Heat-Transfer Model for Toroidal Transformers

Toroidal transformers provide increased design flexibility, efficiency, and compact design when compared to traditional shell- or core-type transformers. In this paper, the steady-state thermal analysis for toroidal transformers is conducted using a lumped parameter model which can be applied to small power and distribution-grade toroidal transformers as well. Two cases are considered: 1) when the transformer is kept in open air and 2) when it is installed in sealed enclosures. The detailed model includes the effects of the number of turns of windings, number of layers, insulation properties, and geometric properties of the transformer. The model is capable of finding the hotspots that are of paramount importance for the designer. The model parameters are calculated from the design (geometrical) information; therefore, it is suitable to be included in the design loop of transformer design software. The results are compared with finite-element simulations and lab tests on prototypes of various power ratings fitted with thermocouples to record internal temperatures. The model can also be used with varied external media and encapsulation, such as air, oil, and epoxy.