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:
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
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