GB2483503A - Variable capacitance machine - Google Patents

Abstract

A variable capacitance machine has at least one phase comprising members 31,32 with dielectric material between them and having electrodes 33,34. Relative movement between members 31,32 varies the capacitance between the electrodes 33,34 and the space between them contains a conductive dielectric material 35. The dielectric can be a fluid such as water or an elastomeric material such as gel or a deformable solid such as an acetal co-polymer. There can be multiple separately energised phases (fig 8) and multiple electrodes on one or both members. Electrodes can move relatively linearly or angularly. The dielectric material 35 in the space can have a higher permittivity than either of dielectric materials 36,37 carried by the respective electrodes. The dielectric materials carried by the electrodes can be insulators such as polyimide or mica. The machine can function as a generator or motor and a switched voltage can be applied to the electrodes, such as an alternating voltage having a period less than the dielectric charge migration time constant.

Description

Operatjon of a Variable Capacitance Machine The present invention generally relates to the conversion of energy in a variable capacitance machine.

Figure 1 shows a simplified representation of a variable capacitor. The variable capacitor consists of two members 11 and 12, each carrying conductive electrodes 13 and 14. Typically one member is stationary and the other member is movable, either in a linear or rotary sense. The space between the members contains a dielectric 15 in which an electric field may be developed by charges on the electrodes. In Figure 1(a), the movable member is positioned so that the conductive electrodes are substantially aligned. In this position the machine has a high capacitance, requiring a large amount of charge to be present on the electrodes for a given voltage between them. In Figure 1(b), the movable member is positioned so that the conductive electrodes are substantially unaligned. In this position the machine has a low capacitance, requiring a small amount of charge on the electrodes for a given voltage between them. Given certain assumptions, the value of the capacitance may be found from the following equation: sA Capacitance = T where A is the area by which the electrodes are effectively overlapped, t is the thickness of the dielectric between the members and is the permittivity of the dielectric. The capacitance is increased if the dielectric material has a high permittivity.

The relationship between charge q and voltage v is shown graphically in Figure 2. The aligned position, with maximum capacitance, is represented by the Cmax line. The unaligned position, with minimum capacitance, is represented by the Cmin line. The capacitance, being the ratio of charge to voltage, is the slope of the line. The inventor has recognised that a practicable variable capacitance machine can be realised using a variable capacitor to provide for the conversion of energy between mechanical and electrical forms by appropriate cnergisation.

According to embodiments disclosed there is provided a variable capacitance machine having at least one phase comprising first and second members with dielectric material between them, the first member having at least one electrode, the second member having at least one electrode, which second member is arranged in a relatively movable relationship with respect to the first member such that there is a space between them, whereby relative movement varies the capacitance between the electrodes, the space containing a conductive dielectric material.

According to some embodiments there is provided a variable capacitance machine having at least one phase comprising first and second members, and first, second and third dielectric materials between them, the first member having at least one electrode carrying the first dielectric material of a first permittivity, the second member having at least one electrode carrying the second dielectric material of a second permittivity, which second member is arranged in a relatively movable relationship with respect to the first member with a space between the first and second dielectric materials, whereby relative movement varies the capacitance between the electrodes, the space containing the third dielectric material which has a permittivity that is higher than that of the first and second dielectric materials.

The first and second dielectric materials may be different, but are advantageously the same, for example a polyimide or mica.

Jn some embodiments the machine has multiple electrodes on one or both of the first and second members.

While the third dielectric may be a fluid, such as substantially water, it may be an elastomeric material or a deformable solid in applications where limited movement between the members is acceptable.

According to embodiments described, there is provided a variable capacitance machine system in which the energy conversion is optimised by the use of high permittivity dielectric materials. More particularly, the present invention relates to the operation of a variable capacitance machine by using an alternating excitation which enables the use of a conducting dielectric between the members. This overcomes the problem of charge migration in a conducting dielectric which would otherwise limit the period for which the effective output of the machine is available.

The machine disclosed herein may be used to provide a mechanical output. For example, as a motor providing motive force between the members, or as a bearing in which the force generated between the members is used to sustain a gap between them. Alternatively, the machine disclosed herein may be operated as an electrical generator for providing an electrical output from a mechanical input.

