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On Friday, June 19, 2015 at 12:19:59 AM UTC-4, wrote:
Just wanted to get back to the group about this AC motor that was running
in reverse after being disassembled and then put back together. It turns
out that reversing the brushes did indeed reverse the direction of the
motor. We can't argue with success, but I still don't fully understand why
this is so. Can anyone please explain this to me? Thanks, Lenny

Thanks everyone for the great explanations. It makes more sense now. The
original owner of this thing apparently tried to replace the brushes
himself. I can only surmise that he didn't realize that the bakelite brush
holders were held into the housing with small set screws. So he did what any
idiot would do. He used a "bigger hammer". Some people should never pick up
a tool. It looks like he tried to pry the holders out without releasing the
screws and cracked them into many pieces.

The field is connected to the brush holders with push on connectors. That's
a good thing too because to get it out he must have ripped it loose from the
remnants of the brush holders. Luckily the field wasn't damaged.

At this point he apparently gave up and gave the tool to my son who has been
working on it since. So in dis assembly the correct orientation of the field
then become unknown, and after replacing the brushes and holders we
evidently switched the brush positions.

So he now has the direction problem resolved but the new brushes arc really
bad. Perhaps the old ones did too and maybe that's why Mr Wizard tried to
replace them in the first place but we don't know.

The commutator does not appear to have worn down much during it's lifetime.
A growler test shows no shorted windings to ground, The areas between some
of the the segments however appear to be a little ragged and opened a bit
from the arcing, no doubt. A dial indicator on the commutator shows an out
of round condition totaling 1.5 thousandth's, on each side for a total of
three thousandth's for the entire piece. According to a machinist we
consulted this doesn't seem like enough to warrant turning the commutator,
but I've been considering something else. With this motor spinning at 9000
RPM would a 1.5 thousandth's out of round condition be enough to "bounce"
the brushes and make them arc? Brushes and holders are new and each brush is
mounted stationary, and it's relationship to the position on the commutator
cannot be altered. I can't figure out what else could be causing this? Lenny

The "ragged" wear and arcing would indicate open circuits in the armature
windings. Put it back on the growler and drag a hacksaw blade across the gap
in each slot. There should be an arc en you do that, if not most likely that
winding is open.