ADJUSTABLE SPEED DRIVES By: Richard D
Service Application Manual SAM Chapter 620-130 Section 6A ADJUSTABLE SPEED DRIVES By: Richard D. Beard P.E. Consultant, RSES Manufacturers’ Service Advisory Council INTRODUCTION Many commercial and industrial machines and processes require adjustable speed. Adjustable speed usually makes a machine more universally compatible and increases its versatility. Adjustable-speed drives also are being used in residential equipment, including air conditioners, refrigerators, heat pumps, furnaces, and other devices driven by motors. These drives optimize speed and torque, making them generally more efficient than non-adjustable-speed drives. An adjustable-speed motor is one in which the speed can be varied gradually over a wide range—but, once adjusted, it remains nearly unaffected by the load. A variable-speed motor is one in which the speed varies with the load, usually decreasing when the load increases. The term "adjustable speed" implies that some external adjustment, which is independent of load, will cause the speed to change. A variable-frequency inverter drive is an example. The term "variable speed" describes a drive in which load changes inherently cause significant changes in speed. A direct current series motor, for example, exhibits this characteristic. An adjustable variable-speed motor is one in which the speed can be adjusted gradually. However, once adjusted for a given load, the speed will vary with changes in the load. A multispeed motor is one that can be operated at any one of two or more definite speeds, each being practically independent of the load. The multispeed motor is neither an adjustable-speed nor a variable-speed drive. Multispeed motors usually have two, three, or four definite operating speeds.
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