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?”


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