Field of the invention

This invention relates to inductively coupled power transfer (ICPT) systems and has particular application to compensation of a primary conductive path of such systems

Background to the invention

In an ICPT system, a common arrangement is the circuitry shown in figure 1. Referring to that figure, a power supply 1 provides an alternating current to a primary conductive path 2. In many ICPT applications the primary conductive path is a litz wire cable made up of two lengths of cable which are short circuited at one end and connected to the power supply at the other end. The cable is often arranged along a rail or provided in a floor and is frequently referred to as a track. Therefore, for convenience the primary conductive path will be referred to as a track in the present document in connection with the embodiments described. One or more pickups generally referenced 3 each have a pickup coil 4 which is tuned by tuning capacitor 5. The pickup coil 4 is inductively coupled to the primary conductive path 2 by a mutual inductance M. Energy is transferred from the primary conductive path to the pickup coil to supply a load 6. A power control section 7 controls the power supplied to the load. More detail on ICPT systems is available in the prior art, for example US patent specification 5,293,308 (Boys et al.) describes an ICPT system for an electric vehicle and the contents of that patent specification are hereby incorporated herein by reference. The primary conductive path 2 typically includes a compensation arrangement 8, which is often referred to as track compensation.

Compensation is required because as a track gets longer its inductance increases and it becomes increasingly difficult to drive. For example, if the track inductance is one microHenry per metre and the track current is 100 A at 2OkHz, then the voltage drop for one metre of track is 12.6 V so that if the maximum possible voltage that the power supply can supply is 400 V RMS then the maximum possible length of track that can be driven is only 31.8 metres. In most ICPT applications of factory automation for example, track lengths of 100 to 300 metres are common so it is apparent that simply connecting a track to a power supply is not a practical solution.

Track compensation techniques are often used at the start of the track as shown in figure 1 , or at the end of the track, or at regular intervals along the track. This compensation usually takes the form of series connected capacitors (with some power supplies parallel capacitors can also be used at the start the track) which have a reactive impedance with the opposite sign to the reactive impedance of the track, and thereby lower the voltage required to drive it.

Using the same numerical example as that set forth above for 100 metres of track the inductive reactance is 12.56 Ohms. If the power supply can supply 400 V RMS then it can drive a reactive impedance of 4 Ohms at 100 A, so the track impedance need needs to be reduced to 4 Ohms by adding a series capacitor with a reactance of 8.56 Ohms at 2OkHz. This could be added as a single capacitor at the start of the track but the voltage drop across it would be 856 Volts. Since this is dangerously high three capacitors spaced 33 metres apart would be a common solution.

A major problem with these existing compensation techniques is that when a capacitor is added to a track it requires both wires of the track to be cut so as to keep the track balanced with respect to ground. The capacitance is always added as equal reactances in each wire or cable and therefore requires four wire joints. Since the track is typically made of litz wire, the joints are very difficult to make "in the field" i.e. on-site. For example, the track is usually placed at some height above the ground and to try to physically join a wire which cannot be stretched or manoeuvred since it is often part of a track structure, and which is located several metres above the ground, is very difficult.

Another problem is that the impedance seen by the power supply with such compensation techniques is now the difference between 12.56 Ohms and 8.56 Ohms, and both of these values are temperature sensitive and subject to ageing, so it is difficult to maintain accuracy over the life of the equipment and with normal temperature variations.

Yet another issue is the accuracy of the capacitors. Commonly available capacitors have an accuracy of typically plus or minus 20% which is completely inadequate for these applications. Capacitors with 5% tolerance are more than twice as expensive as those having a 20% tolerance. Capacitors with 2% tolerance are twice the price again. In practice, 2% variation is still not good enough for this application, especially for very long tracks, so the selection of capacitors is a problematic issue.

Summary of the Invention

It is an object of the present invention to provide an improved compensation method, apparatus or system for the primary conductive path of an ICPT system. Alternatively, is an object of the present invention to at least provide the public with a useful choice.

In one aspect the invention consists in compensation apparatus for a primary conductive path of an ICPT system, the apparatus comprising a transformer having a magnetically permeable core adapted to be magnetically associated in use with a conductor of a primary conductive path such that the conductor provides a primary winding for the transformer, and the transformer having a secondary winding which includes a compensation capacitance.

Preferably the secondary winding comprises one or more turns and is terminated with the compensation capacitance, and the core comprises a toroidal core.

