Electric motors are used extensively to drive fans, pumps, conveyors, printing presses, and many other processes. A majority of these motors are standard, 3-phase, AC induction motors that operate at a single speed. If the process (fan, pump, etc.) is required to operate at a speed different than the design of the motor, pulleys are applied to adjust the speed of the equipment. If the process requires more than one speed during its operation, various methods have been applied to allow speed variation of a single speed motor. These methods include variable pitch pulley drives, motor-generator sets, inlet or outlet dampers, inlet guide vanes, and Variable Frequency Drives. The following section will briefly discuss each of these speed control technologies.

Changing the pulleys throughout the day to follow demand is not feasible, but the use of a "Reeves" type variable pitch pulleys drive was a common application. These drives utilized a wide belt between two pairs of opposing conical pulleys. As the conical pulleys of the driven shaft were brought together (moved apart) the pulley diameter would increase (decrease) and decrease (increase) the belt speed and the process speed. This type of system is still extensively in use in the food and chemical industries where mixing speeds can dramatically effect product quality. These systems are a mechanical speed adjustment system which has inherent function losses and require routine maintenance.

Motor-Generator sets were used in the past to convert incoming electricity to a form required in the process including changes from AC to DC. The DC output could then be used to synchronize numerous dc motors

*Facilities today generally have significant harmonics. Especially when capacitors are used in systems with harmonics and variable frequency drives, professional advice is needed. Also motor capability may be a problem. See (8) page 71 for more information at the required speed. This was the common type of speed control in printing presses and other "web" type systems. The use of an Eddy-current clutch would vary the output of the generator to the specific needs of the system. The windage and other losses associated with motors are at least doubled with the generator and the efficiency of the system drops drastically at low load situations.

A majority of the commercial and industrial fan and pump speed control techniques employed do not involve speed control at all. These systems utilize inlet dampers, outlet dampers (valves) with or without bypass, or inlet guide vanes to vary the flow to the process. Inlet dampers, used in fan applications, reduce the amount of air supplied to the process by reducing the inlet pressure. Outlet dampers (valves) maintain system pressure (head) seen by the fan (pump) while reducing the actual volume of air (liquid) flowing. Inlet guide vanes are used in fans similar to inlet dampers but the guide vanes are situated such that as air flow is reduced, the circular motion of the fan is imparted upon the incoming air. Each of these control methods operates to reduce the amount of flow with some reduction in energy required.

Variable Frequency Drives (VFD) change the speed of the motor by changing the voltage and frequency of the electricity supplied to the motor based upon system requirements. This is accomplished by converting the AC to DC and then by various switching mechanisms invert the DC to a synthetic AC output with controlled voltage and frequency [Phipps, 1994]. If this process is accomplished properly, the speed of the motor can be controlled over a wide variation in shaft speed (0 rpm through twice name plate) with the proper torque characteristics for the application. The remainder of this paper will discuss the various issues and applications of VFD's. Figure 1 shows the percent power curves versus percent load for simplified centrifugal air handling fan application.

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