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RE: (ET) Speed controller



 
-----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