A thyristor-controlled intermediate frequency (IF) unit consists of a controllable or uncontrollable rectifier, a filter, an inverter, and various control and protection circuits. During operation, the three-phase power frequency current is rectified into pulsating DC by the rectifier, then smoothed into DC by the filter before being sent to the inverter. The inverter uses thyristors as electronic switches, converting the DC current into a higher-frequency current to supply the load.
In an IF induction furnace, the IF current delivers energy to the load through the induction coil, which is often a component of the inverter. The power factor exhibited by the induction coil and the furnace charge is very low. To improve the power factor, a tuning capacitor is needed to provide reactive power to the induction heating load. In practice, the capacitor is used in series, parallel, or a combination of series and parallel connections with the induction coil. Based on this, inverters can be divided into three types:
1) When the capacitor and the induction coil form a series resonance in the inverter, it is called a series inverter. The voltage of the series inverter is a function of the induction coil voltage, and the inverter current is the same as the coil current. 2) When the capacitor and induction coil form a parallel resonance in the inverter, it is called a parallel inverter. The voltage of the parallel inverter is the same as the coil voltage, while the inverter current is much smaller than the coil current; the former is a function of the latter. The performance of the parallel inverter is closer to that of a medium-frequency generator set, and it is widely used in induction furnaces.
3) A series-parallel inverter combines the characteristics of series and parallel inverters.
In addition, there are frequency multiplier inverters, which can increase the operating frequency. These are all passive inverters because they do not send the converted AC energy back to the power source. In fact, passive inverters have many circuit structures and can be classified in many ways. For example, according to the number of phases output by the inverter, they can be divided into single-phase, three-phase, and multi-phase; according to the circuit structure, they can be divided into symmetrical and asymmetrical; or bridge type and non-bridge type, etc. The circuit structures of inverters used for induction heating mainly include single-phase bridge parallel, series, series-parallel, and frequency multiplier inverter circuits. These lines each have their own characteristics, making it impossible to clearly define their application scope, let alone simply compare their advantages and disadvantages.
