Variable Frequency Drive Applications

Variable frequency drive systems offer many benefits that result in energy savings through efficient and effective use of electric power. The energy savings are achieved by eliminating throttling, performance, and friction losses associated with other mechanical or electromechanical adjustable speed technologies. Efficiency, quality, and reliability can also be drastically improved with the use of VFD technology. The application of a VFD system is very load dependent and a thorough understanding of the load characteristics is necessary for a successful application. The type of load (i.e. Constant torque, variable torque, constant horsepower) should be known as well as the amount of time that the system operates (or could operate) at less than full speed.

Figures 3 and 4 show the energy savings potential for a variable speed fan and pump, respectively. The VFD pump is compared with a valve control system which would be adjusted to maintain a constant pressure in the system. The VFD fan is compared with both the damper control and the inlet guidevane controls. These curves do not account for system characteristics (i.e., head or static pressure), which would need to be included in an actual design. These figures show that the amount of savings achievable from the VFD is based upon the

Figure 3. Energy savings with VFD fan. (Source: Lo-bodovsky, 1996)
Figure 4. Savings with VFD pump. (Source: Lobodovsky, 1996)

percent volume flow for both cases. The consumption savings would be determined by the percent of time at a particular load multiplied by the amount of time at that particular load.

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