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

What are the factors that affect variable frequency drive (VFD) rangeability for flow control?

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 rangeability of flow control by variable speed pumping exceeded the rangeability of the magnetic flowmeter 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.

ISA Mentor Program Posts & Webinars

Did you find this information of value? Want more? Click this link to view other ISA Mentor Program blog posts, technical discussions and educational webinars.

An excerpt 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. 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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