A contribution to the wireless transmission of power

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Abstract

A resonant transmitter–receiver system is described for the wireless transmission of energy at a useful distance for grid-coordinate power and information. Experimental results are given showing delivery of power of an unmodified Tesla resonator contrasted with a modified version achieving improved efficiency over a 4 m range. A theoretical basis is provided to back up the experimental results obtained and to link the study with previous research in the field. A number of potential routes are suggested for further investigations and some possible applications of the technology are considered.

Highlights

► We present a model of wireless energy transfer and the methodology of its construction. ► We present a mathematical model describing the scheme under consideration. ► We examine the theoretical and experimental performance of this model. ► We contrast this model with other models in the current state-of-the-art. ► We conclude this model demonstrates an additional arrangement for wireless power transmission.

Introduction

Before the success of Marconi’s 1902 radio transmissions to what later became the sole means of the wireless art, various alternative methods were investigated for the transport of energy and information over long distances without wires. The most well-known practical experiments 5 years earlier were performed by Tesla [1], who had discussed wireless transmission as early as his 1892 lectures delivered before the American Institute of Electrical Engineers in New York and the Institute of Electrical Engineers in London [2]. His methods focused on non-radiative means, in stark contrast to the work of Marconi [3]. Despite significant experimental successes, the work was not be taken up by the wider scientific community [4].

While Marconi, supported by the work of Heaviside, would go onto establish the practical “Hertzian” model of radio transmission, Tesla’s work would lay dormant. At the heart of Tesla’s work was the consideration his efforts relied heavily on theories espoused by Poynting and Larmor. A centerpiece of this understanding is the treatment of the space used as the carrier medium [5] was extended from Maxwell’s idea of non-radiative magnetic vortices [6], and of a disturbance contained in a structure [7]. To the present day wireless energy transfer occurs in near or mid-range regions, arguably in some radiative form. Near-field transfer is of the type exhibited in the transformer effect and usually obtained through mutually-inductive coils and capacitive effects. Distances achievable are generally very short, limited to a few times the diameter of the coils. Losses occur due to resistive and radiative effects. To achieve reasonable efficiencies over a few meters, the primary and secondary coils need to be positioned with an accuracy sharing an axial alignment. Work by Kurs et al. [8] and Karalis et al. [9] demonstrated the feasibility of such a concept, using magnetically-coupled coils of large radius.

Such works established theoretical and practical methods of the wireless transfer of energy, distinguished by the broadcast frequency and circuit geometry. Mid and far-field power transmission involve longer ranges and higher frequencies, where distances achieved are much greater than the radius of the broadcast coils. This requires either the use of microwave signals or optical beaming. In general, such techniques require both line-of-sight and complex tracking systems. The purpose of the research described in this paper is to investigate efficient medium-range power delivery using small-scale antennas without tracking systems where the transmitter and receiver can lie at a distance regardless of position and orientation.

An important consideration to reflect is that Poynting, Larmor, and Tesla agreed on two fundamental principles [10]: (1) The Earth acts as a conductor corridor, a charged shell capable as a transmission medium and (2) By designing a suitable circuit, pathways could be created allowing electrical currents to pass between distant locations. A more subtle conclusion is that the coils could be designed in such a manner as their magnetic fields resonate, at least in part, with the Schumann resonance [11]. Haus [12] observed and mathematically incorporated some of these possibilities into his notion of evanescent waves which become more pronounced at higher frequencies in the near-gigahertz range.

In [8], [9] there was a elaboration on Tesla’s work, arguing through an extension from optics, the idea of evanescent waves in the matter of the coupling of modes described by Haus and Huang [13]. The authors [8], [9] focused their investigation on magnetic field resonances for inductively-coupled power transmission [14], discussing the practical usefulness of their results due to the coupling relationship between distant resonant objects. They demonstrated the effect of coupled magnetic resonances using mid-range field induction at megahertz frequencies.

