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Understanding motor temperature rise limits

  • November 2003
  • Number of views: 1142
  • Article rating: No rating

Tom Bishop, P.E. 
EASA Technical Support Specialist 

How do we know if a motor is operating within its temperature rating? The simple answer, and a good one, is that the National Electrical Manufacturers Association (NEMA) has defined temperature rise for electric motors in Motors and Generators, NEMA standard MG 1-1998. In this article we will focus on temperature rise and tem­perature sensing of three-phase induction motors. 

We will begin by identifying some key terms. Temperature rise is the increase in temperature above ambient. Ambient temperature is the tempera­ture of the air (or other cooling medium) in the area surrounding the motor, frequently termed “room temperature.” The sum of the ambient temperature and the temperature rise is the overall, or “hot,” tem­perature of a component. Insulation temperature classes are based on the overall temperature. For ex­ample, a Class B winding system is rated 130ºC. The normal maximum ambient, per NEMA, is 40ºC. The temperature rise limit for the Class B winding would be estimated at 90ºC (130-40). 

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