The relative movement between the members can be either linear or rotary. In embodiments this is achieved by the use of known techniques for providing a machine frame in which the machine operates.

The invention is further described by way of example with reference to the following drawings in which: Figure 1(a) shows a variable capacitor in the maximum capacitance position; Figure 1(b) shows a variable capacitor in the minimum capacitance position; Figure 2 shows a graph of the relationship between charge q and voltage v for a variable capacitor; Figure 3 shows a capacitor with insulating dielectric layers attached to the electrodes; Figure 4(a) shows a variable capacitance machine in which a conductive dielectric contains re-distributed charges; Figure 4(b) shows the force waveform resulting from a constant applied voltage; Figure 5(a) shows the force waveform for an applied voltage which is periodically reversed; Figure 5(b) shows a capacitance/frequency relationship for a variable capacitance machine; Figure 6 shows a power circuit which supplies an alternating voltage to a capacitive load; Figure 7(a) shows a variable capacitor in the maximum capacitance position, in which both positive and negative electrodes are on the same member; Figure 7(b) shows a variable capacitor in the minimum capacitance position, in which both positive and negative electrodes are on the same member; Figure 8 shows a double-sided variable capacitor with two stationary members and one movable member; Figure 9 shows a variable capacitance machine in which the stationary and movable members carry multiple electrodes; Figure 10 shows a variable capacitance machine consisting of a number of double-sided stationary members and double-sided movable members; Figure 11(a) shows a variable capacitance machine with the stationary electrodes grouped into a number of different phases; Figure 1 1(b) shows the capacitance between different electrodes of 11(a) plotted as a function of the position of the movable member; Figure 12(a) shows a variable capacitance machine in which in which each stationary member has phase electrodes connected in parallel; Figure 12(b) shows a variable capacitance machine in which in which each stationary member has phase electrodes connected in series; Figure 13(a) shows an electrode pattern for a stationary member of a rotary machine; Figure 13(b) shows an electrode pattern for a movable member of a rotary machine; Figure 14 shows a variable capacitance machine connected to a controller; and Figure 15 shows a variable capacitance machine configured as a bearing.

The basic operation of the variable capacitance machine in accordance with disclosed embodiments is as follows with reference to Figure 2. The machine is initially placed in the unaligned Cmjn position and a voltage v1 is applied. This requires electrical energy to be supplied, represented by the area of the triangle OAq1 in Figure 2. With the voltage maintained at v1, the electrodes of the machine are now moved to the Cmax position. For the movement from A to B, a mechanical force is developed which provides a mechanical output from the machine. During this movement, the charge must increase from q1 to q2 to maintain a constant voltage. This requires more electrical energy to be supplied, represented by the area of the rectangle ABq2q1. With the machine in the Cmax position, the voltage is reduced to zero by removing the charge q2 This returns energy to the electrical system, represented by the area of the triangle OBq2. The triangular area OAB (shown hatched) represents the net electrical energy which has been supplied to the machine but not returned and is given by the equation: Energy = v1(q2 -q1)= 1v(C -c) Assuming negligible dissipation, this energy has been converted to mechanical form. If the difference between the capacitances Cmax and Cmjn is dC, and the distance moved is dx, then the average force is given by the equation: 1 2dC Average Force = -v1 -2 dx This force is maximised by maximising the voltage v1, maximising the change in capacitance dC, and minimising the distance dx. The change in capacitance is increased if the dielectric material has a high permittivity. Water has a high relative perniittivity (around 80) and is therefore a suitable dielectric fluid for the gap.

By using a prime mover to move the moving member, mechanical energy can alternatively be converted to electrical form. Thus the machine can be operated as a motor or as a generator.

Figure 3 shows an embodiment of a variable capacitance machine with stationary and movable members 31 and 32, respectively, comprising electrically conductive electrodes 33 and 34 defining a space between them.

