Temperature & Insulation – A Brief Guide


IEC 34-1, BS 4999, AS 1359.32.


The “Life” refers to the life of the windings before they require rewinding. The Temperature rise of the windings, (and the insulation materials), of an electric motor is critical to the life expectancy of the motor, and is a function of the design of the motor. The insulation materials age overtime and this aging process is directly related to temperature. Eventually the materials lose their insulating properties and break down causing a short circuit.

The increase in temperature of a motor is due to the losses that occur in the motor. These loses are mainly made up of copper and iron losses. The temperature inside the motor will depend on how effectively this heat can be removed by the cooling system of the motor. It should not be assumed that a motor that appears to be hot externally, is hot internally.

If the cooling system is efficient the thermal gradient through the motor will be small and the difference between the winding temperature and the external temperature low.

Motor Insulation Failure

Some standards estimate the life of the insulation materials as 25, 000 hours if operated continuously at their rated temperature that the life will be reduced by 50% for every 10 degrees of excess temperature.

Western Electric motors are built with Class F insulation and designed for Class B rise, and most of the motors only have a Class E rise. this “Thermal Reserve” greatly increases the life of the motor so that it is not of concern, especially when most motors do not operate at less than full load, and are not in continuous ambient temperature of 40 degrees. A life of 20 to 30 years under normal conditions can confidently be expected.

Insulation ClassAEBFF
B Rise
Temperature Rise105120130155155180
Maximum Temp of the Winding100115120140140165
Ambient Temperature404040404040
Allowance for Hot Spots551015
Maximum Temp Rise of Winding607580100
Thermal Reserve0000200
Motor Insulation Class


The permitted temperature rise of the windings of an electric motor are subdivided into different insulation classes and temperature limits.

The above table applies for motors in an ambient temperature up to 40 degree C and an altitude of less than 1000 meters above sea level.

The difference between the ‘Maximum Temperature of the Winding’ and the ‘Temperature Limit’ is because there will be hot spots in the winding which are not measured by the ‘Resistance Method’ which only measures the Mean Temperature of the whole winding. An Allowance is made for this difference to ensure that no part of the winding is operating at it’s full thermal rating. It is not considered to be practical to try to locate and measure the hottest spot in a winding.

The temperature rise of the winding is measured by the resistance method using the following formula:

DT = (R2-R1) / * (235 + T1) + (T1 – T2)


DT = Temperature Rise in deg.K.
R1 = Cold resistance of the Winding @ T1
R2 = Hot resistance of the Winding @ T2
T1 = Ambient @ Start up deg.C.
T2 = Ambient @ Finish in deg.C.
235 = Reciprocal of the temperature Coefficient of the resistance of Copper at O deg.C.

For a winding to comply with Class F insulation requirements all the materials must be to Class F specification or better.

3-Phase Induction Motor Construction


There are a number of advantages in buying motors with a thermal temperature reserve apart from an anticipated long service life.

* Service Factor. This is really an American, (NEMA), term that is not covered in IEC standards. It means that the motor can be overloaded without serious damage of overheating.

Typically NEMA specifications call for Service Factors of 1.1 or 1.15, meaning a 10% or 15% overload. Service Factor is in fact using up the thermal reserve of the motor and allowing it to operate at its full Class temperature rise. Although IEC does not acknowledge Service Factor in the same way it certainly allows motors to operate to their full class rating and in fact most motors with a generous thermal reserve will easily match the NEMA requirement for 1.1 or 1.15. Service Factor and duty rating, e.g. S1, S2, etc, should not be confused. Duty ratings are clearly covered in IEC standards for different repetitive short term overloads which can be defined and simulated to ensure the motor still meets the requirements for temperature rise.

* Voltage of Frequency Variations. In some installations, especially with their own power generation, or a very weak grid, large fluctuations in voltage and/or frequency are possible, which can cause increases in the temperature rise of the motor. Motors with a large thermal reserve can operate in these conditions usually without exceeding their Class rating by using some or all of their thermal reserve, depending on how large the fluctuation is.

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