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Sunday, August 30, 2009

Test Results of CrazyBike 3.1 Drivetrain

Remember I had been running on 24V, and I upped that to 36V?

So far the performance difference is pretty noticeable. Zippier starts with less current draw, both very important as the idea is to keep me from having to put any load on my poor knees (especially the kind of load needed to start from a stop, with a 120 pound bike plus me and cargo). Plus I need to keep current draw low so as to be able to draw more total power from the batteries over a longer period (Peukert's Law).

I didn't just change the voltage, I also changed the gearing for the motor, so that it still outputs about the same max speed to the rest of the drivetrain, but because of that it has better startup torque and gets to speed faster, and is in the high-current/low-speed region for a much shorter time.

So now, even with less total Ah on the bike (was 2x 12v 31Ah, is now 3x 12V 17Ah) it gets better results than it did before. I have not yet tested it's full range, but I think it will probably wind up slightly better range (was 15+ miles at around 15-16mph average cruising speed including sometimes generous pedalling, is now at least 10 miles at around 17-18mph average cruising speed with no pedalling, and only lost about 1-2mph over the last couple of miles).

I think that higher voltage, causing it to draw less amps for less time during acceleration, is combining with the Peukert effect to allow me to draw more from the lesser-capacity batteries.

At 24V the average cruising current was at a guess 15+ amps (didn't have a way to measure exactly, as I had no meter to do that). Startup currents were enough to pop the little 25 amp breaker I had installed, if I didn't help it start by pedalling pretty hard for at least a few strokes. At 36V the average cruising current is around 6-10 amps. Startup currents are over 20 amps (HF meter now used only goes that high), but it never pops the breaker.

The temperature of the heatsink never went more than about 11°F above ambient. When it was really hot on the way to my friend's place, around 1pm, it was about 112°F in the area, and that's the same thing the sensor read. On the way back home, about 2am-ish, it was around 80F when I started, and rose to 91°F for short periods during and just after high-current acceleration tests (starting up in the highest gear to put the most load on the motor). It cooled by several degrees within about 30 seconds or less, and stayed generally around 86°F.

So I guess I don't need a heatsink any bigger than that little bitty one for the small current draws this motor pulls with this gearing at these voltages. That's good, because bigger heatsinks generally weigh more and they take up more space, both of which are bad for this bike. ;)

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