassembled it, or not. It was a n00b mistake.

Back to debugging.

As I was reassembling the mixer for the SECOND time (the armature needed replacement), I noticed (since I had the head off and upside down) that the motor brushes weren’t making contact with the armature.

🤦‍♂️

So here’s what happened: when I put the motor brushes back in after the initial removal to inspect them, I didn’t think to check their orientation. Each brush has a chamfered edge, which lines up with a tab in the brush holder. If these don’t line up, even though it *feels* like the brush is all the way in, it won’t make contact with the motor armature. So there’s no completed circuit, and the motor won’t run. Of course, the tab is deep inside at the far end of the brush holder, so you can’t tell without looking very carefully.

This is a “feature” of modern KitchenAid mixers that use this motor design; vintage mixers use a square brush holder. I’m sure they had a good reason for wanting to change the brush holder and I can probably come up with a couple of reasonable explanations. That being said, it’s a trap for the unwary.

So, two takeaways:

  1. Always turn the mixer on when it lands on the bench, to make sure that it runs. If it doesn’t, you know you’ll have to figure out why. If it does, then it should also run after reassembly.
  2. If the mixer won’t run after reinstalling the brushes, check the brush orientation. Vintage or modern, it’s important to make sure that the curve on the brush head lines up with the curve of the armature; but in a modern mixer it’s also crucial to line up the chamfered edge of the brush with the tab on the brush older.
  3. *There’s almost never a need to remove the motor brushes, unless you’re planning to completely disassemble the motor.

(And yes, especially on a very old and/or high-mileage mixer, inspect the brushes for wear. If there’s less than about 3/8ths of an inch of graphite left, they should be replaced.)

(* It’s often said that the two most difficult problems in computing are naming things; cache coherency; and off-by-one errors.)