How does a Variable Frequency Drive (VFD) control motor speed?

A VFD converts fixed-frequency AC to variable-frequency, variable-voltage AC to control motor speed. Since synchronous speed Ns = 120f/P, varying frequency changes speed. VFDs use rectifier (AC to DC), DC link (filtering), and inverter (DC to variable AC using PWM). V/f ratio is kept constant to maintain flux and torque capability. Modern VFDs include vector control for dynamic response similar to DC drives. Benefits include smooth starting, energy savings, and precise speed control.