The following technical discussion is part of an occasional series showcasing the ISA Mentor Program, authored by Greg McMillan, industry consultant, author of numerous process control books, 2010 ISA Life Achievement Award recipient and retired Senior Fellow from Solutia Inc. (now Eastman Chemical). Greg will be posting questions and responses from the ISA Mentor Program, with contributions from program participants.

In the ISA Mentor Program, I am providing guidance for extremely talented individuals from countries such as Argentina, Brazil, Malaysia, Mexico, Saudi Arabia, and the USA. This question comes from Muhammad Khalifah in Saudi Arabia.

Muhammad Khalifah’s Question

How fast is a variable frequency drive (VFD) if compared to an industrial control valve, and does this vary from application to application?

Greg McMillan’s Answer

Greg Shinskey in his study Flow and Pressure Control Using Variable Speed Drives (Control Conference, Chicago, 1980, pages 161-167), found that the VFD deadtime was essentially zero and the response time was much faster than for a control valve.

If the drive and motor have a generous amount of torque compared to the inertia of the impeller and rotor, a velocity limit (rate of change of speed limit) or deadband is not unnecessarily introduced, and speed and torque control is done in the VFD, the only deadtime in a flow or pressure response is due to PID scan and execution time and measurement lag and update time. For liquid or polymer pressure control or incinerator pressure control, the use of a VFD and a fast sensor can be essential for tight control creating a scenario where analog control is needed to eliminate digital delays as discussed in Analog Control Holdouts.

However, many times velocity limiting and deadband are introduced in the drive setup making the VFD slower than a control valve because the VFD supplier doesn’t understand the effect of dynamics on control loop performance. Also, to realize the benefit of a faster final control , the user must increase the gain and reduce the reset time per Equation 1 in the resource file Effect of PID Execution Time and Equation 2 in the blog PID Controller Tuning Rules.

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Often users are not accustomed to the much faster tuning settings and operations is concerned that things are happening much faster. If the loop output limits are not judiciously set, the loop can get into a lot of trouble very quickly. If the PID has a dynamic reset limit option (external reset feedback), directional setpoint velocity limits can be added in the analog output block to provide an easily adjustable slower approach to undesirable speeds without re-tuning.

An excerpt on VFD performance (pages 366-379) from the ISA book Essentials of Modern Measurements and Final Elements in the Process Industries provides a more detailed answer to the question on the speed characteristics of valves and variable frequency drives. Click this link to download the free excerpt.

Additional Mentor Program Resources

See the ISA book 101 Tips for a Successful Automation Career that grew out of this Mentor Program to gain concise and practical advice. See the InTech magazine feature article Enabling new automation engineers for candid comments from some of the original program participants. See the Control Talk column How to effectively get engineering knowledge with the ISA Mentor Program protégée Keneisha Williams on the challenges faced by young engineers today, and the column How to succeed at career and project migration with protégé Bill Thomas on how to make the most out of yourself and your project. Providing discussion and answers besides Greg McMillan and co-founder of the program Hunter Vegas (project engineering manager at Wunderlich-Malec) are resources Mark Darby (principal consultant at CMiD Solutions), Brian Hrankowsky (consultant engineer at a major pharmaceutical company), Michel Ruel (executive director, engineering practice at BBA Inc.), Leah Ruder (director of global project engineering at the Midwest Engineering Center of Emerson Automation Solutions), Nick Sands (ISA Fellow and Manufacturing Technology Fellow at DuPont), Bart Propst (process control leader for the Ascend Performance Materials Chocolate Bayou plant), Angela Valdes (automation manager of the Toronto office for SNC-Lavalin), and Daniel Warren (senior instrumentation/electrical specialist at D.M.W. Instrumentation Consulting Services, Ltd.).

About the Author
Gregory K. McMillan, CAP, is a retired Senior Fellow from Solutia/Monsanto where he worked in engineering technology on process control improvement. Greg was also an affiliate professor for Washington University in Saint Louis. Greg is an ISA Fellow and received the ISA Kermit Fischer Environmental Award for pH control in 1991, the Control magazine Engineer of the Year award for the process industry in 1994, was inducted into the Control magazine Process Automation Hall of Fame in 2001, was honored by InTech magazine in 2003 as one of the most influential innovators in automation, and received the ISA Life Achievement Award in 2010. Greg is the author of numerous books on process control, including Advances in Reactor Measurement and Control and Essentials of Modern Measurements and Final Elements in the Process Industry. Greg has been the monthly "Control Talk" columnist for Control magazine since 2002. Presently, Greg is a part time modeling and control consultant in Technology for Process Simulation for Emerson Automation Solutions specializing in the use of the virtual plant for exploring new opportunities. He spends most of his time writing, teaching and leading the ISA Mentor Program he founded in 2011.

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