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Topic: Motor speed controller question from a dummie (Read 1 time) previous topic - next topic


Apr 24, 2011, 02:13 am Last Edit: Apr 24, 2011, 02:24 am by zoomkat Reason: 1
My parents had a 63 dodge dart covertable with a similar dealer instaled AC. My high school hot rod project was rebuilding a 57 dodge D500 engine with 10.5:1 high compression pistons, 320 deg Isky cam, and a 2x4 barrel mainfold with two carter carbs from a chevy 409, all to be put in a 56 dodge coupe that I got for $20. Those were the days! As to your AC motor, you may be able to use some rectifier diodes from radio shack to drop the 12v to slow the fan. Each diode will drop ~.7v across it, so three diodes in series would drop the voltage by 2.1v. If the diodes work, you could connect them in line with the slow side of the switch to either the slow wire, or over to the high speed switch terminal so the current sill go thru the high speed wire if there are suspected issues with the slow speed wire. This could be a tempory workaround that would have minimal impact on the origional equipment ("restore" is much more picky than "repair").
Consider the daffodil. And while you're doing that, I'll be over here, looking through your stuff.   8)


Thank you all for your words of wisdom.

Just to clear up a couple of question marks, the motor is a permanent magnet type, the voltage is indeed 12V in the 1957 Chrysler, and the 8 amp draw is only instantaneous on startup and immediately drops to 4 amps when it reaches speed (less than one second elapsed time).  I've had the motor all apart to resurface the armature and cosmetically restore it.  All appearances inside are fine and the motor is very free.  I do have a service manual for the car, but with this sort of thing they don't go much beyond telling you to replace the motor and see if everything works fine then.

I plan to mount the PWM in an underdash location obscure to all but the greatest scrutiny, and run the controller wires down to the evaporator unit from there.

This is an extremely rare unit of which I have have only become aware of one other in our whole Chrysler 300 Club.  So getting parts or a replacement unit would be next to impossible.  Therefore, if no harm is likely to be done by disconnecting the low speed wire and introducing a PWM on the high speed wire to the motor, I look at the PWM solution as being not only a shortcut to getting around the problem, but also simultaneously giving me more comfortable choices of fan speed.  But, of course, the caveat here is that a PWM has to be able to work on a brush-type motor for this solution to work - and if I read your messages accurately, you all concur that it should.  True?


A logic level MOSFET would be the major PWM component for controlling the motor speed. Search this forum and the old forum for previous info.
Consider the daffodil. And while you're doing that, I'll be over here, looking through your stuff.   8)


Apr 24, 2011, 07:25 am Last Edit: Apr 24, 2011, 04:03 pm by Daanii Reason: 1
You could use one of these, and just hook a potentiometer to it to control the speed. www.pololu.com/catalog/product/1373

EDIT: I did not notice that the motor driver recommended by daveg360 has a potentiometer already on the board. That does make it a very good choice for your application.


The item I posted in my first reply would be absolutely fine for this.  It's got plenty of head room and is extremely simple to use.  If need be you could pop the potentiometer off the board and install it somewhere handy and then run wires back to the board.  My only caveat would be that it might be a touch noisy at 100Hz.
If your system involves lethal voltages/life critical/flamable elements - you probably shouldn't need to ask.
The Arduino != PC.

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