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Saturday, April 25, 2009

Motor Design Flaw

There is a serious design flaw in this powerchair motor, in the form of a plastic disc used to hold the brushes, insulating them from the motor casing and each other. The "clever" designers used a soft plastic to make that disc, and it's not heat resistant. Not a problem in a small light-duty motor, but this one is a heavy-duty motor intended to be run hard and get warm to hot--remember, in a powerchair like this came from, it (along with it's mirror twin on the other wheel) would have directly driven the small diameter wheel at around 4MPH with up to (I think) about 250 pounds of person *plus* the chair/batteries/etc.

It seemed like it might be working a little less well than it ought to even when I started with it, but Friday on a several-mile ride, whenever I was in an area safe to do so in, I tested out how well it would work on motor-only, so I can find out how dependable it is on that if I need it in the future. That of course heated up the motor even more than previously, plus it was a pretty warm day (90-something °F), so the already-warped soft plastic got even more pliable, and the brushes were pulled farther away from the motor until it stopped working at all.

As I have not run the motor much yet, I can tell what wear on the brushes is from prior usage, and what is from me. The brushes were already at an angle away from the motor due to their spring-loading pushing the whole assembly away on that plastic disc, and that left an angled wear on them, curving away from the motor as it slowly pushed farther and farther away, pressing more and more on the forward edge and less and less on the rear edge. When it got even hotter during my tests today, it moved a little further and hit the stop of the front wall of the motor, and stayed there, wearing a little angled notch on the front edge of the brushes.

The plastic disc is simply far too soft for this. It actually melted a little bit right at the point it contacts the brush-mounting plates! They could have used any number of plastics or non-conducting materials that are MUCH harder than this, including the old bakelite-in-fiberglass that most motors I've taken apart have in them. But this Chinese-made motor was done the cheap and dirty way, so now I will have to make my own replacement for the disc, and disassemble the motor to put it in.

Then I have to polish the armature and the brushes so they match again, and properly conduct power to the motor so it runs right and doesn't get damaged by arcing. :(

For the moment, all I could do was replace the thin brass rivets that were no longer holding the plate to the plastic (because of the melting) with tiny screws threaded into the brush holders where the rivets used to be, but those screws will melt into the plastic, too, if I run the motor hard at all--difficult not to do because of that throttle peculiarity I wrote of previously, where it suddenly jumps from around 2/3 power to full power, and usually I need something just above 2/3, so I am stuck with full or nothing most of the time.

This is going to take some serious time to really fix, and I haven't yet had the time to do it, and probably won't until this coming Thursday at the earliest. The risk is that I won't be *able* to fix it, and might leave the motor in an unusable state (right now, even a good bump will pull the brushes away from the motor far enough to keep it from working correctly, and sometimes at all--tapping lightly on the end-casing (the plastic part that formerly held the electromechanical brake) will usually get it started again, but sometimes I have to open that casing and reposition the brush holders carefully.

I am probably not going to fix it this week because there is a group I just discovered called PhoenixPhreaks, a part of FreakBike Nation where others that also build "freaky" bikes gather. PhoenixPhreaks will be having a ride/get together on Friday night, which I'm going to take the bike to even if the motor is not working (I'll leave the batteries and motor home, if it's not, because that's a lot of mass to move by pedal power alone!), as long as I don't get my work schedule changed between now and then.

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