Controlling motor speed with a potentiometer

  1. I'm trying to control the speed of a DC motor by using a potentiometer, but am unsure what value resistance I should use. Having tried 500k, 10k and 500ohms, the motor stalled almost as soon as I turn the dial (a bit more leeway with the 500ohm but not much).

    [​IMG]

    Rated as 12kg-cm (or 1.2 N.m) torque, with an operating voltage of 4.5-18V (rated at 12V).

    No load current is 70mA, fully loaded is 1380mA. Has a shaft speed of 36rpm.

    Not going to be using it for much so it shouldn't get anywhere near fully loaded.

    Any help at determining an appropriate potentiometer? Considering 3V, 6V or 9V, though 9V will probably exceed the power rating pretty quickly.
     
  2. jcsd
  3. Maybe you should consider a switched-mode controller.
     
  4. Run the motor with a variable voltage power supply.
    Record the minimum current and voltage required.
    The maximum voltage across the potentiometer will be the power supply voltage minus the motor voltage.
    Calculate the potentiometer resistance using the maximum voltage across the potentiometer and the minimum current.

    The slider on the potentiometer should be good for 1.38 amp. This is a large potentiometer.
    Be sure the potentiometer meets the necessary watt and current rating.

    Rheostats are frequently used as high power potentiometers.

    Some DC motors will not operate satisfactorily with potentiometers
     
  5. Just messing around at the moment and trying to get my head around it all, hence why I'm using the potentiometer.
     
  6. Not sure if I'm doing something wrong, but using a 3V battery, I get approx. 5rpm, with a current of 40mA.

    1.8V: 0 rpm, 85mA
    3V: 5 rpm, 40mA
    6V: 12 rpm, 41mA
    9V: 20 rpm, 50mA
    12V: 30 rpm, 54mA

    Calculating the resistance required when using a 9V battery gives: (9V - 3V) / (0.04A) = 150 ohms. Is that the correct procedure (bearing in mind I'm limited with regards to voltage range)?

    However, using my 500ohm variable resistor in series, the motor started at:
    3V battery supply: 5 ohms
    6V battery supply: 11 ohms
    9V battery supply: 20 ohms
    12V battery supply: 28 ohms

    It stalls at a higher resistance than it starts due to the back emf, but I'm more interested at when it starts. It seems the voltage across the motor jumps from very low to almost 100% when it starts, instead of gradually increasing.

    Testing it at 12V (quickly, as it'd fried my 0.5W potentiometer), it starts at a resistance of 28 ohms but stalls at 290 ohms, which is a huge discrepancy.

    Am trying to build a rudimentary controller: motor + potentiometer connected to 'wheel', if wheel turned than potentiometer resistance decreases, motor starts and rotates until the resistance increases sufficiently, returning the wheel to its equilibrium. But the much higher stalling resistance makes it seem a near impossible task.
     
  7. vk6kro

    vk6kro 4,059
    Science Advisor

    As this uses over an amp under full load conditions, simple potentiometer methods will not work.

    A better approach is to use pulse width modulation.
    You can read about this here:
    http://en.wikipedia.org/wiki/Pulse_width_modulation
    The potentiometer varies the duty cycle of the voltage reaching the motor.

    Here is a circuit that is simple and looks effective:

    [​IMG]

    The four 4093 gates are all in one cheap integrated circuit.
     
  8. Thanks for that, vk. Was doing some reading on the topic last night and was just about to leave to buy diodes, transistors and an inverter IC to make a PWM.
     
  9. Since 1A is not much, you can also use some sort of proportional controller using an op-amp. In any case, you will need a feedback mechanism.
     
  10. Yeah, that's what I'm getting at. So I could just connect the output of the potentiometer directly to an op-amp (say, an inverting amplifier) and use that as the feed in to the motor? Would that resolve the issue of the motor stalling and starting at different resistances?
     
  11. The July issue of Nuts and Volts magazine just came out and there is an add for "30A PWM DC Motor Speed Control" that might do what you want.
    qkits.com

    Don't take this as a recommendation for QKITS as I have never used their product.
     
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