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Old 10-21-2015, 07:23 PM   #2220 (permalink)
e*clipse
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Quote:
Originally Posted by MPaulHolmes View Post
I've gotten current measurements for all 3 phases before though. They are just sine waves. And not messy sine waves either. Actual clean looking sine waves. Then, the current coming from the capacitor is
currentCap = (duty1*current1 + duty2*current2 + duty3*current3)/3. But that's an average current I guess. But if we pretend that there's no help coming from the batteries, we could look at what happens for one period. We could assume that the current is constant the whole time, and figure out the voltage drop from the current and capacitance.
What i'm looking for is to break down a part of a sin wave so that you can actually see the switching affects. It should be pretty clean because of the high motor inductance. However you should see a "digitization" of the sine wave where the actual current is ramping around the ideal waveform. It seems in a locked rotor test, it would be possible to set things up so the input is in one phase and the output is in the other two; that's just a specific quadrant of operation. I suppose I could make the model work in any quadrant by matching the pwm duty cycles.

Whoah! Wait a minute. Maybe this is good enough. Really - It's purpose was to spec the capacitor. It's working for that. Maybe I'm getting too academic about this. If a good model seems useful for other stuff, I'll be happy to keep going on this. Otherwise I see some rapidly approaching diminishing returns.

*** edit ***
Perhaps I should see about the switching spikes on the bus at turn off. This will probably require non-ideal switches. I've read that the time lag for even a schottky diode to switch can cause some serious spikes.

This would be useful because we could design for these spikes, rather than just building in a 2x safety factor. This directly affects capacitor cost and size.
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Last edited by e*clipse; 10-21-2015 at 07:29 PM.. Reason: move to spikes
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