This article explains the meaning of thermal classes and temperature rises and provides an overview of the half-life expectancy of an insulation system.
The definition of thermal classes as specified by the IEC 60085-2008 and by the IEEE Std1 -2001 are summarised in table 1 . Thermal class refers to the designation that
is equal to the numerical value of the recommended maximum continuous total temperature in degrees Celsius (0C).
Classification of insulation system according to NEMA MG1 where insulation systems are divided into four classes according to the thermal endurance of the system for temperature rating purposes. These classes have been established in accordance with IEEE Std1.
Temperature rises are classified as temperature rises by resistance and temperature rises by ETD (Embedded Temperature Detector).
Table 1: Thermal classes assignment
Rises by resistance are established by measuring temperature at cold and after thermal stability conditions. Temperature rises by resistance state the maximum temperature rise (Delta T) as specified by IEC60034-1 table.7 and NEMA MG1 part 32 table.32-3 and based on 40ºC ambient temperature.
The maximum temperature rises by ETD are specified by IEC60034-1 and NEMA MG1 part 32 and based on 40ºC ambient temperature.
If the total operating temperature is raised by 10°C then the thermal life expectancy of the insulation system is reduced by 50%
Figure 1: Insulation half-life of class H insulation system verses temperature.
Half-life graph of class H insulation system
Underwriters Laboratories (UL) have established operating temperatures and associated life expectancy levels for all the electrical insulation materials available and grouped these materials into the various classes familiar to all those involved with electrical machines and equipment.
UL used Arrhenius Equation to model the relation between time and temperature. Arrhenius equation which describes the temperature dependence on the velocity coefficient of chemical reaction can be used to model the relationship between system test life and temperature. The Arrhenius equation was simplified by taking the natural logarithms to plot the relation between time and temperature.
The half-life curve of class H insulation system as described by the UL standard is illustrated in figure 1.
■ To operate class H insulation system at its maximum operating temperature of 180ºC, the design life expectancy of the insulation system is 20kHrs.
■ To operate class H insulation system at 155ºC, the design life expectancy is 120kHrs.
■ To operate class H insulation system at 130ºC, the design life expectancy is 640kHrs.
■ Similarly, to operate class F insulation system at its maximum operating temperature of 155ºC, the design life expectancy of the insulation system is 20kHrs.
■ To operate class F insulation system at 130ºC, the design life expectancy is 120kHrs.
■ To summarize, operating class H insulation system at 155°C will give similar life to operating class F insulation class at 1300C. Operating class H insulation system at 1800C will be similar to operating class F insulation system at 155°C.
The meaning of the half-life phrase is that after a period of 20kHrs continuously operating at 180ºC (or at 155°C for class F insulation system), the insulations ultimate electrical strength which can be correlated to the dielectric resistance, will have degraded to approximately half of its original value when it was new. It does not mean that it will fail after 20kHrs.
The half-life curves indicate that if an alternator total operating temperature is reduced by 10°C then the thermal life expectancy of the insulation system is approximately increased by 200% (doubled). Conversely, if the total operating temperature is raised by 10°C then the thermal life expectancy of the insulation system is reduced by 50% (halved).
In practice other external factors such as applied load, voltage spikes due to connected load or environmental conditions, contaminations, maintenance practices, all can have an influence on the insulation life and therefore understanding and limiting these factors will help to improve the life of the insulation and alternator.
For more information please contact the Applications team