Let drives steer you toward better performance, with these caveats.

The use of VFDs for control of pumps, fans, and even compressors, have become common on both new construction projects and retrofits. VFDs are a valuable tool in providing both improved control and energy efficiency. Over the last decade, the cost of these devices has dropped, and the reliability has improved, making their use almost a “no brainer.” With that said, careful consideration should still be given to how the drive will be applied prior to selection. This month’s column will focus on the best places to use drives and a future column will focus on how we recommend connecting and integrating drives with the BAS.

BENEFITS OF DRIVES

The primary benefit of using a VFD is that it provides an efficient method to vary the capacity of rotating equipment such as fans and pumps. The fan laws show that reducing the flow in a system effectively can reduce the required power by the cube of the reduced flow. In other words, reducing airflow by half can result in the fan energy being reduced to one eighth of the original power required.

For example, a fan that used 100 kW at full flow would drop to approximately 12.5 kW at 50% flow. Similar results can be expected with pumps and other rotating equipment. While there are other available technologies that provide effective methods to modulate flow, such as inlet guide vanes and variable pitch fans, the use of VFDs is more effective in achieving performance that comes closest to matching the theoretical limits of the fan and pump laws.

APPLICATION CAUTIONS

Given the potential benefits of drives, it isn’t surprising to see them being widely applied. However, there are some cautions we urge in considering use of drives.

• Fix systems control first: For example, if you have a VAV air handler that is not effectively controlling static pressure (often due to VAV box issues, balancing, duct leakage, or air handler control problems), then retrofitting from inlet guide vanes to use of a VFD won’t provide much, if any, benefit. The system must be changed to get proper control in order to get benefit from using the drive.
  
  
• No system control: We are starting to see drives installed on pumps and fans that are not in variable-flow configurations. Typically, these drives are being used not to vary flow, but rather for use in matching pump or fan capacity to system needs. When this is done, the balancer should be using the drive to set flow instead of utilizing the flow balancing valves. We often see drives running at full flow against balancing valves that are 50% closed. An even better solution, however, is to properly select (and balance) the pump or fan and not rely on the drive.
  
  
• Poor system control: While most drives have the ability to run their own control loops, we generally prefer to have the drive be controlled as an output from the BAS. It is important to keep in mind that these control loops must be properly tuned to prevent hunting and control errors. A hunting fan produces system imbalances and may affect equipment life.
  
  
•   Setting limits: While there are benefits to reducing the speed of a fan or a pump, there are also limits to how far you can effectively slow down a device without causing stall or premature motor failure. The general rule of thumb is not run below 20% (12 Hz), but the actual value is dependent on the drive, motor, and application.

We consider the VFD to be a key part of the control system, providing numerous benefits at an affordable cost. The next portion of this series will focus on how we recommend integrating drives, available data, and added benefits, including the ability to get free submetering out of VFDs. ES