New thermometric method of measuring power losses in solid metal elements

Where constructional-steel elements are exposed to a strong leakage, it is desirable that the losses in them should be measured at the prototype-system stage to ensure that overheating cannot take place. The thermometric method has been widely used for this purpose, but it can give gross errors with thick elements when heat is generated in a thin surface and conducted rapidly away to the cold mass of the material. In the paper, an improved method, is presented for setting up the equations for heat flow and electromagnetic-wave propagation in the material that takes account of the nonuniform distribution of heat sources, the variation of the permeability with the field strength and the modes of excitation of the eddy currents. These equations have been solved numerically, using a computer. The results are presented in the form of graphs showing the corrections to be applied to the conventional analysis. A new practical method for measuring losses, based on these results, is described. Instead of plotting a temperature/time curve, a reading of the temperature rise is taken after a specified interval, e.g. 30 s. From the graphs given, the local power loss and the field strength at the surface can then be read off. An example of the measurement of the loss in the tank wall of a transformer is given in which the direct measurement of the surface-field strength compared well with that derived from this method.