Power coupler for rotating capacitive power transfer and electrical machine comprising such power coupler

Abstract

The invention concerns a power coupler for rotating capacitive power transfer, comprising two rotating capacitors 1, each one comprising: A fixed electrode 2 provided to be electrically connected to an AC power source; A rotating electrode 3 facing the fixed electrode 2 and provided to be electrically connected to a rotor of an electrical machine; An electrolyte 4 located in a gap between the two electrodes 2, 3; Said electrolyte 4 comprising a solvent and a dissolved salt, characterized in that the solvent is organic.

Description

POWER COUPLER FOR ROTATING CAPACITIVE POWER TRANSFER AND ELECTRICAL MACHINE COMPRISING SUCH POWER COUPLER

[0001] The invention is related to a power coupler for rotating capacitive power transfer, and in particular in rotating electrical machine comprising such power coupler to feed an electromagnet.

[0002] Wired power couplers usually consist of a stationary conductive part made with wire ending in graphite brush in contract with a rotating ring made of copper. Such systems are, however, known to have problems of wear and material loss.

[0003] To avoid those inconveniences, wireless power couplers have been developed.

[0004] Inductive power couplers provide a good power transfer, generally higher than 10kW. However, such couplers occupy high volume due to shielded magnetic cores in order to avoid electromagnetic disturbances. In addition, those installations are expensive.

[0005] Capacitive power couplers avoid those last inconveniences. A capacitive power coupler comprises two capacitors, each one comprising two electrodes facing each other and separated by a gap. The electrodes are powered by an AC power source.

[0006] In some embodiments the gap between the electrodes is simply filled with air. However, this embodiment does not deliver more than 4 or 5 kW. In addition, the AC current frequency must be higher than 1 MHz: expensive and complex power electronics must thus be used, and the resulting capacitance is very weak, around a few nano-farads, which reduces power transfer efficiency.

[0007] In other embodiments, the gap is filled with aqueous electrolyte. The power transferred is higher than a gap filled with air: from 2 to 10 kW. In addition, the AC current frequency is around 10 kHz and the resulting capacitance around tens of microfarads. However, the electrodes of the power coupler are highly sensitive to corrosion. In addition, since an aqueous electrolyte starts evaporating above 100°C,such capacitive power couplers with aqueous electrolytes are difficult to use with rotating electric machines.

[0008] The aim of the invention is therefore to propose a power coupler for rotating capacitive power transfer which delivers enough power, and which is usable in a wide range of conditions.

[0009] For this purpose, the invention proposes a power coupler for rotating capacitive power transfer, comprising two rotating capacitors, each one comprising:- A fixed electrode provided to be electrically connected to an AC power source;- A rotating electrode facing the fixed electrode and provided to be electrically connected to a rotor of an electrical machine;- An electrolyte located in a gap between the two electrodes;- Said electrolyte comprising a solvent and a dissolved salt, the solvent being organic.