In a further aspect the invention consists in an ICPT system primary conductive path including a conductor and a transformer having a magnetically permeable core magnetically associated with the conductor such that the conductor provides a primary winding for the transformer, and the transformer having a secondary winding which includes a compensation capacitance.

In one embodiment the primary conductive path comprises two or more conductors provided substantially parallel with each other and a core is magnetically associated with each conductor.

Preferably the secondary winding is magnetically associated with both cores.

The secondary winding may comprise one or more turns and is preferably terminated with the compensation capacitance.

The core or cores preferably comprise toroidal cores.

Preferably the conductors are elongate and parallel with each other and the cores are provided about each conductor at substantially the same position along the path.

In a further aspect the invention consists in a method of compensation for a primary conductive path of an ICPT system, the method including the steps of providing a magnetically permeable core about a conductor of the primary conductive path so that the primary conductive path provides a primary winding of the transformer, providing a secondary transformer winding about the core, and providing the secondary winding with a compensation capacitance.

In one embodiment the method includes providing a core about each conductor of a primary conductive path at substantially the same position along the path.

Preferably the method includes associating the secondary winding with each core.

In a further aspect the invention consists in compensation apparatus for a primary conductive path of an ICPT system, the apparatus comprising a capacitance and inductance connected in parallel, the parallel connected capacitance and inductance being connected in series or in parallel with the primary conductive path, one of the capacitance or inductance being controlled so as to provide variable capacitance or inductance to provide a variable volt amps reactive (VAR) source.

Preferably the apparatus is provided in parallel with the primary conductive path.

Preferably the inductor comprises a variable inductor.

In one embodiment the variable inductor may comprise a saturable inductor.

In a further aspect the invention consists in an ICPT system primary conductive path including a conductor and a transformer having a magnetically permeable core magnetically associated with the conductor such that the conductor provides a primary winding for the transformer, and the transformer having a secondary winding which includes a compensation capacitance, and a variable VAR source provided in the primary conductive path to compensate for variations in the compensation capacitance.

Preferably a plurality of transformers are provided at first predetermined intervals along the path.

Preferably a plurality of variable VAR sources are provided at second predetermined intervals along the path, the second predetermined intervals being less frequent than the first predetermined intervals.

Each variable VAR source may be used to correct tolerances or variation in the capacitor or capacitors associate with the one or more transformers.

The or each VAR source is preferably operated in such a way that the impedance looking in to the primary conductive path is purely resistive.

In a further aspect the invention consists in an ICPT system including:

- a primary conductive path connectable to a power source for providing alternating current to the primary conductive path, the primary conductive path in use supplying electrical energy, the primary conductive path having one or more conductors that extend along the path;

- a pick-up having a secondary coil provided about a magnetically permeable core, the secondary coil in use receiving electrical energy from the primary conductive path through inductive coupling; wherein the primary conductive path includes compensation apparatus according to any one or more of the foregoing statements.

Drawing Description

One or more embodiments of the invention will now be described with reference to the accompanying drawings, in which:

Figure 1 is a schematic outline of a known track compensation system,

Figure 2 is a diagrammatic perspective view of a transformer for use to provide track compensation,

Figure 3 is a diagrammatic illustration of an ICPT system track including a transformer as shown in figure 2 and a variable VAR controller,

Figure 4 is a graph of rms voltage (V) against rms current (A) for a saturable inductor,

such as the inductor shown in figure 5, at 1000Hz, for different DC bias currents which are shown in the accompanying legend,

Figure 5 is an illustrative circuit diagram of a saturable inductor which may be used to provide a controllable variable reactance.

Description of Preferred Embodiment(s)

Referring to figure 2, we have found that existing track compensation problems can be solved at least in part by providing a magnetically permeable core to provide a transformer which uses the primary conductive path (i.e. track) of the IPT system as a primary winding, and has a secondary winding which includes a compensation capacitance. As shown in figure 2, the track typically has two conductors 10 and 11 which essentially comprise a loop of conductive material. The preferred material is litz wire, since the frequency that is typically used is in the vicinity of approximately 40kHz. Magnetically associated with each conductor or cable of the primary conductive path is a magnetically permeable core, preferably a toroidal core generally referenced 12. In the preferred embodiment a plurality of individual toroidal cores are used, for example five cores about each conductor as shown in figure 2.

