Patent Application
20170207592
Not applicable.
Not applicable.
1. Field of the Invention
The field of the invention relates generally to systems and methods for transfer of electric current without mechanical contact between conductors and homopolar machines comprising same. More specifically, the field is further defined as the use of microemitter arrays comprising a plurality of microemitters having current transfer features such as microtips or blades to form contactless current transfer structures, and homopolar machines comprising same.
2. Background Art
The ability to conduct a current between two structures in a controlled manner has many useful applications. Some of these applications, such as, for example, homopolar motors, are discussed in exemplary fashion herein. The implementation of a homopolar motor may require the transfer of large DC currents across a rotating or sliding (i.e. translating) mechanical connection forming electrical contact. Such connections are unreliable and may pose limitations on the rotational speed at which the structure operates.
Reliable transfer of large direct currents (DC) without mechanical contact has important applications in many areas. One application where this technology is expected to have a major impact is turbo-electric propulsion, which has been identified as a key technology for future transportation aircrafts. Turbo-electric propulsion technology offers advantages in respect to efficiency and furthermore leads to significant noise and emission reduction. However, the requirements on the propulsion system are very challenging and can only be met by superconducting (SC) machines with unprecedented power density. Fully superconducting synchronous machines are considered as a possible solution, but are very difficult to realize, partially due to high AC losses in the armature windings at the necessary high RPM of the required machines. Homopolar machines, on the other hand, would avoid many of the technical challenges encountered in the realization of fully superconducting synchronous machines, could offer unprecedented power density, but require high current transfer over sliding contacts, which so far has been a showstopper for the development of homopolar machines. However, if a solution can be found for the required transfer of large DC currents into a rotating system with longevity, high reliability and efficiency, homopolar machine technology would constitute a “game-changing” technology.
The present invention comprises an apparatus and method that have one or more of the following features and/or steps, which alone or in any combination may comprise patentable subject matter, and which provide substantial advantages over the prior art.
In accordance with one embodiment of the present invention, the invention comprises an apparatus for conducting electric current without mechanical contact, eliminating or drastically reducing the problems associated with sliding or rotating electrically conductive mechanical contact which are significant drawbacks of the prior art. In an embodiment, a plurality of microemitters, which may be fabricated from diamond or other appropriate materials, are proximal to, but not in physical contact with, at least one electrically conductive electrode having a surface area. The gap between each of the plurality of microemitters and the at least one electrically conductive electrode operates to allow the plurality of microemitters and the at least one electrically conductive electrode to be translated relative to each other, forming a contactless electrically conductive emitter-electrode structure, enabling the conduction of electric current between two mechanical structures that are translating or rotating relative to one another.
An exemplary embodiment of the invention comprises a contactless electrically conductive emitter structure of the invention in which one such mechanical structure is a rotor of a homopolar motor, and the other mechanical structure is the stator of a homopolar motor, and which the stator comprises the emitter structure and the rotor comprises the electrode structure in electrical communication with the emitters of the stator. In this exemplary embodiment electrical current is transferred from rotor to stator across the gap between each of the emitters and the electrode(s), enabling the rotor to rotate relative to the stator while at the same time conducting an electric current across the contactless electrically conductive emitter-electrode structure.
In a homopolar motor of the invention large DC currents in the range of 3,000 to 80,000 A or even higher can be transferred between the edge of a rotating disk or ends of a rotating cylinder and a stationary electrode.
Thus the present invention enables a highly reliable and efficient apparatus for the transfer of large DC currents into a rotating system, such as, for example, homopolar machines. An embodiment of the invention is a homopolar machine, which may be a motor or generator, which contains at least one, or, generally, a plurality, of the contactless electrically conductive emitter structure of the invention.
The accompanying drawings, which are incorporated into and form a part of the specification, illustrate one or more embodiments of the present invention and, together with the description, serve to explain the principles of the invention. The drawings are only for the purpose of illustrating the preferred embodiments of the invention and are not to be construed as limiting the invention. In the drawings:
As used herein, “disc” and “disk” have the same meaning.
As used herein, “cylinder” and “drum” have the same meaning.
As used herein, “SC” means superconducting.
As used herein, “DC” means direct current and “AC” means alternating current.
