Variable Frequency Drives for Centrifugal Pumps

The prices of low-voltage variable frequency drives (VFDs) have declined substantially since they were introduced, while the cost of power has increased. Normal project economics will now justify these VFDs — which go up to 375 kW (500 hp) or beyond — for pumps over the largest part of the range of application of the pumps. To efficiently evaluate and apply VFDs, process engineers need to know certain elementary things about them. They also need to know about the interactions of pumps and hydraulics when the pumps run at variable speeds. 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. Usually, screening level economics — as presented here — must be applied and some uncertainty must be accepted.    Figure 1. A fan is the only moving part of a variable frequency drive Toshiba International Introduction 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. These operations are…