For the purposes of illustration, the movement of this machine is taken to be linear. However, the following description applies to a rotary machine with equal effect as will be described later. The terms stationary' and movable' are intended to indicate the relative movement between the members of the machine. This is determined by the use of supporting structure and bearing arrangements with which the skilled person will be familiar from the construction of other type of electro-mechanical machines. These are disclosed only in outline and largely omitted from the drawings for the sake of clarity.

Typically, the electrodes are made of metal (for example, copper). The space between the electrodes of the capacitor (constituting the dielectric) may be in a vacuum, or contain one or more dielectric materials. The opposing surfaces of the electrodes 33 and 34 in this embodiment are each covered with a layer of insulating dielectric material 36 and 37. The gap 35 between the material on the stationary and movable members may contain an electrically conductive dielectric fluid. Alternatively, if only limited movement is required an electrically conductive elastomerie material such as a gel or a soft solid dielectric (for example, acetal co-polymer), can be used in the gap 35. When the capacitor is charged by applying a potential difference between the electrodes, an electric field E is set up in the dielectric between the electrodes 33 and 34. If the dielectric has a permittivity, then electrical energy is stored at /26E2 joules per cubic metre. The disclosed embodiments use capacitors in which the material of the dielectric layers 36 and 37 are solid insulators, for instance a polymer (such as polyimide) or mica. Suitable polymers typically have relative permittivities in the range 2 to 4, but this will vary according to application. For the purposes of this disclosure the dielectrics on the electrodes can be considered as insulators. In this embodiment, the gap 35 contains a high permittivity fluid, for instance, water. It will be appreciated that this permittivity is significantly higher than that of the dielectric materials of layers 36 and 37. The dielectric fluid is held in place by a boundary (not shown) surrounding the stationary member 31. In effect, the boundary creates a vessel containing the dielectric fluid in which the stationary member is immersed.

Indeed, in some embodiments, the variable capacitance machine disclosed operates in an environment which is made up of the dielectric fluid.

As well as developing the desired shear force at the gap, a normal force will also be developed between the oppositely charged members of the variable capacitance machine. However, with a high permittivity fluid in the gap 35, most of the energy in the dielectric is stored in the solid dielectric layers 36 and 37 of relatively lower permittivity rather than in the dielectric material in the gap 35 between them. The capacitance therefore does not vary significantly with gap length and the normal force is minimised, reducing the loads on the supporting structure.

The dielectric layers 36 and 37 adjacent the electrodes perform two main functions although a variable capacitance machine can be realised using only electrodes and a dielectric in the gap between them. One function of the layers 36/37 is to provide electrical insulation between the electrodes. Another function is to minimise the normal force between the electrode plates. The capacitance varies little with gap size due to the relatively high permittivity of the dielectric in the gap 35. Furthermore, most of the voltage across the electrodes is dropped across the solid dielectric layers 36/37 and the electrodes because of their relatively low permittivity. Thus, most of the energy is stored in these solid dielectric layers 36/37.

Although water has a high permittivity, it is not a perfect insulator. Even pure "de-ionised" water contains ions giving it a small but far from negligible conductivity of around 5 giS/rn at 25°C. Impurities including dissolved gasses from the air further increase the conductivity of water. The conductivity of water can be reduced by chilling it, and refrigerated water (typically containing ethylene glycol) is used in water capacitors for certain applications. Such a solution can be used for the dielectric in the gap 35 in the machines of the embodiments disclosed herein. However the practical advantage of using room-temperature water for the dielectric in the space between the plates is that it needs less refinement and maintenance. *

When an electric field is applied to a conductive dielectric, the charges within the dielectric migrate so as eventually to counteract the entire electric field.

The intrinsic dielectric charge migration time-constant r of this effect is related to the conductivity and permittivity of the dielectric by the following equation:

S

where C and a are respectively the permittivity and conductivity of the dielectric. The intrinsic dielectric charge migration time constant for water is less than 1 ms at room temperature and can be increased to more than 1 Oms using chilled water. The migration of charge and the consequent negation of the electric field reduce the available output from a capacitor motor. Figure 4(a) shows schematically the ultimate re-distribution of the charge within a water dielectric 41 in the gap between electrodes. With the electric field negated, there is no longer any change of capacitance with position and therefore no force. Figure 4(b) shows the rapid decay of force in a variable capacitance machine with a steady voltage applied as in Figure 4(a). As a result, the applied constant voltage is effective at delivering a moving force between the members for only a limited period which is linked to the dielectric charge migration time constant t.

