High frequency solid-state power sources for induction heating

Induction heating and melting applications often require a power source to convert 3-phase mains input power to single-phase output power at a higher and variable frequency. Amongst various power conversion schemes, solid-state power converters using the most modern devices provide the best power control techniques available for this application. In designing for this purpose, careful consideration must be given to the characteristics of the load, which presents a very low power factor and an impedance possibly varying widely as the heating cycle proceeds. From the variety of thyristor commutation techniques commonly employed in high-power inverters, series load commutation is particularly suited to high-frequency applications, as it has an intrinsically high turn-off time for the circuit thyristors (clearly essential at high operational frequencies) and much reduced switching losses. However, series commutation circuits are load sensitive, and therefore require careful design, especially with an induction heating load. Recent developments in power conversion techniques have led to the elimination of the d.c. link in a.c. to a.c. power conversion, enabling both high operational efficiencies and substantial savings in the initial cost of the device to be achieved. This new type of converter (called a cycloinverter) power and frequency control facilities. However, in a cycloinverter, since high-frequency switching is performed simultaneously with rectification, these control schemes are dependent on the operational frequency. The direct power conversion in a cycloinverter causes, unfortunately, distortion currents in the input lines and the output circuit, and it is the designer's task to minimise these undesirable components. The project aims to investigate the potential uses, both of the series inverter in its high-frequency form and of the cycloinverter, as power sources for induction heating. Design criteria are established for each circuit, with consideration given to turn-off time, efficiency, power factor, component ratings and predicted load variations. Computer simulations of the converters are employed to investigate the different voltage and current waveforms in the circuits, and to establish how the performance of each inverter may be optimised and these are verified by results obtained an experimental prototypes.