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Home > Archive > Electrical Engineering > November 2006 > Excessive brush wear on DC motor.
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Excessive brush wear on DC motor.
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| Dawoodi Bohra 2006-11-20, 1:25 pm |
| Greetings!
We have a field excitation DC motor on our SAMP machine. We are
experiencing 1 brush on the DC motor to be wearing out a lot faster
than the other 3. (My understanding is that the brush that carries the
least amount of current gets worn out the fastest)
The specs of the motor are as follow.
500VDC , 49 full load Amps.
Supply: 460V 3 phase.
Running load 20Amps.
The brush contacts are jumped. i.e 1-3 and 2-4 are paired so that equal
current passes through each brush set.
The behavior of brush number 3 is unexplainable: wearing out almost 40%
faster (replaced one on Thursday and down 40% in 3 days approx 40 hrs
of machine runtime).
We have 3 DC motors that are all showing the same problem.
The commutator on 2 of those motors were replaced but we are still
experiencing the same characteristics.
We contacted SAMP (Italian manufacturer) and they have advised us to
buy their brushes($33 a pop) which is about 3 times more than what we
are currently paying. I don't think that their brushes will somehow
magically fix the problem we are experiencing.
The brushes that they use are from the same manufacturer (helwig) that
we are currently buying them from.
I am not able to figure out what the problem is:
I connected a microscope and the signal out of the drive is as clean as
it gets. No problems there.
Any insight would be greatly appreciated.
Best regards
| |
| mike.j.harvey@gmail.com 2006-11-20, 5:25 pm |
|
Is there much dust from the worn brush? DC brush wear is the result of
mechanical friction and electrical erosion. Friction produces carbon
dust; the result of electrical erosion is the vaporization of carbon
with little physical residue.
I am not able to figure out what the problem is: I connected a
microscope and the signal out of the drive is as clean as it gets. No
problems there.
What 'drive'? Please explain.
| |
| mike.j.harvey@gmail.com 2006-11-20, 5:25 pm |
| Dawoodi Bohra wrote:
>
> The specs of the motor are as follow.
> 500VDC , 49 full load Amps.
> Supply: 460V 3 phase.
You are supplying DC to this motor or AC?
| |
| Dawoodi Bohra 2006-11-20, 5:25 pm |
|
mike.j.harvey@gmail.com wrote:
> Dawoodi Bohra wrote:
>
>
> You are supplying DC to this motor or AC?
Supplying 460VAC.
| |
| mike.j.harvey@gmail.com 2006-11-20, 5:25 pm |
|
Dawoodi Bohra wrote:
> mike.j.harvey@gmail.com wrote:
>
> Supplying 460VAC.
Am I missing something here?
1. You have a DC motor, with commutator and brushes, rated at 500v DC,
2. You are feeding it with 460v AC
3. ????
I must be missing something. Has it got a built in rectifier?
| |
| Don Kelly 2006-11-20, 5:25 pm |
| "Dawoodi Bohra" <sahimsf@gmail.com> wrote in message
news:1164046779.359506.12600@f16g2000cwb.googlegroups.com...
> Greetings!
> We have a field excitation DC motor on our SAMP machine. We are
> experiencing 1 brush on the DC motor to be wearing out a lot faster
> than the other 3. (My understanding is that the brush that carries the
> least amount of current gets worn out the fastest)
>
> The specs of the motor are as follow.
> 500VDC , 49 full load Amps.
> Supply: 460V 3 phase.
>
> Running load 20Amps.
>
> The brush contacts are jumped. i.e 1-3 and 2-4 are paired so that equal
> current passes through each brush set.
> The behavior of brush number 3 is unexplainable: wearing out almost 40%
> faster (replaced one on Thursday and down 40% in 3 days approx 40 hrs
> of machine runtime).
> We have 3 DC motors that are all showing the same problem.
>
> The commutator on 2 of those motors were replaced but we are still
> experiencing the same characteristics.
>
> We contacted SAMP (Italian manufacturer) and they have advised us to
> buy their brushes($33 a pop) which is about 3 times more than what we
> are currently paying. I don't think that their brushes will somehow
> magically fix the problem we are experiencing.
> The brushes that they use are from the same manufacturer (helwig) that
> we are currently buying them from.
>
> I am not able to figure out what the problem is:
> I connected a microscope and the signal out of the drive is as clean as
> it gets. No problems there.
>
> Any insight would be greatly appreciated.
> Best regards
>
------
It appears that the problem is position related. Check the tension on the
brush holder spring - too tight or too loose could be a problem. Also
alignment or binding in the holder could be a problem. Does the brush slide
easily in the holder? Does it contact the commutator correctly? Do you
observe any arcing at the problem brush?
Have you had this problem with the original brushes?
