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Entry type: FAQ Entry ID: 39181972, Entry date: 10/30/2009

Thermal classes

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What are the thermal classes, and what are their assigned temperatures?







In earlier publications, thermal classes were referred to as temperature classes or insulation classes.


In electric machines, the temperature is very often the dominant factor in the aging of insulating materials and insulation systems. IEC 60085 therefore defines thermal classes with specified temperatures. These are the maximum temperatures which the insulation may reach at rated power and under other specified conditions. The thermal stability of the insulation system must be suitable for this temperature.  


Rule of thumb:

Each increase of 10 K in the winding temperature halves the statistical service life.  


Thermal classes according to IEC 60085



JC,max: Maximum permissible ambient temperature
DJlimit: Maximum permissible winding overtemperature
DJMF: Overtemperature for the hot spot of the winding. This is an empirical value that takes into account the hot spot, which cannot be measured


There is no temperature reserve available which could be used during operation.


Maximum permissible hot spot temperature of a winding (rated temperature of the insulation system)

    Nowadays, the insulation systems of electric machines are usually designed for thermal classes F and H. However, the actual use of the motors at rated power in mains operation often corresponds to the lower classes B or F.

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