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Development of a Flywheel Energy Storage based Power Supply for Resistance Spot Welding

High Current Interleaved Buck Converter
FEM for Inductor Design (Fluxdensityvectors)

In automated industrial production resistance spot welding is used to joint metal sheets for example during assembly of cars or white ware. In order to melt the metal, a voltage is applied to the sheets resulting in a current flowing through the metal, which lets its temperature rise. The ohmic and thermal resistance of the metals is very low thus high currents are needed to generate the required dissipation power at the welding spot. As longer welding time means that more heat is transferred into the surroundings of the welding spot, higher currents lead to lower heat losses making the process more energy-efficient. In the typical application the welding time is about some tenth of a second while the time between two welding processes is at least four seconds, leading to a pulsating load profile with a small duty cycle.

A major challenge is to generate the extremely high currents of some tens of kiloamperes. State of the art technology is medium frequency spot welding (MF-SW). In this technology an inverter transforms the line voltage into a 2 kHz medium frequency alternating voltage. A medium frequency transformer decreases the voltage to the desired 1...3 V and a diode-bridge rectifier generates the desired direct current. A drawback of this technique is the missing energy buffer in the system meaning that the whole power for the welding process is directly drawn from the mains. This means that the point of common coupling needs to be dimensioned for high pulsating loads, which generate perturbations in the mains. Additionally a medium frequency transformer is heavy compared to high frequency converters and the diode rectifier causes high losses due to its inherent pn junction voltage drops.      

This project aims to develop a novel energy storage capable of providing the output power during the welding process and to smooth the power drawn from the mains. To meet the requirements an energy storage with high output power rating, small energy capacity and highest dynamics is needed. At the same time very good cycle stability for several millions of cycles is mandatory. As a previous project discovered, a hybrid energy storage consisting of a flywheel energy storage and capacitors achieve best results regarding volume, dynamics and lifetime. The requirements for this application differ substantially from specifications of available storages on the market. This is why a unique hybrid flywheel storage is developed. Owing to the complexity of the storage a multi-objective optimization is executed.

In order to obtain power converter with highest power density and increased efficiency at the same time, several MOSFET converters are implemented. A higher switching frequency compared to state of the art technologies is used which decreases the size of magnetic components. To handle the higher frequency a massive parallelization of MOSFETs and interleaved half bridges is utilized. To further decrease size and losses of magnetic components, an interleaved coupled inductor (intercell transformer) will be implemented reducing the magnetic flux inside the inductor cores. Because of the systems complexity several development methods are used, comprising analytical calculations, simulation of electrical behavior on semiconductor level using spice models, finite elements methods to model magnetic components and modeling of control on system level using Simplorer/PLECS.             

In summary, the novel welding power supply is expected to reduce the peak power of the mains and the converter is expected to be smaller and the energy required for welding will decrease making the system more efficient. Furthermore, a highest level of system dynamics and robustness is projected.

Die Universität der Informationsgesellschaft