Re: 4WD EVs - commercial opportunity?

[XURQ03A@prodigy.com ( LEE A HART)]

Peter VanDerWal wrote:
>The two motors will only run at the same speed if they have the same load

Little toy PM motors have low efficiency and are not very "stiff". Their
rpm can change 2:1 from no-load to full-load at a fixed voltage. They do
not fight each other too much, and so can be wired in parallel and used to
independently drive two wheels.

With a high quality PM motor, rpm changes less than 10% from no-load to
full-load. Suppose you go around a tight corner so the turning circle for
the inside and outside wheels is more than 10% different. The inside wheel
delivers ALL of the torque. The outside wheel is turning more than 10%
faster, so it acts as a GENERATOR and is actually slowing you down. Like a
car with a locked differential, it tries to go straight ahead.

>I haven't experimented enough with series motors to be sure but I thought
>torque WAS related to RPM?

It is, of course. But it is quite different from a PM motor.

To see what happens, recognize that a series motor acts like a resistor
(except that it converts electrical power into HP and not heat). Its
equivalent resistance Req = RPM x K (where K is a constant for any given motor).

When you go around a corner so the inside motor is turning 10% slower, its
resistance is 10% less.

If the motors are in parallel, 10% less resistance means 10% more current.
10% more power in means 10% more HP out. Since the rpm went down 10%, the
torque must have gone up 20%. 20% more torque on the inside wheel is not
enough to notice.

If the motors are in series, their currents must be the same. This means
their torques must be the same. If one wheel is in the air (delivering zero
torque), then the other wheel delivers zero torque, too (exactly like a
normal differential).

Clear as mud? At least, now you should know how to wire the motors to get
out of it!

Lee Hart                     If you would not be forgotten
4209 France Ave. N.          Soon as you are dead and rotten
Robbinsdale, MN 55422 USA    Either write things worth the reading
phone (612) 533-3226         Or do things worthy of the writing
e-mail XURQ03A@prodigy.com   (Ben Franklin, Poor Richard's Almanac)