The cores are preferably constructed from a very high permeability material that has low losses at frequencies to greater than the VLF frequencies used in ICPT applications at flux densities to higher than one Tesla. With this material, toroidal transformers with only one turn on the primary, as shown in figure 2, are practical. For example, at 2OkHz, two rows of four or five toroidal cores can operate with a voltage of more than 300 volts using for example cores made of a material marketed under the trademark Fl N EM ET from Hitachi metals.

In practice these transformers are inserted into the primary by threading the conductors through the cores at the time of construction. A secondary winding 14 may have one or more turns to magnetically associate it with the core or cores and is terminated with the single compensation capacitor 16. However, those skilled in the art will appreciate that multiple capacitors may be used. The secondary winding is of known length, so its terminations can be made in the factory rather than on-site thereby guaranteeing quality to higher standards than is available in the field. Accordingly, the transformer solves

problems associated with compensation capacitors as they have been traditionally used, in which difficult connections need to be made in the track conductors. It will be seen that a separate secondary winding may be used on each core, each secondary winding being terminated in a capacitance.

Turning now to figure 3, the transformer arrangement described with respect to figure 2 is illustrated diagrammatically in a primary conductive path (i.e. track) of an ICPT system. Also, further compensation apparatus is shown generally referenced 20. Apparatus 20 comprises a VAR (Volt amps reactive) controller. It comprises a capacitor 22 and a controlled inductor 24 (although those skilled in the art will appreciate that both the inductor and the capacitor may be controlled, or the capacitor may be the controlled element with the inductor being uncontrolled). In this example the capacitor and the controlled inductor are placed in parallel with the track, however they could also be provided in series if desired.

If the inductor 24 is controlled in such a way as to be switched completely out of circuit, the circuit effectively operates as a capacitor. As the inductor is progressively switched into the circuit, it passes through a point where current in the inductor matches the current in the capacitor, and the circuit acts like an open circuit (apart from some harmonic currents that continue to flow). Increasing the conduction through the inductor 24 further causes the circuit to act like an inductor. Therefore, a variable VAR source is available.

The variable inductor may be implemented using a saturable inductor. A saturable inductor is one where the core of the inductor is allowed to saturate such that its relative permeability is reduced. Under these conditions the inductance of a winding on that core will reduce. The degree of saturation may be controlled by a DC current applied through a separate winding. In the simplest application and ignoring any distortion in the waveforms an AC voltage applied to the inductor will cause an AC current to flow giving characteristic curves for AC current versus AC voltage as shown in figure 4. As the DC current is increased the inductance reduces and a given current is achieved with a much lower voltage.

An example of a saturable inductor and associated control circuit is shown diagrammatically in figure 5. In that figure, DC current is applied to a winding on the centre limb 50 of a magnetic core 51 and AC voltages are applied to windings 52 and 53 on the outer limbs. There is theoretically a perfect balance so that no AC voltage appears

on the DC winding. The outside limbs have air gaps 54 and 55 in them to give a more precise inductance when the DC current is zero. As the DC current is increased the inductance of the outer windings is reduced from its maximum value to essentially zero. The circuit may be adjusted to purpose by changing the size and material of the core, the numbers of turns on the windings, and the size of the air gaps.

This means that an application for track compensation such as that shown in figure 3 can be used for long ICPT tracks. For example, for a 400 metre track, the apparatus described above may involve transformer coupled capacitors for each 33 metres of the track, starting at the short-circuit end (i.e. remote for the power supply) and working back towards the power supply. At the 100 metre points the track conductors will be cut and a capacitor would be added directly to the track with the controlled VAR source to compensate for capacitor variations so that errors do not accumulate. The VAR source could be in series with the track or in parallel with it and will be operated in such a manner that the impedance looking into the line towards the short-circuit end is purely resistive.

Using this strategy 20% tolerance capacitors are transformer coupled to the track along its length, and each 100 metres any errors are completely corrected by the controlled VAR source so that the errors do not accumulate. The VAR source at the start of line could be included with the power supply circuitry in a number of ways well known to person skilled in the art.

The compensation strategy disclosed above offers improved levels of performance with low-cost capacitors. It is much easier to install with fewer terminations to be made in the field, and is self-correcting for temperature and ageing variations.

Integers of the invention having known equivalents then such equivalents are herein incorporated as if individually set forth.

Although this invention has been described by way of example and with reference to possible embodiments thereof, it is to be understood that modifications or improvements may be made thereto without departing from the scope of the invention as defined in the appended claims.