Synopsis: With the increasing use of drytype transformers, attention has been focused upon the fact that the difference be tween the average temperature rise by resistance and the hottest-spot temperature rise is much greater than in liquid-immersed transformers.In this paper there are given the results of an extended series of thermal tests on a variety of dry-type transformer coils. Data are presented in regard to the effect of dif ferent heights of coil stacks, different con figurations, and, in addition, some essential precautions in making temperature meas urements are pointed out. Hot-spot tem peratures were investigated in coils cooled by natural draft and by forced-air circula tion at different velocities for both directed flow and random flow.Sufficient data are given to determine the ratio between the average temperature rise of the windings and the hottest-spot tem perature rise. Based upon this relationship, the need is shown for review and modifica tion of the conventional hot-spot allowances for dry-type transformers.

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