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I am designing a drive that outputs an RPM of about ~350, with output torque of about ~10 mNm. I found a cheap motor that provides this torque (plus more for insurance), but the speed at rated voltage is much higher than I want. I calculated that a 30% duty cycle would give me the design torque and RPM I want. I am just concerned that I would be running the motor too slowly. I know that the microcontroller that sends the PWM signal has a limit on the minimum duty cycle I can apply, but I am wondering what, generally, is a good rule of thumb for practical minimum duty cycles applied to a motor (since if a motor moves too slowly, the friction, momentum, etc may change and it may not behave in the expected deterministic manner). To put it short, is 30% too low? Or should I be more concerned about the ratio between rated speed and design speed?

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    \$\begingroup\$ Please post a link to the motor's datasheet. \$\endgroup\$
    – HikeOnPast
    Dec 7 '12 at 17:57
  • \$\begingroup\$ johnsonelectric.com/en/products/motion/automotive-motors/… \$\endgroup\$
    – Max H.
    Dec 10 '12 at 13:08
  • \$\begingroup\$ As you point out, that motor is rated for ~10kRPM loaded and ~12kRPM unloaded. I've seen motors run successfully down to ~20% of base speed, but I think you'll have challenges running it at 3% of base speed. There's already a worm gear on the output shaft...why not use a gear reduction in your application? \$\endgroup\$
    – HikeOnPast
    Dec 10 '12 at 15:18
  • \$\begingroup\$ Limited space. My design has changed now, however, as I have instead incorporated a small & cheap gearmotor instead. \$\endgroup\$
    – Max H.
    Dec 10 '12 at 16:00
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Let me preface my answer with a disclaimer: All of my experience PWM'ing motors is with speed control of cooling fans. Your application will certainly differ, at least a little bit.

The way I find the minimum duty cycle, I look for the minimum that will allow the motor to start spinning. With standard muffin fans, this is about 40%. Then I add a small margin just to be safe. I find this by applying 0% duty cycle and slowly ramping it up until the motor starts spinning.

This startup power is higher than the amount of power required to keep the motor spinning once it has already spun up. With fans, this is somewhere in the 20-30% range. In other words, if I wanted to spin a fan really slow, I would have to apply 40% to get it moving and then I could back down to 20-30%.

Normally, just to be safe, I do not go below the startup power. That way I can be sure that the motor is spinning, although it does limit the minimum speed that I can do.

There are problems with this, however. Many things can affect the startup power requirements. Temperature, motor loading, age, dust, different motor lots, etc. You have to take all this into account, and built in some power margin.

Alternatively, you have to monitor your motor through a tachometer or something similar. Then have some motor control software do the appropriate thing if the motor is spinning too fast or too slow. Good motor control software will automatically take into account the startup power and other things.

If you don't want to write motor control software then you have little choice but to empirically measure what the startup power requirement is for your system and then add some more for margin. And hope that you added enough.

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  • \$\begingroup\$ Also consider whether the motor has an integral cooling fan or otherwise relies on its own motion to cool itself. If running the motor more than ~20% below rated speed, it will likely overhead when running near rated torque. \$\endgroup\$
    – HikeOnPast
    Dec 7 '12 at 17:56
  • \$\begingroup\$ I was originally planning on running it on much less than 20% below rated speed. I was not planning on having a cooling system. \$\endgroup\$
    – Max H.
    Dec 7 '12 at 18:19
  • \$\begingroup\$ In my experience, running a motor continuously at that far below rated speed is a bad idea. Can you use a mechanical reduction of some sort (pulley, gearbox, etc.) so that the motor can run closer to rated speed? You may even be able to use a smaller motor (torque multiplication). \$\endgroup\$
    – HikeOnPast
    Dec 7 '12 at 21:34
  • \$\begingroup\$ @DeanB Bad, how exactly? \$\endgroup\$
    – user3624
    Dec 7 '12 at 21:41
  • \$\begingroup\$ Cooling, primarily. Imagine the worst-case scenario of a motor being used at stall as a torque actuator - it never completes a full revolution. There's no airflow other than convection, but potentially full I2C losses in stator, rotor, commutator (assuming brush DC). There's a reason that inverter-rated AC motors that are made to be run at low speed continuously have external, full speed blowers rather than relying on shaft-powered cooling fans. \$\endgroup\$
    – HikeOnPast
    Dec 7 '12 at 21:51

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