TPT September 2010

A rticle

A novel modular power supply system for induction heating applications 1 Cesano M, 1 Di Carlo F, 1,2 Dughiero F, 1 Griffero G, 1 Natale L

1 R&D Department, Power Electronics Division, SAET Group, Italy 2 Department of Electrical Engineering, University of Padova, Italy

Abstract In this paper a description of a novel power supply for induction heating applications is introduced. Some remarks will be presented as for the topology used, which should be reliable for induction hardening and induction tube welding purposes, and for the particular control technique. Moreover some considerations about the load matching circuit will be proposed together with some novel solutions. Experimental results related to some recent installations will be presented and critically discussed also in comparison with other solutions currently available in the market. 1 Introduction Induction heating is an industrial process which is more and more used in different fields. A typical induction heating installation is mainly constituted by a frequency converter, a load matching circuit, the inductor and the load. The control of frequency and power during the process is mandatory to guarantee a high quality product. The frequency converter can be considered as the core of the installation. The choice of the topology [1, 3, 4, 5] , the control technique [2] , the usage for different applications, and the adaptability to different loads [6] are the main characteristics of a power generator for induction heating applications. In this paper a solution of a modular frequency converter, operating in a wide range of frequencies and reliable for different kinds of applications, has been presented. Some typical applications have been discussed in order to demonstrate the versatility and reliability of the solution proposed. In fact the same modular generator has been implemented for high frequency hardening applications and for high frequency induction tube welding. 2 Converter topology Various converter topologies can be adopted in induction heating application. In the following, a brief comparative analysis will be performed to underline the differences and the effectiveness.

Figure 1 : Voltage fed inverter with series load

2.1 Series Resonant Inverter A possible converter topology is the series resonant inverter, as depicted in figure 1. A rectifying bridge supplies a VF inverter and a series resonant oscillator is considered. A big DC-link capacitor bank is required and should be located as close as possible to the inverter to reduce parasitic inductances. Then the design is normally very compact. A drawback is the reduced immunity to the short circuits on the load: the DC-link capacitor discharges itself through the inverter when a short circuit occurs. Furthermore, a matching transformer is required to match the low load impedance with the high generator impedance. This introduces losses in the circuit and is quite complex to manufacture (more and more at the increasing of the frequency). As the inverter DC voltage is constant, the control board must regulate both power and frequency. A possible solution is the so- called “frequency sweep”, where the power is regulated by changing the frequency and then moving the working point over the load impedance characteristic; the inverter is fired with theoretical duty- cycle 50% and introducing a dead time to avoid leg’s short circuit. Another possibility is the resonant PWM where the frequency is fixed (close to the natural resonant one) and the power is regulated by changing the leg duty-cycle: the maximum output power is so reached at duty 50% and it will decrease together with the reduction of the duty.

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S eptember 2010

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