A VFD (Variable Frequency Drive) controls the speed of an AC motor by adjusting the frequency and voltage of the electrical power it supplies. In simple words, the higher the frequency, the faster the motor spins; the lower the frequency, the slower it runs. This relationship between speed and frequency is the basic principle behind how every VFD works.
Here’s how it happens inside the drive. When AC power enters the VFD, it first passes through a rectifier, which converts it into DC (direct current). Then the inverter section, using IGBTs (Insulated Gate Bipolar Transistors), converts that DC back into AC with a variable frequency. By changing the switching pattern of these IGBTs, the VFD can precisely control how many electrical cycles per second (Hertz) reach the motor — directly controlling its speed.
To maintain proper torque, the VFD also adjusts the voltage-to-frequency ratio (V/F ratio). This means when frequency decreases, voltage decreases proportionally, ensuring the motor produces the same torque at different speeds. The drive can also smoothly accelerate or decelerate the motor, avoiding mechanical shock or sudden starts and stops that could damage equipment.
In short, a VFD controls speed by varying the frequency and voltage of the power going to the motor. This allows smooth, energy-efficient operation while giving complete control over acceleration, braking, and torque. That’s why VFDs are widely used in pumps, fans, conveyors, and other industrial applications where speed flexibility and energy savings are essential.