On 2/9/2012 22:57 PM, DaveC wrote:
A 1/2(?) horsepower 2-brush DC motor was having bearing issues so I
disassembled it and replaced the ball bearings, turned down the commutator on
a lathe, and installed new original equipment brushes. A good clean-up was
done as there was much grease and carbon dust inside.

snip

Thanks,
Dave


A growling sound without turning can be a couple of things.

First, check that commutator bars aren't shorted. If you turned it on a
lathe or something, you should have then cleaned/undercut the slots
between the bars. This cleans out any copper that would short them and
it lowers the mica between the bars so it doesn't interfere with the
brushes sliding across the tops of the bars. If you didn't, take the
rotor out and clean the slots with a thin saw blade or tool that will
just fit in the slot. Don't use a triangular file, that will bevel the
edges of the bars and cause other problems (more sparking/burning at the
brush edge). But a lot of shorting between bars usually just keeps it
from turning at all, it doesn't 'growl' much, the shorts just trip the
supply breaker.

Next, if it has only two sets of brushes, this thing probably has just
two field poles in the stator. If you disconnected the wiring between
each pole, it is critical that you reconnect them together correctly.
If they end up so that both are creating a north pole towards the center
of the machine (or south pole), then the torque created from current in
the rotor windings will just cancel out and it will sit and buzz/growl.
With the rotor removed, connect a small battery (D flashlight cell
would work) to the winding and slowly move a compass near each pole,
noting which end of the compass needle points to the pole. The two
poles should be opposite polarity.

If the brushes are re-installed 90 degrees from where they should be,
that too will cause the symptoms you describe. If you didn't match mark
these before dis-assembly, well experience is a great teacher, isn't it
:-). Move them 90 degrees and try again. If it rotates the wrong way,
swap either the field wires, the armature wires.

Lastly, a damaged winding that shorts across several coils will do this,
but since it worked before you took it apart, you would have to have
bashed the windings with a hammer or something to damage them like this.

This doesn't sound like it's really big enough of a DC motor to have
commutating poles or a series field winding, so I won't bother with
those issues (suffice to say, getting those straightened out takes more)

Good Luck,

daestrom