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
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
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
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.
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)
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
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!
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
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
David A. Smith
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.
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
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,
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.