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What kind of behaviour change should I expect to see, if I feed a 12V 50kHz frequency, with 50% duty cycle, or a 100Hz 50% duty cycle, across a DC Motor?

TheLoneKing
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Sherby
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    Give us a clue about the size or inertia of the motor... –  Sep 14 '13 at 21:48
  • I Just meant a normal 12V DC Motor, or it could be an LED. What effect does the frequency of the source have on it? – Sherby Sep 14 '13 at 21:50

1 Answers1

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This entirely depends on the electrical and mechanical qualities of the motor in question. (and of course the properties of the driver circuit)

The inertia and friction affects how the motor reacts to the incoming signal. It is like a mechanical low pass filter: the higher the inertia, and lower the friction the lower the cutoff frequency is.

The whole setup can be thought of an integrator, whose input is the control waveform, and the output is the RPM of the motor. The "worse" the motor is, the smoother the result will be.

A motor, with large inertia, and llow friction compared to that would have no effect - the frequency would be way beyond the cutoff point of the low pass filter, effectively "smoothing" out the result.

A tiny motor, with very small inertia would follow the waveform closer, meaning

  • fluctuating speed - if the low pass filter just cuts off just a bit below the frequency, the RPM would be like a triangle waveform put on top of a constant value.
  • stopping in between the pulses - if the inertia is small enough (cutoff point above the frequency), theoretically it is possible that the motor even stops between the pulses.
  • an ideal motor, with 0 inertia, and no friction (and of course ideal coils, no loss, no resistance, etc) would follow the input waveform completely: spinning at 100% rpm, when the pulse is on, and stopping when it is off. Issue is: such does not exist... (also, cutoff point would be at infinite frequency...)

In real life situations, using generic motors, I doubt you'd find anything different (but as I can't try this out right now, this is to be taken as an opinion). However, the resulting EM radiation can make a difference: you should shield appropriately. There can also be audible noise - as this clearly falls into the range of human hearing...

With some smaller motors, 100Hz could probably mean "jerkyness" in the operation.

ppeterka
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