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Thread: [PIC]: Motor PWM question & C671JW part question
www.piclist.com/techref/microchip/ios.htm?key=motor
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face BY : Roman Black email (remove spam text)



Bob wrote:
>
> I'm building a project I seen in an electronics mag that is using a C671 PIC,
> that PWM's a hexfet to run a DC motor.  The original ASM code is generating
> about 20ms wide pulse widths (50hz).  I am looking to use this to drive a fuel
> pump at different speeds.  I don't know the exact specs of the pump yet, but I
> assume that it can do something like 40+ pounds of pressure.
>
> The question:  Is there an "optimal" PWM cycle size that I should be using for
> this kind of DC motor?  Does it depend on what speed it is running at?  i.e.,
> could/should I code it so that it has a different PWM frequency at different
> motor speeds for optimal tourqe & minimal motor core loses?  Or would you
> recommend just throwing a certain sized capacitor in there to even out the DC
> pulses?

It doesn't matter that much provided the pwm is fast enough
to give a reasonably regular current in your motor. This
depends on motor inductance, but freq above 10kHz are usually ok.
I must ask a question, does your pwm keep the freq constant
and change the pulse width to vary speed, or do you always
use 20mS pulses, and have changes in freq to change motor
speed?

It will help if you explain more about your end application,
if you are making a fuel injection controller you do know
that normally fuel pressure is regulated by a pressure
return valve, and fuel pump speed won't affect it much?
I'm curious. :o)
-Roman

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