A.C. current distribution system

[0002] THIS INVENTION relates to improvements in or relating to an a.c. current distribution system and more particularly relates to an a.c. current distribution system for minimising the electric field along the current distribution system.

[0003] A typical a.c. voltage distribution system is shown in FIG. 1 of the accompanying drawings. The a.c. voltage distribution system comprises first and second voltage generators which generate, respectively, a.c. voltages V_Aand V_B, V_Abeing equal to and 180° out of phase with V_Bsuch that V_A=V_B. The two voltages are fed down a power bus comprising a pair of conductive tracks which run parallel to one another and are separated from one another. As seen in FIG. 1, various impedance loads may be connected to the tracks along the length of the tracks. Such a voltage distribution system is characterised by the sum of the currents in the adjacent tracks at any one instant in a specific locality along the tracks being zero thereby resulting in a low magnetic field (H-field). Similarly, the sum of the voltages in the adjacent tracks at any instant in a specific locality along the tracks are also zero. This results in a low electric field (E-field).

[0004] In some applications, it is preferable to use an a.c. current distribution system rather than an a.c. voltage distribution system such as a current loop system. An example of such a current distribution system is shown in FIG. 2 of the accompanying drawings.

[0005] A typical a.c. current distribution system Comprises two a.c. current generators which generate, respectively, currents I and {overscore (I)} at voltages V₁and V₂, where V₂=V₁. The current generators are regulated to be constant and precisely antiphase with one another, although the amplitude of the current need not be precisely regulated. The currents are fed to a current loop comprising a pair of conductive tracks which run parallel to one another and are separated from one another. Any impedance loads to be powered from the current loop system are connected in series to one or other of the tracks. At any instant, the sum of the currents in a specific locality along the lengths of the tracks is zero. This results in a low magnetic field. However, in contrast to the a.c. voltage distribution system, the sum of the voltages at any instant along the tracks in a specific locality is not zero and, in fact, increases along the length of the tracks depending upon the number of loads connected in series along the tracks. This results in a worsening electric field along the length of the tracks. For example, in the locality immediately between the current generators and a first load, the sum of the voltages is zero at any one instant. In the locality immediately after the first load and before the second load, the sum of the voltages is: ΣV=V₁+V₁−V_Load. Further, at the tip of the loop, the sum of the voltages, ΣV, equals 2V₁. The increase in the sum of the voltages, ΣV, from 0 to 2V₁results in a worsening electric field along the length of the track.

[0006] It is an object of the present invention to provide an a.c. current distribution system which does not suffer from the above-mentioned disadvantages.

[0007] Accordingly, one aspect of the present invention provides an a.c. current distribution system fed by a current source for providing electrical power to a load, the current distribution system comprising a first and a second conductive means which run parallel to one another, which are connectable, respectively, at one end to the current source and which are connected together at the other end to form a current loop, and coupling means to couple substantially one half of the load in series at a first position along the first conductive means and to couple substantially the other half of the load in series at a second position along the second conductive means, the first and second positions being substantially adjacent one another.

[0008] Another aspect of the present invention provides a method of reducing the electric field in a current distribution system comprising the steps of coupling a load to be powered by a current source feeding the current distribution system to a first and second conductive means which run parallel to one another, which are connectable, respectively, at one end to the current source and which are connected together at the other end to form a current loop, wherein substantially one half of the load is coupled in series at a first position along the first conductive means and substantially the other half of the load is coupled in series at a second position along the second conductive means, the first and second positions being substantially adjacent one another such that the sum of the voltages on the conductive means in the same locality at any one instant is zero.

[0009] Conveniently, the load comprises two distinct half loads, each of which is ohmically connected in series to the respective conductive means.

[0010] Preferably, the load is inductively coupled to the respective conductive means by a transformer.

[0011] Advantageously, the load is ohmically connected across the terminals of one or more secondary windings of the transformer and the coupling means comprises a pair of substantially identical primary windings of the transformer, each of which is ohmically connected in series to the respective conductive means, the voltage drops across the primary windings being substantially identical, such that the load is split substantially equally between the two primary windings.

