I've been gaining understanding about using PWM for a motor speed controller. I'm developing a project (go kart) that uses a 24V DC starter motor and I want to build a motor speed controller that is somewhat rudimentary/simple.

Is it possible to utilize Arduino technology or some kind of PWM connected to a relay that switches the power supply on and off to the motor at the cycle speed that will act as a 'throttle'?

Or, is there a rudimentary means of controlling the motor speed in 4 or 5 stages (less elegant but equally as effective...)?

Any links to other resources or advice is greatly appreciated.

  • 2
    \$\begingroup\$ A relay can't even begin to switch at PWM speeds... \$\endgroup\$
    – Majenko
    Dec 28, 2011 at 23:58

1 Answer 1


You've got the right idea with PWM, but relays are totally inappropriate as the switches in this case. You want to run the PWM at least a fw 100 Hz, and that would be slow. That's fast enough for the motor to work well enough, but could cause audible whine. That is why most motor controllers pulse at 25-30 kHz, just past the audio range.

Probably the best way to switch the voltage to your motor is with a low side FET, or perhap several in parallel. Use good FET gate drivers intended for that purpose, and control those from the PWM output of your microcontroller.

A starter motor is not really a good fit for what you are trying to do, but you should be able to get some results like you expect. In general, start motors take a lot of current, have high torque, but aren't meant for sustained operation in several ways.

  • \$\begingroup\$ Long ago I saw old automotive GENERATORS (not alternators) used as motors with some success. These MAY be available from automotive dismantlers ("wreckers") but may have now got overly dear because of age. \$\endgroup\$
    – Russell McMahon
    Dec 29, 2011 at 0:42
  • \$\begingroup\$ Olin, thanks for the info. Any chance you would share a good source for a similar motor with less draw? We're using it in a pretty rugged environment (b roads, fields, small home made dirt track). \$\endgroup\$
    – Ofeargall
    Dec 29, 2011 at 1:54
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    \$\begingroup\$ @Ofeargall - why not look at motors from toy electric ATVs, motorized bicycles, wheelchairs, etc? You might even be able to salvage the control circuit. \$\endgroup\$ Dec 29, 2011 at 6:58
  • \$\begingroup\$ @ChrisStratton, I think that's a good idea. I guess my concern is they wouldn't ha ve enough torque for our application... \$\endgroup\$
    – Ofeargall
    Jan 2, 2012 at 19:20

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