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What is Adjustable Speed Drive?

T. L. Childree
T. L. Childree

Adjustable speed drive is a term used to describe any type of energy transmission device that allows manual or automatic adjustment of its fixed operating speed to control a process or conserve energy. The fixed speed of an adjustable speed drive motor can be increased or decreased according to the type of application for which the device is being used. Adjustable speed drive devices can be electrical or non-electrical. Non-electrical drives may be either mechanical or hydraulic. Electrically powered drives may use direct current, eddy current, or alternating current.

Electric motors operate at a fixed speed governed by the number of internal windings contained in them. Adjusting the speed of a motor using multiple sets of windings would be cumbersome and expensive. A separate motor speed control mechanism is typically required to cause a fixed speed motor to run at varying speeds. This type of speed control mechanism is commonly known as an adjustable speed drive. These drive mechanisms were originally intended for use in process control, but are also utilized frequently to conserve energy.


An adjustable speed drive aids in process control by providing a different operational speed for each process performed. This mechanism also offers smoother motor operation along with acceleration and torque control. Adjustable speed drives can be used to conserve energy by decreasing the fan speed in heating, ventilation, and air conditioning systems. During heating cycles, the reduced fan speed provides improved circulation of warm air throughout a room. Improved air circulation reduces the number of heating cycles required by the system and reduces energy consumption.

Adjustable speed drives can take many different forms including mechanical, hydraulic, and electrical. Mechanical drives may be a variable pitch, belt, and pulley design or a traction type, metal roller system. Hydraulic methods typically take the form of a hydrostatic or hydrodynamic drive. Hydrostatic drives adjust speed through the use of fluid-powered pumps and motors. Hydrodynamic drives change motor speed by means of an impeller-driven input shaft and a rotor-driven output shaft.

An electrically powered adjustable speed drive may utilize either direct, eddy, or alternating current. Direct current drives adjust motor speed by changing its field current or armature voltage. Eddy current drives utilize a device known as an eddy current clutch to regulate a fixed speed motor. Alternating current drives operate by either decreasing the motor’s applied voltage or increasing its winding resistance. Both alternating and eddy current drives are generally considered to be an inefficient adjustable speed drive mechanism.

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