View single post by Keinokuorma
 Posted: Thu Jul 27th, 2006 01:01 pm
PM Quote Reply Full Topic
Keinokuorma



Joined: Mon Jun 26th, 2006
Location: Finland
Posts: 1248
Status: 
Offline
Every now and then it happens that a machine that relies on an electric motor, will cease to function properly. When everything else has been ruled out, you may find yourself in the conclusion that there's something wrong with the motor. While dryers, many top-loaded washers, dishwashers and many other types of appliances generally employ an induction motor, which is a quite durable and failsafe design, most of the horizontal axle washers, vacuum cleaners etc. that require a variety of different speeds, employ a "brushed" motor.

While the induction motor will produce rotor current through induction, and require no other mechanically wearing parts than the bearings, it will only function with AC, and it is limited to a narrow selection of different speeds without complicated circuitry. Here comes in the brushed design: It can in most cases be operated with DC as well, it can run under a wide voltage range and at various speeds. The drawback in this design is, that the magnetic flux in the rotor must be kept at a steady angle compared to the stator flux. This is done by conducting current into the rotor through two brushes and a commutator. The commutator will therefore wear the brushes gradually, until they are out.

There is a solution to the problem: New brushes. I will try my best to explain the basic steps of changing them and rendering the motor quite like new.

If you just can't find the original brushes with reasonable price, and especially if warranty is out, you can probably find something fit to substitute the original brushes.  There are various types of brushes, like massive copper brushes in some car starter motors, and brass leaf/bristle brushes in some small motors, but mostly you will run into two pieces of black carbon-based substance. In the case of the former two brush types you are best off acquiring genuine parts.

Preparing to change the brushes, examine your need to detach the motor from the appliance. If you can detach it, it will usually be easier to work with it. If you find it easy enough without detaching, don't detach. Always ponder the ease factors of detaching and remounting, versus working on the motor while it is mounted. Sometimes to remove the motor, you may need to take apart an entirely functional assembly - if you can access the brushes without doing that, consider leaving the motor in place for the job.

Examine the brush holder assembly: How it is mounted, and are you able to remove the brushes while it is in place, etc. Check your chances to acquire the whole assembly, and compare the price to original separate brushes, and possible substitutes. You may notice that original parts can be quite expensive, and just about the same amount of work to install.

When you have detached the brush holders form the motor, see how far the brushes extrude from the holders. On a generic washing machine motor, less than 1/2 inch is often barely enough... 3/4 inch or more should be still enough to run for a good while, and new brushes often extrude some 1 to 1-1/2 inches. Some small motors use accordingly small brushes, and as short as 2mm may be still enough, and the new brush might extrude only 5mm or so. Sometimes when the brushes are on the fringe of wearing out, low speeds may still work somewhat, but high speeds can give big trouble. Also you might have noticed that in the evening everything worked nice, and in the morning, no go, as the brushes have cooled down and shrunk a tiny wee bit, just enough to make no more contact.

Sometimes you may also notice that one brush is right out, and the other has a good 3/4 inch to go... some people consider this to be because the motor spins in one direction more than the other, this might be true, but I take it for uneven srping pressure or material hardness. In this case, if the other brush head looks good and it moves freely, you could consider only changing the worn out brush. However, you may need to come back shortly.

If the commutator has been roughed out by sparking from the worn brush, you will need to smoothen the surface, or the new brushes will be worn out soon. The easiest way to do a good job about this, is to find a way to attach the motor spindle to a drill, insert a piece of fine sanding paper between the commutator and old brush, and run the drill in short intervals until the commutator looks smooth enough. Sometimes you will need to detach the motor to attach it to the drill. Or, you could use this girinding stone: http://www.repairclinic.com/referral.asp?R=154&N=1063578 especially if you order brushes from RepairClinic at the same time. In an Asko service manual it says to rotate the motor manually when grinding the commutator with the stone.

If you decide to rebuild the old holder assembly: Remove and examine the brushes, and pay attention on how they are mounted. Whether the original brushes are flattish or square-looking at the end, measure both thickness and width, and you can probably get brushes that match those measures. If you can get brushes with some extra width or thickness, you can grind some off. You need a flat surface where you can mount a suitable stopper that is lower than the intended brush thickness. Put the brush head against the stopper, and grind in steady length-wise motions with a file. Try to apply pressure evenly on the head and butt ends when grinding, and check the measure after every few strokes with a caliper.

Also, measure the brush holder and the load spring entirely compressed if you can. Do the math - the brush must fit inside the holder with the spring almost compressed. If you can find brushes that fit, but not with the spring, you may be easiest off by grinding off some length of the brush. That may be a tough job to do neatly, but a dremel or similar miniature grinder is good for it.

If the closest replacement you can find is just as wide, but flatter than the original, it doesn't matter much. As long as the width is a close match, everything should be fine. A little width-wise play doesn't hurt in a one-direction motor, but on a direction changing machine, it should be matched as close as possible. Remember though, that the brush must have free length-wise travel. Further, if the original brush had the twine mounted in the rear end, and the only considerable replacements have it on one side,you must get flatter than original brushes. Otherwise the twine may not fit.

You may not be able to find any fitting motor brushes cheaply, but you can still get quite lucky with car alternator brushes. They are usually made shorter and of a harder material than the common motor brushes, but they work fine as long as the width and thickness are taken into account. Also they can cost as little as around $5 a pair while original brushes can cost around $50 and a complete holder assembly up to $100. You may even find that the original part or recommended replacement are no longer available!

One thing - check the new brush head that is to come into contact with the commutator. It may be flat, or concave-curved to contour the commutator. If it is entirely flat, you can install it, but the motor will not give optimum output until the curve has formed. If it is curved, I recommend rounding out the edges a bit with a file. If you use a brush that is curved for a smaller radius of commutator, and you don't round out the edges, there may be trouble lurking around when starting the motor. Also here, if the curve is different, optimun output will be obtained only after the curve has settled.

Also, the brush holder probably has a pinch or screw that grabs the copper twine going into the brush. From the old brush, measure the distance of the warp in the twine to the brush, make a mark at that distance to the twine on the new brush, and lock the mark inside the pinch. This will keep the brush from wearing completely out and the spring hitting the commutator. I've seen that happen on motors without such pinches, and usually that means there are a couple commutator lamels stuck into the spring and torn off.


Last edited on Tue Aug 14th, 2007 12:08 am by Keinokuorma



____________________
"There is no reason anyone would want a computer in their home."
- Ken Olson, Digital Equipment Corporation (1977)