View Single Post
Old 01-09-2009, 08:26 AM   #148 (permalink)
MPaulHolmes
PaulH
 
MPaulHolmes's Avatar
 
Join Date: Feb 2008
Location: Maricopa, AZ (sort of. Actually outside of town)
Posts: 3,832

Michael's Electric Beetle - '71 Volkswagen Superbeetle 500000
Thanks: 1,368
Thanked 1,202 Times in 765 Posts
Another Major software change

I am making another pretty major software change. Ian of Zero Emission Vehicles Australia had mentioned on the EVTech list that his 11" direct drive electric motor had sort of a hair trigger throttle at low RPM. At 10% duty, he would already be up to the full 600 amps, at which point his current limiting would kick in! He had the burnout marks in his garage to prove it. I haven't noticed this problem because I've been controlling an e-bike hub motor, which is quite different. It accelerates nice and smoothly by just controlling the PWM duty with the throttle.

So, he reprogrammed his ATMega8 micro-controller so that the throttle controlled the PWM duty AND the current limit. For example, at 10% throttle, no more than 10% of the current limit would be allowed, and no more than 10% of the PWM duty would be allowed. One of those will be less than 10% duty, but so what! Since current is proportional to torque, it makes for smooth acceleration at low RPMs.

The Torque/RPM curve of a DC motor has torque starting really high, and pwm duty starting really low, and ending with torque really low and PWM duty really high. It will be nice to have both of those under control, and not just the pwm duty.

I doubt my Curtis 72v 400amp controller uses the throttle to control current and PWM duty. It generally has a jerk when it takes off from 0 rpm. That has really annoyed me for some time. The home-made controller is going to feel WAY smoother.
__________________
kits and boards
  Reply With Quote