Adjustable speed drive (ASD) or variable-speed drive (VSD) describes equipment used to control the speed of machinery. Many industrial processes such as assembly lines must operate at different speeds for different products. Where process conditions demand adjustment of flow from a pump or fan, varying the speed of the drive may save energy compared with other techniques for flow control.

Where speeds may be selected from several different pre-set ranges, usually the drive is said to be “adjustable” speed.

If the output speed can be changed without steps over a range, the drive is usually referred to as “variable speed”.
Adjustable and variable speed drives may be purely mechanical, electromechanical, hydraulic, or electronic.