Parameters of heating curves of electrical machinery

Abstract

DISCUSSION AT MIDWINTER CONVENTION tion," and it therefore, in general, includes the effect of the reluc tance of the iron as well as of the air. Dr., Slepian's point is that the effect of the reluctance in the iron, when saturation is absent, is not taken into account.The two cases treated are, a. "Special Case," in connection with Fig. 3 of the paper. Here it is specifically stated that, so far as L 0 is concerned, the magnetic reluctance of the iron is neglected. Therefore L 0 in this special case in the industance corresponding to zero reluct ance in the iron. Hence Dr. Slepian's discussion, referring to the equations based on this assumption, is not pertinent.b. Another case, Appendix D, in which no qualifying as sumption is made with respect to L 0 except those given in the definition, quoted above, from the list of symbols. And, there fore, if it is desired to consider the case which Dr. Slepian has in mind, he should use the equations for this case given in Appendix D. PARAMETERS OF HEATING CURVES OF ELECTRICALMACHINERY 1 (KARAPETOFF) NEW YORK, N. Y., FEBRUARY 10, 1926 G. Ε· Luke: As mentioned in this paper, Professor Kennelly derived the heating characteristics of an electric machine by considering it as one "chunk" of metal 2 . This paper gives the thermal.equations on the basis of two "chunks" of metal (core and windings). An approximate solution of this problem based on three "chunks" of metal (core, windings, and frame) was given in another previous Institute paper 3 . As a matter of fact, a correct solution of this problem would have to be based upon an infinite number of "chunks."Professor Karapetoff assumed two thermally connected masses of metal (core and windings), each having a uniform temperature and loss throughout. In practise, the losses and temperatures are usually much higher in the teeth than in the main part of the core ; hence, there will be a heat flow in that direction. In the windings, there may be a considerable heat flow from the em bedded part to the ventilated end portion. Thus, the heating curve of the "hot spot" on the copper will depend not only upon the thermal constants of the embedded copper and core but also upon the thermal conditions of the end windings. Again the large variation in the temperature of the cooling fluid in its passage through the machine will tend to cause all parts of the machine to vary with it.The assumptions which must be made in order to derive the approximate thermal characteristics of a machine are more closely approached in small machines with relatively little forced cooling. On the other hand, in large machines, where long heat and fluid flow paths are found, together with a high velocity of the cooling fluid, the derivation of the heating characteristics on the assumption of uniform core and winding temperatures is likely to be erroneous.The method of solution of the problem as simplified by Professor Karapetoff interested me, as several years ago 1 had occasion to solve the same problem. In my solution, θ\ was eval uated from eq. (2) (also d ...