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In the first of these articles, I introduced the technology of the VFD—an established device that is capable of creating a type of alternating current (AC) that can be used to power AC induction electric motors used to turn centrifugal pumps. Since the VFD generates AC at different frequencies, the induction motor rotates at speeds proportional to the frequency.
In that article, I pointed out that the laws of physics relating to centrifugal pumps, called the affinity laws, show that pump rotation speed and the flow rate of the pumped fluid are directly proportional, whereas the power required varies as the cube of the speed and the square of the pressure head. For a process control flow loop, this means that the output of a PID loop controller can be directly applied to the speed setpoint of the VFD in order to control flow. Of course, the PID output classically has been connected to the position setpoint of a process control valve.
The clear advantage of using the VFD to control pump speed, and therefore, the flow rate, is that it is no longer required to pump with enough head to overcome the pressure drop across the control valve. The energy savings caused by the reduced pump pressure head is significant because of the pump affinity law. In addition, as long as the desired flow rate remains within the speed control range of the VFD with a particular pump, the control loop response will be much faster than can be obtained using an electromechanical control valve. The few users who have actually tried this configuration have reported large energy savings and adequate control loop performance. Unfortunately, there have been no experiments reported to measure the actual performance of such a control loop where the VFD provides the final control element.
In my market research for a VFD supplier, I found that users were generally not aware of the option to use a VFD as the final control element for a process control loop. Use of VFD is rarely if ever specified, except in water and waste applications, where a control valve would be exceedingly expensive because of its size. As it turns out, this is no surprise, since the VFD manufacturers generally do not market their products to process control users, which would require that they supply users with an application data sheet for the use of VFDs as the final control element of a process control loop. Users generally learn about the equipment they specify in their plants from the college classroom, data sheets supplied by their vendors and from direct sales efforts of suppliers. College textbooks, as well as ISA's own textbooks on process control, currently do not show the use of a VFD as the final control element for a control loop. Without the informative and educational
application data sheets, it continues to be unlikely for a user to specify the use of VFD as the final control element of any process control loop.
The suppliers of VFDs tell me that they do not market the VFD as a final control element because users do not ask about it. The supply chain for equipment in most process industry plants is usually divided into these categories:
The organization of the end user's company and its engineering contractors usually separate purchasing, engineering and maintenance along these same lines as well. The result is that VFD suppliers sell to the electrical equipment interests, but not to the control systems interests. While the process industries buyers think of VFD in terms of replacement for DC and AC synchronous motors, they really don't know much about their use in process control systems. Process engineers and control systems engineers generally are unfamiliar with VFDs and their applications. This series of articles published in a process control-oriented publication hopes to change this situation and make process control engineers aware that the VFD/AC induction motor/centrifugal pump is an energy-saving and accurate alternative to the more traditional control valve.
However, not all control engineers read this publication. It seems that the VFD manufacturers would recognize the potential market for VFDs in process control. In fact, most of the VFD manufacturers have been contacted in my survey and are at least somewhat aware that VFDs can be used as the final control element of a process control loop. However, they are currently not trying to develop this market that seems to be "out of their comfort zone." They are not willing to staff their marketing effort with the necessary people to support bids into process control applications, or to create the sales collateral materials (application data sheets) necessary to support active sales into process control applications. Even companies who make both VFDs and process control systems are not willing to cooperate to offer the VFD as an alternative to the process control valve that they often cannot supply themselves. When the VFD drives vendors were asked, they simply state that there is no market for the VFD in process control systems. Likewise, when control systems suppliers were asked if they would quote VFDs as final control elements with their process control systems, they prefer to respond to the user RFQ that specifies only control valves.