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 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
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In article

There is a device called a "growler" that has been used for decades to detect winding problems in dc motors. It is useful, because the low winding resistances in dc motors makes it difficult to detect some flaws.The growler counts on induction to discover winding flaws. I suggest yuou look up growler and put one together.
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Sam

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As expected, orienting the brushes 90 degrees and the motor runs!
Thanks for all the helpful suggestions.
Dave
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Glad you closed the loop here!
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DaveC wrote:

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.
Jamie.
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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.
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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
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On 2/11/2012 1:15 AM, DaveC wrote:

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
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Grind a tool from a hacksaw blade, mount it in the tool post, and use the carriage
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So, just a sharp knife point thing?
Thanks, Dave
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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)
Cheers
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On 2/10/2012 19:36 PM, Martin Riddle wrote:

Some folks advise against beveling the edge of the bars. It doesn't really help the brush slid onto the bar. But it does widen the gap and reduce the surface area for brush-bar contact. This can increase brush sparking/burning.
On large machines, GE and WE always advise cleaning the slots but keeping perfectly straight bar edges.
Using a knife wouldn't be my first choice, it's too easy to actually carve off some of the copper. Someone mentioned a custom ground blade to fit the slot, and that's best. Just wide enough to get all the mica and not leave a thin sliver on either side, but not so wide that it cuts away any copper from the bars. That's what we did in the Navy's motor rewind shop (lot's of DC motors of various sizes on submarines :-)
daestrom
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wrote:

Been there, did that... My wife's grandmother was the only person I've ever known that wore out an old Electrolux vacuum cleaner. When the motor was disassembled, there was a noticeable groove in the commutator. Put it on a lathe and turned it down in very small steps, finishing off with very fine grit on a wet cloth to smooth to a near mirror finish. Complete by removing any copper flash that wound up in the gaps between commutators.
Be careful not to cut too much at a time as the torque drag might twist the commutator on the shaft. Also watch that you don't get too close to where the wires are attached to the commutator.
Everything was marked on disassembly to get the orientation correct on reassembly. If all is right, applying a DC to the motor will cause a rotation speed equal to the speed when the polarity is reversed. If the fields aren't aligned to the commutator, it will run faster in one direction.
With new brushes, worked at least another ten years and Grandma was very happy.
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Oppie wrote:

I used to turn down commutators on lots of vacuum cleaner motors without taking them apart. I would run them on a variable DC supply and use a hard ink eraser, with it running. Some were so bad that I had to hit the worst spots with emery paper, but I didn't have a lathe and it worked. :)
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Michael A. Terrell Inscribed thus:

wrote:

I've done that with a lolly stick and fine glass paper, then I discovered the abrasive sticks made just for this purpose.
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Baron wrote:

This was 30 years ago. No internet, and no industrial suppliers in town. :(
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Michael A. Terrell Inscribed thus:

Similar time frame. Maybe 40... :-) It was "Hoover" that provided the abrasive stick. The idea was to use them to quieten down particularly sparky motors. Even though they had noise suppression capacitors and chokes on the motors they could cause TV picture break up. People did complain about it as well. I'd being using the lolly stick for some time before then to smooth the comm. I'm of the opinion that poor manufacture of some armature left the comm slightly off centre causing the brushes to bounce and thus arc more than they normally would.
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Baron wrote:

A lot of those motors had the air flow over the commutator, and it would get dirt or sand in it when people didn't put the bag in right, or would use it till the bag was full, and ripped. I wasn't working on the vacuums, just the motors so the only parts I saw were new bushes & bearings. A neighbor was in the used vacuum cleaner business and would give me half a van load of bad motors at a time, then buy back any that I fixed.
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Michael A. Terrell Inscribed thus:

That sounds like a nice little earner. ;-)
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Baron wrote:

$10 to $20 for common motors, and $50 for Rainbow/Rexaire. He would stop by in a panic, telling me what he needed. I'd tell him to give me an hour to dig through my good motors and would try to find one. Then i would dig out what he needed, and fix it. A lot had dmagaed fans from metal or glass being picked up, so it was a five minute job to remove them and a set of good ones from a burnt out motor, polish the armature and put it all together. then the scrap metal went into bins for the scrapyard. I got really good with a wood chisel & various punches to remove the copper from the old motors. The segments were easy to remove, with a pipe wrench. Clamp the armature in a vise, and give a twist with the wrench. it was kind of like taking corn off a cob. ;-)
Sometimes I would strip 100 bad motors at a time, for the good parts just to make room.
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