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Welcome to the official blog of the International Society of Automation (ISA).

This blog covers numerous topics on industrial automation such as operations & management, continuous & batch processing, connectivity, manufacturing & machine control, and Industry 4.0.

The material and information contained on this website is for general information purposes only. ISA blog posts may be authored by ISA staff and guest authors from the automation community. Views and opinions expressed by a guest author are solely their own, and do not necessarily represent those of ISA. Posts made by guest authors have been subject to peer review.

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Is a VFD or Valve Faster?

This guest post is authored by Greg McMillan.

In the ISA Automation Week Mentor Program, I am providing guidance for extremely talented individuals from Argentina, Brazil, Malaysia, Mexico, Saudi Arabia, and the USA. We will be sharing a question and the answers each week. If you would like to provide additional answers, please send them to Susan Colwell at ISA. The nineteenth question is from Muhammad Khalifah in Saudi Arabia:

"How fast is a VFD if compared to a control valve and does this vary from application to application?"

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 InTech online article “PID tuning rules.” 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 retuning.

The excerpt Essential Book Excerpt VFD Performance from the ISA book Essentials of Modern Measurements and Final Elements in the Process Industries provides on pages 377-379 a more detailed answer to the question “Which Is Faster: A Valve or a VSD?”

 

Greg McMillan
Greg McMillan
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 digital twin for exploring new opportunities.

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