David W. Spitzer

Which of the following statements is true about using variable-speed drives to operate equipment in a process plant? Variable speed drives:

A. Reduce electrical energy consumption

B. Improve control as compared to a control valve

C. Reduce vibration
D. Decrease power surges


All of these statements are true in a general sense. However, investigation of variable-speed drive operation in the context of the process may yield contradictory results.

It is generally presumed that variable-speed drives operate equipment in a manner whereby the hydraulic output is matched to the actual process load. This evokes a sense of efficiency and implies a reduction in electrical energy consumption. In one application, a variable-speed drive was applied to an air compressor to enable the substitution of compressed air for a more expensive gas. The electrical energy consumption of the compressor increased because more compressed air needed to be produced. However, the cost of the expensive gas was reduced by a greater amount than the increased electrical cost associated with the compressor. The simple payback for this project was only eight months – despite the increase in electrical energy consumption. Answer A is wrong in this application.

It might seem intuitive that reducing the operating speed of equipment will reduce equipment vibration. This may sound reasonable, but reducing the speed of cooling tower fans can cause the cooling tower fan to operate at its resonant frequency. Extended operation in resonance might shake the tower to the point of failure. Many drives have adjustments that allow the drive skip over these frequencies to reduce the time that resonance can occur. Answer C is wrong in this application.

Additional Complicating Factors

There are many complicating factors that can come into play when applying variable-speed drives to processes. While there may be some relatively standard ways in which variable-speed drives are applied to certain process configurations, all processes are different and deserve custom consideration. The benefits of applying variable-speed drives can be substantial and beneficial to the operation, but you should be careful.

David W. Spitzer is a regular contributor to Flow Control with more than 35

years of experience in specifying, building, installing, startup,

troubleshooting and teaching process control instrumentation. Mr. Spitzer

has written over 10 books and 150 technical articles about instrumentation

and process control, including the popular “Consumer Guide” series that

compares flowmeters by supplier. Mr. Spitzer is a principal in Spitzer and Boyes LLC, offering engineering, expert witness, development, marketing, and distribution consulting for manufacturing and automation companies. He can be reached at 845 623-1830.