Several AC motor controller models are presently being used as general purpose drives.  These are the Pulse Width Modulated (PWM), the Current Source Input (CSI), and the Load Commutated Inverter (LCI).  Even though the different models offer specific advantages and characteristics, the PWM type is universally regarded as offering businesses the best combination of simplicity, performance and economy for general purpose applications.

PWM controllers operate by converting AC power source to a fixed DC voltage using a full-wave rectifier.  The resulting DC voltage is smoothed by a filter network and applied to a pulse width modulated inverter using high power transistors.  The speed reference command is directed to the microprocessor which optimizes the carrier (chopping) frequency and inverter output frequency at the same time.  This is necessary to maintain a proper volts/Hz ratio and high efficiency throughout the normal speed range.

The voltage being applied to the AC motor is a pulsed approximation of a true sinusoidal waveform.  This is also known as a PWM waveform due to the fact that both the carrier frequency and pulse width is modulated to change the effective voltage amplitude and frequency.  The current waveform very closely follows the shape of a sine wave and as such provides improved low speed motor performance, efficiency, as well as minimal motor heating.