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Modelling And Control Of Inductive Power Transfer System
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CHAPTER ONE
INTRODUCTION
1.1 Background of the Study
Inductive power transmission has become a more and more popular method to deliver power to mobile electronic devices and small appliances with a power consumption of up to 100W [1]. In its large meaning, inductive power transfer can take the form of electricity, light, heat or even sound and wind. The inductive power transfer involves a starting point (source) and an ending point (receiver) without intermediate material that would be intentionally added to achieve some purpose. In the scope of this thesis, the starting and ending points are both characterized by electric energy, with in between no additional wires or material.
Light energy transfer can be obtained by photons travel from a source such as the sun or a laser, to a receiver that is generally a photovoltaic cell. The advantage of the laser as a source is that it provides a very directional illumination thanks to its high degree of temporal and spatial coherence. However, the efficiency of light energy transfer is very low, because the losses at each step of the transfer processes are important, namely in the source, in the air and in the receiver. Furthermore, the economical aspects are not attractive [45]. For these reasons, light energy is interesting only for applications that require a transfer over a very large distance, for example in the space.
The microwaves can also transfer energy over a relatively large distance that can be greater than the emitter and receiver sizes. They are electromagnetic waves ranging from 1 GHz to 300 GHz and are used for far-field energy transfer [83]. The most common source for domestic applications is the magnetron thanks to its unexpensive cost, but high frequency waves can be also generated by antennas. The receiver is a rectenna which is actually an antenna with a rectifier used to convert microwaves into DC electricity. The main drawbacks of this technology are the health risks and the relatively low efficiency that can be obtained.
The most used way to transfer contactless energy is the magnetic induction whose principle is shown in Fig. 1.1. It uses near-field electromagnetic waves in the range of frequencies between 10 kHz and a few MHz. The characteristic distance of transfer is the same order as the size of the receiver (secondary coil) and the emitter (primary coil). Such inductive power transfer systems are based on the coreless transformers theory. The working principle consists in applying a high frequency current in a primary coil. In the surrounding air, it generates a magnetic field that induces a voltage to a secondary coil if placed in proximity. It is necessary to operate at high frequency because coreless transformers suffer from weak mutual coupling. To enhance the coupling between primary and secondary, both coils must be well aligned and the air gap must be as small as possible. With this description, a distinction can be made between inductive inductive power transfer and RFID systems.
Indeed, the former involves a transfer of power through a relatively short distance as defined above, while the latter is used to transfer information (with power generally lower than 100mW) through a large distance.
Figure 1.1: Principle of Inductive Power Transfer
1.2 Scope of the thesis
1.2.1 Historical context and current state
Nicola Tesla proposed the first theories of inductive power transfer in 1899-1900. He carried out various wireless transmission and reception experiments through air or matter. At his Colorado Springs laboratory, he experimented for example a remote supply of 200 light bulbs through the ground from a distance of about 40 km [14].
The first reported researches on the so-called energy transport by inductive coupling date from the 1960s [71]. Although the principle of inductive inductive power transfer was known for a long time, this technology has remained immature for a long time as the first industrial applications appear in the 1990s with the electric toothbrushes. Even nowadays the number of electric devices supplied by inductive power transfer is relatively low. This absence is probably due to the lack of standards and regulations for this technology, and also to the uncertainty from the average population concerning the inherent health dangers.
In 1998, a scientific committee has published general guidelines [25] to avoid any kind of health risks concerning the exposure of the population to electromagnetic fields. Basically these recommendations provide the restrictions for the public exposure as a function of the operating frequency, the immersed body proportion and the size of the coils. However, in practice, they are not relevant according to [10]. Therein, a study on the maximum power transfer, based on these restrictions, shows that common applications including coils size of 40mmto 100mmcould transfer no more than about 30mW, which is roughly two orders of magnitude lower than existing products on the market.
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