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Originally Posted by bennelson
If I understand correctly, a Citicar uses several contactors to connect and disconnect the various batteries so that all are used at the same time, but sends differing amounts of voltage (12/24/48) to the motor, thus different speeds.
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You are almost correct, sir. The first step is 24v through a fat resistor, then straight 24v, then 48.
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What if I used a 36 volt pwm controller (Cheap! as there are a zillion of them for golf carts) and then had a contactor set to disconnect the controller and instead connect all the batteries in series. A reversing contactor might be able to do both at the same time, right?
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What would happen, is you would be vaulted to revered status among all the cheap.. I mean budget.. EV builders out there.
It's slightly more complex than that however. My limited understanding of the issue is that when you switch BACK from 72v to 36v, there needs to be something to:
1) prevent the contactor from arcing if you're under load when you open it (I think this is done with a big resistor in parallel with the contactor); and,
2) prevent "back EMF" (high voltage in the motor that exists briefly upon disconnecting the power) from flowing backward and hitting the controller, thus frying it.
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Having full voltage running through the controller, then doing a hard disconnect from it, doesn't sound healthy for the controller.
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I'm not certain about this part, but it seems to make sense.
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Does this seem like a reasonable idea for a cheap experimenters way of not having to buy a 72 Volt or more controller?
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Definitely.
THE Yoda of the EVDL has recommended this approach, but nobody has done it yet, as far as I know.
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It would also allow max amps at 72 volts, would that help with hill climbing?
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Absolutely, compared to 36v.
Do eeeet!