Three phase AC motor speed controller is a three phase sine wave power inverter with variable frequency. Since the motor is synchronized with the line frequency, to control the speed of three phase AC motor can be done by controlling the frequency of the power line supply. Even thought three phase power inverter is a complex circuit, there is an integrated circuit to fulfill this purpose. The schematic circuit diagram is as follows :

3 phase ac motor - 1

3 phase ac motor - 2

The component values shown only that for OB2 channel. The others are similar. Please be careful when designing the layout, because this motor controller use optical isolation. Low and high voltage network have to be separated clearly. This is to minimize the risk of hazardous electrical shock.