12 Volt PMDC motor armature

Hi Guys hope you can help me?

My mother in law has to be taken for walks in a wheel chair, getting old now and after struggling pushing her around the hills of Sheffield we came across a POWERSTROLL at the right price and it transformed the walks. One day SHMBO said it's not working. Checking it over found one of the brushes had a burnt out spring!!! I replaced the brush with one from my spares for the minilathe and although the motor spins it doesn't have the same torque it used to have. I stripped down the motor to check it over and noted that if I hold the DMM probe on one segment and the put the other probe on all the others in turn the resistance value is about the same about 0.4 ohm. This doesn't seem right to me. The commutator was slightly discoloured so I cleaned it with emery paper (sorry Kwill) pulled a knife blade through the slots but no debris came out. The brush that had burnt out was a different type to the one that looked ok so I think someone has been there before.

So three questions:

Is the armature pooped?

Is there a differance between mains voltage and 12 volt brushes?

How does the brush pick up the DC?

Hope you can help, Mrs Sims would be very pleased!

Cheers Michael.

You have probably got to get 10A through the brush to the armature so it needs to be in good contact. This means a strong enough spring and the brush shaped to the curve of the commutator. Some brushes have a seperate copper braid inside the spring if they are high current ones. You might find some from an old alternator that fit.

The windings are all connected together, it is rather difficult to get meaningful results froma resistance measurement but if it is running it is probably ok.

Brushes are made of different mixes for different uses. A low power mains motor can use a mainly carbon brush, a low voltage high current motor may have 90% copper or silver content so that it has a much lower resistance than an all-carbon brush. So fitting a mains brush to a low voltage motor is not a great idea.

The brush is intended to contact the commutator across the whole of the brush end, if the contact is only along a narrow area and a lot of current is sent through the brush then rapid wear will occurr, arcs to the copper commutator will cause years of wear in seconds. So brushes are shaped before the motor is heavily loaded. Please NEVER emery! the emery can cause extensive damage to the commutator, brushes and bearings. Garnet paper around 220 grit with the paper against the commutator will do a good job of shaping the brushes. The commutator is best skimmed with a lathe, the motor shaft often having countersinks for turning between centers.

Billy.

Michael, one important thing is that the new brush should be the cprrect fit in the brush holder, if the brush is a bit large, it's easy to bring it down to size with a file, or even a bit of sandpaper (just messy). A good place to get brush material of a suitable grade is an old starter motor, or you might buy a pair at a car spares shop. Its best to change both at the same time. All this if you can't get brushes from the place that sells/serviced the Powerstroll, most places around here, have service depots so you can get your machine serviced regulary, ie check tires, battery, oil in gearbox/ diff, and the brushes in the motor. Ian S C

I'm sorry but in case like thiis why mess about trying to do a " make do and mend". Have you been in touch with the makers of the unit - they're in Brighouse (http://www.medicaretechnology.com/), be much better to get the right bits and if necessary get the unit checked over by them.

Keith

Hi Guys thanks for the replies so far it looks like it may be worth trying to source the right type of brush because I would say the motor works barring full power. The resistance across the brushes was about 2 ohms a 12 volt fan motor for a car I have measured about 0.5 ohm that kind of ohms on the STROLLER motor would be heading in the the right direction for 100Watts which is the power rating of the motor.

Keith I did ring the company you mentiontioned but the don't deal with the public, they found me agent in Sheffield but he didn't answer his phone. What I am trying to do is get this working as quick as possible so trying different avenues even got a bid on a unit on ebay.

Cheers Michael

Michael

PM me the details of the unit with any serial numbers etc. According to the makers website I've got at least 4 stockists of these units close by - one's even in walking distance. From my dealings with them in the past they've been very helpful. If I have the details I'll see if they can come up with the goods.

Keith

I think Keith's right for perhaps another reason: In the event of an accident or failure causing injury, your MIL's (and your) legal position may be seriously compromised by "non-approved" repairs (whether or not they were causally related to the accident.

Years ago I did some freelance engineering for a company that made motorised wheel chairs. That business was one of the largest single sources of law-suits (this is in Canada - don't know about the UK). I didn't spend very long at it because I felt that, working freelance I was too personally exposed to the same thing.

Hi Keith I have sent you a PM.

Sid this motor is for a carer assist device, Mrs Sims is beyond taking responcibilty for herself! If you can find her a dog / cat to stroke or some books she is happy. The brush repair is pretty benign but the UK is going down the route you describe.

Cheers Michael.

Michael

PM sent

Keith

Hi Guys, thanks for all your input. I bought some alternator brushes from JCR in London because they specified two dimentions that ment their brushes would go down the whole, I suspected they would be too long but it was easy to cut them down with a junior hacksaw. I tried to get the curve in the end but failed miserbly.

The motor kicks like a mule now, even pulled a wheely but not with Mrs Sims onboard

This has highlighted another fault in that I think the power should come in gradually but it is all or nothing. The pot control looks special. Going to leave it as it is for now, try and teach SWMBO to get the chair moving before applying the power else I can see the gearbox going next.

Once again thanks for all your input.

Michael.

Michael

Creating the correct profile on the brush is usually done by trapping a longish strip of sandpaper/wet&dry between the brush and commutator (abrasive side out) and alternately pulling the paper strip ends. Easier to do than describe, clean the carbon dust off before running.

I would not advise using the chair without a correctly functioning speed controller. straight On/Off sounds positively scary!

Ian

Take care Michael,

This is the sort of thing that can happen if you are careless about speed control on mobility equipment. A sober warning for us all.