It is an object of the invention to enable the contactless transfer of electric current from a first structure to a second structure, the electric current being transferred across a gap between the first and second structures, and the current being passed through an array of microemitters as described herein and as described in the referenced and incorporated documents. Thus, the contactless transfer of current between two structures which are translating or rotating, or both, relative to one another is achieved with the benefit of eliminating the problems associated with physical or mechanical contact wear-out.
It is a further object of the invention to claim electric machines that comprise the contactless transfer of electric current as described herein. One such class of machines is homopolar machines.
In the simplest form, homopolar electrical machines consist of an electrically conductive disc or cylinder rotating in a plane perpendicular to a uniform static magnetic field. In case of a generator a potential difference is created between the center of the disc and the rim (or ends of the cylinder) with an electrical polarity that depends on the direction of rotation and the orientation of the field. In case of a motor the potential difference is applied, leading to a current through the disk or cylinder, which creates a Lorentz force in the magnetic field, causing the disk or cylinder to rotate. The two cases consisting of a disk or a cylinder or drum, are schematically shown in
Referring now to
In any of the embodiments shown and described, the current transfer between rotating surfaces may be accomplished by the contactless current transfer structured described herein and in the references that are incorporated by reference. Furthermore, any of the homopolar machines described herein may operate as a motor, wherein an applied magnetic field causes a disc or drum to rotate when current is passed through the disc or drum, or may operate as an electric current generator in which a rotating conductive disk or drum, in the presence of a magnetic field, generates an electric current.
The governing equations of a homopolar generators i.e., the induced voltage and power, Faraday's conductive disk, where angular velocity Ωm=2πn, for a rotationally symmetric magnetic field B(r), are presented below. Eq. 1 provides the relationship for the induction of motion; Eq. 2 provides the relationship for an induced DC voltage.
Power conversion for B(r) with a mean value B estimated is provided by the relationship shown in Eq. 3:
U
i=Ωm·B·R2/2→Pi=UiI Eq. 3
The excitation windings can be, but are not necessarily, produced of superconducting material.
The homopolar concept overcomes key issues inherent to superconducting synchronous machines and therefore has the potential to be the preferred solution of future superconducting machines. While the field in the air gap of a synchronous machine is practically limited to 2 Tesla, because of saturation of the required back iron, homopolar machines benefit from the highest field possible, and fields of 6 Tesla or even much higher are feasible. In this respect the homopolar concept makes full use of superconductivity, which enables high fields without a penalty in efficiency. Also AC losses in the armature winding, a major issue of fully superconducting synchronous machines, are non-existent in homopolar machines, and applications like various propulsion systems typically experience strong vibrations (car, airplane, etc.) that can cause significant eddy current losses in cryostat walls and any surrounding metal surfaces which increase with the RPM. In contrast, for a homopolar machine the current and resulting Lorentz forces are practically constant and the higher the RPM the better for the operation, since the currents needed for a required power level are reduced. Furthermore the field winding configuration of a homopolar machine is much simpler than those of synchronous machines, i.e., solenoidal windings instead of saddle or racetrack coils. Tape conductors like REBCO (rare earth barium copper oxide) are well suited for homopolar machines, enabling operational temperatures in the range of 30 K or even approaching 77 K. Relatively small cryo-coolers with high efficiency are therefore sufficient to produce the required operational temperature. Additional advantages of the homopolar concept include reduced torque ripple, increased mechanical robustness of the windings, and simpler handling of various fault conditions like quench protection of the field coils.
Homopolar machines in contrast to most other electrical machines require that all of the machine power has to be transferred through a current collection system, typically consisting of brushes. This necessarily involves current transfer across a sliding interface. Such current transfer system with sufficient longevity, high reliability, low friction, and low electrical losses constitutes a major technological hurdle that has so far hampered the widespread use of homopolar machines. Sliding/rolling contacts constitute a technical issue for homopolar machines. Such contacts put limitations on the RPM, i.e., the relative speed of the sliding contacts and also limit the amount of current that can be transferred per contact. If moving contacts could be eliminated, homopolar machines would be preferred over synchronous machines for many applications.
