What do i need to create a control circuit to regulate an AC engine speed?

I want to use PWM techniques to regulate the speed of a relativly small AC engine. I know there are integrated circuits that can performe this task, but this is a college proyect so my friends and I need to create the contorl circuits ourselves. First we need to now how PWM technique work (Specifically the mathematical behavior of the wave shape in time). Now the control circuit will be attached to a three-phase inversor that consits of six IGBT's. The load conected to the inverso will be of course an AC engine. The engine speed will change in time, so we need to control the voltage at the bornes using PWM techniques. Remember we cannot use integrated circuits available on the market other than IGBT's and maybe some operational amplifiers, it is important to build the control circuit ourselves, so the more specific the answers the better.What do i need to create a control circuit to regulate an AC engine speed?
You should use pulse width modulation (PWM). PWM is essentially an on-off sequence in time, like a square wave alternating between 0 and 1.





Clearly full voltage will give you maximum speed. This happens when the voltage is always on. Now, suppose you only wanted to go half the maximum speed. You could send a a square wave with 50% duty cycle (on half the time and off half the time) at high enough frequency that the motor will spin at a constant 50% maximum speed. The motor is like a lowpass filter and therefore, a square wave at high enough frequency will produce a steady response without oscillation.





If you want the motor to go slower, say, at 25% maximum velocity then send a pulse that is on 25% of the time and off 75% of the time.





In reality, if you want 25% speed you may have to send something like a 27% duty cycle because of the time delay between commanding 'on' and the system actually turning 'on'. In this application, though, I don't think you will have this porblem.