Classic cars forum & vehicle restoration.

peppiB · Posted: Tue Jul 21, 2015 9:34 pm Post subject: Electric motor help needed

Sorry this is not a car, but it is transport - my wheelchair!

I had to install a new brush holder on the nearside motor of my wheelchair. Motor apart, new holder in place, reassemble. Pushed the control lever forward and the chair shot backwards. Oh dear, or words to that effect, I have put the magnetic sleeve on the wrong way round. Motor apart again ... and the magnetic sleeve will only fit 1 way - the way I had it. No other wires were touched, nothing else moved. What could have caused the reverse in polarity of the motor?? It has been apart previously for cleaning and there have been no problems.

Problem urgent as I rely on wheelchair to get me around and help me look after the girl/woman in my care

Yes, there is a local wheelchair engineer, but £75 call out and £100 an hour labour is just a tad expensive for this pensioner

DM · Joined: 21 Dec 2008 Posts: 212 Location: North Cornwall

If the magnet will only fit in one position, you must have reversed the two brush connections.

If the brushs holders are mounted on the end plate of the motor you may have re-fitted the end plate 180 deg out of position.

Or is it possible that the wires from the brushes are long enough for you to have reversed the position of the brushes ?

peppiB · Posted: Tue Jul 21, 2015 10:19 pm Post subject:

Hi DM

Only 1 brush holder was changed, and the wire was just transdfered to the new unit. The other brush holder/wire wasn't touched or moved. It is the inner (ie the one pointing towards the middle of the chair) brush and the end plate was refitted accordingly

Got me puzzled .... and grounded!!

Just thinking - would fitting the magnetic cover 180 degrees out have the same effect?

DM · Joined: 21 Dec 2008 Posts: 212 Location: North Cornwall

Yes, if you fit the field magnets 180 deg out the motor will run backwards.

Peter_L · Posted: Wed Jul 22, 2015 12:21 am Post subject:

Hello Peppi. I hope you can get the problem resolved.

I have become an avid user of my digital camera for a wide range of repairs, restorations and renovations. At zero cost per image, (after camera cost) it is very reassuring to have an image of what it looked like before it became pieces.

Regards

Peter

peppiB · Posted: Wed Jul 22, 2015 8:44 am Post subject:

Thanks Peter - one of these years I will get wise enough to mark/photograph things! Spent 5 decades earning a crust taking photos but rarely touch a camera now, only the odd occasion I manage to get out along the river to photograph feathered birds (although some of the hen parties who inhabit this area on a weekend fit into that category)

DM - I will try rotating the magnets and see what happens. Presumably the alignment of the magnets determines whether the motor is a left hand or right hand unit.

Her ladyship medicated and fed so as soon as I am up to it, back to the challenge of the motor

DM · Joined: 21 Dec 2008 Posts: 212 Location: North Cornwall

Fitting the magnets 180 deg out swaps the north and south poles so the motor will run the opposite direction.

There are three ways of reversing a fixed magnet dc motor -

Reverse the supply.
Reverse the connections to the brushes.
Reverse the field magnets (on a 2 pole motor which is what you will have).

You have managed to find the third option by accident.

peppiB · Posted: Wed Jul 22, 2015 2:56 pm Post subject:

Hi DM

Have found another way to reverse the polarity. If the brushes aren't exactly on the same alignment then the motor will run backwards! Managed to shear one of the long retaining bolts, so called in at a small motor repair place and he guided me through the charcteristics of the things. Came home and checked and the brush I had been working on was ever so slightly off square. Sadly he didn't have a bolt so am still screwed! Can get threaded rod but have to buy 10 metres and I only need 17 cm!

Bitumen Boy · Posted: Wed Jul 22, 2015 5:25 pm Post subject:

peppiB · Posted: Wed Jul 22, 2015 7:34 pm Post subject:

MikeEdwards · Joined: 25 May 2011 Posts: 2470 Location: South Cheshire

10m sounds like it would present more problems than it solves - you'd need a pretty big van to take it home.

Bitumen Boy · Posted: Thu Jul 23, 2015 10:49 am Post subject:

Peter_L · Posted: Thu Jul 23, 2015 1:05 pm Post subject:

Don't the likes of B&Q or Wickes, sell threaded rod by the metre ?

Here, all the box DIY stores carry such things. a little pricey but cheaper than driving 50Km. Then we have "Princess Auto", a big boy's paradise.

http://www.princessauto.com/en/, check out their car lifts and calculate the price now that there are 2$ Cnd to a UK £1

peppiB · Posted: Thu Jul 23, 2015 7:25 pm Post subject:

Checked both Wickes and B&Q here and the smallest threaded rod they have is M4 - too big. Sad

Don't want to drill out the threaded part of the motor and re - tap

goneps · Posted: Thu Jul 23, 2015 11:20 pm Post subject:

Threaded rod should be available down to M3. Standard lengths are 1m and 2m; anything longer than that is generally for industrial applications. Try engineering fastener suppliers rather than retail outlets.

Richard