Variable Speed Drives (VSDs), also known as adjustable speed drives, are large industrial electric motors whosespeed can be adjusted by means of an external controller. They are used in process control and help saving energyin plants that use many powerful electric motors.The use of adjustable speed in process control matches the motorspeed to the required tasks and may compensate for changes in the process’s variables.

The use of adjustable speed for saving energy is exemplified by the adjusting the speed of a cooling fan motor to match the temperature of the machinery
parts it is cooling.VSDs are effective in energy savers in pump and fan applications; “they enhance process operations, particularly where flow controlis involved. VSDs provide soft-start capabilities, which decrease electrical stresses and line voltage sags associated with full voltage motor start-ups, especially when driving high-inertia loads.”

Adjustable frequency drives are a specific type of VSDs; they are controlling the rotational speed of an alternating current (AC) electric motor by controlling the
frequency of the electrical power supplied to the motor.AC drives are used to bring about process and quality improvements in industrial and commercial applications’ acceleration, flow, monitoring, pressure, speed, temperature, tension, and torque.Fixed-speed loads subject the motor to a high starting torque and to current surges that are up to eight times the full-load current. AC drives instead gradually ramp the motor up to operating speed to lessen mechanical and electrical stress, reducing maintenance and repair costs, and extending the life of the motor and the driven equipment.

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