The selection of induction motors designed to match the load requirements of any particular application need to ensure that the temperature inside the motor windings does not rise to a level that exceeds the critical temperature.
The critical temperature is the temperature in which the stator and rotor winding insulation can withstand without permanent damage. Insulation damage can shorten the useful life of the motor and eventually results in electrical faults.
IEC 60034-1 classifies the temperature rise limits of insulation materials and specifies the maximum permissible temperatures that the various classes of insulation materials could withstand. The temperature rise of the induction machine is the permissible increase in temperature, above this maximum ambient, to allow for the losses in the motor when running at full load.
A safe temperature is the sum of the maximum specified ambient temperature and the permitted temperature rise due to the mechanical load. This IEc standard specifies a maximum ambient temperature of 40oC.
1. IEC 60038-1 Rotating electrical machines - Part 1: Rating and performance
2. Practical Variable Speed Drives and Power Electronics - Malcolm Barnes