Do VFD Energy Savings Outweigh Potential Mechanical Problems?

Abstract

The use of Variable Frequency Drives (VFD's) for controlling rotating machinery has become relatively commonplace over the past ten to fifteen years. This sudden increase can be attributed to considerable strides in the development and reliability of power electronic components. Commercialization has naturally followed and many motor applications which were traditionally fixed speed are now operating on variable speed. As with any equipment addition or change that has a technical impact on its associated elements, a period of time is required to assess the full scope and degree of this outcome. In the case of VFD's, it would appear that there has been a significant amount of research, analysis and documentation concerning the application and compatibility issues related to electric motors. Many of the electromechanical problems evident in the early products have been eliminated or mitigated by addressing them with appropriate modifications either in the motor or the VFD. However, the adaptation of VFD's to a motor system affects much more than just the motor. Driven equipment designed and installed at a time when the engineer was only considering fixed speed as a drive possibility, is frequently being converted to variable speed. Although much research has been done on variable speed generally, a literature search would soon indicate that only a small portion has been focused on the application issues associated with the driven equipment. It is this area of interest which is being addressed by this brief overview. Fans, pumps and blowers fall under the general classification of turbomachines. It is estimated that collectively, they comprise 60% of electric motor applications. Historically, most turbomachines have operated on a fixed speed basis because of cost, the lack of a variable speed product suitable for the particular application or because the system didn't require it. Of the few which were installed with variable speed capabilities prior to VFD' s, typical technologies employed were DC motor controllers, eddy current clutches and fluid couplings. Due to the site specific nature of a particular installation, some of these technologies still have applications for which they may be the most suitable. The decision to use a VFD raises a number of important turbomachine application issues. Unfortunately, many of these technical issues are frequently ignored or dismissed without a proper analysis and the success of a project is seriously jeopardized. While a detailed review of the subject is beyond the scope of this paper, the intent is to introduce some of the more pertinent considerations.