PulseWidthModulation comments

[Roger Heflin]

On Thu, 4 Feb 1999 rauscher at icst.com wrote:

> 
> Ludis wrote:
> >Years ago as a summer intern I worked on a motor controller which used
> >PWM.  A brushless DC motor ran on up to 120 to 170 volts at up to 10 to
> >15 amps.  A switching (PWM) power supply controlled the voltage applied
> >to the motor.
> <snip>
> >An improved version used a microcontroller controlled digital PWM.  I
> >think the PWM frequency was about 20 KHz and the PWM had 5 bits of
> >range.  The same microcontroller monitored the motor position/speed to
> >form a closed loop system.
> 
> Was this motor designed specifically for this freq? When I first developed
> this PWM motor controller, I was trying to run it at 20KHz also. But the
> motor was down on power. Motor was an AstroFlight cobalt, 9.6V at 35Amps,
> such as those used in R/C model aircraft. Going to the lower freq brought
> the power back up. I looked at possibly skin effect, but the wire used in the 
> winds wasn't that large.
> 

It could also be inductance on the windings was causing the power to
be down more than predicted.  I would think using a lower frequency
would correct a problem in this area over using a high frequency.
There may be enough windings that prevent the current from ramping up
fast enough at 20khz.

			Roger