Innovative heating concept using IGBT multi-converter technology

    1.      Introduction
    The technological requirements for the forging process are forced by the needs of a modern production cycle. So high productivity, accuracy in quality and very high flexibility of different parts force the production process to come up with new strategies and ideas to recover the different technical needs.
    The whole forming process is strongly influenced by the quality and repeatability of the heating process. So the first target is to obtain the desired enthalpy content in the billets. Furthermore the axial and radial temperature differences in the billets should be as small as possible. However this conflicts with a low scaling and sticking rate. Another important aspect under the view of increasing energy costs is an energy consumption optimized production. Furthermore the length of the equipment is limited by the space of the production facility. So it is very important that the heater respectively the heating process is well adapted to the customers’ requirements in order to get reliable production and high productivity.
    2.      The concept of a modular heater system
    The quality of the heating process strongly depends on the total length of the installation, the number and power rates of the coils as well as their frequencies. For a single converter installation the coils can only be optimized for a single throughput and billet dimensions with respect to small temperature differences in the billet with a low scaling and sticking rate and at low energy consumption. This optimum results from the right choice of the winding number for each coil and the operation frequency
    which must the same for all coils, since they are fed only by a single converter. So this frequency also can only be a compromise. At the heater inlet, where the billets are still cold and therefore ferromagnetic, a low frequency is appropriate to heat up not only the billet surface but also its inside.
    For the centre section of the heater where the surface temperature of the billets is approximately between curie temperature and the desired forging temperature a higher frequency is necessary to keep the energy consumption small. In the last section basically a temperature compensation between the surface and the centre should be effected to get a homogeneous axial and radial temperature distribution within the billets. The highest frequency is necessary in order to compensate the thermal losses on the billet surface in essence. Due to that new technology with separately fed coils at different frequencies by separately controlled converter units it is possible to make the winding numbers of all coils mostly equal in spite of different required power rates for the respective heater sections. Therefore every coil can be placed at any position so that the number of required spare coils can be reduced substantially.
    This multi converter concept is based on IGBT converter technology. Therefore modular converters are designed in such a way, that every IGBT unit is a single converter of 250kW.
    The modules can be combined in such a way that each coil gets the necessary power for the specially designed heating process. Mostly a combination of maximum 3 modules per heating region is combined to supply the power to that coil for the heating region. The heating task leads to a reduction of the needed power in the following regions so that there an installation of fewer modules  is required.
    In the end of the heater the holding zone is fed with 250kW or in fact with one converter for the power supply. The principle is designed to scale the heating process in the different regions independently from the coil design.
    The modules are integrated in the converter board. Therefore a combination of the single modules to the power supply is adapted to the overall heating process.
    The power control is realized with an IGBT control board with an own CPU via fibre cable connectors.
    That system is very reliable and well proofed in different heating installations world wide.
    3. THERMPROF© – a software package for controlling the heating process
    Since it is possible to control separately every single coil respectively every single converter unit the user needs a tool to find out how to do this to avoid overheating or melting of the billets and to get the best heating process as possible for his special task without any time and cost intensive attempts for an optimal heated billet with respect to uniformity of temperature, low energy consumption and low sticking and scaling rate.
    For that task THERMPROF© has been created. It is an end user software package designed for the pre-calculation of the heating process of multi-converter technology heaters for an enthalpy controlled zone heating. The user can quote the desired billet diameter and the throughput or the billet length and its cycle time. Furthermore the averaged temperatures (enthalpy) of the billet cross section area at the ends of every separately controlled coil or coil unit must be defined. The result is a chart for the core, surface and averaged temperature of the billets and the scaling within the heater.
    Converter load and temperature are supervised in order to avoid converter overload or local overheating of the billets where the admissible maximum local temperature for each separately controlled coil or coil unit section can be defined by the user. If a converter unit fails this unit can be switched off in THERMPROF© to find out the highest throughput which is still possible.
    For more details about the temperature another window can be opened where the temperature distribution within the billet is shown in a coloured 3D visualization. That can be done for any position of the billet by moving the billet along the heaters length. Since it is possible to define the time from leaving the heater to the first forming the temperature within the billet during and after that cooling down time can also be monitored in that way. So the user has the total control about the billet temperature from the beginning of the heating process up to the first forming.
    Another feature of THERMPROF© is the cost calculator. It considers the energy consumption, the material costs with respect to the scale losses and the die wear against the uniformity of the temperature distribution within the billets at the first forming. Now with THERMPROF© the user is able to modify the heating process in order to optimize it respectively to minimise the piece costs with the help of the cost calculator.
    A further advantage of the enthalpy controlled zone heating by IGBT multi-converter technology is an
    even heating up curve of the averaged temperatures of the billet cross section area independent of throughput and billet dimensions. This avoids overheating within the heater at low throughputs and leads only to small billet temperature fluctuations at the heater outlet after throughput changes.
    3.   PRODAPT-FX - Heating processor for converter control system
    The heater processor PRODAPT®-FX is designed for the use in the environment of a forging shop. It helps the blacksmith to find the optimum setting values for the heater for the respective billet programs and to store the values once they have been found, so that they can be reproduced any time.
    The hardware is based on the industry controller AC800 soft PLC without any rotating parts. The processor is able to take over the parameters from the software package THERMPROF and to control all the necessary functions out of that information. It has proved its value many times over in other industrial sectors.
    Operation and visualization are performed by an operator panel which is user-friendly and suitable for the forge environment. By means of the panel, the heater can be operated via a touch screen and function keys without a mouse. The controller itself has no vulnerable hard disk or active ventilation system and is insensitive to vibrations.
    The control during the process is visualized on the control panel. So the user can follow the process all the time and the process can be controlled in a very short time. Also changing of parameters can be done online during the process is in operation in different steps.
    In the events of faults in upstream or downstream installations of the forging line, the feed is slowed down to the minimum possible value to reduce the scrap (Holding function). To reduce the scrap when starting up the installation the “Cold Start-up Function” is used. If the coil lining is still cold, the feed is initially reduced to ensure heat-soaking of the coil lining.
    The “Cycle Operation” is used to supply the heated billets to the extraction unit with accurate cycle-timing. The feed is controlled in such a way that there are no undefined cycle times if changes are made to the cycle time.