[0010] The power coupler of the invention may also include the following optional characteristics considered individually or according to all possible combinations of techniques:- The electrolyte remains in a liquid state between -30°C and 150°C.- The electrolyte resistivity peis at most 150 Q.cm.- The viscosity of the solvent is at most 3 mPa.s, in that the relative dielectric constant of the solvent is at least 35, and in that the dipole moment of the solvent is at least 3.5 Debye.- The solvent is a member of the following families: carbonate ester, lactone, amide or a mixture thereof.- The solvent is propylene carbonate, N,N-dimethylformamide, gammabutyrolactone, gamma-valerolactone or a mixture thereof.- The salt is a combination of an anion and a cation, wherein the cation is a member of the following cation families or a mixture thereof: 1 R-3R’- imidazolium, 1 R-2R”-3R’-imidazolium, lithium, or a mixture thereof, wherein R is a methyl, ethyl, propyl, isopropyl, butyl or tert-butyl group, wherein R’ is a methyl, ethyl, propyl, isopropyl, butyl or tert-butyl group, wherein R” is a methyl group, and wherein the anion is a member of the following anion families or a mixture thereof: non-oxidizable inorganic anions, notably CIO4-, alkylsulfates R-O-SO3, wherein R is a methyl or ethyl group, methanesulfonates R3C-SO3, wherein R is H or the methyl group, fluorinated methanesulfonates, notably F3C-SO3, and fluorinated sulfonylimides (R-SO2)-N-(SO2-R’), wherein R and R’ are chosen among F or the trifluoromethyl group -CF3.- The salt is one of the following compounds: 1-ethyl-3-methylimidazolium methanesulfonate, 1 -ethyl-3-methylimidazolium bis(fluorosulfonyl)imide, 1 -ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, 1 -butyl-3- methylimidazolium bis(trifluoromethylsulfonyl)imide, 1 ,3- dimethylimidazolium bis(trifluoromethylsulfonyl)imide, 1 -ethyl-2,3- methylimidazolium trifluoromethanesulfonate, lithium perchlorate, lithium bis(trifluoromethylsulfonyl)imide or a mixture thereof.- The electrolyte is maintained in the gap between the electrodes of each capacitor with at least a sealing ring.

[0011] Another object of the invention concerns an electrical machine comprising a stator, a DC power source, a DC-to-AC converter electrically connected to the power source, a power coupler as described above, the fixed electrode of each rotating capacitor of the power coupler being fixed to the stator of the electrical machine and electrically connected to the DC-to-AC converter, and the rotating electrode of each rotating capacitor being electrically connected to an AC-to-DC converter which is electrically connected to an electromagnetic rotor of the machine which is mounted on a rotational axis, the rotating electrode of each rotating capacitor being mounted on the rotational axis.

[0012] Other characteristics and advantages of the invention will be apparent in the below descriptions, by way of indication and in no way limiting, and referring to the annexed figures among which:- Figure 1 , which represents a scheme of a rotating capacitor of the power coupler of the invention,- Figure 2, which represents variation of the module and the phase of the impedance of the conductivity cell with the electrolyte of the invention versus frequency of the AC voltage source,- Figure 3, which represents variation of the real part and the imaginary part of the complex impedance of the same conductivity cell with the same electrolyte of figure 2 versus frequency of the AC voltage source- Figure 4, which represents variation of capacitance versus frequency of the AC voltage source, of the conductivity cell for four electrolytes,- Figure 5, which represents variation of capacitance of the conductivity cell for two electrolytes of the invention versus salt mass percentage, at a predetermined AC voltage source frequency,- Figure 6, which represents variation of the electrolyte resistance of the conductivity cell for the two electrolytes of figure 5 versus the salt mass percentage, at a predetermined AC voltage source frequency,- Figure 7, which represents variation of real impedance for two different capacitors made with flat steel plates, having two different areas, using the same electrolyte as in figure 2 versus frequency of the AC voltage source,- Figure 8, which represents variation of capacitance for the same two different capacitors made with flat steel plates, having two different areas, using the same electrolyte as in figure 7 versus frequency of the AC voltage source,- Figure 9, which represents a scheme of the circuit for estimation of the power transfer from the power source to the motor of the electrical machine.

[0013] First, it is noted that on the figures, the same references designate the same elements regardless of the figure on which they feature and regardless of the formof these elements. Similarly, should elements not be specifically referenced on one of the figures, their references may be easily found by referring oneself to another figure.

[0014] It is also noted that the figures represent mainly one embodiment of the object of the invention but other embodiments which correspond to the definition of the invention may exist.

[0015] Elements in the figures are illustration and may not have been drawn to scale.

[0016] The invention refers to a power coupler for rotating capacitive power transfer, and to an electrical machine comprising such a power coupler. The invention thus allows wireless capacitive power transfer for electrical machines, in particular Wound Field Synchronous Machines.

