Variable frequency PWM

[umka]

I am trying to build a variable frequency PWM for to control the speed of either a single of three phase electric motor but am having trouble being able to set the period as a variable so its speed can be constantly changed.

I did a search and found something a while back but cannot find it now so any help would be appreciated.

I have a pic 18f4431 for power pwm if needed which I would think it is.


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[PCM programmer]

Does this mean you want two separate frequencies, based on the motor
type ? Then you vary the speed by changing the duty cycle.

[umka]

With an AC motor do you not have to vary the frequency to vary the speed? then the duty ratio will determine the current limit?

I may have this total wrong but i want to be able to have a PID speed controll on and ac motor either single or three phase.