However, if the polarity of the applied voltage is reversed, then the electric field is reversed and the force is restored, as shown in Figure 5(a). Moreover, the partially re-distributed charge now increases the electric field rather than reduces it. The second force peak 52 is higher than the first 51 because the partially re-distributed charge is here augmenting the electric field. Once again, however, the charge migrates and the force is reduced. Figure 5(a) shows that repeated reversal of the applied voltage allows the force to be developed continuously albeit with some ripple.

This effect is further illustrated in Figure 5(b) which shows the variation of is capacitance with frequency for the device shown in Figure 3. At low frequencies denoted by region A, the reversal of the excitation is slow compared to the intrinsic dielectric time-constant. Under these conditions, the dielectric 35 is effectively a conductive layer within the capacitor. The electric field is confined to the solid dielectric layers 36 and 37, resulting in a high total capacitance. Moreover, the capacitance does not vary with position so no force is developed.

At high frequencies denoted by region B, the reversal of the excitation is fast compared to the intrinsic dielectric time-constant. Under these conditions, the permittivity of the dielectric 35 is more important than its conductivity. The capacitance at high frequency is reduced because of the finite permittivity of the dielectric layer 35. Moreover, the capacitance does now vary with position so a force is developed between the stationary and movable members.

Alternating the field at high frequency results in less force ripple. However, every reversal requires energy to be removed from the capacitor and then re-applied. Handling this reactive energy can incur losses which would increase with increasing frequency. There are also resistive losses within the conducting dielectric, and these too may vary with frequency. The optimum frequency is a compromise between several factors and could be between a few kHz and several hundred kHz. For a water dielectric in the gap 35, having a charge migration time constant of about 1 ms, the frequency of alternation of the applied voltage should be preferably greater than 1kHz (for instance, 2kHz). The intrinsic time constant could be substantially shorter than 1 ms, for example if the water contained impurities which increase its conductivity. In practice, therefore, the operating frequency could be in the range of 2kHz to 100kHz or more.

The power circuit driving the variable capacitance motor must be capable of applying bipolar voltages across the electrodes. Furthermore it is not always appropriate to apply a stepped voltage waveform to a capacitive load as this may result in large current spikes. In some applications a resonant circuit is appropriate to supply the reactive energy. Also, when the supply is bipolar, a transformer can be used to provide a high voltage across the capacitor from lower voltage circuitry.

An example of a power circuit is shown in Figure 6. This is a transformer-coupled Hartley oscillator and may be used to apply an alternating voltage to the variable capacitor machine 61. The oscillator contains a resonant circuit formed by an inductor 62 coupled to the machine 61. In this embodiment, the inductor is magnetically coupled to a secondary winding 63 which together form a step-up transformer. The inductor has a centre-tap 64 which is connected to the positive power-supply terminal 67. The ends of the inductor are alternately connected to the negative power-supply terminal 68 by transistors 65 and 66. The system operates at a resonant frequency determined by the capacitance and the inductance. The tuning and functioning of a Hartley oscillator will be well understood by the skilled person and will not be described in further detail here.

The output of the power circuit of Figure 6 is an alternating voltage across the electrodes of a phase of the variable capacitance machine containing a variable capacitance as described herein. Multiple phases can be arranged in the same machine each of which is separately excitable by a power circuit. The frequency is chosen to be fast compared to the intrinsic dielectric time-constant, giving operation in region B of Figure 5(b).

Figure 7 shows another version of the variable capacitor machine. This has stationary and movable members 71 and 72, respectively, with a gap 73 between them in which the conductive dielectric referred to above resides. The stationary member 71 carries both positive and negative electrodes 74 and 75.

The movable member carries an electrode 76 which has no electrical connection to it. Thus, this embodiment is particularly useful as there is no need for electrical connection to the moving member. In Figure 7(a), the electrodes of the capacitor are in the aligned, maximum capacitance, position.

