Abstract:
In many applications it is necessary to transfer power to a rotatable electrical load. This has been achieved using slipring assemblies. Sliprings have been widely used in electrical generators, packaging machinery, robotics arms, and wind turbines. For centuries mechanical sliprings have been the only viable option for these industries; however due to high friction between rotatory and stationary parts, frequent maintenance is required which increases the operating cost of these applications. Moreover, they are not a safe option to be placed in hazardous places or in the presence of explosive gases. Thus recently, the Rotating Transformer-Based Contactless System utilizing Inductive Power Transfer (IPT) technology has been considered as an alternative option for mechanical sliprings. Since in this contactless solution there is an air gap between the primary and the secondary sides, direct electrical contacts are eliminated and a more reliable and safe option as compared to mechanical sliprings is achieved. However, in some high power applications, a single phase contactless system within a constraint size may not be adequate to supply the required amount of output power. Hence, more single-phase units often need to be installed in order to transfer the required power to the load. To overcome the associated problems with the mechanical slipring and increase the power capability of the single-phase contactless slipring topology, a poly-phase contactless slipring based on axially travelling magnetic field theory has been proposed and researched. Generating such a travelling magnetic field requires a poly-phase primary power converter to provide proper phase shift between the primary side phases and synchronize the individual converters. The aim of this research is to design and practically built a poly-phase high frequency primary power supply to drive a poly-phase contactless slipring system. First, the literature and theoretical study of the available single phase and multi-phase primary power converters are conducted. It is found so far that a viable option for poly phase configuration is the three phase inverter which is a hard switched converter. However in order to increase the frequency, reduce the loss and EMI and increase the efficiency, a resonant converter is preferable. Next, using a push-pull current-fed resonant converter two individual methods to achieve soft switched three phase primary side power converter are proposed and implemented. The first method is a fixed frequency control and the second technique is quasi soft switched control; in the former case the gate signals are given to the switches from the controller, and for the latter case, one of the phases is operating with ZVS (Zero Voltage Switching) and is considered to be as the driving phase for the other two phases. In fact the other two phases follow the driving phase and their switching frequency is defined by the resonant frequency of the driving phase. The time delays between the three phases are obtained based on the period of Phase-1; initially, CompactRIO controller calculates the period of Phase-1, and then generates the delays for the other phases instantly. These methods are implemented and verified practically on a three phase slipring system. The practical and simulation results have demonstrated that the total harmonic distortions of the resonant tank current waveforms of these techniques are below 4%. Furthermore, the system achieved a maximum efficiency of 80.7%, and the optimum converter efficiency stays at 91.3%.