There are differences between brushes- some are softer than others and there
may be minor dimensional differences- particularly with "no-name" brushes.
--
Don Kelly dhky@shawcross.ca
remove the X to answer
----------------------------
| |
| Don Kelly 2006-11-20, 8:25 pm |
| "Dawoodi Bohra" <sahimsf@gmail.com> wrote in message
news:1164057093.132688.34710@k70g2000cwa.googlegroups.com...
>
> mike.j.harvey@gmail.com wrote:
>
> Supplying 460VAC.
HUH!
I assume that you have some sort of rectifier in between so that the motor
is actually getting DC and that you have limited the voltage to be sure that
the motor rating is not exceeded.
I suspect that this is what Mike wants to know.
--
Don Kelly dhky@shawcross.ca
remove the X to answer
----------------------------
>
| |
| Dawoodi Bohra 2006-11-21, 9:25 am |
|
Don Kelly wrote:[color=darkred]
> "Dawoodi Bohra" <sahimsf@gmail.com> wrote in message
> news:1164057093.132688.34710@k70g2000cwa.googlegroups.com...
> HUH!
> I assume that you have some sort of rectifier in between so that the motor
> is actually getting DC and that you have limited the voltage to be sure that
> the motor rating is not exceeded.
>
> I suspect that this is what Mike wants to know.
> --
>
> Don Kelly dhky@shawcross.ca
> remove the X to answer
> ----------------------------
>
Sorry for the late reply.
The machine has a digital DC drive that converts 460VAc to DC.
We are currently running the machine at 75% load and therefore pulling
345VDC.
We have replaced the springs on the brushes and also confirmed using
the Oscope the waveform at the armature. (clean();)
Also connected a AMP-probe to find out that the brush set that is
wearing faster is pulling 10amps while the other is pulling 9 amps.
I don't know whats going on.
| |
| daestrom 2006-11-21, 5:25 pm |
|
"Dawoodi Bohra" <sahimsf@gmail.com> wrote in message
news:1164046779.359506.12600@f16g2000cwb.googlegroups.com...
> Greetings!
> We have a field excitation DC motor on our SAMP machine. We are
> experiencing 1 brush on the DC motor to be wearing out a lot faster
> than the other 3. (My understanding is that the brush that carries the
> least amount of current gets worn out the fastest)
>
> The specs of the motor are as follow.
> 500VDC , 49 full load Amps.
> Supply: 460V 3 phase.
>
> Running load 20Amps.
>
> The brush contacts are jumped. i.e 1-3 and 2-4 are paired so that equal
> current passes through each brush set.
> The behavior of brush number 3 is unexplainable: wearing out almost 40%
> faster (replaced one on Thursday and down 40% in 3 days approx 40 hrs
> of machine runtime).
> We have 3 DC motors that are all showing the same problem.
>
> The commutator on 2 of those motors were replaced but we are still
> experiencing the same characteristics.
>
> We contacted SAMP (Italian manufacturer) and they have advised us to
> buy their brushes($33 a pop) which is about 3 times more than what we
> are currently paying. I don't think that their brushes will somehow
> magically fix the problem we are experiencing.
> The brushes that they use are from the same manufacturer (helwig) that
> we are currently buying them from.
>
> I am not able to figure out what the problem is:
> I connected a microscope and the signal out of the drive is as clean as
> it gets. No problems there.
>
Ah, brush wear questions....
Rapid brush wear in general can be caused by a lot of things (off-neutral,
atmosphere contaminants, light load, poor commutator surface, the list is
long...).
But you only have trouble with one position. Has any other maintenance been
done? (for example, remove/replace brush-holders, disconnect/reconnect any
wiring).
What's the appearance of the brush face when you remove it? You can tell a
lot from this. Parallel grooves imply rough commutator surface.
Burning/pitting on leading or trailing edge imply commutation problems
(spacing, interpole). Chip/flaking imply 'chatter' vibration (bad springs,
high-mica, un-even commutator). Two perfectly polished surfaces, but at
different angles indicate the brush is cocking at different angles in the
holder.
Perfectly smooth, polished face but rapid wear mean 1) no/low current
density 2) atmospheric contaminant (unlikely to affect just one) 3)wrong
grade.
Some thoughts about *one* brush wearing differently. Check the spacing of
all brushholders. We used to do this by wrapping cash-register tape around
commutator, set down all the brushes, draw a line on the paper along the
leading edge of each brush (pay *attention* to rotation and leading edge),
remove paper and measure the spacing between marks (you have to also mark
where the tape overlaps). If it's off even a little bit, this can cause
some brushes to spark/arc much more than others.
Spring tension off. If the one 'bad guy' is in a difficult place, your
workman may have 'cheated' a bit and not checked this. Some types of
holders have 'constant pressure' springs, while others have adjustable
tension.
