Structured Protection Measures for Better Use of Nanocrystalline Cores in Air-Cooled Medium-Frequency Transformer for Induction Heating

Medium-frequency segment constitutes the bulk of energy consumed by induction-heating loads where transformer is an integral part for power delivery. Traditional water-cooled transformer is less efficient, bulky, and involves complex manufacturing processes. Its large leakage hinders efficient power transfer to load. Availability of litz wire conductors and low-loss high-flux-density magnetic cores helped popularize energy-efficient air-cooled transformers in several high-power induction heating applications. For optimal design, the energy efficiency of transformer should be high; moreover, costly conductors and cores need to be optimally used. For constant-current variable-power loads, the copper loss is minimized first, which inherently, core loss permitting, minimizes the core area. The cores are forced to operate at large flux density where this article elaborates that nanocrystalline cores are more suitable. Such a configuration needs to be safe and functionally reliable, particularly, when the operating point of transformer has two dynamic variables-voltage and frequency. For reliable operation, this article proposes structured protection measures where, under genuine fault conditions, the controller is switched off, but for magnetic saturation under healthy conditions (e.g., overload), the frequency is momentarily increased to take cores out of saturation. The proposed concept has been validated in a 60-kW controller designed for forging applications.