On Thu, Oct 06, 2011 at 08:17:11PM -0700, Femm wrote:
>
>
> Ahhh Gerald I hate to tell you this but you are still feeding 24 volts to that
> motor...
True enough... but it doesn't actually matter, since motors don't really
care about voltage. What matters is the total power dissipated across it
- which is what PWM is intended to control. In a DC application, the
voltage would determine that P(d) by working across the resistance
presented by the motor, which would result in a given current - but PWM
is not DC. You could, theoretically, apply 100kV across a motor; as long
as you pulsed it at a low enough duty cycle, that motor would work
perfectly, last just as long, etc. (Granted, 100kV would have its own
problems - spark gaps, for one - but the overall point remains valid.)
I used to design pulsed power systems for Hughes Aircraft way back when.
If we had tried driving those Gunn and IMPATT diodes that we were using
with DC to the output levels that we needed, we would have sent them up
in a cloud of smoke in literally fractions of a second. With PWM, we got
incredible power densities, and enough signal strength that we routinely
bounced our millimeter-wave signals off the Moon as a test. Great heat
transfer characteristics, too (although I don't recall the physical
basis for why that was the case.)
Voltage/resistance ratios - i.e., Ohm's Law - aren't all that applicable
once you start chopping the drive power. Especially around inductors,
which motors are.
--
Ben Okopnik
-=-=-=-=-=-
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