This thesis describes various modeling techniques, analyses and prevention methods for imbalanced and inhomogeneous distribution of magnetic flux inside the magnetic components of high power DC-DC converters. Two techniques for detailed modeling of the discussed magnetic components are presented in detail. In the first technique the finite element method is used, where the detailed finite element model is adequately coupled with the circuit model of the DC DC converter, and where also the properties of active and passive converter components are described adequately. The direct coupling between both models based on modified nodal analysis is presented. The second technique for detailed modeling of magnetic components is based on the lumped parameter method. This way the topological as well as hysteresis properties of the magnetic subsystem are taken into account. Further, a magneto-dynamic model of soft magnetic steel sheets is presented, where the interdependence between eddy currents and magnetic field inside soft magnetic steel sheets is taken into account. The parameters of circuit models that are difficult to determine, are determined using optimization algorithm differential evolution. This thesis also presents overviews and analyses of sources regarding imbalanced and inhomogeneous distribution of magnetic flux inside the magnetic cores of transformers inside DC-DC converters. An imbalanced magnetic field is a consequence of the drift of the magnetic flux inside the transformer core. Inhomogeneous distribution of magnetic field is however a consequence of inadequate distribution of transformer windings along the transformer's magnetic core. These discussed phenomena can cause both saturation of the transformer core, and generally leads to lower efficiencies, power densities and operating reliabilities of such devices. Further, active and passive methods for prevention of the discussed phenomena are presented in detail. It is recognized that the imbalance of the magnetic flux can be prevented both with active as well as passive methods, where inhomogeneous distribution of magnetic flux can be reduced only by passive measures. Methods and sensors for determining magnetic flux imbalances and saturation inside the magnetic components are also presented, where the sensor in the form of a magnetic bridge is presented in detail. Based on this sensor an active technique for preventing of saturation of transformers' cores is presented, which uses hysteresis control of magnetic flux inside the transformer core. At the end of the thesis the discussed problems are analysed with the numerical calculations based on the derived models, which are validated with measurements.
D.09 Tutoring for postgraduate students
COBISS.SI-ID: 18058518Monitoring system for welding transformer is capable of taking measurements and recording data of electrical variables of a welding transformer. The collected data can be transferred to the PC, converted into DEC code and evaluated. Based on the obtained data, the operation of welding transformer during its life span can be evaluated whiles in the case of failure a diagnostic procedure can be performed. Monitoring system consists of microcontroller (PIC), FLASH memory, thermistors with adjustment circiuts and additional circuits for voltage measurements on rectifier diodes, circuit for measurement of transfomer's output voltage, circuit for measurement of welding current, input filter, energy capacitor, voltage regulator and two signal LED diodes. The microcotroller is connected with FLASH memory chip through SPI bus. Computer can be connected with microcontroller via USB/SCI bus.
F.08 Development and manufacture of a prototype
COBISS.SI-ID: 17217046Winding placement that enables uniform magnetic excitation in the resistance spot welding transformer iron core has been developed and patented, in collaboration with Indramat d.o.o. (Bosch Rexroth Group). Patent application is the result of the research project. Proper design of the winding placement has numerous positive implications: - reduction of the non-uniform magnetic flux in the core - reduction of the core’s cross-section - reduction of the length of the winding - reduction of the leakage flux in the transformer window - reduction of the transformer iron core and winding losses, as well as the losses of the entire device - reduction of the required flow and the amount of cooling liquid - greater utilization of the materials used - increased device operational reliability - increased power density and the efficiency of the device - possibility of faster robotic arm movement - high efficiency and welding quality
F.07 Improvements to an existing product
COBISS.SI-ID: 17031446Patent consist of two parts: - The first part of the patent describes the iron core saturation detection with a measuring bridge. The measuring bridge detects the displacements of magnetic field in the welding transformer iron core, when the core is saturated. The patent describes the discrete electronic circuitry of the measuring bridge, capable of detecting the iron core saturation in a very short time, despite the present interferences. The signal from the circuit can be used for the control of the spot resistance welding transformer system in such a way that iron core saturation can be avoided and allowing the iron core to be to fully exploited. Hence, the dimensions of the iron core, for the same power, can be smaller. - The second part of the patent describes the improvement of the Advanced Hysteresis Control (AHC) algorithm already protected by patent EP 2 097 912 B1. The disadvantage of the existing AHC algorithm is that it may cause the short pulses, which may have the negative impact on the resistance spot welding system. The proposed enhancement of the algorithm completely excludes the possibility of the occurrence of these pulses. The algorithm constantly compares the values of welding current and magnetic flux. Considering the concurrent states of both variables, algorithm predicts the possible irregularities and using the predefined switching patterns controls the inverter in such a way to prevent apearance of short pulses. In this way, harmful short time pulses can be avoided.
F.07 Improvements to an existing product
COBISS.SI-ID: 18119958The doctorate thesis describes various methods for Joule loss calculation in windings of resistance spot welding transofrmers. Due to the proximity effect between the individual coils of the windings, the current density distributes non-uniformly at the given operating frequencies. Non-uniformly distributed current density caused by the consequnces of the proximity effect,increases Joule loss in windings in comparison to the case, where the current density distributes uniformly. It is clearly shown that the proximity effect increases with frequency. Therefore, it is significant to determine thecurrent distribution in windings applied in further Joule loss calculation.An advanced analytical method of successive approximation, which does not take into consideration the iron core of the transformer, is developed to enable such calculations. Within the developed method, all mutualifnluences among the transformer coils are taken into consideration. Calculation of Joule loss is based on the general equation that includes current density and specific conductivity. The comparison of two methods for analytical calculation of Joule loss in transformer windings respectively the advanced analytical method of successive approximation and Dowell method showsthe significant advantages of the former. The advantage of the developed analytical method is confirmed with numerical results obtained by finite element method and experimental tests. The presented advanced analytical method of successive approximation is based on the analytical equations and, therefore enables higher level of results accuracy and less time-consuimng analysis. Moreover, it is suitable for implementation of the optimization procedures. The described analytical calculation of Joule loss is based on theparametric winding model and enables quick calculations for any transformer dimensions and operating frequencies.
D.09 Tutoring for postgraduate students
COBISS.SI-ID: 266391296