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I know just enough about electronics that I think it's a wonder I haven't died from touching both ends of a flashlight battery, but here is my problem and I'm hoping someone out there can guide me.

I am restoring a 1957 vintage Chrysler dealer-installed air conditioning system in my 1957 Chrysler.  The evaporator unit that mounts on the transmission hump has a little fan on it that has a two speed motor.  But apparently there must be a short in the motor that causes it to run at high speed no matter which speed you switch it to.  When you energize either one of the speed wires the motor runs, and the other speed wire also tests energized.  Even if both speeds were working fine, you'd really want one somewhere between the two, so I want to go for a better solution than fixing what may be wrong in the motor.

My plan is to put what I understand is called a PWM in one of the speed wires and disconnect the other one.  That way I will have infinite control of the range of speed for the fan.  Now, all the info I have dredged up on PWMs says they are made to work on brushless motors.  I don't know if brushless motors were even born in 1957 yet, but this certainly isn't one.  And for the life of me I can't figure out if a PWM is supposed to work on a brush type motor as well.

This 12V motor is drawing about 8 amps on startup and 4 amps running.  Can I buy just any old PWM of perhaps 10-15 amp capacity and hook it up inline to this motor to control the speed?  Or do I have to use use a resistor type controller for this brush-type motor?  What should I look for and where should I buy one ready to plug in?
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Brushed motors are fine for PWM control.  Something like this: http://store.qkits.com/moreinfo.cfm/MXA066 would probably be perfect for your application.  I take it you are looking to control the speed with some form of knob?  if you're looking to control the speed with a microcontroller you'll need to modify that or look elsewhere.
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I know just enough about electronics that I think it's a wonder I haven't died from touching both ends of a flashlight battery, but here is my problem and I'm hoping someone out there can guide me.

I'm not a mechanic, nor do I play one on TV, but I did fix my wife A/C blower motor on her 93 Taurus this morning (some piece of foam rubber got in the squirrel cage and made it vibrate - damn, but they sure do put the screws for the cover in odd and difficult to reach spots!). I try to do what work I can on my autos, so I am going to try to help you a bit here...

I am restoring a 1957 vintage Chrysler dealer-installed air conditioning system in my 1957 Chrysler.  The evaporator unit that mounts on the transmission hump has a little fan on it that has a two speed motor.  But apparently there must be a short in the motor that causes it to run at high speed no matter which speed you switch it to.  When you energize either one of the speed wires the motor runs, and the other speed wire also tests energized.  Even if both speeds were working fine, you'd really want one somewhere between the two, so I want to go for a better solution than fixing what may be wrong in the motor.

Personally, I would look into seeing what is wrong with the motor; why it isn't switching speeds correctly. Even if you do put in your modification for PWM as you mentioned (though depending on whether this is a show-car or such, you might want to think about this), you'd be doing so on a motor that has a problem which could come back to haunt you at any time. Fix that issue first, then work on the PWM control. Likely it is either a burned-out resistor pack (basically resistors of different values switched in to control current - the resistors will be high-wattage wire-wound with sand/ceramic, or possibly heavy duty heating element wire wound on some kind of heat resistant core), or if the 57 is older than resistor packs (like I said, I'm not a mechanic!), then it might be some kind of weird switching arrangement on a series/shunt wound DC motor, to switch portions of the coil in/out to change the strength of the magnetic fields (and thus the speed of the motor). I suppose that there could also be other methods as well.

You might want to get in contact with a local or nearby vintage automobile club in your area; they could put you in contact with someone to help you get this diagnosed and fixed. I would also suggest looking into finding (it will be difficult) a repair manual for that year/make/model - look into old back issues of Popular Science magazine (on Google Books) to find advertisements on the auto-repair manuals (I am not sure Haynes or Chilton were doing it back then, but I have read that decade of PopSci, and there were other publishers of such repair manuals - so you might look into those; check via Amazon, Alibris, and Abe Books, among other online book sellers - oh, also Ebay). You might also find in the pages of PopSci some articles or "tips-n-tricks" type blurbs on fixing your blower motor; A/C units were a fairly new thing of the time period, and I do remember seeing a few articles about them in the magazine (on how to install third-party units, how they worked, etc). Lastly - Popular Mechanics magazine is also in Google Books, so check those out as well.

My plan is to put what I understand is called a PWM in one of the speed wires and disconnect the other one.  That way I will have infinite control of the range of speed for the fan.  Now, all the info I have dredged up on PWMs says they are made to work on brushless motors.  I don't know if brushless motors were even born in 1957 yet, but this certainly isn't one.  And for the life of me I can't figure out if a PWM is supposed to work on a brush type motor as well.

First off, understand that PWM is a control method (not just for motors) - it stands for "Pulse-Width Modulation"; basically it's a method of switching current to a device (whether a motor, an LED, a lamp, a speaker, or something else) at a set frequency, while varying the time the current is "off" vs the time it is "on" (if you are familiar with welding, it is similar to a "duty cycle" - the amount of time you can use the welder before you have to let it "rest" - though a duty-cycle is a much longer and drawn out form of "PWM").

What you were probably looking at (and here's another thing I have noticed - in auto mechanics, they use one terminology to explain another - and confuse the hell outta people who know both) were "motor controllers", which use PWM to control the speed of a motor.

