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Home > Archive > Electrical Engineering > August 2006 > Replacing brushes
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| People,
I'm not sure if this is the best group for my query but I'lll give it a
try. I have an electric lawn mower that was made by Sears way back in
1975. The nameplate rating is 120VAC - 10A. No other information is
available. The mower has an electric brake feature when shut off and
the mower has worked fine for many years (obviously). Yesterday while
mowing, the motor started to slow down and then died. I suspected worn
out brushes. I took the motor apart and indeed one of the brush wires
had disconnected causing the brush spring to carry the current until it
lost it compression due to extreme heat.
I didn't bother even checking Sear for replacement brushes as they
stopped supporting this tool many years ago. So I figured, how hard can
it be to find some replacement brushes. I went to my local ACE hardware
and found a set of brushes that were the correct size including the
brush wire guage.
I noticed that the motor commutator was very black probably from brush
dust and heat. I pulled the armature and chucked it in my drill press
so I could clean up the commutator. I used 600 grit emery cloth on a
flat stick followed by 1500 grit and then the paper side of the cloth
to clean up and polish the commutator. I put the new brushes in and
fired it up. It seemed to run fine although there was sparking from the
brushes. I proceeded to mow the lawn but after about 5 minutes of
mowing the motor started to bog down. Now it runs very rough with lots
of sparking from the brushes.
Questions:
1. Was there a basic flaw in my repair process?
2. If so, what should I have done differently?
3. Is it possible the brushes are not of the right type even though
they are the correct size (e.g. not the correct composition of carbon
for that motor type)?
4, What do you suggest I try next?
Thanks in advance for your assistance.
John
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| Palindr☻me 2006-08-18, 8:25 pm |
| john wrote:
> People,
> I'm not sure if this is the best group for my query but I'lll give it a
> try. I have an electric lawn mower that was made by Sears way back in
> 1975. The nameplate rating is 120VAC - 10A. No other information is
> available. The mower has an electric brake feature when shut off and
> the mower has worked fine for many years (obviously). Yesterday while
> mowing, the motor started to slow down and then died. I suspected worn
> out brushes. I took the motor apart and indeed one of the brush wires
> had disconnected causing the brush spring to carry the current until it
> lost it compression due to extreme heat.
>
> I didn't bother even checking Sear for replacement brushes as they
> stopped supporting this tool many years ago. So I figured, how hard can
> it be to find some replacement brushes. I went to my local ACE hardware
> and found a set of brushes that were the correct size including the
> brush wire guage.
>
> I noticed that the motor commutator was very black probably from brush
> dust and heat. I pulled the armature and chucked it in my drill press
> so I could clean up the commutator. I used 600 grit emery cloth on a
> flat stick followed by 1500 grit and then the paper side of the cloth
> to clean up and polish the commutator. I put the new brushes in and
> fired it up. It seemed to run fine although there was sparking from the
> brushes. I proceeded to mow the lawn but after about 5 minutes of
> mowing the motor started to bog down. Now it runs very rough with lots
> of sparking from the brushes.
>
> Questions:
> 1. Was there a basic flaw in my repair process?
> 2. If so, what should I have done differently?
> 3. Is it possible the brushes are not of the right type even though
> they are the correct size (e.g. not the correct composition of carbon
> for that motor type)?
> 4, What do you suggest I try next?
>
> Thanks in advance for your assistance.
>
> John
>
Did you clean out (undercut) the commutator gaps? If not, you could have
a build up of cu/c dust which is bridging between windings.
Did you shape the brush faces to the commutator diameter?
Yes there are different brush compositions.
You need to strip it down, see how much the brushes have worn, whether
they have worn equally and evenly, how much dust is present, whether
the gaps have been bridged.
What could have happened is that the new brushes only had a small point
of contact when first fitted, that part of the brushes have worn down
quickly and the brush is now spanning adjacent windings. So a bit of
brush tip profiling may be called for.
Or it could be that there was a build up of copper and carbon in the
gaps. Because the composition is too soft..
