Variable frequency drives (VFDs) used to be expensive, long-lead time items that were only used in systems where the air flow rate or water flow rate was varied or modulated. Now that competitively priced VFDs are readily available, most systems can benefit from this technology and you could be saving energy even in applications you may not have considered.

It is obvious why VFDs are beneficial and an integral component of variable flow systems, but why install VFDs on constant flow systems?

  1. VFDs can provide energy savings by reducing the speed of a pump or fan, instead of simply adding restriction to the system using balancing valves, flow control valves, dampers, and other similar devices.
  2. The “soft-start” feature on VFDs reduces the electrical demand by limiting the in-rush current as the motor starts. This not only can reduce the demand charge on your electric bills, but can eliminate nuisance breaker trips on start up and extend the life of belt drives and bearings.
  3. VFDs save time during the testing and balancing (TAB) process by allowing the technician to easily adjust the flow at the control pad of the VFD or their laptop computer. Saving time for the technician not only speeds up the TAB process but also reduces the cost for these services.
  4. If specified properly, VFDs also serve as an electrical disconnect; simplifying and reducing the cost of the electrical installation.
  5. If the component is properly sized, a VFD provides flexibility in adjusting the operation of that component to accommodate changes in the system configuration during construction and/or future system changes. For example, a fan or pump could be sized for a future need but operated efficiently at lower capacities by slowing the rotational speed.
  6. For water systems, VFDs can be less expensive than automatic flow control valves and can more precisely balance water flow rates at pumps.

VFDs are not just for variable flow systems anymore. On most systems, they can reduce installation costs and operating costs, improve the flexibility and functionality of the system, and make the system easier to balance and adjust. Next time you’re replacing an old component, or starting a new project, call your engineer to see how VFDs can work for you. Once you experience the benefits, you will likely require VFDs on all of your system components!

VFDs on the pumps, chiller, cooling tower, and supply air fans at the Mockingbird Station Office tower were instrumental in achieving the $100,000 in annual energy savings generated by the new central plant system.

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