I broke the Mountain Wheelchair Motor :(

So, the other day I was over the moon when I wired up one of the mountain wheelchair motors and witnessed it spinning for the first time. It’s taken so long to find a suitable motor that this felt like quite an important milestone in the project.

However, and it goes to show that I don’t really know what I’m doing, I’ve managed to partially destroy said wheelchair motor…

In the video below, you can see that the motor spins in the forwards direction without problem, but starts to make a funny noise when reversed. Initially I thought this was a software feature of the controller which was limiting reverse to half speed. That is until, part way through the video, the motor stops turning in reverse at all, a warning light appears on the controller, and then suddenly, although not caught on camera, smoke starts to bellow from the motor! I quickly disconnected everything from the batteries but was unable to capture the smoke on film:

As soon as I put my camera down I placed my hand on the motor and was astonished at how hot it was. In an attempt to reduce the damage I decided to open up the motor so that it would be easier for the heat to dissipate, but it was too late; as you can see by the burnt coils in the image below, the damage was already done.

The Cause of the Burn-Out

There are three wires which run between the controller and the motor. Although they are from different manufacturers, they appear to have adopted the same standard; both have a Yellow wire labelled “Phase A”, a Green wire labelled “Phase B”, and a Blue wire labelled “Phase C”. Yellow goes to Yellow, Green goes to Green and Blue goes to Blue, it’s obvious right? wrong!

Because I’d done the obvious thing and made the mistake of wiring the motor according to the manufacturer’s specifications, it meant that as I was trying to reverse the motor, it was actually fighting against itself (think of two powerful magnets both trying to pass through each other). This is what caused it to overheat so quickly.

It was quite frustrating at the time because it highlighted the fact that I don’t really know what I’m doing. I had no idea the manufacturer’s wiring could be wrong. I had no idea it would burn out so quickly. What else will I mess up further down the line?

The Solution

With a little digging around I found that wiring the motors is a matter of trial and error, and completely ignoring the manufacturers’ diagrams. As well has the three phase wires, there are three matching hall wires, so in total, there are 36 possible wiring combinations. You just have to try each one until you find a combination that works.

Anyway, much trial and error later, and not only is the burnt motor working perfectly in both forwards and reverse directions, but I’ve also added an additional fail safe…

Preventing the Wheelchair Motors Burning Out Half Way Up a Mountain

Seeing how fast the motor burnt out put a bit of a dent in my ego, but that was nothing compared to the potential consequences of getting stuck in a wheelchair half way up a mountain because a motor had suddenly burnt out.

In light of this I’ve been able to add a “thermometer” into the circuit which automatically cuts power to the motors if it gets too hot, and then lets the motors resume normal operation once it has cooled down again:

This way, when Ada is driving up a particularly difficult section of mountain in the wheelchair, and one motor suddenly gets too hot (110°C), either the power of the other wheels will keep propelling her forward, or, the wheelchair will come to a stop and she’ll have to let the motors cool down for a moment (to 95°C) .

This does mean having to modify the inside of the motors to accommodate the thermistor, but looking on the bright side, at least I now have a guinea-pig motor to run tests on!

Phase / Hall Wiring for Kelly KBS48101X and Mac 536HF

And finally, just in case somebody else stumbles upon this page because they found the same combination of motor and controller didn’t work for them, here’s a working phase / hall combination of a Kelly KBS48101X controller and Mac 536HF motor:

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