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(11) | EP 0 555 560 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Low leakage transformer |
| (57) Transformer including a first and a second winding between which no mechanical contact
exists, characterized in that said first winding consists of a hollow tube, and that
said second winding is composed of n electrical conductor turns each realized by pulling
said electrical conductor from a first to a second end of said tube through said tube
and then leading said conductor from said second to said first end outside said tube. In a preferred embodiment n equals 1 and the transformer comprises as primary and secondary winding, two cilindrical tubes mounted in a substantially concentric way. |
[0001] The invention relates to a transformer including a first and a second winding between which no mechanical contact exists.
[0002] Such a transformer is already known in the art and is e.g. described in the article "Non contacting power transformer" by J. Kiedrowski from the Sperry Corporation and presented at the NASA Space-station Power Management and Distribution Workshop, April 24-26, 1984. Herein a transformer is described with classic 'barrel' wound primary and secondary windings coupled by a closed magnetic path. Unlike the primary, the secondary coil is not attached to the core.
[0003] A considerable leakage flux of the transformer is present partly in the inner-winding space and partly, in a non-uniform manner, in the core which results in considerable core losses.
[0004] An object of the invention is to provide for a transformer of the above type but with lower core losses than the known transformer.
[0005] This object is achieved due to the fact that said first winding consists of a hollow tube, and that said second winding, is composed of n electrical conductor turns each realized by pulling said electrical conductor from a first to a second end of said tube through said tube and then leading said conductor from said second to said first end outside said tube.
[0006] In this way the second winding can move freely, with respect to the tube, within the free space created between the tube and the second winding.
[0007] The leakage flux lines generated by the new transformer are circular and hence well spaced, i.e. when permeating the magnetic core, a high concentration of flux lines is avoided, thereby eliminating localised core saturation and indeed resulting in lower core losses.
[0008] According to another characteristic of the invention n equals 1, and said hollow tube and the part of the one conductor pulled from said first to said second end through said tube, are cilindrical tubes which are mounted in a substantially concentric way.
[0009] An air gap is maintained between the two tubes such that the second winding may move a give distance in any direction.
[0010] Since the leakage flux of a transformer is proportional to the square of the number of turns of a winding, the number of turns of the above transformer being one, and because for the latter transformer parasitic losses are even more reduced due to the circular symmetry op the 2 tubes, its leakage inductances are lower than for the transformer as described in the prior art, whilst losses due to high flux concentration in the magnetic core are practically eliminated.
[0011] It can also be shown that the forces, in the new transformer, between the primary and the secondary winding are lower than in the known transformer, and that, by making the tubes long compared to their diameter, fringe flux effects may be reduced such that forces between tubes will change in a negligible manner when the secondary is moved a small distance with respect to the primery in any axis.
[0012] According to yet another characteristic of the invention at least one magnetic core is slid over said hollow tube which is the primary winding of said transformer, whilst said second winding is the secondary winding thereof.
[0013] In this way the magnetic core can he fixed together with the primary winding and the secondary winding can move with respect to both. Magnetising inductance is increased by the large available length of the hollow tube on which the magnetic core or the series of cores is placed, and hence magnetic coupling is improved.
[0014] According to still another characteristic of the invention said primary winding is connected to output terminals of a power supply circuit whilst said secondary winding is connected to a floating arrangement thereby transferring power from said power supply circuit to said arrangement.
[0015] As a result only the secondary winding, coupled to the floating arrangement, imposes a weight on the latter arrangement and since the primary winding exerts a very small force on the secondary one, as explained above, the transformer has a substantially small influence on the stability of the position of the floating arrangement.
[0016] It is to be noted that the invention is not restricted to one-phase transformers and can easily be applied to n-phase transformers, in which case there are n secondary windings, all, a few or even only one of which can be mounted freely with respect to the primary winding.
