VFDs on Process Fans - When Are They a Good or a Bad Idea?

Variable Frequency Drives (VFDs) are an established technology on process fans that can save power by utilizing the Fan Laws. VFDs can be a more effective control device for controlling flow on a fan than an inlet damper when utilized correctly. This is especially evident in the case of low voltage motors and/or fans that are operating at a low damper position. This paper will explore various projects at a cement plant in Kansas that have had varying degrees of success. In one case, the VFD worked beautifully and had a very low payback period. In another case, process conditions changed over time and all of a sudden, the power savings from installing VFDs were negligible. This paper will look at how to identify good cases for installing VFDs and how to properly estimate the payback of a VFD project. It will also look at other considerations that need to be made when selecting a VFD, including installation, electrical and automation issues.