A VFD controls the speed, torque and direction of an AC induction motor. It takes fixed voltage and frequency AC input and converts it to a variable voltage and frequency AC output. In very small VFDs, a single power pack unit may contain the converter and inverter.
Fairly involved control circuitry coordinates the switching of power devices, typically through a control board that dictates the firing of power components in the proper sequence. A microprocessor or Digital Signal Processor (DSP) meets all the internal logic and decision requirements.