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Trade press article — Electrical Construction & Maintenance

A Simple Approach to Duty Ratings of AC Machines

How to ensure the correct duty rating for each application

  • April 2022
  • Number of views: 1154
  • Article rating: No rating

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.

  • NEMA duty classifications
  • IEC duty types with "S" ratings
  • Turndown ratios
  • Questions to consider

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