The working principle of a VFD (Variable Frequency Drive) is based on controlling the frequency and voltage of the power supplied to an AC motor to adjust its speed and torque. In simple terms, a VFD converts fixed-frequency AC power from the mains into variable-frequency AC power. By varying this frequency, the VFD can make a motor run faster or slower, depending on the process requirement.
The operation happens in three main stages — Rectification, DC Link, and Inversion. First, the incoming AC power (usually 50 or 60 Hz) passes through a rectifier circuit, which converts it into DC (direct current). This DC power is then filtered and stored in capacitors on the DC bus to provide a stable voltage. Next, an inverter section made of IGBTs (Insulated Gate Bipolar Transistors) rapidly switches the DC on and off to generate a Pulse Width Modulated (PWM) AC output with variable frequency and voltage.
The frequency of the inverter output determines the motor speed. For example, if a 50 Hz motor is supplied with 25 Hz, it runs at roughly half speed. The voltage is also adjusted proportionally to maintain the correct torque, a method known as the V/F control principle (Voltage-to-Frequency ratio). This ensures smooth motor performance at all speeds.
In short, the working principle of a VFD involves converting AC to DC, filtering it, and then converting it back to AC at a controlled frequency and voltage. This electronic control allows soft starting, smooth acceleration, precise speed regulation, and significant energy savings — making VFDs an essential part of modern automation and motor control systems.