Monday, July 18, 2011

DirectDrive v1.0: 2.4kVA Test

After my ducted fan load tester turned out to be basically unworkable, I decided to go back to lower-RPM motors. This way I'm not simultaneously testing the controller's basic functionality and the ability of my FOC algorithm to handle greater than 1,000Hz electrical frequency (it can't).

The first candidate: a re-wound and sensored Turnigy 8085-170 motor:

Well this pretty much didn't work at all. There's some issue with the sensors that makes it sound horrible, and also it's too small to be able to sink a lot of power internally.

I decided to stop when the hot glue holding the sensors in started melting.
Anyway, that was just a little distraction before the real show:

The Mars PMAC (brushless Etek) is probably my favorite load-test motor. It's so big that you can't really hurt it, at least not in the time scales of a bench test. I put the DirectDrive controller on its edge to give the heat sink a tiny bit of natural convection, but no active cooling for this test. I wanted to see the temperature rise. Nice, steady temperature rise, and no spontaneous FET destruction.

Now here is where it gets interesting. Short of coupling it to a second Etek, there's not much I can do to apply mechanical load to the motor. (At least with the ducted fan, I had a way of doing so.) So, instead, I brought the motor up to speed under ideal timing conditions, i.e. manipulating the phase of the drive voltage until the current draw is at a minimum. Then, at nearly full voltage amplitude, I threw the timing way off so that the motor begins to draw 100A (phase peak).

And I let it sit that way for one minute. The phase voltage is a sine wave with an amplitude of 16V. The phase current is a sine wave with an amplitude of 100A. But...they're not in phase. So the real power is still low, but the apparent power is 2.4kVA (1.5*100A*16V). The good thing about this load is that it can all be run off a bench power supply, no need to break out the giant batteries yet.

But is it actually a representative test of the controller? It doesn't really test anything outside of the bus capacitor, where the DC current is determined by real power only. But what I'm still not sure about is how it looks to the inverter itself. Here's what it comes down to:

The RMS current is the same regardless of the phase angle. The only thing that changes is the phase of the sine wave that the duty cycle traces out. But, the MOSFETs are still either blocking 36V or carrying their share of the current at any given instant. It makes sense, then, that both the conduction losses and the switching losses should be the same regardless of the relative phase of the current with respect to the duty cycle. Anyone care to confirm or refute this?

If that is the case, than I'm happy with the test results. A minute of running at 100A with passive cooling yielded a temperature rise of 15ºC. I would guess that the heat sink mass is about 0.25kg. If it was mostly just absorbing power (with no active cooling and very little temperature gradient, this is true), then a quick estimate of the power dissipated in the inverter is 24W, which is exactly where it should be for 100A.

More testing to come.

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