Saw a guy on you tube with a EV bike conversion. Sounds like he used a high power DC-DC converter to power the controller and motor. Buck converter, 72v batt voltage stepped down to 48v for the Etek. This way he could use smaller AH cells. Trading Volts for Amps as he but it. Sounds interesting. Would have to be very eff to be worth it. Bike was a Derbi GPR-50.
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That is not a good idea. Adding an extra DC-DC converter would generate losses since it's not 100% efficient. It's no idea to have an extra step down converter when a motor controller itself is a variable step down converter.
The correct way to do this would have been to have just connect the 72v pack to a controller that can handle that voltage and then limit the output voltage from the controller. On many chinese controllers this could be done by adding two resistors between the gas handle and the "throttle input" on the controller, so that it never reaches maximum throttle.
Only problem with that is a speed controls isn't a step down converter, its a pulse width modulators, the motor would see the full 72v and a reduced average current.
youtube link?
The motor windings don't see the full 72V due to the inductor smoothing the PWM. It won't be 48V either, but eteks should be able to take it for a little while at least.
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Travis Gintz
1986 Honda VFR Conversion
www.evfr.net
http://www.youtube.com/watch?v=xrFPMLAfs3s
Here is the link. Can't find much more on it.
I've watched that video before.. I've always thought that guy was totally confused when he said "72 volt 450A DC-DC converter". In that bike as set up he's no doubt having a 72V/450A Alltrax controller. Maybe by some stretch of technical jargon specificity it's a "DC-DC converter" but the common name for it is "Controller".
- David Herron, The Long Tail Pipe, davidherron.com, 7gen.com, What is Reiki
pgt400:
a DC/DC step down converter also use pulse width modulation or pulse frequency modulation, the other parts are just not needed in this case. The motor will not see the full 72v since the inductance of the motor will smooth the PWM voltage spikes. There would not be a problem for a 36v motor to be run from a 72v battery pack if the controller is limiting the PWM duty cycle to max 50%.
Yes I know, I've designed several "boost & buck" battery chargers in the past. The only question was IF he was using a DC/DC converter to step down the voltage as he claims.
pgt400:
Ok, I did not see the question mark. Based on your first post I thought we were discussing the fact that he used a DC/DC-converter in the motorcycle. And since you said it sounds interesting, I gave my point that it is not so interesting, and the fact why it is not.