Homopolar machines have some unique features that make them of great interest for many applications. One of these is that there is no armature reaction of the applied current on the field windings, since they are orthogonal, which is of particular interest for a superconducting field coils, where such interaction can cause quenching of the superconducting coils. Additionally there is very low electrical noise generated by the machine, which also is of interest for particular applications. While synchronous machines typically require back iron, which saturates at a maximum of two Tesla and therefore limits the field that can be used in the airgap between rotor and stator, for homopolar machines the higher the field the better. If the current transfer onto the rotating system does not compromise the operation of the homopolar machine, very high RPMs are feasible and no special electronics or starter is required to start the homopolar machine as needed for synchronous or induction machines.
Some of the advantages of a homopolar machine can be summarized as follows: simplified machine design; fixed DC superconducting excitation coil; simple coil design with HTS conductor; high excitation fields possible 5 Tesla and higher; DC output, no rectification required, direct feed to power transfer line; and a room temperature rotor system.
One possibility for the current transfer to the rotor of a homopolar machine is the application of electrical brushes. However, brushes introduce mechanical and electrical losses and also introduce lifetime issues. As a general rule several performance factors need to be balanced or compromised. The electrical voltage drop multiplied by the current transferred by a given brush defines the electrical portion of the power dissipation and needs to be minimized. The friction coefficient of the sliding interfaces depending on sliding speed (proportional to the RPM) and the brush pressure defines mechanical losses and needs to be minimized Typically the brushes are designed with equal amounts of electrical and mechanical losses. Brush life and slip ring life need to be optimized. Brush environment which can depend on temperature, surrounding atmosphere, wear debris management need to be considered.
Liquid metals or other liquids with high electrical conductivity constitute an alternative solution for the current transfer problem. In fact, the first electrical machine invented by Faraday around 1821 used liquid mercury to make contact between the circumference of a rotating disk and its shaft. Various liquid metals have been considered as a replacement for the mercury, which due to its toxicity makes it nowadays unacceptable. However, all potential liquids require careful control of the operational temperature and the surrounding atmosphere. Additionally, the unavoidable losses caused by the viscosity of the liquid constitute the main problem and limitation for high RPM.
The present invention utilizes microemitter arrays comprising a plurality of, for example, micron-size electron field emitters resulting in contact-free current transfer of electric current to a nearby electrode, forming a contactless electric current transfer structure with high longevity, high reliability and are insensitive to temperature and if needed ionizing radiation. In the invention, depicted in exemplary fashion in
Thus, in a most general form, the contactless electric current transfer structure of the invention is for communicating electric current between two structures that are able to be translated or rotated relative to one another, and has a first structure which may be, for example, a rotor or stator electrode, having a plurality of microemitters forming a microemitter array, each microemitter defined has having a current transfer feature such as a microtip or blade tip; a second structure, such as a rotor or stator electrode, having a conductive surface forming an electrode; and a magnetic field B defined by magnetic lines of flux; wherein a predetermined gap G exists between the current transfer feature of each microemitter and said electrode such that contactless electric current transfer is able to occur between the microemitter current transfer feature and the electrode; and wherein said first structure (e.g. rotor 002 or 200) is capable of being rotated or translated relative to said second structure such that, during said rotation or translation, said predetermined gap G between the current transfer feature and said electrode is maintained.
Likewise, in its most general form, a homopolar machine of the invention comprises a rotor portion, a stator portion, and a magnetic field defined by magnetic field lines B(r), in which the rotor is in electric communication with a rotor electrode through a contactless electric current transfer structure of the invention and the stator is in electric communication with the rotor through a contactless electric current transfer structure of the invention, and in which an electric current, which may be but is not necessarily a DC current, is passed through the rotor electrode into the rotor, and along the rotor and into the stator, and wherein the electric current has a direction of current path that is substantially orthogonal to the magnetic field lines. In a homopolar motor of the invention the current is caused to flow by applying a voltage differential across the rotor electrode and the stator electrode, causing Lorentz forces to act on the rotor, in turn causing the rotor to rotate about a longitudinal axis. In a homopolar generator of the invention, the rotor may be mechanically rotated by a rotating force, causing the generation of current I as the conductive rotor is rotated in the presence of magnetic field B(r).
