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-----Original Message-----
From: wombat [mailto:wombat RealNS com] Sent: Saturday, December 11, 1999 7:41 PM To: electrak list Subject: (ET) Speed controller "
I wonder why some of the Electrak people
can't come up with an idea for a speed controller we could build from parts, the
prices o commercial ontrollers are ridiculus. Should be
able to do it with several power FETs is parallel and a simple pulse width
modulator. Leave the reversing circuit in to change polarity, just control
the power into the motor.
Any suggestions or ideas ?
Wombat
"
There are several ways to do it. FET's (or
IGBT's) of suffecient power are available WITHOUT going parallel. PWM is
fine for FET or IGBT
but not quite as good for the more common
Darlington or
bi-polar methods of PWM; you don't really want
to drop .6V at 100+ amps just
for the control which
is exactly what any bi-polar scheme (like Darlington) does
it. Darlington based PWM is common, but for an EV it isn't
too great. It's the way
the blower speed control in your
Ford would do the job. One problem with PWM or IGBT
for this application is that your
control voltage for the way FET's are
normally wired must be at a higher potential than the
device; for a 36V tractor, you need a small control at
say 37V to get the FET fully "on". Without that,
the FET isn't running very effeciently. This isn't a trivial
problem. I have a circut that works
without
the extra control voltage, but the patent
is not yet filed and belongs to my employer,
Ford. I doubt it will be filed before late
next year. If anyone
want to play around at these currents on
the cheap, go to
Marlin P. Jones web site www.mpja.com, and pick up some
cheap surplus/used power
IGBT's or power Darlington's... they have
them to 800 amps for a few bucks (they also have some
13 amp/15 V output 42/60 V imput D/D converters
for $25 as that seemed to be pretty
interesting to some folks that wanted to do radios a
few months ago) and try some control. I get to use newer
and nicer designs at work but these should
work fine. PWM is NOT a problem for this application because the
inductance of the ET's motors is so high.
Larry
Elie
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