These devices have been available for about 20 years and have a wide range of applications ranging from single motor driven pumps, fans and compressors, to highly sophisticated multi-drive machines. They operate by varying the frequency of the AC voltage supplied to the motor using solid state electronic devices.
These systems are fairly expensive but provide a higher degree of control over the operation and in many cases, reduce the energy use enough to a least offset if not more than pay for the increased cost.
ASDs allow precise speed control of a standard induction motor and can result in significant energy savings and improved process control in many applications. ASD can also control the speed of a standard squirrel cage NEMA type B induction motor and they are suitable not only for new applications, but also for retrofit on existing motors.