DC motor problems

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.

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

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

Dave posted that he'd solved the problem, 15 days ago.

Brushgear wrongly aligned on reassembly.

I would suspect that the replacement brushes are a little short or that the spring force on the brushes is too weak. I used to go as far as slightly contouring the brushes to the commutator to make max contact (and reduce arcing).

David_J

Wossgoinon?

This was satisfactorily resolved ages ago.

Read the thread.

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.

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. :)

You need high speed (~200 ft/min) and a *very* sharp tool. Commutators are a bitch to machine to a decent finish without ripping the segments off..

Cheap commutators sometimes have very thin segments, maybe .025" thick.

When in doubt I use as close to a zero rake as possible to avoid pulling out the copper bars. Most of the motors I've worked on are pretty old and they didn't skimp on the thickness of the copper. I would use a pointed cutting tool and a really slow feed rate to get minimum cutting forces. Last pass would be with a more rounded tool just to smooth it out and finally the fine grit to polish it out.

Michael A. Terrell Inscribed thus:

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

This was 30 years ago. No internet, and no industrial suppliers in town. :(

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.

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.

Michael A. Terrell Inscribed thus:

That sounds like a nice little earner. ;-)

$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.

Michael A. Terrell Inscribed thus:

A very profitable exercise nowadays with the price of copper being so high. I just wish the thieves would stop stealing the iron grates and aluminum road signs. A whole load of them have vanished overnight. Not to mention the theft of copper wiring from the railway system.

Next door neighbor came round today and asked if I knew that water from the kitchen sink was pouring out of the wall and onto the yard !!! The swine had pinched the two meters of lead drain pipe between the wall and grate,

They are destroying central air condition equipment around here, for the couple dollars worth of copper tubing. Some businesses were hit twice within weeks. Their insurance only covered it once. Some of the thefts were from non profits that had no coverage and had to do without it in Central Florida. :(

The problem is pretty easy to solve, given that the legislators *want* to solve it. That they haven't tells us a lot.