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darryl
10-28-2014, 06:51 PM
This would probably be more related to hobby stuff, but for what it's worth-

Needed some brushes recently for a rotary contactor, for which I needed 12 contacts and ended up using springy wire to make the contacts. All is well on that project, but the thought continued as to how to produce brushes for various projects. Today it occurred to me that I have some copper coated carbon rod laying around somewhere. I found it and did some testing- I sliced off a 1/8 thick slab from this 3/8 diameter stuff, then sliced that into two half discs. After polishing up the copper, I formed a curve matching that of the slab on a strip of brass. I squared up one half disc, leaving it looking like a motor brush, but with a semi-curve of copper stuck to one end. To this I soldered the brass tab. The other end of the tab gets rolled into a loop so it can be restrained by a pin, which is where power would come into it, and where a spring would apply some pressure to keep the brush in contact with a commutator.

I've done this before, making brushes for a model motor, and it worked well. In that case I made three brushes side by side for each side of the commutator, figuring that arcing would be minimized, which is the case. That motor runs really well, even when relatively high currents are being drawn. The fabrication of the brushes was relatively crude then- I used brass channel from the hobby shop, relying on a mechanical pinching plus some conductive paint to hold the carbon brush in place. Today the brush 'arm' is soldered directly to the brush, which is neater-looking, doesn't increase the width of the brush, and is lighter and will probably be easier to implement. The copper end of the brush being curved almost matches the curvature on the commutator, so the brush could be worn down to almost nothing before any copper or brass would come into contact with the commutator.

I'm convinced that a multiple brush per side arrangement is the way to go, especially where currents might be higher, and this way of making the brush allows for virtually all of the commutator length to be touched by brush material. Of course, I'm free to use this method to make single brushes of any length, so I do have the option to make a brush an inch or two wide if I choose to. I would be limited in the 'wrap around' size of the brush as would be measured by how far it goes around the commutator, and in this case using 3/8 diameter rod I would be able to get about 5/16 of size after squaring up the brush. That's as much or more than most carbon brushes I've seen, so it's good on that account. The other limit is what the length of the brush would become, which would be 3/8 minus what it would take to trim it up. Using a larger diameter copper coated carbon rod would increase all possible dimensions of course.

Anyway, just thought I'd pass this idea on. Seems like an ideal way to make a brush for a rotary contactor, as it's easy to do, gives a solid electrical connection to the brush, and could be made very compact- especially given that the number of rotations of a rotary contactor might be very low over a long period of time.