Worth reading: AC motor protection

A new, concise handbook, "AC Motor Protection," by Stanley Zocholl, a masters-degree engineer with Schweitzer Engineering Laboratories (SEL), intends to reverse the observation that "too often, little attention is paid to critical ac motor protection factors.


A new, concise handbook, "AC Motor Protection," by Stanley Zocholl, a masters-degree engineer with Schweitzer Engineering Laboratories (SEL), intends to reverse the observation that "too often, little attention is paid to critical ac motor protection factors. This 90-page, illustrated handbook promises to supply critical data for prevention of untimely ac motor failures.

Written for professional engineers, "AC Motor Protection" covers factors such as thermal limit curves and their relation to winding temperatures in an induction motor, which can result in premature motor and system failures that are costly and avoidable. The handbook includes a review of synchronous and induction motor principles, along with electrical, mechanical, and thermal models needed to provide proper relay protection for these motors operating under various conditions. A section of the handbook focuses on such motor protection requirements as pullout/stall protection, high-inertia starting, phase and ground protection, differential protection, running overload protection, and drive protection.

"AC motor protection presents unique requirements that are too often not met," says Zocholl. "In addition to faults and insulation failures, proper ac motor protection requires overcurrent protection against short circuits from external faults in connecting cables and from internal faults in motor windings. Plus, because ac motors are thermally rated and limited, protective relays must be applied to prevent overheating where no faults are present."

"AC Motor Protection" handbook is available now through SEL's web site , priced at $34.95, plus shipping.

—Frank J. Bartos, executive editor, Control Engineering, fbartos@reedbusiness.com

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