Cooling of Litz Wires in Industrial Induction Heating Applications

Induction heating has the potential to be a very efficient heating method and by using litz wires instead of copper tubes the losses can be significantly reduced. The drawback with litz is the fact that it comprises insulation materials with low temperature resistance, typically in the range 150 – 200 °C. In order to solve this problem a solution is to create channels in the litz structure in which a cooling medium can flow. In order to investigate the possibility with cooled litz wires a number of litz wires were manufactured with different types of internal cooling channels and infiltration materials. The experiments showed that it was possible to maintain a steady state temperature of the litz wire below 25 °C at a current density of 10 A/mm2 and frequency of around 20 kHz. A thermal FEM model was developed in order to calculate the equivalent thermal conductivity of the litz structure based on the measurements. The induced losses in the cooling tube was also investigated with an electromagnetic FEM model showing that the choice of dimensions and material for the cooling tube might be crucial for the heating efficiency.