Now - your motor is a brushed motor (whether it is a permanent magnet motor, or some form of shunt/series/combo wound - that's a wholly different question); while brushless motors did exist (ie, 3-phase AC motors, and/or shaded pole AC motors) in 1957, they were not likely used (as you would had to convert the DC of the battery to AC - they had to electro-mechanically do this just to get radio to work, because of the tubes in the radios needing the higher voltages). Likely the motor is either a permanent magnet motor, or possibly a shunt/series wound.

You will want a motor controller with PWM (and it doesn't need reverse capability) for a brushed DC motor, at the voltage the motor uses (you are claiming 12 volts, and you know your car - but 6 volts was pretty common in the period; your car was in the "transition" from 6 volt to 12 volt systems - heh, reading PopSci, they were also thinking about using hydraulics everywhere for power accessories!), for the current it needs.

This 12V motor is drawing about 8 amps on startup and 4 amps running.

Something tells me this isn't a permanent magnet DC motor, given the current draw - probably shunt or combo (I suppose it could be series wound, but those were prone to "runaway" with light loads, and I think a fan would be considered a "light load" - dunno).

Can I buy just any old PWM of perhaps 10-15 amp capacity and hook it up inline to this motor to control the speed?

Such a motor controller would probably work OK, though I think on the low end I would go with 12 amps (2 amps extra doesn't seem like enough headroom to me).

Or do I have to use use a resistor type controller for this brush-type motor?  What should I look for and where should I buy one ready to plug in?

Nothing that you buy will just "plug in" - if you want to install something like this into your system, you are going to have to do some "hacking" here to get it to work. If you wanted to go with a resistor controller (they use them in automobiles for blowers even today on modern cars; they are cheap, relatively robust, but you have to mount them to something to act as a heatsink for the extra heat they produce so they don't burn out), I would look into how modern cars have them hooked up, generally (IIRC) they are simply a couple large high-wattage power resistors with three "taps" - the lowest tap connects direct to the power input from the fuse block (high speed), then next tap is across the first resistor (medium speed), and the last tap is across both resistors (low speed); you could add a third or fourth resistor (and taps) to add a few lower speeds if you need them. You would also need a multi-position switch, of course (rated for the current); the resistors would likely be 20-25 watt or higher power resistors, probably 5-15 ohms each - something like that. Get ones with metal heatsinks attached, then mount those to a larger piece of metal or to the firewall of the vehicle (or if you can manage it without damage or modification, to the evaporator of the A/C unit via a mounting screw, so that the A/C carries away the excess heat).

Note that all my numbers above are "guesses" - I don't know much about resistor packs as used on vehicle A/C blower systems, so do a lot of research beforehand if you take this route instead of a motor controller. You might also look into buying a used but working resistor pack for a vehicle with similar blower specs from a pick-ur-part place. They likely have tons of them (and might be able to recommend you one).

Finally - I want to re-iterate to you the importance of getting that motor fixed -first-; like I said, look into local vintage automobile clubs in your area, you might find a person who was a mechanic of the era (heh - maybe you'll find a "Gus Wilson" old-timer who knows exactly what to do!) who can help you out, and get it working right. You might also find somebody who can help you install or set up a motor controller with PWM or a resistor pack, and do it in such a way so-as not to ruin the value of the vehicle (especially if it is a show car).

Good luck, and I hope the above helps you out!

smiley
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Is it possible that the two wires you are connecting to are downstream of where the resistor packs would normally be?

Cr0sh - I agree that double the current at startup sounds odd (but I guess it could it be a little "sticky" after all these years) in terms of practical application - can you treat a shunt motor in the same way as a permananent magnet motor in terms of PWM?
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Is it possible that the two wires you are connecting to are downstream of where the resistor packs would normally be?

Cr0sh - I agree that double the current at startup sounds odd (but I guess it could it be a little "sticky" after all these years) in terms of practical application - can you treat a shunt motor in the same way as a permananent magnet motor in terms of PWM?

To be honest, I've never tried it, but I can see why it might be an issue (because in either shunt/series/combo - you'd be switch both the field and rotor coils on/off - hmm). If you could re-wire it so-as to keep the field steady (ie, much like a permanent magnet motor), while only applying the PWM to the rotor, it would probably be OK.

Hmm - maybe PWM wouldn't work well or at all with such motors (short of modification of the internal connections, of course). In that case, resistor packs or some other method might be called for (if you could vary the field coil and/or the rotor coil strength separately you could effect speed changes that way).

Interesting...

smiley
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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").
« Last Edit: April 23, 2011, 07:24:52 pm by zoomkat » Logged

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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?

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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.
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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.
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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.
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Quote
Finally - I want to re-iterate to you the importance of getting that motor fixed -first-; like I said, look into local vintage automobile clubs in your area, you might find a person who was a mechanic of the era (heh - maybe you'll find a "Gus Wilson" old-timer who knows exactly what to do!) who can help you out, and get it working right. You might also find somebody who can help you install or set up a motor controller with PWM or a resistor pack, and do it in such a way so-as not to ruin the value of the vehicle (especially if it is a show car).
I agree with this approach and I remember the Gus Wilson stories as well.  Keep in mind that the resistor(s) are generally in the shunt field circuit and the motor speed increases when the resistance increases.  This is not a mistake, you really have to decrease the shunt field current (by increasing the resistance) to increase the speed.  

Don

« Last Edit: April 24, 2011, 12:21:35 pm by floresta » Logged

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I think that the OP has stated that it's a permanent magnet type motor......
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Thanks for all your help folks.  I just ordered the MXA066 controller from Qkits and should see it this week yet.  It looks like it should be easy enough to wire in.

Kebwood
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Good choice. Look forward to hearing how it works.
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