VFD'S for Centrifugals?
Can you put a Variable Frequency Drive on a multi-stage centrifugal air or gas blower? Absolutely! Will it reduce your power costs? Maybe. Will it hurt the motor or blower operation? Probably not.
Centrifugal blowers have been in service long before VFD's came on the market. For the most part, they were all designed to operate directly driven at a speed of 3600 rpm. Each blower is designed for a specific volume and pressure. Changing the performance was accomplished by changing the impeller configuration. In some instances, the blowers' performance was improved by belt driving it to a higher speed, but not much more than 4000 rpm.
Positive displacement blowers on the other hand, are usually belt driven, and their performance is changed by varying the speed(interchanging the sheaves). VFD's revolutionized the PD blower market by making it so easy to vary the blower performance with the turn of a dial on the variable frequency drive.
Controlling the centrifugal blowers with VFD's is like day and night compared to PD's. Their performance changes with the speed, but in a much different way. As the speed of the centrifugal decreases, the pressure capabilities are decreased by the square! The attached logarithmic curve shows the steep loss of pressure in relation to speed. A minor speed adjustment of 100-200 rpm could reduce the blower pressure to less than your system pressure resulting in no air!
If a blower system is operating at its' design performance, then the motor is most likely operating at peak efficiency. Lowering the speed slightly would result in only minimum $$ savings. VFD's help better if your actual system pressure is much less than design. The lower you can decrease the speed, the more savings in power costs.
From a maintenance point of view, the speed of the blower has no effect. Although there are still some motors out there that are not suited for VFD control, the majority of motors on centrifugal blowers can be operated safely with VFD's. Installing a VFD is generally just a matter of replacing the motor starter. If you are considering the switch, contact us for competitive pricing, and assistance with calculating your ROI.
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