All PWM drives contain an input converter, dc bus, and output inverter. For simplicity, only one phase of input and output are shown for a 3-phase drive.
Selecting the right drive is essential to getting the best performance and efficiency from an electric motor. A motor drive controls the speed, torque, direction, and resulting horsepower of a motor. Dc drives typically control a shunt-wound dc motor, which has separate armature and field circuits. Ac drives control ac-induction motors and, like their dc counterparts, control speed, torque, and horsepower.
For instance, take a simple application of a fixed-speed motor driving a fan. Replacing a three-phase motor starter with variablefrequency drive (VFD) permits the fan to operate at variable speeds. One benefit is energy savings, because the system varies airflow by controlling motor speed instead of with an air outlet damper.