A guide to selecting adjustable-speed drive systems: ASDs can save money and energy.(Fundamentals: understanding current industry technologies)

Commercial and industrial firms use adjustable-speed drive (ASD) systems to control pumps, fails, and blowers. ASDs may also be found controlling hoists and cranes, conveyors, and machine tools. These applications present a wide range of requirements and characteristics, and so drive vendors have developed a wide variety of ASD. Combined, the variety of application and the many combinations of drives available has made choosing the optimum drive for any given application a challenge.

Advancements in solid-state power electronics have led to a new generation of electronic ASD, enabling commercial and industrial facilities to dramatically reduce energy use and operating and maintenance costs, while improving operations. ASDs can be applied to ac motors regardless of motor horsepower or location within a facility. They can be used to drive almost any motorized equipment from a small fan to the largest extruder or machine tool.

ASD units consist of three basic parts. The rectifier converts the fixed frequency ac input voltage to dc. The inverter switches the rectified dc voltage to an adjustable frequency ac output voltage; it may also control output current, if required. The dc link connects the rectifier to the inverter and may also contain an inductor as well as a capacitor.

ASDs are described and classified by their inverter sections, which can use either current-regulating or voltage-regulating techniques to switch dc power to ac power. Drives are classified as current-source inverters (CSI), variable-voltage inverters (VVI), or pulse-width-modulated inverters (PWM).

The PWM type is the most common design, available in sizes up to several thousand horsepower. Some manufacturers provide CSI and VVI units in the higher horsepower sizes, but competition has caused them to develop PWM designs to meet present day demands for improved efficiency and performance.

The PWM design uses a diode bridge in the rectifier to provide a constant voltage dc bus. The inverter section uses a pulse-width-modulation algorithm to produce near sine-wave output to the motor, which reduces motor heating.

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