[0017] The above mentioned and other objects and features of the invention will become more apparent and the invention itself will be best understood by referring to the following description of an embodiment taken in conjunction with the accompanying drawings wherein:
Fig. 1 shows a transformer according to the invention; and
Fig. 2 represents a section II' of the transformer of Fig. 1.
[0018] The transformer represented in the figures 1 and 2 is used to supply electrical power to a not shown floating microgravity isolation mount platform. The primary of the transformer is connected to a power source (also not shown) whilst the secondary is connected to the platform. Such a platform must be supplied with electrical power without exerting significant force on it. How this is realized is explained later.
[0019] The transformer represented in the figures comprises two concentric, cylindrical copper tubes L1 and L2. The outer tube L1 is the primary and the inner tube L2 is the secondary winding of the transformer. A toroidal magnetic core C is slid over the primary winding L1 and fixed to it. The secondary winding L2 can move with respect to the primary winding L1 and to the core C. The tubes are electrical conductors made of copper which are large enough to ensure their mechanical rigidity while ensuring that the majority of copper carries current, taking into account the skin effect of the current at the given operating frequency in this case 100 kHz. In the considered embodiment these considerations result in a transformer of overall diameter of 3.6 cm and a length of 14 cm and which weights less than 500 g.
[0020] It has to be noted that the magnetic core can consist of a series of cores and that the dimensions of the transformer of course change with environmental requirements such as the frequency of the current, the maximum allowed weight of the transformer and so on.
[0021] The construction of the transformer in this way ressembles to that of an already known transformer, which is described in the article "Design considerations for high power high frequency transformers", by H.H. Kheraluwala, B.W. Novotny, D.M. Divan, IEEE PESC, 1990 Record, pp. 738-741, except for the fact that the secondary winding of the known transformer can not move with respect to the primary, for the shape of the outer tube which is the known transformer is U-shaped, and except for the secondary of the known transformer, which is realized by a series of turns of Litz wire rather than a single tube as in the embodiment. The advantages of the new structure with respect to the old one will be made clear in the following description of its working.
[0022] An alternating voltage U1 (not shown) with a frequency f, applied to the primary, winding, i.e. to the outer tube L1 causes a current I1 (not shown) to flow from one end of the tube to the other end. The current I1 alternates with the same frequency f as the voltage V1. I1 produces a magnetic field H (also not shown) outside the primary, the fluxlines of which are concentric around the tube. According to Ampere's law :
Herein, r is the distance perpendicular from the point where the magnetic field is calculated in, to the axis of the tubes. The magnetic field also alternates with the frequency f. The magnetic field H induces a magnetic induction B and a magnetic flux PHI (not shown), both of which are concentrated in the magnetic core C and both of which are also alternating with the frequency f. Due to the alternating magnetic flux PHI, a voltage is induced over the primary, which is the voltage U1, and a voltage U2 is induced over the secondary (the inner tube). According to Faraday's law :
Herein, w1 and w2 are the number of turns of the primary and the secondary winding, respectively. Hence,
.
[0023] Due to the voltage U2 a current I2 (not shown), with frequency f, flows through the secondary if both ends of the secondary are connected via a finite impedance. Since the voltage U1 over the primary is constant, the total magnetic flux PHI, caused by currents I1 and I2, must also be a constant according to equation (2).
[0024] As a consequence, since according to Lenz's law, I2 produces a magnetic flux which is opposed to the causing flux PHI, the flux caused by the current I2 must be compensated by an increase in current I1.
[0025] To keep PHI constant, I2 must be equal to I1 plus the magnetizing current, where the magnetizing current is the current which flows through the primary winding, when no current flows through the secondary.
[0026] The working of the new transformator is thus similar to that of a normal transformer where the primary and the secondary winding are fixed solenoids. The way in which the latter type of transformers operates can e.g. be found in "Elektrische machines, deel 1, Transformatoren" by W. Geysen, A. Vandenput, ea, Acco, 1986, pp. 10-27.
