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 PWM speed control of DC motor

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pebe Posted - Dec 01 2009 : 4:04:13 PM
When using PWM is there a lower limit for the pulse repetiton frequency? The 4QD site suggests 20KHz is probably the optimum and ideally the period should be shorter than the L/R time constant of the motor. But I seem to recall a small AC/DC motor being driven by a mains frequency lamp dimmer.

I want to control one from a PIC where the minimum PW would be 8ms.

Any info would be welcome.
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audioguru Posted - Dec 02 2009 : 6:05:30 PM
If you overload a motor it will run at a slow speed. It will smoke and burn.
But if you have a reasonable load then slow it with PWM it will be fine. It receives less power when PWM reduces its average power. Less power is less heat.
pebe Posted - Dec 02 2009 : 2:01:14 PM
Yes, I realised that but are there any ill effects from running at low speed, like overheating?
audioguru Posted - Dec 01 2009 : 9:04:58 PM
At low PWM frequencies a motor will pulse and vibrate.
At over 20kHz it will be smooth and silent.

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