[0012] In order that the present invention may be more readily understood, embodiments thereof will now be described, by way of example, with reference to the accompanying drawings, in which:

[0013] FIG. 1 is a schematic representation of a known a.c. voltage distribution system;

[0014] FIG. 2 is a schematic representation of a known a.c. current distribution system;

[0015] FIG. 3 is a schematic representation of a first embodiment of an a.c. current distribution system according to the present invention;

[0016] FIG. 4 is a second embodiment of an a.c. current distribution system according to the present invention;

[0017] FIG. 5 is a further embodiment of an a.c. current distribution system according to the present invention incorporating a balancing transformer;

[0018] FIG. 6 is a schematic representation of the embodiment of FIG. 2 incorporating a balancing transformer; and

[0019] FIG. 7 is a schematic representation of the embodiment of FIG. 2 provided with a control coil.

[0020] Referring to FIG. 2, the problem associated with known a.c. current distribution systems is that the electric field worsens as loads are connected in series along the length of the track. As previously mentioned, referring to FIG. 2, a typical a.c. current distribution system comprises two a.c. current generators which generate, respectively, currents I and {overscore (I)} at voltages V₁and V₂, where V₂=V₁. The currents are fed to a current loop comprising a pair of conductive tracks which run parallel to one another and are preferably separated from one another.

[0021] Any impedance loads to be powered from the current loop system are connected in series to one or other of the tracks. At any instant, the sum of the currents in a specific locality along the lengths of the tracks is zero. This results in a low magnetic field. However, in contrast to the voltage distribution system, the sum of the voltages at any instant along the tracks in a specific locality is not zero and, in fact, increases along the length of the tracks depending upon the number of loads connected in series along the tracks. This results in a worsening electric field along the length of the tracks. For example, in the locality immediately between the current generators and a first load, the sum of the voltages is zero at any one instant. In the locality immediately after the first load and before the second load, the sum of the voltages is: ΣV=V₁+V₁−V_Load. Further, at the tip of the loop, the sum of the voltages, ΣV, equals 2V₁. The increase in the sum of the voltages, ΣV, from 0 to 2V₁results in a worsening electric field along the length of the track.

[0022] Referring to FIG. 3, an a.c. current distribution system embodying the present invention incorporates a conventional current source as previously described in relation to the current distribution system shown in FIG. 2. The current source feeds the current loop comprising two conductive tracks 10,11.

[0023] An impedance load L_Tis to be powered from the current loop. The load L_Tis split into two equal half loads L_A, L_B, which are connected in series to respective tracks 10,11 substantially adjacent one another in the same locality—i.e. distance along the tracks from the current source. Thus, half the load L_Ais connected in series with the first track 10 and halt the load L_Bis connected in series with the second track 11. The voltage on track 10 immediately before the first half load L_Ais V₁and the voltage immediately after the first half load L_Ais V₁−V_LA. Similarly, the voltage on track 11 immediately before the second half load L_Bis V₁and the voltage immediately after the second half load L_Bon track 11 is V₁−V_LB. By locating half the load L_Ton each of the tracks 10,11, the sum of the voltages immediately preceding the half loads L_A, L_Bon tracks 10 and 11 is zero (V₁+V₁) and the sum of the voltages on the tracks 10,11 immediately after the half loads L_A, L_Bis also zero (V₁+V_LA)+(V₁−V_LB), where L_A=L_Band V_LA=V_LB. In this manner, not only are any voltage drops across the impedance load L_Tmatched, but also any phase changes. Thus, should the impedance load incorporate a reactive component, these too will sum to zero.

[0024] In contrast to the conventional a.c. current distribution system, the current distribution system embodying the present invention maintains a substantially zero electric field not only along the tracks 10,11 before any impedance loads but also after any loads since the impedance loads are split evenly at substantially the same localities along the tracks 10,11 around the current loop.

[0025] An example of a load L_Twhich can be split into equal parts as described above would be a double incandescent stop lamp comprising two separate 5 ohm bulbs. The first bulb could comprise the first half load L_Aon the first track 10 and the second bulb of the pair could comprise the second half load L_Bon the track 11. Alternatively, if only a single 10 Ohm incandescent bulb is to be used as part of a cluster, two separate 5 Ohm bulbs could be connected to respective tracks 10,11 rather than using a single bulb. In this manner; the load is evenly split in the same locality between the tracks and the electric field along the tracks is thus maintained at substantially zero.

