VARIABLE SPEED CONTROL OF HVAC FANS BASIC INFORMATION


Since fan capacity is directly related to fan speed for all types of fans, fan speed selection has always been a part of fan system design. Belt drives with adjustable or exchangeable sheaves have been a traditional method for adjusting a given fan to a specific system capacity requirement.

Two-speed or multispeed motors have provided a means of fan speed and capacity adjustment on a high-low or high-medium low basis. For small motors, a manually adjustable speed switch similar to a lighting dimmer switch has been available.

Speed adjustment for fans to modify capacity to exactly meet the load has great potential benefit for fan energy conservation. If volume flow is proportional to fan speed, and developed fan pressure is proportional to the square of the fan speed, fan power requirements are proportional to the cube of the fan speed.

This means that a 50 percent reduction of fan speed results in a reduction of fan power (theoretical) to one-eighth of the original power requirement. Where air-handling systems serve loads of varying intensity, an opportunity to directly control fan volume by varying fan speed is of great potential benefit.

Many variable air volume (VAV) systems run in a range of 40 to 80 percent of design capacity most of the time. Fan speed adjustment can be in addition, and a corollary, to modulation of a primary heating and cooling medium for the supply air stream.

If inlet vane dampers or adjustable belt drives have been common mechanical-type volume adjustment techniques, with industrial fluid drives as a high-priced option, recent development of competitively priced variable-frequency drives, also derived from an industrial market, are proving to be a great addition to the HVAC system configuration.

These units, which vary in size from fractional up through several hundred horsepower capacity, use transistor technology to rectify alternating current in the standard 50- or 60-Hz format, and reconstitute it at any desired frequency. Since induction motor speed depends on frequency of the power supply, varying voltage frequency determines output of the fan, which can be automatically adjusted to match the connected load.

While variable or adjustable speed drives (VSD or ASD), also called variable frequency drives (VFD), are electrical in nature, they are directly involved in the mechanical duty. Selection and specification may be a joint mechanical/electrical assignment but should be abrogated by neither.

Of many VFD brands available on the market, there is wide variation in character and configuration. There are many purchase options. Different brands are better or worse in terms of powerline ‘‘harmonic’’ generation. Nearly all can be programmed and can accept 4 to 20 mA, 0 to 5 V dc or 0 to 10 V dc input signals.

Common voltage ratings are 208/230 or 460 V, three-phase. Motors must be carefully selected to be able to withstand higher winding temperatures, as motor cooling is typically reduced with lower fan speed. VFDs usually obtain a power factor of 0.9 or higher while imposing a 5 percent power consumption penalty on the motor use. (The VFD penalty is recovered, and more by the power savings in reduced motor speed.)

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