On 8 Jun 2013 at 17:29, Geof Thompson wrote:In terms of the way they drive the wheels, DC motors in series will act like
> If two motors were connected in series, but with opposite polarity.
> Then mounted end to end, what would happen if they were used as a rear
> axle? would they act like a differential or would they work like a
> posi- trac. or would they just not work at all?
a conventional differential. DC motors in parallel will act more like a
limited slip differential.
The amount of "slip" with parallel motors will depend on the type of motors.
I think that series motors might be more tolerant of differing rpm going
round corners, but PM motors tend to be more rigid about voltage vs rpm and
might fight you more. Not having tried it, though, that's just a guess.
How you make this idea into a usable drive axle is another matter. I don't
see how you could just bolt two motors together and make a drive axle.
Other folks have mentioned the issue of reduction gearing. Even if the
speed were correct, I'm not sure that conventional motor housings would have
the mechanical strength necessary to handle supporting vehicle weight over
rough ground.
Another concern in a road EV would be the large unsprung weight at each
wheel. That probably wouldn't matter much, if at all, with something like a
tractor.
What would be more likely to work would be to forge an axle and mount the
motors outboard, driving the wheels through chains or belts. Bob Beaumont
did something like this in the Tropica roadster in the 1990s. The rear
wheels were on trailing arms. It used two motors, each with its own
controller. Each motor drove one rear wheel through cogged-belt reduction.
It worked, but it was a fairly expensive setup compared to one motor driving
a conventional differential.
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