A rectifier is a non-linear component that converts ac to dc. Rectifiers typically use non-linear devices such as diodes of thyristors. These devices permit the flow of current in only one direction. Three-phase ac drives typically use a 6-pulse bridge rectifier with thyristors.
Both dc and ac drives need a rectifier. In a dc drive, the rectifier converts ac to dc for the motor. In the ac drive, the rectifier converts the ac to dc for the inverters that eventually produce the variable frequency, variable voltage ac for the motor.
The rectifier takes 3-phase sinusoidal ac and chops out the positive sections for each phase to produce dc. The dc voltage has triangular shaped sections. The dc current has sine shaped portions.
The dc voltage is 1.35 times the RMS ac voltage. To conserve power in and out, the dc current is 0.816 times the ac line current.
Some ac drives use active (intelligent) bridges which are capable of regenerating into the ac power grid.