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Old 09-28-2014, 09:06 AM   #27 (permalink)
mechman600
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Yes, AC systems are much safer because it is impossible for them to fail full-on, unless of course it is a potbox failure, but allegedly Toyotas were known to fail that way a few years back too.

DC motors have a limited RPM range because as their speed increases, so does back EMF, which is self-induced voltage in the opposite direction that it is spinning. Therefore, voltage necessary to maintain a given amperage (amperage is proportional to torque) is proportional to RPM. At zero RPM it takes almost no voltage to reach max controller amps, and at, say, 5000 RPM it may take full pack voltage to achieve the same amperage/torque. On my 72V Electric Booger, the crossover where full pack voltage was put to the motor to maintain max amperage was at 3500 RPM. Above 3500, while pack voltage remained at the motor, amperage began to fall as the motor speed increased above that. Thank-you back EMF!

AC motors do not have back EMF to deal with, which is why you see a single speed Tesla S with a motor that delivers 300 ft/lbs of torque and revs to 14,000 RPM.

As for the above question about starting off in higher gears, here is a video I posted a long time ago about first gear vs second gear starts in the Electric Booger. As you can see, having no clutch really slows the upshift down, which is why I will do a clutch next time. This was more or less a test to show that. A third gear start with the car would have been PAINFULLY slow, so I didn't even bother to show that.
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