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Re: (ET) Speed controller



I've been looking forward replacing the controls in an Elec-Trak for a while.  Problem is that I don't have the time, yet.  I have an E20 which has lots of electrical problems, most caused by mice.  My first attempt will be to just replace the logic relays with a processor, leaving the power contactors and the resistors to get the different speeds.  Once I get the processor in control, I'm planning to replace the contactors with a contactor-free, resistor-free design, like the PWM/FET/IGBT stuff outlined.  (I may keep the resistors for the field, though.)
Any suggestions, just in case I do get to it this winter?

I would like to replace "Card 4" in this E20.  Among other things, this card determines (I believe) if the motor current is low enough to allow higher speeds to be enabled.  Does anyone know at what current level the motor must be limited to the lower speeds only?  If not, does anyone know what the original components are on the board?  (On this E20, this board has been repaired countless times!)

Thank you,
Allan

"Elie, Larry (L.D.)" wrote:

 
-----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 IGBTbut not quite as good for the more common Darlington or bi-polarmethods of PWM; you don't really want to drop .6V at 100+ amps justfor 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 notyet 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