Adjustable-speed drive

Adjustable speed drive (ASD), also known as 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. Adjustable speed drive (ASD), also known as 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 (termed variators), electromechanical, hydraulic, or electronic. AC electric motors can be run in fixed-speed operation determined by the number of stator pole pairs in the motor and the frequency of the alternating current supply. AC motors can be made for 'pole changing' operation, reconnecting the stator winding to vary the number of poles so that two, sometimes three, speeds are obtained. For example a machine with 8 physical pairs of poles, could be connected to allow running with either 4 or 8 pole pairs, giving two speeds - at 60 Hz, these would be 1800 RPM and 900 RPM. If speed changes are rare, the motor may be initially connected for one speed then re-wired for the other speed as process conditions change, or, magnetic contactors can be used to switch between the two speeds as process needs fluctuate. Connections for more than three speeds are uneconomic.