The Complete Handbook of Variable Frequency & Adjustable Speed Drives

Chapter 1: Understanding Variable Frequency & Adjustable Speed Drives

A variable frequency drive, or VFD, is an electronic device that controls the rotational speed of an alternating current (AC) electric motor by varying the frequency and voltage of the power supplied to it. The term is often used interchangeably with "adjustable speed drive" (ASD) or "inverter," though "inverter" usually refers to a specific stage inside the drive. In this handbook, we use "VFD" to refer to the complete unit you buy and install, and "ASD" as a broader category that also includes other speed-control technologies, though the focus here is exclusively on the electronic VFD.

Think about a standard industrial motor running at a fixed speed. It spins at a speed determined by the AC line frequency (50 or 60 Hz) and the motor's pole count. For decades, if you needed to change the speed of a machine, you had to change a gearbox, swap pulleys, or use a mechanical clutch. A VFD eliminates that entirely. It takes the fixed-frequency power from the wall, converts it to DC, and then synthesises a new AC waveform at a selected frequency and voltage. This allows the motor to turn at any speed from near zero up to its design maximum, with full torque available across a wide range.

Why does this matter? A VFD is the single most effective tool for controlling a motor's power consumption and its mechanical output. For loads that don't need to run at full speed all the