Still referring to
In
Referring now to
Referring now to
In an embodiment, the entire rim edge 002a of the disk at the disk edge may serve as an electrode area. As an exemplary implementation of the invention, assuming a disk radius Rdisk of 250 mm, a thickness W of 25 mm and a radial extension L of 10 mm yields a total electrode area of about 70,000 mm2. For larger dimensions with Rdisk of 500 mm, a thickness W of 50 mm and a radial extension L of 20 mm, the total available area increases to about 280,000 mm2. For these dimensions, for example, a current of 80,000 A yields current densities of 1,100 mA/mm2 and 300 mA/mm2 respectively. For the lower current scenario of 3,000 A the required current densities are only 40 to 10 mA for the assumed electrode sizes.
The required current transfer can be facilitated by using any of the contactless current transfer structures as described in patents U.S. Pat. No. 6,762,543 and U.S. Pat. No. 7,256,535, which are both herein incorporated by reference in their entirety. The emitters may, but do not necessarily, comprise diamond material. As an example of one embodiment of the emitters of the invention, U.S. Pat. No. 6,762,543 describes emitter structures having current transfer features, such as a microtip current transfer feature, made from diamond or similar materials to establish ultra-sharp tips for efficient electron emission. For the current invention, the most desirable properties for an electron emission cathode are low operating voltage, for example 0.5 to 0.7 Volt, high emission current density, for example 100 A/cm2 to 400 A/cm2, uniformity, and emission stability, longevity and reliability. Contact-free, high current transfer for homopolar machines requires the same properties for best performance. The ranges shown are exemplary in nature, and are not intended to be limiting of Applicant's invention, but rather as preferred ranges. Based on these requirements it is understood that the emitter structures of the invention are not limited to diamond material, and that emitters comprising any other materials meeting the above specifications are within the scope of Applicants' invention.
The operational voltage for maintaining a current transfer may be any voltage, but a voltage of a few volts is sufficient, due to the very small dimensions of the microemitter discharge surface which may be a peak, blade or other structure, which give rise to very high electrical fields in their vicinity.
As for any current transfer technology the effective path resistance, i.e., the resistance across the current transfer has to be extremely small. The total losses due to the current transfer have to be less or equal 2.5×10−3. For a 1 MW system this would constitute a power loss of 2.5×10−3*1×106=2.5×103 W or 0.25% . The total resistance for the 80 kA current transfer has to be (P=R*I2) R=Ploss/I2, i.e., R≦2.5×103/80,0002=3.9×10−7. For the low current version of 3,000 A the required resistance is R≦2.5×103/3,0002=2.8×10−4. Typical operational parameters are summarized in the following table.
For very high RPM it is necessary to house the rotor (cylinder, drum or disk) inside of a vacuum to avoid large windage losses due to friction with any gas surrounding the rotor. For lower RPM a noble gas in the current transfer region could be advantageous, which would lead to a kind of plasma current transfer.
Referring now to
Referring now to
Referring now to
Referring now to
For wireless transfer of AC currents, an embodiment of the invention is depicted in
In any embodiment of the invention, gap G between the current transfer feature (e.g. the microtip) of an emitter and an electrode may be any predetermined gap that enables contactless conduction of current from a microemitter to an electrode. The value of G may be a function of the applied voltage, whether the gap is contained within a vacuum, noble gas, or inert gas, and the acceptable loss. Typically, but not necessarily, gap G may range from 0.5mm to 2.0mm
The various embodiments of the invention shown and described herein are exemplary only and are therefore themselves not exhaustive of the invention as described and claimed.
This non provisional patent application is a non-provisional of and claims the benefit of U.S. provisional patent application 62/278,980, filed in the United States Patent Office (USPTO) on Jan. 14, 2016 titled ELECTRICAL CURRENT TRANSFER APPARATUS, which is hereby incorporated by reference in its entirety, and this application also claims the benefit of U.S. provisional patent application 62/435,146, filed in the United States Patent Office (USPTO) on Dec. 16, 2016 titled ELECTRICAL CURRENT TRANSFER APPARATUS WITH DIAMOND AND OTHER EMITTER STRUCTURES, which is hereby incorporated by reference in its entirety. U.S. Pat. Nos. 6,762,543 and 7,256,535 are both herein incorporated by reference in their entirety.
| Number | Date | Country | |
|---|---|---|---|
| 62278980 | Jan 2016 | US | |
| 62435146 | Dec 2016 | US |
