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Reference standards IEC 34-1, BS4999, AS 1359.32

'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 over time 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 losses 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 not 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.

Some standards estimate the life of the insulation materials as 25,000 hours if operated continuously at their rated temperature and the external temperature low.

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, are not not in a continuous ambient temperature of 40 degrees. A life of 20 to 30 years under normal conditions can confidently be expected.

Insulation class





B rise


Temperature rise







Max temp of the winding







Ambient temperature







Allowance of hot spots







Max temp of rise of winding







Thermal reserve







The permitted temperature rise of the windings of an electric motor are subdivided into different insulation classes and temperature limits. The above table applies to motors in an ambient temperature up to 40C 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 its full thermal rating. It is not considered practical to try to locate and measure the hottest spot in the 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 atT1
R2 = hot resistance of the winding atT2
T1 = ambient at start-up deg C
T2 = ambient at finish in deg C
235 = reciprocal of the temperature coefficient of the resistance of Copper at 0 deg C.

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

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 Factor 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 (eg 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 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 the size of the fluctuation.

Effects of temperature and insulation on motor performance and life

It is a fact that you could take just about any electric motor and hook it up to any appliance and it would work. As would be expected though, some motors would perform better and last longer than others.

The one fundamental characteristic which determines whether a motor survives or dies is quite simply how hot it gets. Motor overheating can be caused in a number of ways including:

  • overloading
  • ambient temperature too high
  • incorrect supply voltage and/or frequency
  • frequency of starting

All of these can seriously affect motor performance and/or life.

On the other hand, a motor operating in an ambient temperature below freezing could be overloaded and run quite happily for years.

The controlling factor is always the temperature of the motor.

Perhaps the best starting point is to understand that any motor is designed such that when operating at its rated output under known conditions, its windings will experience a set increase in temperature (temperature rise) above the ambient temperature around the motor.

The temperature rise is a function of two criteria:

1. The amount of heat generated in the rotor and stator per unit time
2. The efficiency of heat transfer from the motor by the type of cooling system employed.

Providing the motor output, power supply, location and other relevant conditions remain constant, then the resulting temperature rise will also remain constant.

The motor manufacturer having established the design temperature rise and assuming a maximum ambient temperature that is likely to be encountered, then decides upon the use of particular grade or 'class' of insulating material which will be sufficient to ensure no thermal breakdown that would lead toshort circuiting between phases or between phase and earth as well as providing for an acceptable lifetime for the motor.

Each 'class' of insulation has its own maximum recommended temperature. If this critical temperature is drastically exceeded, failure will occur in a very short time. If it is marginally exceeded, then the lifetime of the insulation (and the motor) will be reduced by the order of 50%, leading to premature burn-out.

An increase in load of about 4% will result in an increase in temperature rise of 10%, which makes selection of the correct motors for the job absolutely vital for long term cost-effective operation.



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