Or, if the brushes don't conform to the comm profile, you may have
bridged the gap with extruded copper - extruded by the very high brush
pressure on the small area that the brushes act on..Especially, if the
brush composition is too hard.
So, all sorts of possibilities...
--
Sue
| |
| George Jetson 2006-08-19, 3:25 am |
| "Palindr?me" <me9@privacy.net> wrote in message
news:12eclrqb7f7jpf9@corp.supernews.com...
> john wrote:
> Did you clean out (undercut) the commutator gaps? If not, you could have a
> build up of cu/c dust which is bridging between windings.
>
> Did you shape the brush faces to the commutator diameter?
>
> Yes there are different brush compositions.
>
> You need to strip it down, see how much the brushes have worn, whether
> they have worn equally and evenly, how much dust is present, whether the
> gaps have been bridged.
>
> What could have happened is that the new brushes only had a small point of
> contact when first fitted, that part of the brushes have worn down quickly
> and the brush is now spanning adjacent windings. So a bit of brush tip
> profiling may be called for.
>
> Or it could be that there was a build up of copper and carbon in the gaps.
> Because the composition is too soft..
>
> Or, if the brushes don't conform to the comm profile, you may have bridged
> the gap with extruded copper - extruded by the very high brush pressure on
> the small area that the brushes act on..Especially, if the brush
> composition is too hard.
>
>
> So, all sorts of possibilities...
>
> --
> Sue
>
>
"I used 600 grit emery cloth "
Real emery cloth residue is conductive, if used it has to be cleaned out
thoroughly.
--
They can have my command prompt when they pry it from my cold dead fingers.
| |
|
|
> Did you clean out (undercut) the commutator gaps? If not, you could have
> a build up of cu/c dust which is bridging between windings.
>
> Did you shape the brush faces to the commutator diameter?
>
> Yes there are different brush compositions.
>
> You need to strip it down, see how much the brushes have worn, whether
> they have worn equally and evenly, how much dust is present, whether
> the gaps have been bridged.
>
> What could have happened is that the new brushes only had a small point
> of contact when first fitted, that part of the brushes have worn down
> quickly and the brush is now spanning adjacent windings. So a bit of
> brush tip profiling may be called for.
>
> Or it could be that there was a build up of copper and carbon in the
> gaps. Because the composition is too soft..
>
> Or, if the brushes don't conform to the comm profile, you may have
> bridged the gap with extruded copper - extruded by the very high brush
> pressure on the small area that the brushes act on..Especially, if the
> brush composition is too hard.
>
>
> So, all sorts of possibilities...
>
> --
> Sue
Sue,
Thanks for your quick reply. I did not undercut the commutator contacts
but I did visually check for copper smears/dust as a result of my
cleaning/polishing. I didn't see anything but I ran the edge of my
fingernail through each slot. Perhaps I should have concentrated more
on that area (i.e. used a magnifiying glass and a toothbrush).
I did not check the match of the new brush surface to the commutator
but the new brushes did have a concave contact surface and were the
same thickness (1/4 ") as the old brushes.
It was of course hard to tell if the brush composition is to hard or
soft. Maybe a teardown will help answer that question (boy, where is
that CSI lab when you need it).
One thing I didn't mention in my original post was that I'm not
entirely sure, but it may be that just one of the brushes was sparking
when I first fired it up. If that was the case (I just don't remember),
is it possible that there is a problem with the windings, either
armature or field?
John
| |
|
| George,
Thanks for your reply. I wasn't really emery cloth, it was silicon
carbide wet/dry sandpaper. I just used "emery cloth" as a generic term.
Nevertheless you are right, I probably should have done a more thorough
job of cleaning.
I'm going to break it down again and see how much damage is present. If
it still looks repairable, I will be sure to thoroughly clean it to
eliminate any residue.
After I wrote my post, I started wondering if perhaps the brushes
weren't the only problem. Is it feasible that the windings are damaged?
The mower was not bogging down when it died and I did not detect any
burned wire smell when I first took it apart, but maybe the "damage" is
subtle.
