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
- 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
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.