[0027] The forces on the windings can be calculated from the law of Biot-Savart, which in its simplest form reduces to:
Herein, F is the force exercised on a conductor of length 1 carrying a current I and placed in a magnetic field with an induction component B. This force is perpendicular to the conductor. The magnetic induction B to be considered in the case of the transformator is a leakage induction since only the leakage fields occur at the places where a current flows. Hence, in order to see how the flux losses and thus also the forces between the windings can be minimized, the leakage flux is now considered.
[0029] A first part, called ideal leakage flux, is due to the magnetic field between the two tubes, whereby the tubes are supposed to be infinitely long. Due to the symmetric form of the transformer the radial components of the forces resulting from this ideal leakage flux on each winding add to be zero, independently of the radial position of the inner tube inside the outer tube. In other words, for a given axial position, a radial movement does not cause an opposing force. In principle, the axial components of the forces on each winding are negligible since the force is perpendicular to the tube, and therefore, if the tubes are moved in an axial direction this force will not vary, that is no opposing force is generated.
[0030] A second part of the leakage flux is a parasitic flux which occurs due to the fact that the tubes are not infinitely long, i.e. the so-called "fringe effect" occurring at the ends of each tube. The parasitic flux lines do not couple significantly with their corresponding winding and have therefore little effect on the system. Therefore, they do not result in significant force on the windings.
[0031] From measuring the leakage inductance experimentally and comparing the result thereof with the theoretically calculated ideal leakage inductance for a transformer behaving as an ideal solenoid, i.e. without fringe effect, it was concluded that the parasitic "fringe" leakage flux is much larger than the flux occurring between the tubes. The fringe leakage inductance may be as much as 100 times as large as the leakage inductance, which is very small due to the design of the transformer.
[0032] Hence, in order to reduce the leakage inductance, the parasitic effect must be kept as low as possible. In the considered embodiment this is achieved by means of a symmetrical design. The tubes are indeed straight cylindrical and concentric. Bent tubes, like in the earlier mentioned article, tubes with non-cilindrical cross-sections or a non-concentric mounting all cause larger parasitic effects than cilindrical, concentric tubes.
[0033] From the above follows that due to the symmetrical structure of the transformator the "ideal" leakage flux is very small and does not alter with relatively small axial or radial movement of one winding with respect to the other one. Therefore the transformer is ideal for the transfer of power to the earlier described floating platform.
[0034] The "fringe" leakage flux however is significantly larger than the ideal leakage flux and appears therefore in the electrical equivalent circuit of the transformer as the dominating factor. This leakage inductance does however also not alter with a transformer movement since it is independant of the corresponding winding position.
[0035] Consequently, when feeding the floating platform via the described transformer, very small forces are exerted on the latter platform.
[0036] It is to be noted that an alternative for the feeding of the platform could be to use very flexible wires, exerting little or no force on the platform. However, experiments show that such wires would have to be very thin and very long in order to be flexible enough, thereby having a high resistance and causing too much energy to be dissipated to be a practical solution.
1. Transformer including a first and a second winding between which no mechanical contact
exists, characterized in that said first winding consists of a hollow tube, and that
said second winding is composed of n electrical conductor turns each realized by pulling
said electrical conductor from a first to a second end of said tube through said tube
and then leading said conductor from said second to said first end outside said tube.
2. Transformer according to claim 1, characterized in that n equals 1, and that said
hollow tube and the part of said one conductor pulled from said first to said second
end through said tube, are cilindrical tubes which are mounted in a substantially
concentric way.
3. Transformer according to claim 1, characterized in that at least one magnetic core
is slid over said hollow tube which is the primary winding of said transformer, whilst
said second winding is the secondary winding thereof.
4. Transformer according to claim 3, characterized in that said primary winding is connected
to output terminals of a power supply circuit whilst said secondary winding is connected
to a floating arrangement thereby transferring power from said power supply circuit
to said arrangement.