Induction Heating

We investigate in this paper a numerical method to control the temperature of an induction heating process through electric current parameters such as frequency and current voltage imposed to the terminals of the inductor. We seek a sinusoidal voltage function whose amplitude varies in time. For this, we use a two-dimensional induction heating model that involves a phase transition in which the magnetic field has only one nonvanishing component. The procedure can be extended to more sophisticated models without major modification of the optimal control problem. We present a numerical control algorithm and discuss numerical experiments that prove the efficiency of the procedure.

Quench and tempering heat treating operations for tubular products are relying more on induction equipment. The reasons for this can be traced to the lower energy costs for operating induction equipment compared to gas furnaces and the greater flexibility that the induction lines offer compared to their furnace counterparts in regards to recipe control and product mix. However, there are limitations and special considerations for induction heat treating equipment and the induction coils used for these operations. This paper reports on the design and operation of a new induction heat treating line for API 5CT grade L80 and P110 casing and tubing with upset ends. Upset ends pose special technical challenges for induction heating; the generation of a uniform temperature distribution relies heavily on proper coil design as well as line layout and heating time. Simulations of induction heating have provided predictions of heating profiles, and on-the-line testing allowed recipe refinement and validation of simulation models. Results from this case study help to increase confidence in this heat treating process as well as create an improved induction heating line layout for future applications.

This paper presents a series-resonant inverter (SRI) with an extended topology using a pulse-density modulation (PDM) control method. Theoretical analysis shows that the use of the SRI with extended topology and the PDM control (extended PDM-SRI) allows reducing the fluctuation of the SRI output current by more than 40% at the quality factor of 5 compared to a conventional SRI with the traditional PDM control. To eliminate drawbacks of the extended PDM-SRI, specificities of PDM switching sequences to ensure the zero-voltage-switching (ZVS) operation are discussed in detail, and a solution to the voltage imbalance across the capacitors of the extended topology is proposed. Simulation analysis of the extended PDM-SRI confirmed the effectiveness of using the proposed solutions to eliminate the drawbacks. The feasibility of the proposed SRI is confirmed by testing on a 2.2 kW experimental setup.

Artificial thermal damage to gear steel material was produced using two heating methods: induction heating and laser irradiation. In the present work, the aim was to obtain in-depth knowledge about the possibilities of producing controlled thermal damage to calibration samples for the Barkhausen noise (BN) method used in gear grinding burn detection and to detect this damage with the BN method. Laser irradiation was found to be a suitable method for controlled production of thermal damage. The results show that both the laser input power and laser traverse rate must be optimized to avoid the rehardening or magnetic saturation of the samples. A framework for the optimal laser parameters was constructed for the steel material studied here.

This paper deals with implementation of a multi-output Series Resonant Inverter(SRI) for induction heating applications, which uses pulse density modulation(PDM) control for full bridge Series resonant inverters for output voltage and power control. It ensures better efficiency performances than conventional control strategies. The proposed converter can be considered as a two output extension of a full bridge inverter. This full bridge inverter can control the two outputs, simultaneously and independently, up to their rated powers, which reduces the usage of number of components as compared with conventional method. It also ensures higher utilization of switches used for its operation. A two output full bridge series resonant inverter is simulated and implemented. The Experimental results are compared with the simulation results. 1. INTRODUCTION Resonant inverters are now used in many applications to convert the available dc energy into ac energy such as DC-DC resonant converters, induction heating systems for industrial processes or home appliances, electronic ballasts for lighting, radio transmitters and others. The Control of output voltage or power in many resonant inverters has been obtained by varying the switching frequency, which has several disadvantages such as a wide noise spectrum which makes it difficult to control electromagnetic interference (EMI), more complex filtering of output-voltage ripple, a poor utilization of magnetic components. These problems can be eliminated with the use of a fixed-frequency or a narrow frequency range control technique. Induction cookers are low power, maximum output induction heating system usually less than 3KW per load. An induction heating cooking appliance is basically made up of a flat-type induction coil on which the pan to be heated is placed. Between the pan and the coil an insulator (usually a ceramic glass) is placed. The heat is generated at the bottom of the pan due to eddy currents and hysteresis losses. These induced currents are caused by an alternating magnetic field generated by a medium frequency (20-100Hz) current through the coil. Pan-induction coupling is usually modeled as the series connection of an inductor and a resistor, based on the transformer analogy. The values of the equivalent inductance and resistance depend on the operating frequency and the required maximum power. A general block diagram of an induction heating cooking appliance is shown in Figure 1. Induction cookers take the input energy from the ac mains voltage, which is rectified by a diode bridge rectifier. A bus filter is designed to allow a big voltage ripple getting a resultant power factor close to one. Then the inverter topology is used to supply the high-frequency current to the induction coil. Due to this ripple all components

A device based on a high pressure die-casting(HPDC) machine was constructed, with a capacity to produce thixo-casts from steel. After inductive heating to the required semi-solid temperature range, the samples were transported in a protective argon atmosphere to a cylinder of modified HPDC and injected into a pre-heated die. Bearing steel 100Cr6 (after forging) was used as the feedstock material. The metallographic analysis of the steel showed a homogenous structure within a whole volume characterized by a grain size between 2 µm and 8 µm. Differential scanning calorimetry (DSC) analysis allowed to estimate the process temperature for thixoforming at 1 390 ˚C which was attributed to about 45% of a liquid phase. With such technological parameters after the semi-solid processing the microstructure consisted of globular grains of size from 15 µm to 40 µm. The microstructure of globules indicated predominant presence of martensite (about 76% volume fraction) with residual austenite between plates. X-ray analysis confirmed that the thixo-cast samples contain mainly martensite and residual austenite. The hardness of the initial forged sample was HRC 27 and after the thixoforming process it was HRC 60, which confirms the presence of martensite and carbides. The thixo-casts were subjected to low tempering, at temperature of 150 ˚C for a period of 2 h, in order to remove stresses created during the process of thixoforming and to increase plasticity. The hardness after tempering was HRC 62.3. Additionally, in order to identify phases present in the thixo-casts, transmission electron microscopy (TEM) was carried out.