[0026] Of course, there are some loads which are either impossible or impractical to split. In such circumstances, the same concept as described above is implemented but the load is inductively coupled to the tracks 10,11 of the current loop using a transformer. Such an arrangement is shown schematically in FIG. 4. The unsplitable load L_Tis connected to the terminals of a secondary winding S of a transformer. The transformer has a pair of primary windings P₁, P₂. One of the primary windings P₁is connected in series with the track 10 and the other primary winding P₂is connected in series the same locality along the lengths of the tracks 10,11 to track 11. The primary windings are adjacent one another and are inductively coupled to the secondary winding S and thence to the load L_T. P₁and P₂are substantially identical primary windings which cause identical voltage drops either side thereof such that the sum of the voltages at any locality along the track 10,11 within the distribution system at any one instant is zero. Accordingly, the electric field is maintained at substantially zero.

[0027] Transformers which are used for other purposes such as isolation, voltage/current matching to a load or, indeed, control purposes can be easily integrated for use in an a.c. current distribution system embodying the present invention.

[0028] Embodiments of the present invention are particularly well suited to operation at frequencies of the 20 kHz or greater range.

[0029] Preferably, the primary windings P₁and P₂have an identical number of turns and are perfectly matched and result in a 1:1 ratio with perfect coupling. However, in some circumstances, the coupling between the primary windings is not perfect and can, therefore, lead to slight discrepancies between the voltages present immediately before the primary windings on the tracks 10,11 and those present immediately after the primary windings. A similar problem can arise if the load described in FIG. 3 is not split exactly equally when connected in series on tracks 10 and 11.

[0030] In circumstances where the load has not been split equally or when the primary windings do not exhibit perfect coupling, it is possible to remedy the situation by connecting a balancing auxiliary transformer T_xacross the tracks 10,11. The auxiliary balancing transformer could be a tightly coupled bifilar wound toroid. The centre of the transformer coil is centre-tapped to zero volts. This arrangement serves to balance the voltages at the point of connection of the balancing transformer T_xto the tracks 10,11 to be exactly opposite one another such that the sum of these voltages at the locality at any instant will be zero. Little power is transferred between the primary windings P₁and P₂so any current in the balancing transformers would be low.

[0031] Referring to FIG. 6, the existing primary and secondary windings P₁, P₂, S₁, S₂of an E-type core transformer connected to a load L_Tcan be easily incorporated into an a.c. current distribution system according to the present invention by simply connecting the terminals of the first primary winding P₁in series to track 10 and the terminals of the secondary primary winding P₂in series to the track 11 at substantially the same locality along the tracks 10,11.

[0032] The auxiliary balancing transformer T_x, previously discussed in relation to FIG. 5, can be implemented as shown in FIG. 6. The balancing transformer T_xhas been wound around the central core of the E-type core. Respective pairs of primary and secondary windings P₁, P₂, S₁, S₂are wound in conventional positions on the other arms of the transformer.

[0033] As previously mentioned, existing transformers used for other purposes, such as control purposes, are easily implemented in an a.c. current distribution system embodying the present invention. In one such embodiment, shown in FIG. 7, the central core of the transformer shown in FIG. 6 can be wound with a control winding C to replace the balancing transformer T_x. The primary windings P₁, P₂are split as previously described in relation to FIG. 4 and connected respectively in series to the tracks 10,11 such that any voltage drop or phase shift across one primary winding is matched by one identical voltage drop or phase shift in the other primary winding. For example, for power lines or the like. When energised, the control winding saturates the core thereby limiting the voltage generated across the secondary windings S₁, S₂and provided to the inductance load L_T. If the current to the control winding C around the saturable core is terminated, then the core becomes substantially unsaturated enabling the normal output voltage on the secondary windings S₁, S₂to power the load L_T. Such an arrangement allows ready control and switching of the load by appropriately altering the current supplied to the control winding C, whilst maintaining an equal voltage drop across the primary windings connected in series to the respective tracks 10,11. In one embodiment the tracks 10,11 are made from copper and run parallel to one another and are spaced apart by a small distance in the order of 10ths of millimetres. These tracks 10,11 are separated by an insulating plastics layer 12 such as a polyester, polypropylene or polyphenylene sulphide. The thickness of the insulating layer 12 is in the order of 0.1 mm.

[0034] Whilst previously described embodiments are on a small scale, it is envisaged that the same concept can be easily implemented on a larger scale.