In Figure 7(b), the electrodes of the capacitor are in the unaligned, minimum capacitance position.

This geometry is further developed in the embodiment of Figure 8. Here, the movable member 82 is double-sided and a second stationary member 86 has been added to supplement the stationary member 81. This has the effect of doubling the output force from the machine. The compositions of the solid and fluid dielectrics are as before. The electrodes 87 and 88 of the second stationary member are energised in synchronism with the electrodes 84 and 85 of the first stationary member.

The geometry is further developed in the embodiment of Figure 9. Here, the electrodes 91 and 92 are interleaved and repeated many times along the first stationary member 93. The electrodes 96 on the movable member 94 are similarly repeated, as are the electrodes on the second stationary member 95 corresponding to those on the first stationary member. These electrodes are electrically conductive elements, such as copper foils, carried on insulating sheets, which may be manufactured using printed-circuit tecimiques. Although not shown, it will be appreciated that the opposing surfaces of the members are also coated with the insulating dielectric material and that a conductive dielectric fluid is arranged in the gaps as described above.

Many such sheets may be stacked as shown in Figure 10. The stationary and movable members 101/102 are interleaved. The stationary members 101 are mechanically coupled, and the movable members 102 are also mechanically coupled. This arrangement increases the force as there are now more active gaps. The stationary electrodes are electrically connected as shown in Figure 9.

The variable capacitance machines described so far are all single-phase machines in that they have a single capacitance value which varies with position. Multi-phase machines are also possible. Each phase is energised using its own leg of a power circuit of the type shown in Figure 6 above.

Figure 1 1(a) is a schematic representation of a 4-phase machine embodiment.

It is based on the construction shown in Figure 7. Here the stationary member 111 carries electrodes connected to form different phase circuits. In the position shown, the capacitance between electrodes A and B will be high because one of the electrodes 113 on the movable member 112 completely overlaps them both. The capacitance between electrodes B and C will have some intermediate value because the overlap of the movable electrode is only partial. The capacitance between electrodes C and D will be low because the movable electrode does not overlap them at all. Thus as the movable member moves past the stationary members, the capacitance of the different phase circuits AD, BC, CD and DA will vary cyclically as shown in Figure 11(b).

Thus, the phases can be energised sequentially to coincide with the movement of the moving member 112.

Figure 12 shows two forms for the connection of the multiple electrodes. This is illustrated in relation to the four-phase embodiment of Figure 11 and, for clarity, only the connection of the A and B electrodes is shown. In Figure 12(a), the electrodes are connected in parallel. Parallel connection of the electrodes ensures that the phase A electrodes are all at the same voltage and the phase B electrodes are all at the same voltage. The capacitive elements between the terminals 121 and 122 are connected in parallel, each element carrying the full terminal voltage but only a fraction of the current.

Series connection shown in Figure 12(b) may be preferable in some applications. Series connection of the electrodes ensures that the phase A electrodes all carry the same charge and the phase B electrodes carry a similar charge of the opposite polarity. The capacitive elements between the terminals 123 and 124 are connected in series, each element having a fraction of the terminal voltage but carrying the full current.

The connection of greater or fewer numbers of phases or electrodes can be derived from the circuits of Figures 12(a) and (b).

Figures 13(a) and 13(b) show a stationary member 131 and a movable member 132, respectively, for a single-phase rotary variable capacitance machine. Each member is made by the printed-circuit techniques described above and consists of a pattern of copper electrodes covered by an insulating dielectric layer of polyimide. In this embodiment the stationary member has a number electrodes 133 connected in parallel as an inwardly facing array to a radially outer track 134 and to a first terminal 135. Interleaved between the electrodes 133 is a number of electrodes 136 arranged as an outwardly facing array connected in parallel to an inner point 137 and to a second terminal 138. The movable member 132, supported on bearings, is held close to the stationary member 131 with a small gap between them to allow movement of the movable member.

This gap contains the dielectric fluid, such as water, as discussed above.

