Parameters of Heating Curves of Electrical Machinery

When a body is being heated by a uniform addition of a constant quantity of heat per unit time, its temperature above the ambient air (the latter being assumed to remain at a constant temperature) increases approximately according to an exponential law. The exponent is proportional to the ratio between the heat capacity of the body and the coefficient of thermal dissipation into surounding medium. In a paper read before the Institute´s Midwinter Cnvention, 1925, (JOURNAL, Vol. 44, p. 142) Doctor A. FE Kennelly has proposed to include such a coefficient among other characteristics of an electrical machine. In the present paper it is pointed out that for thermal purposes an electrical machine cannot be considered as a single body, since the stator consists of two metal bodies (the winding and the core) between which there may be a considerable heat interchange, and that the rotor is also such a composite body. Differential equations of heat flow in a combinational body are established and solved. The stator winding is thermally determined by its heat capacity and its heat dissipation coefficient, and so is the stator core; further, there is a coefficient of mutual flow. The rotor also requires five similar coefficients. Thus, while an electrical machine could be defined by its thermal coefficients, and the temperature rise of the different parts predicted for a given operating regime, the number of required parameters is much larger than for a single body.