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Carbon Brushes

03/23/2017 11:07 AM

Hi can anyone tell me what the affect would be if you had slightly different sized carbon brushes in a dc motor. The motor is from a mobility scooter. I would like to know if it would effect the power.

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#1

Re: carbon brushes

03/23/2017 11:17 AM
  • If the brushes are too small, the heat dissipated in the brush can be expected to rise above that dissipated by the correct brush, thereby shortening the equipment life until some failure indicates the folly of this course of action.
  • If the brushes are too large, then they may not actually come into contact with the commutator once fitted, again indicating the folly of this course of action.

The best source of reference would be the motor manufacturer; its telephone number is easily available following a brief internet search.

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#2

Re: carbon brushes

03/23/2017 11:38 AM

Carbon brushes can be filed down, with care (they are brittle) until they fit and move properly.

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#3

Re: carbon brushes

03/23/2017 11:53 AM

Nope. Without knowing what the size of the old and new brushes are I cannot tell you anything. It would also be helpful to know the dimensions of the contact area, too. Naturally the new brush should show less wear than the old brush.

The key problem here is the nebulous, imprecise modifier of "slightly different". Slightly different might be a trivial amount for one design but catastrophic for another design.

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#6
In reply to #3

Re: carbon brushes

03/23/2017 12:37 PM

When I took the brushes out on a recent service one was 10x6x16 and the other was 11x7x16. The motor is a Pihsiang M4-9MNF made in Japan. The suppliers are a comany in UK called Surisemedical, I asked them for the right size but only offered to sell me the correct ones for £50.00. Way to expensive. Can't find out the right size for the model. It is running ok. Model Sterling Diamond. Just worried it might do harm to the motor over time.

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#10
In reply to #6

Re: carbon brushes

03/23/2017 2:05 PM

This always puzzles me. If you have no intention of taking our advice, why did you bother to ask?

For your sake I hope this medical device is not being used in a litigious society. If this mobility scooter damages anything or anybody after you've knowingly used unapproved parts then...

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#11
In reply to #10

Re: carbon brushes

03/23/2017 3:40 PM

What advice ?? Are you saying I should pay £50.00 for 2 carbon brushes that I can get for under £5.00.

As regards to hurting people with the scooter I fail to understand what on earth that's got to do with my simple question about carbon brushes. Can do wheelies though.

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#13
In reply to #11

Re: carbon brushes

03/23/2017 6:39 PM

as you have seen, you can only file brushes of the same general composition to fit smaller brush holders. The motor makers have gone to great lengths to secure and end market for a consumable item.

For small fractional HP motors, you can carefully file or sand larger brushes of the same composition to fit the smaller housing. Those with embedded wires are more difficult. The other aspect is the diameter of the commutator, where you need to make a sanding mandrel of the same diameter to make them contact over the area of contact. A short period of operation will fit them quite well. As small scooter motor should be amenable to this fitting process. I have repaired a few dozen small motors this way with success.

In the case of large motors, say 2000HP with very high brush currents, as per the example of 7 repeated failures, the brush will be specially made and matched to the motor, so I would not suggest careless replacement based on price. I would have looked more closely with the first or second failure.

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#16
In reply to #10

Re: carbon brushes

03/24/2017 8:03 AM

The motor is from a mobility scooter. He did not say it is still in the scooter. It may be a can opener motor now.

1. Of course the UK is part of America, we sue each other silly just the same as you guys do. The lawyers taking the lions share of course.

Mr, Rushton

2. If you have the correct shore hardness of the brushes, they can be filed/sanded down to fit the holder and they will work just fine.

3.http://www.carbonbrush.co.uk/

4. http://www.bargainbrushes.com/

£50.00 is total rip off. I guess the store owner wore big glasses and saw you coming.

Next time try your local Halfords.

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#17
In reply to #16

Re: carbon brushes

03/24/2017 10:21 AM

Hi there, I tried both your links but to no avail. I just need to know what is the correct size to fit. This information obviously is not available. Every website offers replacement brushes for my model but all of them include the dreaded phrase SHOULD fit. And all vary in size. They do not provide any spec on the product, so they could be fake rubbish. Ive emailed Pihsiang in China but I'm not expecting a reply about two carbon brushes from a huge global company. This problem is unsolvable.

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#18
In reply to #17

Re: carbon brushes

03/24/2017 11:33 AM

Do you not have the original brushes you removed for profile measurement with a digital caliper? Bad on you for that, if so.

If not, what about the brush holders on the motor, surely those are still in place. You need to figure out what tolerance exists between the holder and the actual brushes to be placed in service, if not having the originals. As to length, consider the back springs that apply a more or less steady pressure of the brushes to the commutator, and do not have them uncompressed, or compressed to the point the spirals are all mashing together.

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#23
In reply to #17

Re: carbon brushes

03/25/2017 5:51 AM

Editing this as I posted before reading your #20. Just out of curiosity, how much did they cost?

You said in #6 the motor was from Japan.

I would use the largest brush that will go in the brush holder but still slide freely. I'd assume that's the 11 x 7. The holders aren't different size by any chance? Unlikely as they carry same current.

But if it's worked OK in the past, and you imply it has, sounds like either size would do. If they're 16mm long do they need replacing? Why not just refit them.

