What is a four-quadrant drive and how does it operate?

A four-quadrant drive operates in all combinations of speed direction (positive/negative) and torque direction (motoring/braking). Quadrant I: forward motoring, II: forward braking (regenerating), III: reverse motoring, IV: reverse braking. Standard voltage-source inverter drives are inherently four-quadrant capable. Power flow reverses during braking - energy must be absorbed by DC link capacitor, returned to grid (active front end), stored in battery, or dissipated in resistor. Required for servo applications, cranes, elevators, and electric vehicles.