Not the same current flow. Wiring of the brush 'pigtail' wrong. Check to
see where it goes, and is it securely connected. With the machine shutdown,
use an ohmmeter and contact the back side of the brush (not the holder, not
the pigtail, not the bus bar, the *brush*) and measure back to the common
bus-bar. Compare to readings for other brushes.
An interpole is bad. The commutation of armature segments as they pass
under the brush is aided by 'commutating' or 'interpoles' between the main
magnetic poles of the machine. If one is not carrying current, the brush
associated with it will 'arc/sparc' like crazy. That would burn through the
brush pretty darn fast. But it would have to be some wiring work that was
done. These poles are normally all connected in series, so for *one* to not
carry the same current as others means 1) maintenance reconnected it wrong
2) the pole winding developed a short. Before you jump on this one (it can
be an expensive repair, and if it's not the problem you waste a lot of
money/downtime), check the brush face for burning/pitting (bad commutation
will leave this tell-tale sign). Simple resistance measurements of the
interpole are not always conclusive since they are very low resistance
normally.
Any inspection windows? Can you see any excessive sparking at this holder
when running?
Are the replacements the right grade/hardness?
Oh, check the gap between holder and commutator. Too large and the brush
will cock forward/back in the holder causing chipping/cracking.
Hope this helps...
daestrom
| |
| Dawoodi Bohra 2006-11-21, 5:25 pm |
|
daestrom wrote:
> "Dawoodi Bohra" <sahimsf@gmail.com> wrote in message
> news:1164046779.359506.12600@f16g2000cwb.googlegroups.com...
>
> Ah, brush wear questions....
>
> Rapid brush wear in general can be caused by a lot of things (off-neutral,
> atmosphere contaminants, light load, poor commutator surface, the list is
> long...).
>
> But you only have trouble with one position. Has any other maintenance been
> done? (for example, remove/replace brush-holders, disconnect/reconnect any
> wiring).
>
> What's the appearance of the brush face when you remove it? You can tell a
> lot from this. Parallel grooves imply rough commutator surface.
> Burning/pitting on leading or trailing edge imply commutation problems
> (spacing, interpole). Chip/flaking imply 'chatter' vibration (bad springs,
> high-mica, un-even commutator). Two perfectly polished surfaces, but at
> different angles indicate the brush is cocking at different angles in the
> holder.
>
> Perfectly smooth, polished face but rapid wear mean 1) no/low current
> density 2) atmospheric contaminant (unlikely to affect just one) 3)wrong
> grade.
>
> Some thoughts about *one* brush wearing differently. Check the spacing of
> all brushholders. We used to do this by wrapping cash-register tape around
> commutator, set down all the brushes, draw a line on the paper along the
> leading edge of each brush (pay *attention* to rotation and leading edge),
> remove paper and measure the spacing between marks (you have to also mark
> where the tape overlaps). If it's off even a little bit, this can cause
> some brushes to spark/arc much more than others.
>
> Spring tension off. If the one 'bad guy' is in a difficult place, your
> workman may have 'cheated' a bit and not checked this. Some types of
> holders have 'constant pressure' springs, while others have adjustable
> tension.
>
> Not the same current flow. Wiring of the brush 'pigtail' wrong. Check to
> see where it goes, and is it securely connected. With the machine shutdown,
> use an ohmmeter and contact the back side of the brush (not the holder, not
> the pigtail, not the bus bar, the *brush*) and measure back to the common
> bus-bar. Compare to readings for other brushes.
>
> An interpole is bad. The commutation of armature segments as they pass
> under the brush is aided by 'commutating' or 'interpoles' between the main
> magnetic poles of the machine. If one is not carrying current, the brush
> associated with it will 'arc/sparc' like crazy. That would burn through the
> brush pretty darn fast. But it would have to be some wiring work that was
> done. These poles are normally all connected in series, so for *one* to not
> carry the same current as others means 1) maintenance reconnected it wrong
> 2) the pole winding developed a short. Before you jump on this one (it can
> be an expensive repair, and if it's not the problem you waste a lot of
> money/downtime), check the brush face for burning/pitting (bad commutation
> will leave this tell-tale sign). Simple resistance measurements of the
> interpole are not always conclusive since they are very low resistance
> normally.
>
> Any inspection windows? Can you see any excessive sparking at this holder
> when running?
>
> Are the replacements the right grade/hardness?
>
> Oh, check the gap between holder and commutator. Too large and the brush
> will cock forward/back in the holder causing chipping/cracking.
>
> Hope this helps...
>
> daestrom
Thanks a lot for the great insight.
I will keep you posted on my findings.
Best regards
S.
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