Temperature rise limits for large motors-i.e., those above medium motor ratings-differ based on the service factor (SF). In the most common speed ratings, the NEMA designation of medium motors includes ratings of 1.5 to 500 hp for 2- and 4-pole machines, and up to 350 hp for 6-pole machines. Table 1 shows the temperature rise limits for NEMA medium electric motors, based on a maximum ambient of 40 C. But NEMA also includes a safety factor, primarily to account for parts of the motor winding that may be hotter than where the temperature is measured. Since NEMA’s maximum ambient temperature is normally 40 C, you would expect the temperature rise limit for a Class B system to be 90 C (130 to 40 C). A Class B insulation system, for example, is rated 130 C, while a Class F system is rated 155 C. NEMA rates winding insulation by its ability to withstand overall temperature. Put another way, the sum of the ambient temperature and the temperature rise equals the overall (or “hot”) temperature of the motor or a component.Īmbient temperature + Temperature rise = Hot temperature The difference between the ambient temperature and that of a motor operating under loa d is called the temperature rise. Brief definitions of a few commonly used terms will make it easier to follow the various procedures.Īmbient temperature (often referred to as “room temperature”) is the temperature of the air (or other cooling medium) that surrounds the motor.
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