Variable Speed Drive & Variable Frequency Drive Applications

Today’s compressor technology has evolved greatly for the controls of the rotary screw compressor. Most notably is the introduction of variable frequency drives (VFD) and variable speed drives (VSD). Although there are differences, both types of controls work to conserve energy. When utilized properly, they are very energy efficient. When undersized efficiency is lacking, and when oversized they have major component issues.
This subject can be very complex. To keep it simple let’s review location and application overview.
Proper installation and location are critical. In facilities that have extreme contaminants, visible and invisible, a VFD could fail prematurely. They can attract particulates that could cause short in the drive itself. Dust, water, heat and other conditions can and will cause expensive failures. These units need relatively clean and cool operating conditions.
Application is just as critical. Under sizing the unit makes it run less efficiently above 90%. Under sizing the unit will cause drive failures, motor failures, and oil carryover. Most compressor manufacturers say the optimum range of consistent operation of a VFD is 50-80%. Operating them below 50% can be beneficial if they are set up properly, but in many cases they aren’t or the load fluctuates, and this causes problems.
Air system audits can be very beneficial in determining the size and application of base load units and VFD’s and it is strongly recommended that experienced auditors looking at the supply and demand side of every application to determine the effectiveness of a VFD.


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