DC Motor Question

I have a question about the durability of small permanent magnet motors.

Many years ago, I had a small, home-made board for running a stepper motor off my old Commodore 64. I did some experimenting with a geared DC motor from an old toy. The motor was connect to the card as if it were a single coil of the stepper motor. I wrote a short basic program to send pulsed current to the motor and discovered that by varying the proportions of the pulses and pauses I could change the speed of the little motor. Many years later, I learned that this technique was called pulse-width modulation, and was being used for digital sound encoding.

But I wondered then, and still wonder, if the pulsed current was bad for the motor. Specifically, is it bad for the brushes?

It is not good for the brushes because the peak current goes up as you reduce the speed. That is because the back emf falls so the voltage the coil sees gets higher the slower you go.

Having said that, it is a widely used technique. The GM3 used in the extruder is only a toy motor and I found the 6V version's brushes wear out very quickly when fed for 12V PWM. The 12V version seems to last much longer, as one might expect.

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[www.hydraraptor.blogspot.com]

Would it be possible to smooth out the current with capacitors?

Not really, capacitors smooth voltage. You could use an inductor in series to convert PWM to an almost constant current though.

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[www.hydraraptor.blogspot.com]

nophead Wrote:
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> Not really, capacitors smooth voltage. You could
> use an inductor in series to convert PWM to an
> almost constant current though.

Wouldn't that tend to conteract the whole speed-regulation effect; i.e. the inducer would draw extra power when the power is on, then release it when the power is off, and keep the motor running at the same speed?

The current is smoothed out to be nearly constant, but that constant is still proportional to the mark space ratio of the PWM so can still be varied to control the speed. Or more correctly it controls the torque and the speed change is a side effect.

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[www.hydraraptor.blogspot.com]

Sounds good. I'll have to do some research on this.