John
>
> "I used 600 grit emery cloth "
>
> Real emery cloth residue is conductive, if used it has to be cleaned out
> thoroughly.
>
> --
> They can have my command prompt when they pry it from my cold dead fingers.
| |
| daestrom 2006-08-19, 1:25 pm |
|
"john" <jensenj6@comcast.net> wrote in message
news:1155943960.968333.159070@b28g2000cwb.googlegroups.com...
> People,
> I'm not sure if this is the best group for my query but I'lll give it a
> try. I have an electric lawn mower that was made by Sears way back in
> 1975. The nameplate rating is 120VAC - 10A. No other information is
> available. The mower has an electric brake feature when shut off and
> the mower has worked fine for many years (obviously). Yesterday while
> mowing, the motor started to slow down and then died. I suspected worn
> out brushes. I took the motor apart and indeed one of the brush wires
> had disconnected causing the brush spring to carry the current until it
> lost it compression due to extreme heat.
>
> I didn't bother even checking Sear for replacement brushes as they
> stopped supporting this tool many years ago. So I figured, how hard can
> it be to find some replacement brushes. I went to my local ACE hardware
> and found a set of brushes that were the correct size including the
> brush wire guage.
>
> I noticed that the motor commutator was very black probably from brush
> dust and heat. I pulled the armature and chucked it in my drill press
> so I could clean up the commutator. I used 600 grit emery cloth on a
> flat stick followed by 1500 grit and then the paper side of the cloth
> to clean up and polish the commutator. I put the new brushes in and
> fired it up. It seemed to run fine although there was sparking from the
> brushes. I proceeded to mow the lawn but after about 5 minutes of
> mowing the motor started to bog down. Now it runs very rough with lots
> of sparking from the brushes.
>
> Questions:
> 1. Was there a basic flaw in my repair process?
Some oversights....
> 2. If so, what should I have done differently?
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.
> 3. Is it possible the brushes are not of the right type even though
> they are the correct size (e.g. not the correct composition of carbon
> for that motor type)?
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.
> 4, What do you suggest I try next?
>
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.
| |
|
| George,
An update. See my latest response to deastrom's post. Thanks again for
your input.
John
john wrote:[color=darkred]
> George,
> Thanks for your reply. I wasn't really emery cloth, it was silicon
> carbide wet/dry sandpaper. I just used "emery cloth" as a generic term.
> Nevertheless you are right, I probably should have done a more thorough
> job of cleaning.
>
> I'm going to break it down again and see how much damage is present. If
> it still looks repairable, I will be sure to thoroughly clean it to
> eliminate any residue.
>
> After I wrote my post, I started wondering if perhaps the brushes
> weren't the only problem. Is it feasible that the windings are damaged?
> The mower was not bogging down when it died and I did not detect any
> burned wire smell when I first took it apart, but maybe the "damage" is
> subtle.
>
> John
| |
|
| Sue,
An update. See my latest responst to deastrom. Thanks again for your
input.
John
Palindr=E2=98=BBme wrote:
> john wrote:
> Did you clean out (undercut) the commutator gaps? If not, you could have
> a build up of cu/c dust which is bridging between windings.
>
> Did you shape the brush faces to the commutator diameter?
>
> Yes there are different brush compositions.
>
> You need to strip it down, see how much the brushes have worn, whether
> they have worn equally and evenly, how much dust is present, whether
> the gaps have been bridged.
>
> What could have happened is that the new brushes only had a small point
> of contact when first fitted, that part of the brushes have worn down
> quickly and the brush is now spanning adjacent windings. So a bit of
> brush tip profiling may be called for.
>
> Or it could be that there was a build up of copper and carbon in the
> gaps. Because the composition is too soft..
>
> Or, if the brushes don't conform to the comm profile, you may have
> bridged the gap with extruded copper - extruded by the very high brush
> pressure on the small area that the brushes act on..Especially, if the
> brush composition is too hard.
>=20
>=20
> So, all sorts of possibilities...
>=20
> --=20
> Sue
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