Adjustable Speed Drive

Types of adjustable speed drives - Speed adjustment techniques have been used in transmitting mechanical power to machinery since the earliest use of powered machinery. Before electric motors were invented, mechanical speed changers were used to control the mechanical power provided by water wheels and steam engines. When electric motors came into use, means of controlling their speed were developed almost immediately. Today, various types of mechanical drives, hydraulic drives and electric drives compete with one another in the industrial drives market.

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".

Mechanical adjustable speed drives
There are two types of mechanical drives, variable pitch drives and traction drives.

Variable pitch drives are pulley and belt drives in which the pitch diameter of one or both pulleys can be adjusted.

Traction drives transmit power through metal rollers running against mating metal rollers. The input/output speed ratio is adjusted by moving the rollers to change the diameters of the contact path. Many different roller shapes and mechanical designs have been used.

Hydraulic adjustable speed drives
There are three types of hydraulic drives, those are : hydrostatic drives, hydrodynamic drives and hydroviscous drives.

A hydrostatic drive consists of a hydraulic pump and a hydraulic motor. Since positive displacement pumps and motors are used, one revolution of the pump or motor corresponds to a set volume of fluid flow that is determined by the displacement regardless of speed or torque. Speed is regulated by regulating the fluid flow with a valve or by changing the displacement of the pump or motor. Many different design variations have been used. A swash plate drive employs an axial piston pump and/or motor in which the swash plate angle can be changed to adjust the displacement and thus adjust the speed.

Hydrodynamic drives or fluid couplings use oil to transmit torque between an impeller on the constant-speed input shaft and a rotor on the adjustable-speed output shaft. The torque converter in the automatic transmission of a car is a hydrodynamic drive.

A hydroviscous drive consists of one or more discs or connected to the input shaft pressed against a similar disc or discs connected to the output shaft. Torque is transmitted from the input shaft to the output shaft through an oil film between the discs. The transmitted torque is proportional to the pressure exerted by a hydraulic cylinder that presses the discs together.