AC motors are designed to run at constant speeds. However, the load demands of most applications require the motor speed be variable. A new article by Emerson Network Power Surge Protection and available exclusively at www.plantengineering.com, addresses the proper application of devices to protect variable frequency drives. The article also provides an overview of VFDs and harmonic currents, and offers recommendations on ways to limit harmonic current on the ac power system and protect a VFD from externally-generated transients that can adversely affect an ac power system.

The article, written by Jim Tiesi, manager of product development for Emerson, looks at the issue of harmonic currents on electrical distribution systems. Such harmonics create failures to power-factor correction capacitors, electrical conductors, transformers and other equipment connected to the ac power system. They can also cause faulty operation of uninterruptible power supplies or emergency generators through false tripping of overcurrent protective devices.

The article and graphics are available now at www.plantengineering.com as the lead article in the Engineer Resource Guide under Electrical Engineering.

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