Variable speed drives are devices that control the rotational speed of an electric motor. They may be mechanical, electromechanical, hydraulic or electronic depending on the demand and are compatible with alternating current (AC) or direct current (DC) motors, though the vast majority are for AC motors only.
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Variable speed drives are necessary because many industrial processes operate at varying speeds for different products or materials, and these devices control the conditions that match the product. Process control and energy conservation are two benefits gained by using a variable speed drive; other advantages include smooth operation, accurate positioning, acceleration control, varying operating speeds and more. Variable speed drives are generally fairly small and are sometimes held in a rectangular plastic enclosure. The components are metal, usually stainless steel or aluminum. If the speed for the electric motor is chosen from a preset range, the drive is called adjustable. If the speed can be set at any level then it is a variable speed drive; these devices are also sometimes called variable frequency drives, AC drives, inverter drives or variators. Variable speed drives are used predominantly in manufacturing to adjust the flow rate of pumps and fans such as those used in ventilation systems. They are also used to control the speed of conveyor belts, assembly lines or belt dryers, for instance.
Variable speed drives may be an electric motor, a diesel engine, a gear box, an electrical switch gear, a hydraulic coupling or another option. Though there is quite an assortment of types, these drives are relatively similar in their basic operation. Traction drives utilize the friction between a speed-adjusting medium and special input and output plates whereas gear boxes use a system of gears to reduce the output. Hydraulic drives rely on a hydraulic pump and motor and couplings use either fluid or magnets to replace the physical connection between motors and loads with air. Variable speed drives may be oriented horizontally or vertically with input and output shaft arrangements including inline, offset parallel or right angle. These drives are able to maintain motor speeds with very close tolerances; a tenth of a percentage is an average margin whereas conventional AC motors can vary up to 3% above or below the desired speed. The differences in electricity also cause variations. AC motors run at speeds determined by the number of poles in the motor and the frequency of the electricity. Speeds are given to the poles so the more poles, the more speeds a motor can have. DC motors are controlled by adjusting the voltage or the shunt field current. The addition of a variable speed drive allows for more precise settings than the existing controls.
