On 2/10/2013 7:27 AM, Jeffrey Jenkins wrote:
Drewcifer wrote
I thank you for the link am I have read it twice and as a non scientist I
don't understand how it applies to the below information suggesting that
pmdc motors should not be run in parallel. I apologize for my ignorance.
...
I really don't know how else to explain this without repeating what I, Lee
Hart or Bill Dube have already wrote. Put as simply and succinctly as
possible, PMDC motors have a fixed and fairly "stiff" relationship between
voltage and RPM, and because no two motors are exactly alike, no two motors
will have exactly the same Volts/RPM ratio (usually referred to as "Kv").
Thus, unless both motors are allowed to spin at different RPMs, the motor
that spins faster at a given voltage will hog all the current, with some of
that current coming from the external supply, and some coming from the other
motor (acting as a generator).
This is pretty much bad news no matter how you slice it, hence, the advice
to not drive two or more PMDC motors from one controller.
...or when the motors are wired in series.
The situation the original poster has proposed (two PMDC motors, each
driving one wheel) isn't optimum, but it isn't hopeless either. If you
have the motors, try them. Wire them in parallel, and measure each one's
RPM to see how closely matched they are. If they are pretty close (say,
within 5%), I'd give it a try.
Your controller can switch the motors in series/parallel. At low speeds
or whenever you're turning sharply, run them in series so they won't
fight each other.
Include an ammeter in series with each motor. Then you can *see* when
the current are unequal, and whether they are fighting each other. If
there's too much fighting, then you'll need to add a second controller.
--
Don't worry about people stealing your ideas. If your ideas are any
good, you'll have to ram them down people's throats. -- Howard Aiken
--
Lee A. Hart, http://www.sunrise-ev.com/LeesEVs.htm
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