Figure 14 shows such a single-phase rotary variable capacitance motor 141 connected to a microprocessor-based controller 142. The controller 142 controls the supply of power from an electrical energy source 143 (for instance a battery, or other constant voltage source) through a switching circuit 142 such as that shown in Figure 6. A stator 131 (as shown in Figure 13(a)) has first and second terminals 135 and 138 connected to the controller by wires 145. A rotor 132 (as shown in Figure 13(b)) is supported on bearings (not shown) and has a shaft 147 providing mechanical connection to a load. Note that the gap between the rotor and stator has been exaggerated for clarity. In practice, the gap is small (for instance about 100 i.tm) and is filled with a high permittivity fluid (for instance water) as described above. A rotor position sensor 148 provides a signal on line 149 to the controller 142 depending on the angular position of the rotor. When the rotor is in a position where positive torque is available) the controller supplies an alternating voltage to the motor at a frequency that is optimised according to the dielectric time constant of the fluid in the gap. The system is controlled by one or more control inputs 1410 which, for instance, vary the magnitude of the alternating voltage. When the rotor is not in a position where positive torque in a particular direction is available, the controller does not supply a voltage to the motor.

Figure 15 shows another version of the variable capacitor machine. This has stationary and movable members 151 and 152, respectively, with a gap 153 between them. Both members carry positive electrodes 154 and 155, which are adjacent each other, and negative electrodes 156 and 157 which are also adjacent each other. The pairs of electrodes are carried on blocks of insulating dielectric material, of the types discussed above, and the gap 153 carries the conducting dielectric. The arrangement of the applied voltages between the electrodes in this embodiment means that the flux path is now generally along the gap. The force generated by excitation of the electrodes in this way is across the gap 153. In a dielectric-filled environment the capacitance between the positive electrodes and the negative electrodes increases as the gap 153 is increased. Thus, when the electrodes are energised, a repulsive force is developed between the two members. Thus, according to the magnitude of the alternating excitation, as described above, a gap of predetermined size can be maintained for a given load. This arrangement of variable capacitance machine has various applications. In particular, this embodiment of the machine can be used as a bearing. The bearing may be contactless, being supported by the generated force urging the members apart. This is essentially frictionless and, therefore, smooth in operation and substantially wear-free. Alternatively, the force created can be used to support a contact-type bearing, thereby reducing the load on the bearing to enable a smaller capacity bearing to be used or to increase the life of a given bearing. An application for such bearings is in prosthetic joints, such as the knee joint of the human body. In normal operation as a bearing, the electrostatic force is orthogonal to the movement so no energy would be converted. Nonetheless, an electrostatic force is developed between the members and alternating the polarity of excitation allows the use of conducting dielectrics (for instance water in the gap 153). The use of water as a fluid dielectric in medical applications is particularly advantageous.

The disclosed embodiments utilise an alternating excitation to enable the use of a conductive dielectric in a variable capacitance machine. The ability to use a conductive dielectric enables the use of commonly found substances, such as water, in forms that are otherwise unsuited as dielectrics. The term "conductive" is intended to mean that the material of the dielectric is such as would not be suited for use in a conventional variable capacitance application because its conductivity would inhibit its ability to retain charge. For example, the use of water as the conductive dielectric refers to water having a conductivity of greater than 5pS/m, However, the invention is capable of using substances of lower conductivity.

The skilled person will appreciate that variation of the disclosed arrangements are possible without departing from the invention. The dimensions of the components of the disclosed machine will depend on the application.

Accordingly, the above description of several embodiments is made by way of example and not for the purposes of limitation. It will be clear to the skilled person that minor modifications can be made to the arrangements without significant changes to the operation described above. The present invention is intended to be limited only by the scope of the following claims.

