Influence of temperature on insulating materials used in electrical machinery

Laboratory tests, as well as the condition of machines which have been running a number of years, show that the temperatures that can be withstood without damage to the insulating materials are considerably higher than those prescribed by international rules. The present values have probably been chosen so as to guard against too high temperatures caused by defective construction of certain parts of the machines. If exploring coils are placed suitably and with care at the points where overheating is most likely to occur, the value of the permissible temperature, as recorded by the coils, should be 10 to 20 per cent higher than the rules permit, in order fully to utilize the materials of construction. Windings for over 5000 volts should not contain any air pockets.