What is the difference between a variable speed drive and a VFD?
The main difference between a Variable Speed Drive (VSD) and a Variable Frequency Drive (VFD) is that a VSD is a broad term for any device that controls the speed of a motor, while a VFD is a specific type of VSD designed to control AC motors by changing the frequency and voltage of the electrical supply. In simple words, every VFD is a VSD, but not every VSD is a VFD.
A VSD can include different types of drives — such as DC drives for DC motors, mechanical drives, or hydraulic systems that vary motor speed. A VFD, however, works only with AC motors and uses power electronics to convert fixed-frequency AC into variable-frequency AC. VFDs are widely used in industries because AC motors are more common, efficient, and reliable. So, when someone says VSD, it refers to the general concept of speed control, while VFD refers to the most popular electrical method used for modern AC motor speed control.