![]() Readers wishing to review the fundamentals may consult Ref. ![]() Except for what is necessary to discuss VFDs, this article does not cover the fundamentals of pumps and hydraulics. A simplified diagram of how a variable frequency drive works. When a VFD is used, the control valve is deleted, and the speed of the pump is adjusted to provide the exact head that is required, thereby saving the energy that would have been dissipated across the control valve. B, and the scheme for using a VFD is shown in Figure 3. These schemes are shown in a simplified manner in Figures 3.Ī and 3. This method is only mentioned here but would have to be considered in the evaluation of a specific case if it is the conventional method that would be used. An alternative method, which places the control valve in a bypass from the discharge of the pump to its suction, is typically used for positive displacement pumps but is also used for centrifugal pumps sometimes. ![]() The control valve consumes the excess head that the pump produces at the desired flowrate, thereby setting the desired flowrate. ![]() Varying the speed of a centrifugal pump controls the head and capacity of the pump. For this reason, VFDs are sometimes called variable speed drives (VSDs) or adjustable speed drives (ASDs), but the more common and fundamental term VFD is used in this article. The frequency of the alternating current that is produced is set by the inverter, and when that current is fed to an electric motor, its frequency controls the speed of the motor. These operations are depicted in a simplified manner in Figure 2, which represents a single- phase current or one phase of a three- phase current. A VFD (Figure 1) comprises a rectifier, which converts an alternating current into a direct current, followed by an inverter, which converts that direct current into a coarse version of an alternating current. The purpose of steam generation process in gas processing plants is to supply steam to all consumers and then recover condensate. Further economic evaluation beyond the approximate method must be performed, but because low- voltage VFDs require only a small incremental capital investment, their application cannot justify a very detailed evaluation. They also need to know about the interactions of pumps and hydraulics when the pumps run at variable speeds. To efficiently evaluate and apply VFDs, process engineers need to know certain elementary things about them. Normal project economics will now justify these VFDs. The prices of low- voltage variable frequency drives (VFDs) have declined substantially since they were introduced, while the cost of power has increased. Variable Frequency Drives for Centrifugal Pumps. ![]()
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