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
- Pulse Width Modulation to improve low speed performance reducing torque pulsation (cogging)
- 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
- Totally enclosed water cooled (TWEC) motors for high temperature fluids to prevent overheating
- NEMA frame B motor to prevent steep torque curve
- Proper pump sizing to prevent operation on flat part of pump curve
- Use of recycle valve to keep pump discharge pressure well above static head at low flow (see article Watch out with variable speed pumping)
- Use of low speed limit to prevent reverse flow for highest possible destination pressure
- Twelve bit or more signal input cards to improve resolution limit of signal to 0.05% or better
- 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
- 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.
- 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.).