On Feb 9, 7:57*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.

Now when I apple power it just growls. If I turn it by hand with power
applied it will turn 1 or 2 revolutions then stop. Before disassembly the
motor ran as expected.

The field and armature are separately terminated at the outside of the motor
and wired to a motor speed control PCB.

The commutator segments are well-separated. I put an ohm meter on the brush
terminals and turned the motor slowly. I see 10 ohms across the armature with
each commutator position (each brush contacts 3 segments). The field measures
1000 ohms. I thought that a bit high, so I found where the 2 separate field
windings are connected (in series) and measured each winding separately: each
is approximately 500 ohms.

There is no mechanical reason the motor should not spin at speed. By hand, it
turns freely and there is no interference between the armature and field
laminations(?).

The motor is rated at 180 volts DC, 1.5 amps. There are 2 separate field
windings (wired in series) and the commutator has 36 segments.

I've had this motor apart several times but still can't find a reason it's
not operating like it should.

Any suggestions would be greatly appreciated (and the sooner the better ;-)
).

Thanks,
Dave

After you turned the commutator, did you undercut the mica between the
commutator segments? It is critical that the insulation be below the
surface of the copper to ensure good contact between the brushes and
the copper. If there is still sufficient undercutting [you only
skimmed the surface of the comm] then be sure that there is no copper
shorting between commutator bars. The copper often 'smears' a bit when
being turned and will bridge the gaps here and there. A "pole growler"
will show up any such shorts.

Neil S.