Last month, we started this series on the use of VFDs as part of a control system. We talked about the many benefits and some recommended cautions in applying the use of this technology. This month’s column is focused on some recommendations for how to apply and integrate VFDs as part of the building control system.


There are several different options for how you can connect a VFD to a control system. Options include:

•Direct control from VFD: Since VFDs are microprocessor-based devices, they have the ability to not only control output frequency but also to implement control loops. This allows the drive to monitor a variable (typically static pressure) then directly vary the drive speed to hold a setpoint. Using the drive as a controller makes the most sense when no other controller is available. For example, we see this used as a solution to add a drive to an existing pneumatically controlled air handler.

• Control VFD from an external controller: In this approach, the controller that is used to control the air handler (or pump) monitors the control input, providing an output signal to the drive. Using this approach allows the control contractor to utilize a single controller to control a number of interlinked unit functions including the control of the drive. There are a number of advantages to this approach, including programming consistency, single-source responsibility, and the ability to easily replace the drive.

This raises the larger question about how we get the connection between the controller and the drive. There are basically two approaches for how to connect these devices. One is to use a hardwired analog interface (typically 4-20 mA or 0 – 10 VDC) to provide the control signal and a second digital input to tell the drive to run or stop. This approach is considered the most failsafe and is the one that we prefer to use.

The other option is to integrate to the drive using an open protocol and to send the commands using digital signaling. Most drives come standard with the ability to communicate with a BAS using open protocols including BACnet®, LonTalk®, and Modbus®. We strongly encourage integration to the drive — even if you decide to send control signals via hardwired inputs. Here are some of the benefits that come out of this integration:

• Energy usage: Drives provide the ability to readily provide information about the power usage of the load they are connected to. This includes amps (A), Watts (kW), and usage (kWh). Since rotating equipment comprises a large portion of a building’s HVAC energy usage, knowing the consumption from a fan, or pump becomes a valuable tool in doing energy management as well as measurement and verification.

• Other data: There is a wealth of information available from most drives, including diagnostics, frequency, temperature, etc.

One of the big questions faced by design engineers is whether or not to specify drives to be provided with an external bypass. The reasoning is that the bypass will allow for the motor to be used (at full speed) even if the drive is to fail. The use of a bypass is really a matter of confidence in the drive electronics. Over the years, these devices have gotten to be more reliable and readily available, reducing the risk of system downtime due to a failed drive. We don’t typically specify bypasses for drives, but instead advise keeping a spare drive on hand if you have a system that cannot tolerate downtime.

To conclude, drives are a great extension of a control system and provide the ability to efficiently control and monitor key building loads. ES