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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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What Are the Factors that Affect VFD Rangeability?

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 eighteenth question is from Muhammad Khalifah in Saudi Arabia:

“What are the factors that affect Variable Frequency Drive (VFD) rangeability for flow control?”

Greg Shinskey in his study “Flow and Pressure Control Using Variable Speed Drives” (Control Conference, Chicago, 1980, pages 161–167) found that rangeability of flow control by variable speed pumping exceeded the rangeability of the magnetic flow meter in the test.

In general, the rangeability of VFD can be 40:1 or better if the following guideline is followed:

Guideline to maximize VFD rangeability for flow control

  1. Pulse Width Modulation to improve low speed performance reducing torque pulsation (cogging)
  2. Totally enclosed fan cooled (TEFC) motors with constant speed fan or booster fan as necessary with class F insulation (inverter duty) and 1.15 service factor to prevent overheating
  3. Totally enclosed water cooled (TWEC) motors for high temperature fluids to prevent overheating
  4. NEMA frame B motor to prevent steep torque curve
  5. Proper pump sizing to prevent operation on flat part of pump curve
  6. Use of recycle valve to keep pump discharge pressure well above static head at low flow (see article “Watch out with variable speed pumping”)
  7. Use of low speed limit to prevent reverse flow for highest possible destination pressure
  8. Twelve bit or more signal input cards to improve resolution limit of signal to 0.05% or better
  9. For tachometer control, gear teeth for magnetic pickups and discs with holes or bands with mirrors on the shafts for optical pickups to provide more pulses per revolution
  10. For tachometer control, keep the speed control in the VFD to prevent violation of the cascade rule where the secondary flow loop should be 5 times faster than the primary (flow) loop as discussed in “More Fun with PID Controllers
  11. To increase rangeability to 80:1, consider fast cascade control of speed to torque in VFD to provide closed loop slip control as described in The Control Techniques Drives and Controls Handbook, IEE Power and Energy Series 35, Cambridge University Press, 2001

The excerpt Essential-Book-Excerpt-VFD-Performance from the ISA book Essentials of Modern Measurements and Final Elements in the Process Industries documents these and other considerations in maximizing the performance of variable frequency drives for process control.

 

Greg McMillan
Greg McMillan
Greg McMillan has more than 50 years of experience in industrial process automation, with an emphasis on the synergy of dynamic modeling and process control. He retired as a Senior Fellow from Solutia and a senior principal software engineer from Emerson Process Systems and Solutions. He was also an adjunct professor in the Washington University Saint Louis Chemical Engineering department from 2001 to 2004. Greg is the author of numerous ISA books and columns on process control, and he has been the monthly Control Talk columnist for Control magazine since 2002. He is the leader of the monthly ISA “Ask the Automation Pros” Q&A posts that began as a series of Mentor Program Q&A posts in 2014. He started and guided the ISA Standards and Practices committee on ISA-TR5.9-2023, PID Algorithms and Performance Technical Report, and he wrote “Annex A - Valve Response and Control Loop Performance, Sources, Consequences, Fixes, and Specifications” in ISA-TR75.25.02-2000 (R2023), Control Valve Response Measurement from Step Inputs. Greg’s achievements include the ISA Kermit Fischer Environmental Award for pH control in 1991, appointment to ISA Fellow in 1991, the Control magazine Engineer of the Year Award for the Process Industry in 1994, induction into the Control magazine Process Automation Hall of Fame in 2001, selection as one of InTech magazine’s 50 Most Influential Innovators in 2003, several ISA Raymond D. Molloy awards for bestselling books of the year, the ISA Life Achievement Award in 2010, the ISA Mentoring Excellence award in 2020, and the ISA Standards Achievement Award in 2023. He has a BS in engineering physics from Kansas University and an MS in control theory from Missouri University of Science and Technology, both with emphasis on industrial processes.

Books:

Advances in Reactor Measurement and Control
Good Tuning: A Pocket Guide, Fourth Edition
New Directions in Bioprocess Modeling and Control: Maximizing Process Analytical Technology Benefits, Second Edition
Essentials of Modern Measurements and Final Elements in the Process Industry: A Guide to Design, Configuration, Installation, and Maintenance
101 Tips for a Successful Automation Career
Advanced pH Measurement and Control: Digital Twin Synergy and Advances in Technology, Fourth Edition
The Funnier Side of Retirement for Engineers and People of the Technical Persuasion
The Life and Times of an Automation Professional - An Illustrated Guide
Advanced Temperature Measurement and Control, Second Edition
Models Unleashed: Virtual Plant and Model Predictive Control Applications

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