This paper is concerned with the construction of a high-efficiency transformer [15] that facilitates the stable transmission of power and information at useful distances. For the coils discussed, the Tesla notion of velocity-inhibition would suggest this type of wireless transmission device is non-radiative where the exchange of energy is along a series of standing planar waves existing as a power series along the distance between the coils. The series is supported by the power transformation in family time of magnetic fields in the distant circuits at resonance.

This paper hypothesizes that the space between the transmission coils can be regarded as containing an electromagnetic field object contained on a standing wave which perpetuates the transmission of low-frequency signals—less than 100 MHz—yielding smooth power transmission over long-range distances, where the distance of transmission can be many times larger than the largest dimension of both coils involved in the transfer. In this way, we compose the field object of a series-progression of plane waves behaving as a virtual transmission line. Irrespective of any larger implications, the method still relies on the well-understood concept of inductive coupling.

The authors assessed various [8], [9], [16], [17], [18], [19], [20] methods of wireless energy transfer as contrasting examples to facilitate the analysis of our own model which differs in a fundamental way and contains a significant innovation. While we are similarly pursuing inductive coupling of magnetic fields between resonant coils to allow the transmission of electrical currents at a distance, we have recreated Tesla’s circuit illustrated in his 1900 patent A System of Transmission of Electrical Energy, notably, the arrangement of four tuned circuits exhibited as two concentric spirals. In this paper, we propose three contributions:

  • 1.

    an analysis of this model of wireless power transmission and constructing a circuit;

  • 2.

    comparing the performance of the circuit with the those described in [8], [9]; and,

  • 3.

    introducing an innovation wherein modification of the circuit links its model with the field model.

The wireless transmission of energy through signaling [21] is accomplished by the creation of an inductive link which is maximized at a particular resonance frequency ω0 given the physical characteristics of sets of tuned coils. It is commonly understood that energy transfer in the transformer model discovered by Faraday, permits the transfer of energy between circuits in close proximity. What is not so clear is what schemes are appropriate for longer distance transmission using the same or similar mechanisms. Some will argue that the near-field for these schemes do not exist, rather, the energy is expressed in the far-field exclusively [22], dependent on the type of antenna which typically are loops of a few turns, receiving energy from an externally driven sinusoidal source. Energy is stored in the field by the exchange between the inductive and capacitive components contained in the circuit, through damping of the frequency [23]. Energy harvesting is accomplished using a similar approach.

The Tesla circuit comprises four tuned circuits: a transmission and receiving pair of circular coils each containing a thick-wire primary of a few turns and a thin-wire secondary of many turns. Illustrated in Fig. 1, (C) represents the primary coil (A) the secondary coil, (B) one free end of the secondary coil, and (D) the other free end of the secondary coil of the transmitter. For the receiver, these identical components are labeled with primes so as to distinguish the transmission side from the receiver side of the complete circuit.

The circuit windings, sensitively dependent upon the position of the coils relative to each other, perform ideally when the distance between the primary and secondary winding is no greater than the thickness of the primary, similarly to the transformer effect at close proximity. This ensures maximum current displacement in the outer winding of the secondary. When a constant current is maintained on the secondary, it is possible to enhance the characteristic of this coil. An innovative enhancement will be discussed later in this paper which introduces a connection of a third coil with passive components such as capacitors and resistors between (B) and (D), and between (B′) and (D′) used to manipulate the order of the magnetic field, compounding the power series, projecting away from the secondary coil. Applying a sinusoidal current (G) to the ends of the primary coil at the transmitter, the input current is observed at the ends of the primary coil at the receiver. The current can be utilized via a load represented as lamps (L) and motors (M) and observed on a spectrum analyzer when transmitting at low power levels or using attenuators.

