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Old 09-07-2010, 06:05 PM   #311 (permalink)
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If you convert to power (watts) then yes to muliply by 3, then convert back to V*A. This is important because what's going to each leg will be RMS values, and the input is a DC bus.

so a straight simplistic 60Hz conversion for 360VDC 100A: 36kW input
/3=12kW
12kW / (360V*.707) = 47A RMS per leg (67A peak)

Of course this will change based on frequency, efficiency, waveform shape, etc.. but a good enough ballpark to pick out parts.

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Old 09-08-2010, 12:48 AM   #312 (permalink)
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Quote:
Originally Posted by Automcdonough View Post
If said box would do any good to protect you from an unstable stick of dynamite, then yes. But it could occure anywhere in the system, off the batteries, failed wire insulation, etc.

Basically the safety factor is much more serious when you have stored energy at high voltages. On a 48V pack a short will give you some fireworks and perhaps a fire, but not a full-on EXPLOSION.

Sorry for the derail, I'm done w/ this, just wanted to point out that it is a factor I consider when choosing voltage. The average DIY hobbyist may not even realize what they are getting into.. some reading on wiki or something on arc-flash should clear up any questions or we could start a seperate thread on it.
The voltages are still too low to jump any significant distance. You could provide shielding to protect against exploding components, although the metal car would already provide a good amount of shielding since the inverter would probably be installed under the hood. Avoid putting batteries in the passenger compartment if possible, although the LiFePO4 batteries used in EVs are a lot safer than the Li-ion batteries in consumer electronics.
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Old 09-08-2010, 10:45 AM   #313 (permalink)
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Wow. Single phase AC blows my mind let alone 3-phase. You send a voltage out and then receive an opposite voltage back and yet the load somehow used power. It's like black magic (or maybe white magic cause it's good). That Tesla guy was amazing.

So basically you just need to know if the AC voltages are peak or RMS values. If you had a 144V RMS ac voltage source that could supply 300 amps per phase it sounds like from the 36kw example you would end up with 144*300 (this is an rms value) = 43.2 kw per phase or 129.6 kw total. The peak to peak voltages would be -204 and +204.

If your 144V was a peak value you'd end up with 102V rms at 300 amps per phase or a total of 91.8 kw.
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Old 09-08-2010, 11:04 AM   #314 (permalink)
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Wait, what is rms current? Does ac current follow a similar waveform to ac voltage? I guess it would since the voltage drives the current. So 300 amps rms per phase would really be 424 peak amps? So RMS Power = RMS current * RMS voltage? Would that mean that peak power is twice the RMS power? I guess it would since sqrt 2 times sqrt 2 is equal to 2. So I guess you would need to know if the 300 amp rating would be RMS current or a peak current. If you had a peak current of 300 amps per phase and a peak voltage of 144 volts your peak power would be 43.2 kw per phase and your effective power/rms power/ equivalent dc power would be half that or 21.6kw per phase for a total controller power of 64.8kw or 86.4hp. Not too bad but not as good as cougar. If the 144 and 300 values are both rms though you have 172.8hp, now that's more like it.
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Old 09-08-2010, 11:28 AM   #315 (permalink)
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One thing I know for sure is that each phase is not capable of going out of the range [-300, 300] (or [-x, x], where x is anything you want), since the hardware overcurrent circuit will trip for each phase. So, there will be no currents ever higher than 300 amps in a phase, even for an instant. Now, the voltages of the phases will have a peak voltage of 144v, so the rms voltage per phase will be 102v. The rms current will be 212 per phase I guess? So 102*626 is the rms power? Around 64kW like you were saying?
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Old 09-08-2010, 11:39 AM   #316 (permalink)
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I think that sounds right. Unless you can manipulate the wave form to be fatter and less sine wave shaped so you get more area under the curve. I have no idea if that can be done...sounds like the 300v mosfets would allow you to get around 200v peak for 144 volt rms and 212 rms amps per phase. That would give you closer to 92 kw. Would that mean you need a 200 volt battery pack?
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Old 09-08-2010, 11:44 AM   #317 (permalink)
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Yes. In fact, I think you could get away with a 215v or 220v battery pack with 300v mosfets, but I'm not sure. 144v was OK for 200v components, so x is OK for 300v components, where x is around 220 I think.

144/200 = x /300

Of course none of this would be necessary if there was an infinite supply of cheap high powered igbts, but I didn't want to have to depend on Ebay for everything.
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Old 09-08-2010, 12:22 PM   #318 (permalink)
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Quote:
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The rms current will be 212 per phase I guess? So 102*626 is the rms power? Around 64kW like you were saying?
It's really: 102V * 212A * SQRT(3) = 37450W, all 3 phases are never pushing at the same time, so they don't add directly.
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Old 09-08-2010, 12:33 PM   #319 (permalink)
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Thanks Zach! That makes sense.
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Old 09-08-2010, 01:15 PM   #320 (permalink)
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Hm. Doesn't make sense to me. We already took the wave attributes into account when we calculated rms current and rms voltage right? If each phase is isolated, and you have a given power for each phase, why don't you just have 3*given power. Where does the extra power go? Does it bounce back into the batteries? I knew AC was magic! If you got rid of 2 phases how much power would you have? 144V*sqrt(2)*300A*sqrt(2)*sqrt(3)/3? What does adding 2 more phases do to each individual phase?

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