Design and Production of Encapsulated Coils of 5 to 50 Watts Rating

Abstract

The sponsoring company wanted to optimise the coil shape and improve their encapsulation process. This led to four objectives: a) To develop a mathematical model to enable coils and contactors to be optimised: A simple magnetic circuit model was developed, containing iron, air gap and leakage and incorporating the B-H curve to give magnetic effect in terms of shape. Comparisons with practical measurements had inaccuracies but addition of airpaths surrounding the device improved this to 5%. The existing products were shown to be near optimum shape, although higher fluxes were attainable by increasing magnetic frame thickness. b) To solve production problems leading to a high failure rate: Controlled tests to define the process boundary conditions were made by varying temperature, vacuum pressure and time and resin mixing. The results showed temperature to be above 60°C and resin mixing good. Comparisons with the shop floor practice indicated poor heating, mixing and temperature control and changes made eliminated these failures. c) To improve production methods: Studies of the existing process, mould design, heating and vacuum dispensing were made. The existing process work showed a bottleneck existed, that 40% resin was wasted due to incorrect equipment setting and that the encapsulation skin did not affect coil temperature. Nylon moulds, PTFE coatings, gasket sealing and non-screw terminations were evaluated. The nylon failed because of mechanical heating and releasing properties, although gasket sealing and non-screw terminations were successful. Heating by induction and coil resistance were made but both were unable to gel the resin. Vacuum dispensing, however, indicated quick and reliable impregnating. d) To design an automated encapsulation process: A design proposal of a dispense-in vacuum system incorporating conveyor ovens and a microprocessor based controller was made. Cost analysis showed significant savings would be made.