Abstract:
Inductive Power Transfer (IPT) systems use magnetic fields at VLF frequencies in the range of 10-50 kHz to transfer power from a static primary power supply to one or more movable secondary power pick-ups, across relatively large air gaps of up to several centimetres, without any wired connections. This technology is now established as a viable technique for transferring power using electromagnetic induction. IPT systems have found many practical applications in materials handling systems, electrical vehicles, battery chargers and clean rooms. However, a significant problem with IPT systems is sizing of the power supply such that the power requirements of the pick-up · loads may be met by managing the peak loads so that the supply is reliable and the IPT system integrity is not compromised by a lack of power. In this way the system cost is reduced by reducing the cost of the power supply while guaranteeing the integrity of the total system. The thesis is concerned with the development of new power management technologies in large IPT systems, with multiple pick-ups. The optimal power supply size (as low cost as possible without compromising the stability and integrity of the system) is sought to give a truly distributed multi-functional IPT system without compromising its performance. The thesis presents a statistical analysis of the power flow from the power supply to the pick-ups in a multiple pick-up IPT system and the required size of power supply is determined using known statistical distributions to balance cost against the chance of failure of the IPT system. As pick-up power is controlled by essentially random 'on' and 'off' switching cycles of a 'decoupling' switch in each pick-up, the analytical theory to determine any parameter of the power supply size, number of pick-ups, average power of all the pick-ups in an IPT system, and chance of failure in this system is developed on the same basis as the methods of Engset and Erlang B distributions which are used in telephony systems. A statistical load scheduling strategy with output voltage controlled over a communications channel to change the on-board energy stored in the pick-ups is proposed and a deterministic load scheduling strategy is shown to prevent system collapse in the cases where the system lacks power. Load scheduling strategies are common in power systems ( demand-side management) but not in telephony and here it requires a communications system to implement it. Consequently the thesis first discusses analyses of the pick-up controller and possible communications channels before undertaking the statistical analysis. In practical IPT systems, the decoupling switch of an individual pick-up is controlled 'on' and 'off from one instant to the next to follow load changes on the pick-up. The analysis of IPT pick-ups even under ideal switching conditions is intractable due to the presence of high order AC resonant circuits, and switch-mode controllers. The thesis proposes a novel equivalent DC circuitry method to allow the analysis of the dynamic response of the high frequency AC resonant converter pick-up circuit and its switch-mode controller in an IPT system. The validity of the DC circuit approach is verified with Pspice simulations. A pickup controller may then be developed based on the analysis of the equivalent DC model in a parallel-tuned pick-up circuit. The method allows simple closed-form analytical expressions to be developed for different controller options. The equivalent DC circuit philosophy may be extended to the analysis of series tuned pick-ups and other switch-mode controllers but this development is beyond the scope of this thesis. Because of the nature of electrical isolation between the power supply and pick-ups in IPT systems, a functional dynamic interaction between those two parts must be established for stable operation of the system. This is crucial in a multiple pick-up system. In this thesis, likely communications channels in IPT systems are evaluated and the results of radio propagation experiments conducted at 300 to 1000MHz with a coupled mode leaky feeder as the radio distribution medium are presented. The thesis concentrates on the characteristics of whip antennas as the antennas on the other end of the communications channel because of their practical simplicity and low cost. A full characterization of the coupled mode leaky feeder as a possible physical communications channel operating in an IPT environment is carried out. It is shown that a practical low-transmitting power, even coverage, bi-directional communications channel with a wide-coverage capability along the leaky feeder may be achieved by careful handling of the deep fades caused by the immediate surrounding environment of the IPT system. With these analytical methods and a communications system all the power flows in a large IPT system can be monitored and controlled. Thus the stability of pick-up controllers can be guaranteed and the power of each pick-up can be determined easily and limited or measured with the communications system. Therefore the capability of an IPT system is significantly increased while the cost can be reduced by reducing the rating of the power supply without compromising stability. In the thesis these cost savings may be achieved by combining all the aspects covered - pick-up control, communications, and a statistical analysis. Thus periods where the power supply may be overloaded are easily detected and averted by transiently modifying the switching points in the controllers. The shedding of load is small and for very short periods but allows significantly smaller power supplies to be used. As such, a multiple pick-up IPT system is more economically viable but the stability and integrity of the total system is not compromised.