The windings, so constructed and oriented as described, each form one-half of a tuned circuit. By placing each half of the circuit at a distance greater than the radius of its secondary coils, the total circuit expresses cavity effects occurring along the trajectory of energy transfer. It is assumed the observed behavior and the emission of plane waves is uniformly present, though perhaps in different quantities, with circuits of different sizes but having the same relative geometry. It is a fairly trivial exercise to artificially create resonance cavities, magnetically-coupled wherein energy and information are transmitted bi-directionally. These cavities, while homogenous in free-space, maintain a total internal reflection implying each individually or multiply-connected cavity is a holomorphic manifold [24]. In plain terms, it demonstrates that at the extreme limit of mid-range transfer, the transmitter and receiver can be placed at separations in all three Cartesian directions and can be designed in such a manner as to be a planetary transmitter, which is not unlike Tesla’s original description [1], [3], [25], [26].

Considering the radius of the secondary coil and the length of the conductor consisting the spiral, a peak-transmission frequency is established when both halves of the circuit are built to exacting specifications yet wound in the opposite directions, which, for the purposes herein, is 27.50 MHz ± 5%. By applying an external sinusoidal steady-state signal to the primary coil (C) at (G), the identical signal is observed at the primary coil (C′) at (L) and (M) without drift or distortion. By inspection, it is clear the circuit has a complex symmetry; applying external sinusoids to either half—at the transmitter or receiver—the signal is observed at the other half. Experiments have shown that in general that the higher degree of symmetry in the circuit, the greater the resonance and the better the performance.

The two pairs of self-resonant coils allow the transportation of quantities of energy at significant distances comparable to the sizes of the windings involved. Previous attempts at circuits of this type [1], [8], [9], [14], [27], have used large coils (relative to the distance of transmission) and rely on fixed coil qualities. These limitations are overcome by imposing control of the tuning of the resonator cavity through ordered magnetic fields.

Section snippets

Theoretical model of the scheme

The efficiency, η, is a measure of performance of a circuit to catalog the success of this particular experimental design. It is expressed as a fractional quantity, as,η=Useful power outputTotal power input.

Another measure of performance is the energy stored in the magnetic field Hϕ utilized for magnetic-resonant coupling [8], [9]. The circuit consists of an oscillator driving a loop of wire LT coupled to a capacitor CT and a resistor RT at the circuit’s resonance frequency ω0 representing the

The propagation of magnetic currents

In consideration of the work of Poynting [10], we are interested in plotting the magnetic field intensity and the magnetic flux density of the magnetic currents emanating between the coils Lb and Lc. It is insightful to map these magnetic fields between coils Lb and Lc when a source of 1 ampere is applied. Using a computational software package Comsol Multiphysics, we are able to plot the magnetic flux density and magnetic intensity between the distance coils. The plot is shown in Fig. 7.

In

Experimental results

A reconstruction of the Tesla resonator, illustrated in Fig. 1, was built with the physical specifications shown in Table 1. The inductances were measured on a Hewlett–Packard hp4192A into 50 Ω, shown in Table 3. The first test was to plot the results of applying a steady-state sinusoidal source at the peak resonance frequency of 27.50 MHz and chart the measured differential power between input and output. The efficiency, η, as a function of the distance between the coils is shown in Fig. 9.

To

Discussion of results

The intention of this paper was the reconstruction of Tesla’s signaling apparatus from his 1900 patent A System of Transmission of Electrical Energy in order to examine his scheme of the wireless transmission of power contrasted with other attempts. In papers [8], [9] the authors concerned themselves with Tesla’s 1914 patent Apparatus for Transmitting Electrical Energy focusing primarily on the transmission of electrical currents given large coil loops of a few turns. Here, we have attempted to

Concluding remarks

The method here describes medium-range wireless power transmission by means of low-frequency radiating waves, as opposed to either near-field inductive direct coupling or far-field microwave transmission. We have proposed a type of wireless power transmission based on the notion of a transmission-line resonator without any physical containment other than the fields themselves. It suggests that the existence of a pair of tuned circuits, as illustrated in Fig. 1 and schematically demonstrated in

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