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A Simple Approach to Duty Ratings of AC Machines

How to ensure the correct duty rating for each application

April 2022
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Matthew Conville, MBA, PE
EASA Technical Support Specialist

Before putting an AC machine into service, make sure its duty rating matches the application requirements. Otherwise, there’s a good chance excessive heat will degrade the machine’s windings.

Applications are not all created equal. For example, a crane motor doesn’t require the same duty rating as a punch press motor of equal horsepower that runs continuously. Likewise, a chop saw motor wouldn’t need the same duty rating as a pump motor that operates 24-7-365. If the motor must operate at variable speeds other than its nameplate base speed, its turndown ratio is another consideration.

NEMA Std. MG-1-1.40 and IEC Std. 60034-1, Clause 4 describe the duty classifications for the respective standards.