Claims (29)

1. Claimsi 1. A variable capacitance machine having at least one phase comprising first and second members with dielectric material between them, the first member having at least one electrode, the second member having at least one electrode, which second member is arranged in a relatively movable relationship with respect to the first member such that there is a space between them, whereby relative movement varies the capacitance between the electrodes, the space containing a conductive dielectric material.
2. 2. A machine as claimed in any of claims 1 in which the dielectric is a fluid, for example at least substantially water.
3. 3. A machine as claimed in any of claims br 2 in which the dielectric material is an elastomeric material, such as a gel, or is a deformable solid..
4. 4. A machine as claimed in any of claims 1-3, comprising multiple separately energisable phases.
5. 5. A machine as claimed in any of claims 1-4 in which the first member has multiple electrodes.
6. 6. A machine as claimed in any of claims 1-5 in which the first and second members have multiple electrodes.
7. 7. A machine as claimed in any of claims 1-6 in which the electrodes are arranged to be linearly relatively movable.
8. 8. A machine as claimed in any of claims 1-6 in which the electrodes are arranged to be angularly relatively movable.
9. 9. A variable capacitance machine having at least one phase comprising first and second members with dielectric material comprising first, second and third dielectric materials, between them, the first member having at least one electrode carrying the first dielectric material of a first permittivity, the second member having at least one electrode carrying the second dielectric material of a second permittivity, which second member is arranged in a relatively movable relationship with respect to the first member such that there is a space between the first and second dielectric materials, whereby relative movement varies the capacitance between the electrodes, the space containing the third dielectric material which has a permittivity that is higher than that of the first and second dielectric materials.
10. 10. A machine as claimed in claim 9 in which the first and second dielectric materials are the same.
11. 11. A machine as claimed in claim 9 or 10 in which the first andlor second dielectric materials is/are an insulator, for example a polymer, such as a polyimide, or mica.
12. 12. A machine as claimed in any of claims 9-11 comprising multiple separately energisable phases.
13. 13. A machine as claimed in any of claims 9-12 in which the first member has multiple electrodes.
14. 14. A machine as claimed in any of claims 9-12 in which the first and second members have multiple electrodes.
15. 15. A machine as claimed in any of claims 9-14 in which the electrodes are arranged to be linearly relatively movable.
16. 16. A machine as claimed in any of claims 9-14 in which the electrodes are arranged to be angularly relatively movable.
17. 17. A machine as claimed in any of claims 9-16 in which the third dielectric is a fluid, for example at least substantially water.
18. 18. A machine as claimed in any of claims 9-16 in which the third dielectric material is an elastomeric material, such as a gel, or is a deformable solid, such as an acetal co-polymer.
19. 19. A variable capacitance machine system comprising a machine as claimed in any of claims 1-18 and switch means connected to control the application of a voltage across the electrodes of the or each phase.
20. 20. A system as claimed in claim 19, including control means operably connected to control the actuation of the switch means to apply a voltage across the electrodes of the or each phase.
21. 21. A system as claimed in claim 20 in which the control means are operable to control the switch means to apply an alternating voltage across the electrodes.
22. 22. A system as claimed in claim 21 in which the period of the alternation of a voltage is less than or equal to the dielectric charge migration time constant of the dielectric material, for example the third dielectric material.
23. 23. A system as claimed in any of claims 20-22 in which the voltage is applied across the electrodes on respective first and second members.
24. 24. A system as claimed in any of claims 20-22, in which the voltage is applied across the electrodes on the same member.
25. 25. A system as claimed in any of claims 20-24 in which the control means are arranged to operate the machine as a motor, providing a mechanical output.
26. 26. A system as claimed in any of claims 20-24 in which the control means are arranged to operate the machine as a generator, providing an electrical output.
27. 27. A method of driving a variable capacitance machine as claimed in any of claims 1-18, comprising applying an alternating voltage across the electrodes of the or each phase.
28. 28. A method as claimed in claim 27 in which the period of the alternation of the voltage is less than or equal to the dielectric charge migration time constant of the dielectric, for example the third dielectric.
29. 29. A method of driving a variable capacitance machine having at least one phase comprising first and second members with dielectric material between them, the first member having at least one electrode, the second member having at least one electrode, which second member is arranged in a relatively movable relationship with respect to the first member such that there is a space between them, whereby relative movement varies the capacitance between the electrodes, the space containing the dielectric material which is conductive, the method comprising applying an alternating voltage across the electrodes of the or each phase.