Changing motor brushes

[Keinokuorma]

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.

[Keinokuorma]

Decided to put here some visuals, now that I had my camera ready. these pics are from a Nippon denso car alternator job, but most of these techniques are more or less applicable in the motor brush job.

In the upper pic, I have already removed the brush holder from the alternator, desoldered the old brushes, and blown the solder holes clear. You can see the drips of solder on the table. In the lower pic, you will see the old and new brushes for comparison, as well as the load springs.

[Keinokuorma]

Here again the brushes and springs. In the upper pic you will see how the curvature on the old brush heads have settled to the radius of the commutator. On the new brushes, the curvature is much shallower. This kind of formation is normal, but the old brushes have worn in length over time so much that the spring cannot give enough pressure. The contact is weak and the function will become flaky.

Also you will see the material looks like copper. these brushes contain copper as well as anthracite. The material is harder than the usual motor brush. It will take longer to wear down and it can take higher temperatures and more friction.

In the lower pic, there is a shot of the commutator, which the brushes come into contact with. Note that this area in the alternator is different to most motors. Here there are two circular, insulated copper contacts, that connect to the ends of the rotor winding. The usual layout of the motor has a lamelled commutator, and the brushes aren't laid on top of each other like here, but right on opposite sides of the commutator.

Note that although this is an alternator from a car, and not a motor, very smilarly designed motors are used in some cases. The design consists of a stator very similar to an induction motor, and a rotor very similar to this. It can be used as a synchronous or asynchronous motor. In asynchronous mode, the brushes are shorted together by a high current switch, or there is a variable resistor between them, that can be used to adjust the output torque (and speed) of the motor. In synchronous mode, the brushes are supplied constant DC voltage... torque and speed can be adjusted by altering this, although mostly synchronous motors are adjusted by the stator field frequency.

Also this motor type can be switched between the modes easily, and often it is started in async mode, then swithched to sync mode when it is close to top speed. On some really geek systems, during async startup, power is taken out of the rotor circuit, rectified, and fed back to the power network through an inverter. This isn't exactly a perpetual motion machine, but probably one of the closest electrical solutions you can get... anyway it will give smooth startup while saving a lot on peak startup load on the network.

[Keinokuorma]

In these pics, the twines of the new brushes have been routed throught the solder holes of the holder, and the springs have been inserted. The twine is pulled tight enough to keep the spring from extruding the brush all the way so that it would flip out. This will keep the brush from wearing out all the way to the point where the twine is connected. If that happens, the spring will become part of the circuit, heat up, and lose its elasticity. Note that I have wrapped the twines into the mounting ears. this will hold them in place until the soldering is done.

[Keinokuorma]

Here the twines have been soldered to the holder at the right length, and the excess has been cut off. In the lower pic, the silicone "gasket" is mounted, although this "gasket" doesn't seal much anything.

[Keinokuorma]

Here the rebuilt brush assembly is being remounted to the commutator. There is little room in this case to work with the fingers, so the brushes must be pressed in by a wooden stick during the process. In the case of a motor, you probably have more room, and you may be remounting one brush and holder at a time, so the stick trick is not necessarily required. Also if you buy a complete brush assembly for a motor, it may contain a plastic clip that compresses the brushes until the holder is seated.

[Keinokuorma]

Finally, the wooden stick is removed, the holder mounting screws are back in and the cover back on. This is approximately a half-an-hour job after some practice.

[Extech2003]

I see by the last pic that it’s a Nippondenso motor but

what is it out of? Just out of curiosity what’s the model on the motor itself?

[Keinokuorma]

It's a car alternator from my 1991 Toyota Carina II.

Not a motor, but as it has been written, here are listed many technologies applicable to a motor brush job.

[Keinokuorma]

I will post pics from a motor job when I get to do one and am allowed to take pics.

[Extech2003]

Thought this was interesting,

But I don’t know if they sell to individuals.

 

http://www.wai-wetherill.com/products/sresult1bom.cfm?ManufName=NIPPONDENSO&item=100211-611

 

 

[mmtaylor]

Can you buy an assembly where the brushes are already installed or do you what to get a new motor, if you do not want to install the brushes individually.

[Keinokuorma]

mmtaylor,

your question has been answered in your original thread:

http://applianceguru.com/forum2/12571.html

[Keinokuorma]

OK, here we go.

for those who have more or less patiently waited for pics from a motor brush job (not an alternator), I have got some more visuals. This is a Ford blower motor, which an automotive mechanic was so kind to detach for me.

So, in the upper pic, you see one possible method of mounting the brushes. These are on pivoting holders. In the lower pic (yea, that's my well manicured hand there) you see me pivoting one of the holders away from the commutator. The concave curvature at the tip of the brush is well visible.

[Keinokuorma]

And here, the upper pic shows the brush from the other holder. The twine was crimped on the wiring, and I had to cut it off, leaving enough to twirl and solder the new twine on the assembly when properly fixing the motor. this can be done because the brush is immobile in the holder, and there's no strain to the twine.

The lower pic shows a temporary brush that I fitted to test the motor. It is actually one of the old brushes from the alternator job above. The motor worked purr-fectly with that (I plugged it into the 5V output of an old computer PSU for trying it out) so now I know that I only need to get some brushes tomorrow.

[Keinokuorma]

Here's closeups of the brushes.

Upper pic: this brush has some tread to go with, no need to change it for trying out, although I will once I get hold of fitting brushes. the other one was worn so short that the holder rim touched the commutator, and the brush made no more contact.

Lower pic: The temporary brush, which I just twirled on to the twine, fits the holder sideways but is much flatter than original. It also has the wrong curvature, and when I get the replacement brushes, I will probably have to round out the hind edge of the new brush. Otherwise it will make excess noise and wear the commutator. The front edge will take care of itself when the brush wears down.