Laws of heat transmission in electrical machinery

In considering the effect of various conditions on the temperature rise in electrical apparatus, it is necessary to distinguish between the three modes of heat transmission; namely, conduction, radiation, and convection. The laws governing each of these factors are considered. Conduction. A table of all available reliable data on heat conductivity of materials used in electrical apparatus is given. The effect of air spaces is considered. The temperature coefficient of the conductivity differs considerably for different classes of bodies. Methods are given for calculating the heat transmitted through bodies of various shapes. Radiation. The Stefan Boltzman fourth power law is used to calculate the energy radiated from a “black body.” A table for the relative emissivity of other bodies is given. Convection. The laws of convection of heat from wires and plane surfaces at various temperatures are considered. The film theory of convection agrees well with experimental results and makes it possible easily to express the variation of convection with change of air temperature and pressure. For forced convection, Russell´s equation gives reliable results. The effect of viscosity and other factors is of great importance in convection in liquids. Combined Effects. Radiation and convection are usually about equally effective in carrying heat from a surface. The effect of these two factors can often be conveniently taken into account by assuming a certain surface resistance. It is very important to carefully distinguish between radiation and convection from surfaces when the walls of a room are at different temperature from the air contained in it.