Electrical Engineering - Re: Replacing brushes

Author

Re: Replacing brushes - final chapter

john

2006-08-19, 5:25 pm

daestrom,
Thank you very much for the very informative treatise on brushes and
commutators. It is refreshing to have someone answer a post with so
much information. I'm a very detail guy myself, (I answer user posts on
another newsgroup), and appreciate a thorough response.

Unfortunately, it is all academic at this point. I tore down the motor
again and found that half of two adjacent contacts on the commutator
are now missing - broke off half way across their length and went who
knows where. I guess they must have broken off in the brief period when
it ran after my brush replacement. The commutator looked fine when I
dressed it but then it might have had some stress fractures that I just
didn't see. Just for reference, the commutator orignially had a brush
wear groove, (not surprising after some 30 years), I would guess about
..001 to .002, and that's part of the reason I honed it down. The
insulator was well undercut so the only issue is that there might have
been some conductive debris that I didn't properly clean out.

I'm now off to buy a new mower, but thanks again for your support.

John
daestrom wrote:
> "john" <jensenj6@comcast.net> wrote in message
> news:1155943960.968333.159070@b28g2000cwb.googlegroups.com...
>
> Some oversights....
>
>
> First of all, a 'dark commutator' is not necessarily wrong or a bad thing.
> After running a while, a good DC motor's commutator will have a dark
> 'chocolate brown' appearance. A 'shiny brass' commutator is actually a
> *bad* thing. A newly refurbished commutator *should* build up a thin
> coating after running a short time. This coating (a combination of copper,
> oxides from the air, and brush material) actually provides lubrication for
> the brushes. Of course, too much of a good thing can turn into a bad thing.
> A simple canvas cloth can be used to wipe off excessive material. A
> commutator that is totally 'black' however may be a sign of oil/hydrocarbon
> contamination. The heat of tiny pin-arcs under the brush can breakdown
> hydrocarbons and produce a tar-like substance on the commutator. This can
> actually be 'sticky' and cause brush vibration as the brushes stick/slip
> along the surface.
>
> When grinding/polishing, we generally do *not* just rub the surface with
> emery cloth. This can develop an out-of-round condition that must be
> corrected before assembly. If you have the right tools, you can put it in a
> lathe and slowly turn it while a micrometer rides over the bars to measure
> the 'roundness'. Even very large machines, the max run-out allowed is only
> a few thousands of an inch.
>
> If you don't have all those tools, put it back in your drill press with the
> commutator 'up' (put just the stub of the shaft into the chuck, but don't
> damage the bearing surfaces). Don't turn on the drill press, but arrange
> some sort of stand next to the rotor and a steel ruler or something to
> lightly ride on the bars. Turn it slowly by hand to see if the bars are
> out-of-round. Even a millimeter of movement is too much for that size
> commutator.
>
> I hate to say it, but I suspect this is your real problem. And you may have
> caused it by your emergy cloth and drill press.
>
> A dirty commutator can be cleaned with just wiping it down with a piece of
> canvas while it's spinning (even in a drill press / lathe). But if there
> are grooves or ridges, then yes, the commutator did need to be resurfaced.
> Emery cloth can leave conductive dust on the surface and in the slot between
> commutator bars and must be cleaned out. After grinding/polishing a
> commutator, you must inspect the insulator between the slots to see if it is
> still recessed below the surface. If it is flush with the surface, then it
> must be 'undercut'. On large machines, we use a thin hacksaw blade type
> tool to cut down the insulation between the bars. Your machine is probably
> quite a bit smaller and you would need a very thin blade. Do *NOT* try to
> do this with a triangular file. Such a file will bevel the edges of the
> bars and that is *not* what you want.
>
> After grinding/polishing a commutator, it is also important when
> reinstalling it to look for brush-holder to commutator clearance. The brush
> holders have to be close enough so the brush can't cock or vibrate as the
> commutator turns, but far enough away to avoid physically touching the
> commutator bars. (1/16" to about 3/32" is about the max).
>
> Brushes should be seated to the contour of the commutator when first
> installed. To do this, wrap a layer of fine grit *sandpaper* around the
> commutator (rough side out) then install the brushes. Turn the rotor by
> hand in the correct direction several turns. Remove a brush and the face
> should show sanding across the entire face. Small brushes sometimes come
> with a contour pre-machined into them, but ideally you still do this step.
> When the brushes show that the entire face is in contact with the sand
> paper, remove the brushes and sandpaper, and vacuum out any carbon dust
> (better to vacuum than 'blowing it out' with compressed air, as compressed
> air can force carbon dust into places you don't want it). Use a plastic
> bristle brush to clean the slots, always brushing *away* from the windings.
>
> Brush springs need to hold the brush face snugly against the commutator. If
> the bars are uneven, the insulation has 'high spots', or the rotor can
> vibrate a lot in the bearings, the brushes will bounce up off the commutator
> surface as it rotates. That will cause a lot of sparks and rapid
> wear/heating of the brushes.
>
> Small sparking at the trailing edge of a brush can be normal. Severe
> sparking, or sparks that 'trail around' the commutator is bad and needs to
> be fixed. Causes to check....
>
> Loose springs allow brushes to bounce
> Excessive out-of-round requiring the brushes to run in/out as the commutator
> rotates
> Insulation 'fins' sticking up out between the bars
> Contaminants on the bars that are 'sticky' such as 'gums' or tars built up
> from oil/grease
> Improper assembly putting the brushes at the wrong position around so they
> aren't in electrical neutral
> Bad commutating poles (also known as inter-poles), but I doubt such a small
> machine has these.
> Badly seated brushes only making contact at a small percentage of the brush
> face.
>
>
> Absolutely. The carbon grade used in the composition of the brush varies
> with design. Higher surface speeds and lower current densities use a
> 'harder' grade that is much like the graphite in a hard pencil, while lower
> speeds can use a softer grade. But check the other things first before
> looking for different grade of brushes.
>
>
> Well, you need to figure out what is causing the sparking (see above for
> things to check). If you took very much material off the commutator, or
> it's been run a long time (you did mention it is pretty old machine), then
> definitely check the insulation between the bars. One material often used
> is the mineral 'mica'. It is harder than copper, so it doesn't grind down
> equally well. After a lot of wear/grinding, the mica will actually be
> higher than the copper, causing brushes to skip over the bars as they bounce
> over the mica 'fins' sticking up. That is why it is deliberately 'undercut'
> so it is lower than the bars.
>
> When you 'yanked the rotor', did you have to remove / shift the brush
> holders, or are they fixed and only can be in one position. (small motors
> they are often fixed, but larger ones routinely have an adjustment to allow
> them to be turned forward/backward around the commutator a small amount).
> If this is the case, are you absolutely *sure* you put them back in exactly
> the same spot. Being off by just a few degrees of rotation can cause a lot
> of arcs/sparks.
>
> When you remove the brushes this time, check to see how the face looks. Was
> most of it in contact with the commutator, or just some edges. Just edges
> means they weren't seated properly (the sandpaper on the rotor to give them
> the right contour). It can cause very high current densities that will lead
> to sparking.
>
> Look at the sides of the brushes to see if there are signs of them running
> in/out rapidly in the brush-holder. Out-of-round can cause this sort of
> brush movement. Then the brushes just sort of 'float' and only contact the
> high spots and skip over the low spots causing a lot of arcs/sparks.
>
> Also look at the corners/edges. If there are chips and broken edges, this
> can be a sign of 'chattering'. Either the brush is to small for the holder,
> or the holder is too far away from the commutator. So the brush cocks at an
> angle and breaks off edges when the rotor is spinning. Severe bouncing on
> an out-of-round rotor or mica 'fins' can also cause chattering/breakage.
>
> Good luck, let us know what you find....
>
> daestrom
> P.S. As you may now realize, motor repair can be a pretty interesting bit
> of skill.

