Abstract :
The paper contains equations applicable to machines cooled by forced air convection currents. It is to be hoped that the A. I. E. E. Standards Committee will find some helpful suggestions in the paper. Ignoring differences in ambient temperature, the effect of altitude may be considered from two standpoints: (a) The change in temperature rise, the rating remaining the same; or (b) the change in rating, the temperature rise at a given altitude equaling that at sea-level. The applications of both are considered, and equations are solved in both ways. The difference in ambient temperature at sea-level and at altitude is also taken into consideration in other equations. The present A. I. E. E. rule is known to be faulty, applies only on the basis of temperature change, and starts at 1000 meters. These and other objections are believed to be met in this paper. Several of the equations are plotted in families of curves, which should be of assistance to the reader. By assuming other values for some of the factors, other similar curves can be drawn. In Appendix I, the derivations of equations are given. Appendixes II and III cover discussions of two of the factors used in the equations. In Appendix IV, some cases other than a slotted core are discussed. A number of assumptions were made in the derivations of the equations: (1) The temperature coefficient of resistance was neglected. (2) The heat that flows transversely through the insulating wall is the same percentage of the heat generated in the copper at altitude as at sea-level. The equations are intended to apply to a slotted core, although they may be used for other parts of an electrical machine. Some examples are given in Appendix IV.
