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Saturday, July 12, 2008

Torqued Out

I've been experimenting and thinking on how to use the ex-treadmill-motor on the bike most efficiently, which includes a stall current test at various voltages.
@12V it can be held with visegrips but it is difficult. For that I just used a battery across the motor supply wires, since the controller won't run at that low a voltage.
@24V I cannot hold it myself with the visegrips. They are pulled out of my hand as the speed increases, perhaps at 1/2 throttle, using the controller already on the bike. If the motor wasn't bolted to the worktable, I might've been able to hold onto the visegrips but it would have then tried to spin around the shaft.
@36V I felt it safer to just put the visegrips on in such a way that they'd be pinned to the table surface by the shaft's trying to turn, so I wouldn't risk any possible injury, and I'm glad I did. As I increased past about 1/2 throttle, it suddenly started squeaking as the visegrips lost their hold on the shaft, then they somehow moved parallel to the shaft and it began to spin and flipped them thru the air.

The shaft got pretty hot from the current the motor had sucked up in the second or so of doing the last two tests, but cooled quickly once it was rotating. The controller's MOSFETs didn't even hardly warm up from that, since they're on that giant aluminum plate. BTW, the stall current is about 21Amps. I posted the label saying 12Amps before, but I somehow got dyslexic on that figure.

That makes my previous list of calculations off by quite a lot:
@12V (one battery) it could run as 252 Watts max
@24V (two batteries) it could run as 525 Watts max
@36V (three batteries) it could run as 777 Watts max
So that is why the shaft heated up so quickly--at stall it was putting a heck of a lot of power thru the motor for that second-or-so. But not as much as I would've thought....

I discovered with the help of a new multimeter that can measure frequency & duty cycle (I don't have an oscilloscope yet, and need to build the interface to let my PC's soundcard be one) that the controller is never going higher than 75% for some reason. So I never get more than 27 volts into the motor system right now, no matter which motors I use on it. That's 567 Watts on this new motor at full stall load, assuming full throttle is putting out 27 volts instead of 36. No idea why it's happening yet, but will troubleshoot whatever day off I have next week, I guess.

So with these power levels, it's pretty likely that I could use just 12V to power the system, and use all three batteries in parallel for more current capability, or just one battery for times I need it to be lighter (but I will not get much range out of it). Currently I have to use all three in series just to get enough power out of the ex-fan-motors to do anything useful. Part of that might be because of the 75% duty cycle problem, but it might also just be because the fan motors are not designed for torque, just for speed (unlike the treadmill motor).

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