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I have read a lot about using high frequency square/rectangular wave pulsed DC for motor speed control aka PWM-ing, but my question is a bit different.

What if I run a DC motor with a low frequency unfiltered sine wave pulsed DC? Let's say that I want to run a 12V linear actuator motor from the low frequency 50/60Hz AC mains voltage, using a suitable step down transformer and rectifying diodes but no capacitors or any sort of filtering. My assumption is that full wave rectification should work better than half wave.

What are the possible outcomes regarding energy efficiency and longevity of the motor with this setup?

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  • \$\begingroup\$ Which 'linear actuator', and what will it be used for? \$\endgroup\$
    – Bruce Abbott
    Aug 3, 2020 at 6:33
  • \$\begingroup\$ Should be no issue. Some audible 50/60 Hz hum perhaps if you load it hard. \$\endgroup\$
    – winny
    Aug 3, 2020 at 6:41
  • \$\begingroup\$ @Bruce Abbott it's just an example scenario, any medium sized DC motors that are supposed to drive a load, for example, a linear actuator like I mentioned before (smaller, cassette player motor doesn't count). \$\endgroup\$
    – Yudhi G.
    Aug 3, 2020 at 7:17
  • \$\begingroup\$ you can just use the RMS voltage to run your figures about efficiency and longevity. \$\endgroup\$
    – dandavis
    Aug 3, 2020 at 7:39
  • \$\begingroup\$ The reason I asked is that some linear actuators have built-in electronics that might not like unfiltered DC. But if you are just considering a mechanism (brushed DC motor with gears) then it's similar to a regular DC motor/gearbox. \$\endgroup\$
    – Bruce Abbott
    Aug 3, 2020 at 8:55

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The main effects of running a brushed DC motor on unfiltered DC are increased loss and heating due to the higher ratio of rms to average current, and increased noise and vibration caused by the motor changing speed in time with the rectified waveform.

This will reduce efficiency and may reduce longevity, but probably not enough to worry about in most applications. It is often done in mains powered appliances such as hair dryers and heat guns, which use PMDC motors powered via a bridge rectifier.

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