Adjustable speed drives are well-known but often misunderstood because little attention is paid to the system as a whole. Note I used the terminology “adjustable speed drives” (ASDs) and not “variable frequency drives” (VFDs). Most engineers associate speed changes with an inverter-based VFD. That’s true in most applications, but in the process industries we have a significant amount of steam demand that allows us to deploy back-pressure, extraction and condensing steam turbines. These turbines are inherently ASDs and can accommodate shaft-horsepower and process-flow-rate demand very efficiently. I’ll cover steam turbine drives specifically in a future column, but for now, just realize that we have several options to adjust operating speed of rotating equipment.
So why should we be interested in adjusting the speed of any rotating equipment? The answer is simple — we design and build processes and units for a certain specified condition. At these design operating conditions, when the system is started for the first time, the system works at its optimized configuration. However, as time goes on, processes change, demands alter and our system needs to adjust to accommodate the off-design and the “new normal” operating conditions. As engineers, we also over-design our systems by using safety factors to compensate for any shortcomings due to equipment or oversights. Lastly, we design for reliability of operations and redundancy and that should never be sacrificed. Hence, a strong need exists to control our systems at their optimized operation over the range of operating conditions.
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