Application Note: Operating a VFD on a DAB System

Solution

On traditional zoned systems with constant volume RTUs, a bypass damper is introduced to keep the duct from overpressurizing during low load conditions.  Many old systems are being retrofitted by removing the Bypass Dampers and installing a VFD on the RTU.  75F has a solution to this with our Bypass Damper configuration.

In the past, we have used the Generic PI Loop profile for duct static pressure control with a bypass damper.  The problem was our loops only recalculated once per minute with this profile, leading to hunting or unstable VFD operation.  The new Bypass Damper as a System Profile pushes the PI Loop to the Smart Node so that loops run faster and VFD operation is stable.

Wiring Information:

Please find more information here:

https://support.75f.io/hc/en-us/articles/28552061478035-Bypass-Damper-as-System-Device

Notes:

This is only for DAB implementations.  For a VAV implementation, refer to VAV Advanced Hybrid AHU v2 with Advanced AHU Controls:

https://support.75f.io/hc/en-us/articles/29849661963283-VAV-Advanced-Hybrid-AHU-v2-with-Advanced-AHU-Controls