108 PRINCIPLES OF ELECTRICAL DESIGN 



being carried away without requiring very great differences of 

 temperature between the internal portions of the material 

 where the losses occur and the surfaces in contact with the air. 

 The thermal conductivity of all materials used in construction, 

 and of the combinations of these materials, must be known before 

 accurate calculations can be made on the internal temperatures; 

 but, as an indication of how the insulation tends to prevent the 

 passage of the heat to the cooling surfaces, the following figures 

 are of interest. The figures in the column headed " Thermal 

 conductivity" express the heat flow in watts per square inch of 

 cross-section for a difference of 1C. between parallel faces 1 

 in. apart. 



Material Thermal conductivity 



Steel punchings, along laminations 1.6 

 Steel punchings, across laminations 



(8 per cent, paper insulation) 0.038 



Pure mica . 0091 



Built-up mica 0.0031 to 0.0026 



Empire cloth, tightly wrapped (no 



air spaces) 0.0063 



in.. 0.0042 



The maximum temperatures to which insulating materials 

 may be subjected should not exceed the following limits: 



Asbestos 500C. or more 



Mica (pure) 500C. or more 



Micanite 125 to 130C. 



Presspahn, leatheroid, empire cloth, 

 cotton covering, insulating tape, and 



similar materials 90 to 95C. 



If ventilating ducts are provided at sufficiently frequent 

 intervals to ensure that the internal temperatures will not be 

 greatly in excess of the surface temperatures, it is merely nec- 

 essary to see that the cooling surface is sufficient to dissipate 

 the watts lost in the iron and copper of the armature. 



Temperature Rise of Self-ventilating Machines. In calculating 

 the losses and the cooling surfaces of the armature, we shall 

 assume that the current density in the conductors has been 

 so chosen that the end connections will not be appreciably hotter 

 than the armature as a whole. If this density does not exceed 

 the value as calculated by formula (51) of Art. 29, it may be 

 assumed that the temperature rise of the end connections will 