daestrom

2006-08-20, 1:25 pm

"john" <jensenj6@comcast.net> wrote in message
news:1156014162.629290.238380@m79g2000cwm.googlegroups.com...
> daestrom,
> Thank you very much for the very informative treatise on brushes and
> commutators. It is refreshing to have someone answer a post with so
> much information. I'm a very detail guy myself, (I answer user posts on
> another newsgroup), and appreciate a thorough response.
>
> Unfortunately, it is all academic at this point. I tore down the motor
> again and found that half of two adjacent contacts on the commutator
> are now missing - broke off half way across their length and went who
> knows where. I guess they must have broken off in the brief period when
> it ran after my brush replacement. The commutator looked fine when I
> dressed it but then it might have had some stress fractures that I just
> didn't see. Just for reference, the commutator orignially had a brush
> wear groove, (not surprising after some 30 years), I would guess about
> .001 to .002, and that's part of the reason I honed it down. The
> insulator was well undercut so the only issue is that there might have
> been some conductive debris that I didn't properly clean out.
>
> I'm now off to buy a new mower, but thanks again for your support.
>

Ah, loose commutator bars. Something I didn't mention before. The
individual bars are held down using various techniques on machines.
Sometimes just a tight winding of cord. Larger machines often use steel
wire (insulated from the bars), or a steel ring (with an interference fit
and insulated from bars). Small machines sometimes use just epoxy or other
glue. Obviously if they work loose, they can shift and start to bounce
brushes, vibrate the wires connected to them, and literally fly-apart (as
yours has apparently).

Repairing one with missing bars is not worth it except in large machines.
Oh well, you did get 30 years out of her.....

daestrom

sQuick

2006-08-20, 8:25 pm

"daestrom" <daestrom@NO_SPAM_HEREtwcny.rr.com> wrote in message
news:sp_Fg.40039$1Z5.26474@twister.nyroc.rr.com...
>
> "john" <jensenj6@comcast.net> wrote in message
> news:1156014162.629290.238380@m79g2000cwm.googlegroups.com...
>
> Ah, loose commutator bars. Something I didn't mention before. The
> individual bars are held down using various techniques on machines.
> Sometimes just a tight winding of cord. Larger machines often use steel
> wire (insulated from the bars), or a steel ring (with an interference fit
> and insulated from bars). Small machines sometimes use just epoxy or
> other glue. Obviously if they work loose, they can shift and start to
> bounce brushes, vibrate the wires connected to them, and literally
> fly-apart (as yours has apparently).
>
> Repairing one with missing bars is not worth it except in large machines.
> Oh well, you did get 30 years out of her.....
>
> daestrom
>

Not really work repairing, but thanks to 'daestrom'
for an interesting read.

sQuick..

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