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How to control DC motor speed?

  1. Jul 3, 2012 #1
    Hi everyone,

    I have a 24V, 15A D.C motor, and need to control its RPM.
    I know RPM is directly proportional to voltage, and so the answer lies in a technique to vary the voltage across the motor.

    I have looked into potentiometers, variable resistors, and pulse-width-modulation, but since I am only a beginner in electrical engineering, I need help to select the simplest method.

    With regards to potentiometers, I think they will not work because the amperage is too high.
    For variable resistors, I've heard I will also need to use a transistor and some other things, and I am not sure how to set up the circuit either.
    For PWM, I really like the benefits, and the ease of control, but the circuit is really complex. Are there any PWM controllers out there suitable for my situation without excessive modification?

    Help would be really appreciated...
  2. jcsd
  3. Jul 3, 2012 #2


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    24V and 15A looks like the motor could be up to 360 watts or 1/2 horsepower (but probably delivers less mechanical power than that).

    conceptual a very large pot (same as a variable resistor) would be simplest, but you would need a very large one, because to slow down the motor, you would need to lose a lot of power into the resistance.

    PWM is better and they make controllers. if you make it out of big power transistors, rather than SCRs or triacs, then the PWM circuit can be pretty simple.

    using SCRs in a "chopper" circuit for DC control is, admittedly, a b itch. don't try to do that (once and SCR is kicked "on", it's pretty hard to turn it off in a DC context because you need to reverse bias the SCR to turn it off). but you don't need that for a motor as small as yours. a PWM controller for an electric car, well that's different.

    so can you build, with an op-amp, a simple triangular waveform generator? and with a nice triangle wave, do you know how to generate a PWM signal from that? (takes another op-amp.)

    then what you need to do is figure out how to build, the current amplifier with power transistors. think a little about the Darlington circuit. you can put some of these transistors in parallel to get to your 15 amps. and you can put the DC motor in between the collector (or common collector) of the power transistor(s) and the V+ power supply.

    is the motor spin in the same direction at all times? or do you need to be able to reverse it? is this motor for a servo system?
  4. Jul 3, 2012 #3

    jim hardy

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  5. Jul 3, 2012 #4


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    I had a service call at a local packing plant and they used 90 v dc motor controls, and I am positive you can get them for 24 v use, as the questions from the manufacturer included 24 v , 90v ,or 180v. Is this a perm, magnet motor, or other. Based on the small size I am assuming it is a perm. magnet. If you can source (from an old theater or such) an old 2000 to 3000 watt rheostat lighting control, you can use that to control a xfrmer and a ac-dc power supply to vary it down to 0v (most rheostats have an off position). I know it sounds complicated but it can be the least expensive option depending on you scavenging skills.
  6. Jul 5, 2012 #5
    PWM is a good way to go because it is one of the more efficient methods of controlling voltage to the motor. However you shouldn't use a PWM frequency much higher than the minimum you need for smooth operation of the motor. The higher the frequency, the more eddy currents are generated, the more the motor heats up, and the more the efficiency drops.
  7. Jul 6, 2012 #6
    write a simple prog. in pic assembler using a pot with a/d conversion as an input value for your duty cycle register. output to a mosfet and voilá...
  8. Jul 6, 2012 #7


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    This is essentially true....but slight correction.

    24V X 15 A will yield 360 VA. Amps are real whether they are partially "imaginary" or not. An amp meter will read real and imiginary amps as real amps....and they are.....if you put 15 imaginary amps thru a 10 amp wire...it will fry it relatively soon.

    Assuming a .8 power factor....this motor will likely yield 288 watts....watts can directly be converted to HP at the shaft. VA is the actual power consumption (real and reactive) that it will take to yield your watts.

    I'm sure you meant to say this....but just clearing up, doing your "light work" so to speak.
    Last edited: Jul 6, 2012
  9. Aug 4, 2012 #8
    Simplest way I know of to do what you want is to just get a 24vdc electric scooter controller that uses a 5k Pot for the throttle control. You will be able to set it to whatever speed you want. A 24v 350w or 500w watt controller would work and probably cost you about $40- $50. Put a 20/25 amp fuse in it and it should work fine.
  10. Aug 5, 2012 #9


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    That's a thought! A lot of readers would be envious of your cheap prices; but maybe could get something from a wreckers.
  11. Aug 5, 2012 #10


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    psparky, this thread is about a DC motor, so none of what you wrote (about real and imaginary) applies.
  12. Aug 15, 2012 #11


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    OH ya, my bad. I was wondering if DC motors have reactive power, but now that you mention it....how could they with a zero frequency input!!!!!
  13. Aug 24, 2012 #12

    i say, do not but for emergencies use variable potentiometers or variable transformers to control your DC motor load. the stall mode will cook your motor.

    with PWM, the motor will will have time during the off phase of the circuit to diffuse any generated heat.

    a dual full h-bridge circuit with PWM (dual 555) is my favorite circuits.

    since you would also like to have your control interface isolated from the motor driver, i will suggest something for you. there are three components, a plastic straw form a fast food restaurant, an IR photodiode of the package size that would fit in the straw and a photo-transistor of that size.

    into a short section of straw insert one of each, facing each other.

    electrical tape for the wrap and resistors where necessary and your opto-isolated interface circuit is complete
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