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Variable Speed Drive Harmonics

7 months back I was involved in a harmonics evaluation study at Budweiser. This was one of many I've participated in over the past 20 years, basically because in facilities like this not one or two, but hundreds of variable speed drives (VSD) are in use. They are but one of many plants using variable speed drives for nearly all of their production controls. Capacitive reactance is inescapable when capacitors are in use in a circuit, as is inductive reactance when motors are operating, or any other component incorporating electro-magnetic coils. This reactance is the issue when dealing with harmonics, as can be the power supply to the facility itself. All variable speed drives employ capacitors.

As the order of harmonics (frequency) increases, the inductive reactance also increases, while the capacitive reactance decreases. At a particular frequency of harmonics the inductive and capacitive levels become equal, allowing harmonic resonance to set in. This is when problems occur, and remediation becomes necessary.

My particular involvement in the Budweiser plant regarded the overhead crane controls. They employ Gozuk drives, as does nearly if not all of the plant operating machinery.

At another facility with another manufacturer, the cranes I was involved with are also VSD controlled. As equipment and operations changed in the facility the cranes remained, but a host of other VSD controlled machinery in a variety of applications was installed and came online. Problems with erratic operation of some of the machinery began occurring. I was called by the service personnel at the plant to discuss what was going on, and what the causative factor might be. After several conversations with descriptions of the operating issues, it was revealed they had power improvement cap banks installed on the line feed to the building. By bypassing the cap banks as a test, the capacitive discharge occurring was reduced. This isn't the first time I've run across this. Taking the cap banks out of service significantly dropped the distortion levels, and all has been fine.

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