DC motor problems

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
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 ;-)
Reply to
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Actually, yes. The rear casing of the motor that contains the brush holder can be mounted to the main motor frame (containing the field) at 90 degree increments. So 4 possibilities.
I reassembled the motor as I thought it was originally configured but the possibility that it's incorrect is large.
I'll try other orientations tomorrow.
Thanks John!
Reply to
Two--one for each direction! :-) Whenever I disassemble a motor, I scratch a mark across the "parting line(s)". Then I know how to re-assemble it.
Reply to
Some 'brush sets' are 180 degrees apart, and that right on the center axis of the brush 'monolith'. Some are oriented slightly off center (but still 180 degrees apart). Usually on a motor only meant to spin in one direction. The result is that the brush face has the arch abraded across it slightly off center,and may only have one 'correct' re-installation orientation that will preclude chipping and unneeded wear-in sessions.
Reply to
The two brushes are mounted exactly 180 degrees apart. But the brush holder is adjustable (in rotation). I haven't touched that adjustment, so if I just get the gross orientation correct (90 degrees from current setting) all should be well.
Thanks, Dave
Reply to
Use a battery and momentarily connect the field winding. This "magnetizes" the armature (do this with the motor un powered). You only have to do this once.
Reply to
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.
Reply to
Thanks for that info.
I did not undercut mica. I presumed that since, before turning on the lathe, the motor was functioning well enough (sans bearing noise) with not-undercut mica that its purpose was not important.
What's the best way to undercut? Using an exacto or utility knife in my hands gets quickly messy...
I looked closely and eliminated any segment shorts on the commutator.
Thanks, Dave
Reply to
You didn't by any chance unwire the coupling in the field windings? If so, it sounds like you have one winding reversed. This will give you the effect you are seeing. But you did say that you had to hunt for them and the F1 and F2 legs are on the outside?
Other possible problem is brush alignment.. if you don't have the brushes orientated so that one crosses over to the next winding while the other is lagging behind, it'll sit there and hum and some times a spin of the rotor will start it but most likely will come to a stop.
Are you sure you have field voltage? And also, have you ohmed out the armature to make sure it isn't shorting to the rotor? It is possible you could of damaged it while it was in the lathe.
But in any case, what I've done in the past was to use a induction meter to measure the cross over point on the armature to help align the brushes.
P.S. May want to check to make sure you didn't over turn the commutator and also, it is very possible you had a shorted motor to start with.
Also check the field for ground shorts. YOu need to use a megga meter for this. Or if you can get your hands on a IR bridge that will work too.
Reply to
I was going to mention cleaning between the communicators. Use a round cutter, to scoop all the surface crap out. Some also bevel the edges of the communicators so they don't tear up the brushes. For small motors a ball point pen is usually used ( that should give you an idea)
Reply to
Martin Riddle
The idea is good, but there's a better way. Use a punch and put matching dimples across the 'parting lines'. Put one set on one end and two sets on the other.
Reply to
m II
Using a hacksaw blade the idea is to grind off the 'set' on the teeth to a thickness that will be smaller than the commutator bar gaps. Then it is used by having the teeth point backwards towards you, the action is then pulling rather than pushing which IMHO gives more control. Whilst doing this try to hold the blade at an angle so that it cuts against the sides of the bars, usually you can see the material flake off when done correctly. This is important as it removes any insulation that might be on the side of the gap you are creating and then can interfere with brush contact. While grinding the blade snap off the end and make it about 45deg away from the teeth to make it easy to get to the insulation at the connection end. However I doubt the commy is your problem :-)
Rheilly P
Reply to
Rheilly Phoull

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