Heat transfer through an electrical coil

Abstract

A theoretical "basis is laid down for determining the radial conductivity for electrical coils of cylindrical construction made from electrical grade, round copper wire covered with a thin layer of plastic insulation, both with and without paper interleaving between the windings. A novel technique is used to give the radial conductivity of a coil in the form of equations which are functions of the thermal conductance of the insulation on the wire, the conductivity of the air, the maximum compression of the insulation under load, the relationship between the thermal conductance of paper and the applied load, and general dimensions. The technique approximates the lines of heat flow through the coil by a system of lines that can be analysed by one-dimensional heat transfer theory. Methods are described for determining the properties of the constituents of the coil that are required for the calculation of the conductivity, and the results of experiments are given that show that the accuracy of the theoretical prediction of the conductivity is better than + -15% of that obtained by direct test upon the coil. The analysis for the radial conductivity of non-interleaved coils is extended to cover axial conductivity but no experimental verification is given. The use of the two conductivities, axial and radial, in obtaining the temperature distribution by numerical analysis is shown and the limitations of the analysis is discussed. Finally a computer programme is provided for calculating the conductivity of paper interleaved coils.