The relation of overload to the inner temperature of machines

OVERLOAD is generally regarded by users of electrical machinery as a convenience. Very few consider the meaning of a rating containing overload allowances (double rating) from the point of view of the comparability of different makes of machines and of the effects that overload allowances may have on the duration of the insulation of the machine. The object of this paper is to show, in an elementary way, the process of heating going on in electrical machines and to consider what is the fundamental difference between single rating (no overload allowed) and double rating (overloads allowed) and to supply a clear and sound basis for an eventual discussion. I shall here only refer to continuous rating and the word machine will indicate generators, motors, converters and transformers cooled by natural ventilation.