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Since the invention of the AC VFD in May of 1967, the electronics required to rectify the incoming three-phase AC to DC and the regeneration of a different frequency AC current has depended upon high power-bipolar semiconductors. High-power semiconductors have evolved over time to become more reliable and more efficient. However, they have not been able to match the pace of development of low-power semiconductors for use in general electronics. Today's AC VFDs all use multiple insulated gate bipolar transistors (IGBT) for each of the three phases of the AC line to regenerate the variable-frequency AC power. Multiple IGBTs are used because the rated power level cannot be handled by a single IGBT, and use of multiple IGBTs provides redundancy when one fails. These are power-handling semiconductors and do fail occasionally due to heat generated within their circuits. Builders of VFDs have compensated by making failure detection and replacement of VFD circuits rapid and easy—but they do fail. Our question is: would VFD maintenance impact flow control as often as control valve mechanical maintenance to overcome wear and stickiness?
AC VFDs do not generate perfect sine waves at the new frequency. As a result, the power supplied to electric motors contains higher frequency components called harmonics that are filtered out by the windings of the AC induction motor. Harmonics can cause vibration in the AC motor windings as well as heat, as the harmonics power component is dissipated by the reluctance of the motor windings. When a new installation is made knowing that a VFD is to be used, motors can be supplied that are designed to withstand the vibration and heat generated. However, in a retrofit installation using an existing electric motor with a new VFD, the motor will be oversized and will probably work at the new reduced speed and power levels in spite of the harmonics and overheating, but should be closely monitored. Furthermore, when that motor is replaced, it should not be with the same ratings, but with a smaller motor designed for use with a VFD.
It seems clear to this author that using a VFD/pump combination to replace the use of a AC line-powered electric motor and a control valve is a superior way to do flow control and reap large benefits in lower energy consumption with better energy efficiency. However, it is not clear that the human factors will be easy to overcome. Our traditions are all bound by use of control valves as instructed by our textbooks. Suppliers of VFDs have not focused on control valve replacement as an alternative in their marketing efforts. Instrument departments have not cross-trained their repair technicians to repair failed VFDs. There is also the potential for improved process control from elimination of the problems of control valves. We are still waiting for a suitable number of users to actually test these factors in real plants.
Dick Caro is president of CMC Associates.