DC motor controller help please?

Hi all,
I'm a Mechanical Engineering senior, and as you all probably recall, that means it's senior project time. My question pertains not to my own
project, but to the project of a good friend of mine who doesn't have Usenet access.
the basics of the question are these that follow, with a more detailed project explanation later if anyone is interested or if that helps define the problem better.
He's got a 12 Volt DC motor which is driving a shaft. He needs to have speed control on this motor. The motor has a rating of "amps at load = 800 milliAmps; amps at stall amperage = 3300 milliAmps".
He thought, as did I, that a big potentiometer would work. However, he's not finding them anywhere rated more than 2 Watts.
Is the wattage the packaging on the pot referring to not the wattage of the motor it's controlling? If so, what IS the wattage referring to on the pot?
Using Ohm's Law, V= IR, and P = I^2 R, it sure seems that 2 Watts divided by 3.3 Amps squared gives a really small resistance.
Electrical theory isn't my forte, obviously, but I told my buddy that I'd ask the vast knowledge base that exists out here, and see what came back.
Details of the project (insofar as much as I know them, this may not be 100% accurate) follow if anyone's interested. * * * * He and his team are building a 'table-top' size wave generator to be used in high schools, middle schools, and demonstrations. We have an amazing wave research center at our school, and they're sponsoring this development and build project. It's a long, skinny tank with a piston, basically, on one end to generate the waves. AFAIK, they did a whole bunch of research into different methods of wave generation and this seemed to be a good workable and buildable model. They can also create wind waves by pulling air across the surface of the water with some sort of vacuum, but that's not the part they're having issues with controlling. The piston is driven by a motor. The motor speed needs to be able to be varied. We know that there are plenty of power tools that run on DC that have the ability to vary their speed... What do they use? For a 12-V DC motor, what is appropriate?
Any and all advice/insight/comments appreciated! We all have only a few weeks left to really pull these projects together, so anything that's gonna help his team get it going right now will be really, really useful.
TIA, k wallace OSU
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Dear k wallace:
...

Your formulation of I^2 * R is the correct one. The potentiometer is only rated to *dissipate* 2 watts (in this case) indefinitely.

...
If your "buddy" isn't studying electronics, why not just have him use a variable DC power supply? He doesn't get graded for reinventing these... only for the resulting machine... Alternatively, Radio Shack likely has plans for making one from "bits", but will likely be more expensive.
David A. Smith
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N:dlzc D:aol T:com (dlzc) wrote:

So I'm assuming from this that the wattage this motor is putting out is 9.6 at 'peak' and 39.6 at 'stall' which I don't really understand. (my project is developing a heat sink, nothing to do with motors, I just told him i'd try and look up some folks who might have some ideas).
Therefore, a pot that dissipates 2 watts sounds like it would be too small? I'll ask him about the variable DC power supply but i believe that they already have the motor; they would then just hook the motor up to the variable power supply and control it there, with some sort of built-in dial or adjustable readout I assume.
thanks for your advice! k wallace

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For a more sophisticated control you may want to use PWM (pulse width modulation) with a full or half H-bridge circuit (depending on if you need reversing and braking). You can find a lot of info on this type of scheme all over the 'net. also, take a look at www.seattlerobotics.org for a lot of good info for motor control.

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Dear k wallace:

(sorry to reach over your shoulder "ms"...)
The braking part is really important. If the motor speed is changed to lower speeds faster than the load can naturallt brake it, the power supply will be *receiving* power, so it needs to be somewhat sophisticated.
David A. Smith
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For prevent this, you only need one or two rectifiers (diodes).
Stefan
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Dear Stefan Brrring:

... and a really big resistor...
David A. Smith
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says...

Speed regulation with varying load is going to be awful if you control speed with a resistor. A better solution, if you want to build a simple controller, is a variac (variable autotransformer) > 120 to 12 transformer > rectifier bridge.
Simple controllers with better speed regulation usually use what is sometimes referred to as "IR compensation". They look at the counter emf produced by the motor, which is a function of speed, and adjust the output accordingly. This type of controller is available for under $100 for small 90V and 180V motors, look at KB Electronics for examples. I'm afraid I'm not familiar with what may be available for a 12V motor, though I'm sure there's plenty out there.
If you decide to go ahead with the variable resistor, you'll have better luck searching for "rheostat" rather than "potentiometer". They're available over 200W - try Mouser or Digikey.
Ned Simmons
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Also ask at sci.electronics.design...
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Here is a PWM controller to look at. However, it is for one direction only.
http://www.criticalvelocity.com/item.php?itemid=spdc01
Dave
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wrote:

Here's a clue: what is the max power at the motor? 12 volts X 3.3 amps at stall thats 40 watts.
And you were going to control this 40 watt device with a pot capable of 2 or 3 watts? Come on now!
Try a light dimmer to a step down transformer to a full wave bridge. Simple but effective.
Brian Whatcott Altus OK
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no, no. He didn't think that would be a viable option, but while we were looking around the internet neither of us saw any pots rated higher than 2 or 3 watts, so didn't know what exactly to look for next. I sent him some links and advice from folks here and a search page for rheostats, and he asked me to give y'all a big thanks for your time. So, thanks!
Like I said, electrical theory isn't my strongest suit, but I should learn more about it than they teach us in the few elec fundamentals classes we took as juniors or sophomores- it seems like the interface b/w mechanical and electrical is only getting more important as tech develops; I will invest some future research (in or out of school) time in this area. regards, k wallace

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Hot tip-- invest in a copy of "The Art of Electronics" by Horowitz & Hill

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