High power transformers used in induction heating are traditionally conduction-cooled, they are poorly coupled and efficiency is not great. Unlike other power electronics applications, in induction heating, the electrical circuits of transformer are always loaded with rated current, the magnetic circuit is loaded when heating of job taking place. Copper loss is always at peak and core loss is load dependent. There is hardly any work reported on optimizing such critical component. For its optimization, there needs to be a paradigm shift in design approach. The article proposes that the starting point of design is to optimize its energy efficiency by minimizing its no-load loss i.e. the total copper loss and make it air-cooled. For constant current windings it is natural to minimize the length of litz-wire conductors used, it puts constraints on magnetic circuit where the core is forced to operate at high flux density. The transformer would be inherently compact. For such configuration the core material plays an important role. For choice of suitable core to reliably operate at large flux density, this article comparatively studies different soft magnetic materials for practical design of extremely efficient (99.78%) transformer for induction heating.
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