Insulation Class of Electrical Machines

There are insulation papers kept between phases of the winding and in the slots to keep insulation between windings and lamination core. Those insulation papers are designated with a class which determines the maximum temperature those insulation papers can withstand without an effect to the motor life time. Following image shows specifications of each insulation class

           

So it is very important to keep the hot-spot temperature (Point where the temperature is highest) below the maximum permissible temperature for relevant insulation class used in that particular machine.

           

If the motor is operated above rated temperature of insulation class the lifetime of the insulation hence the motor winding will be reduced drastically.

A common rule of thumb is that the life of the motor insulation will be halved every tie motor is operated above 10°C of rated insulation temperature. This diagram shows how motor winding life time depends on excess temperature rise which motor is operated.