If a brush is much too small so it's loose there might be a problem, and shorter life, but in any case it won't affect the motor power.

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#19
In reply to #6

Re: carbon brushes

03/24/2017 11:59 AM

FYI, Pihsiang is Chinese, not Japanese.

Big difference.

It shouldn't be any surprise that Chinese try to sell you defective s-hit at overinflated prices.

They even claim (Patented).

Though what exactly is patented is secret of course.

http://www.pihsiang.com.tw/GM_PDF/TE-888SL(P)(88-025-GM01)-EDITION=2(2012-08-13).pdf

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#4

Re: carbon brushes

03/23/2017 12:19 PM

Brushes are normally made to fit into brush holders. If the brush is too small, it won't be held properly, and obviously, if too big it won't even fit.

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#5

Re: carbon brushes

03/23/2017 12:36 PM

It probably won't make any difference, as long as it fits and is the same material and has the same current rating...Generally speaking brushes are underrated...

http://www.argointl.com/wp-content/uploads/2014/03/necp-How_to_Select_Brushes_for_Motors_and_Generators2.pdf

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#7

Re: Carbon Brushes

03/23/2017 1:34 PM

I can tell you flat out, the wrong brushes will not work correctly. Too small and the brush holder will not function correctly. Excessive wear to the commuter and to the brushes. Too large and they will not fit into the brush holder, and/or will not advance in tension with the commuter ring. Motor will not correctly operate, and could bring about another hazard, depending on DC voltage.

Take the effort and time to always install the brush number recommended for that motor. They are not all that expensive, compared to the sideways results of mis-applied brushes.

If you had two brushes installed such that one is smaller (say thinner), then there can be issues with resistive contact that reduces power flow (current flow).

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#8

Re: Carbon Brushes

03/23/2017 1:42 PM

A brush that is loose in the brush box will move about causing problems with commutation.

The brush has to be the same grade otherwise damage to the commutator with burning or feathering. Dust is also a problem, the wrong grade can create excessive carbon deposits leading to flashover. I found out the hard way by blowing up a 2000HP motor seven times before finding out the company buyers had found a cheaper source.

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#9
In reply to #8

Re: Carbon Brushes

03/23/2017 1:52 PM

It is always just lovely to go through that evolution on a so-called large megawatt generator exciter.

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#12

Re: Carbon Brushes

03/23/2017 6:13 PM

This might be considered medical equipment. Is it for you or an immediate family member? If yes, you might try it. If no, don't even think about it, the potential for a lawsuit is far too high. The price is undoubtedly due to lawyer fees. Ever see those commercials for "bad drugs", bad "medical equipment"? That is how law firms are able to afford offices on 57th St NYC.

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#14

Re: Carbon Brushes

03/24/2017 1:11 AM

Your question is valid, but it also tells me you know little about DC motors and carbon brushes.

Yes you can file/reduce the overall size of the brush to fit, BUT you have to make sure its really good fit. This has ben mentioned several times by my learned CR4 friends, and they are right.

I wonder do you understand the importance of the brushes, their function and what effect they will have if

1. they are hard or soft enough?

2. contain the right mix that is suitable?

3. can they be fashioned to fit?

Maybe you need to contact Morgan Advanced Materials to get an answer!

If you do however decide to "go it alone", do you know how to "bed in" carbon brushes?

The technical question and problems you will face by using "not fit for purpose" brushes far outweigh the electrical power problems in the short term.
But that's not to say your scooter will not face immediate power problems if you do install your "cheap" brushes, it might work fine for a while, who can tell..

In the long term, the results will be replacing the drive motor, replacing batteries or buying a new mobility scooter, as you would have FUBAR the one you have now!

So to select the right brushes.. try MorganAM&T

And while you're at it follow this LINK to give yourself some schooling on brushes.

hope you get your scooter going!

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#24
In reply to #14

Re: Carbon Brushes

03/25/2017 8:37 AM

Morganite are the idiots that supplied us with the wrong grade of brushes.

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#15

Re: Carbon Brushes

03/24/2017 4:23 AM

Brushes are most important part of electrical machine and due consideration should be given to replacement process. I hope you have complete technical information of original brush material from the motor supplier. First get confirmation from the present brush supplier that their supply would meet all technical requirements of the original brush material. Do not accept any deviation. Even mechanical properties would affect the motor performance. Harder brush would harm commutators where as softer material would result in frequent replacement.

As regards size, loose brush would jump and may result in sparking thereby spoiling commutator surface. A tighter brush would make bad contact after some operation time as springs would not be able to push it down.

A costly but right brush would go a long way in longer trouble free service.

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#20

Re: Carbon Brushes

03/24/2017 12:13 PM

Problem solved. Eventually I managed to find someone who actually knows the right size carbon brush for my motor. 6.5x11. That's two months of my life I won't get back. Thanks guys for all of your help.I have learned a lot.

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#21
In reply to #20

Re: Carbon Brushes

03/24/2017 1:44 PM

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#22
In reply to #20

Re: Carbon Brushes

03/24/2017 1:48 PM

repco metric carbon brushes

Here is a link (who knows if a match) to metric size brushes from REPCO, as usual, sorry I am a day late and a dollar short (actually it is your dime and your time).

Absolutely true, we don't get time back, just have to learn from it and move along.

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