[0017] The power coupler comprises two rotating capacitors 1 (one is illustrated in figure 1). Each rotating capacitor 1 comprises one fixed electrode 2 (attached to the stator of the electrical machine via an axis 6), a rotating electrode 3 (attached to the rotor of the electrical machine via an axis of rotation 7), and a space or gap between the electrodes 2, 3. As a non-limitative example, the electrodes 2, 3 can be flat plates parallel to each other or concentric cylinders.

[0018] As a non-limitative example, the electrodes 2, 3 can be made of pure metal or of a metal alloy, or any type of material known to have sufficient electrical conductivity, such as glassy carbon, conductive metal carbides, or metal-coated ceramics.

[0019] The gap between the two electrodes 2, 3 is filled with a liquid electrolyte 4, said electrolyte 4 comprising a solvent and a dissolved salt. In addition, to allow the rotation of the considered electrode 3 and to avoid liquid electrolyte 4 leakage, one or more sealing rings 5 are needed in the design of such capacitors 1 (see figure 1).

[0020] According to the invention, the solvent is organic. One of the advantages of using an organic solvent is that it remains in a liquid state over a wide temperature range, making electrolyte 4 based on said organic solvent an excellent candidate foruse on rotary machines where the temperature of the electrodes 2, 3, of the electrolyte 4 and of the sealing rings 5 can exceed 100°C.

[0021] More precisely, the electrolyte 4 should stay liquid at least down to -30°C, a value agreed for several outdoor applications, including vehicles. On the other hand, the electrolyte 4 should stay liquid at least up to 150°C, since high speed rotating motors can attain such temperature during operation, a value agreed for motors rotating at high speeds, including those used for electrical vehicles.

[0022] The capacitance C of this type of electrolyte capacitive rotating capacitor 1 is known and it is related to the separation of the ions of opposite charge coming from the salt dissolved in the solvent (the electrolyte being the solution of such salt in such solvent) upon the application of a voltage between an electrode and the liquid electrolyte phase.

[0023] This type of rotating capacitor 1 is also known as double layer capacitor. Indeed, in a complete device (that is, with the two electrodes 2, 3), there is one capacitance per electrode, both being in series, the total capacitance of the capacitor 1 being approximately half of the capacitance of each individual electrode / electrolyte interface. The capacitance of the capacitor 1 cannot be predicted, and, hence, it should be measured. Concerning the geometrical dimensions, it is known that the capacitance of this type of capacitor 1 is approximately proportional to the geometrical surface of the electrodes 2, 3 in contact with the electrolyte 4.

[0024] According to the electrochemistry field, for smooth surface electrodes 2, 3, the capacitance per cm2of smooth surface is usually in the range of tens pF / cm2. The capacitance of these capacitors 1 does not depend on the gap between electrodes 2, 3 since the concentration of salt in the solvent is high enough that the so-called diffuse capacitance becomes negligible. On the other hand, these capacitors 1 are also characterized by the electrolyte resistance, Re, which represents a loss for the power transfer, estimated as the well-known ohmic loss. The electrolyte resistance, Re, depends on the geometrical dimensions of the capacitor 1 and it can be estimated with the known formula Re=peg / A, where peis the resistivity of the electrolyte 4, g the gap or distance between the electrodes 2, 3, and A the geometrical surface of the electrodes 2, 3 in contact with the electrolyte 4.The electrodes may be parallel and / or concentric to each other. A constant depending of the shape and edge effects might be added to correct such formula. Therefore, the loss due to the electrolyte resistance, Re, can be reduced by increasing the geometrical area of the electrodes 2, 3 or by reducing the gap between electrodes.

[0025] The electrical rotating machine of the invention comprises a stator, a DC power source, a DC-to-AC converter connected to the DC power source. People skilled in the art perfectly understand that the DC power source connected to the DC-to-AC converter form an AC current power source.

[0026] The electrical machine also comprises a power coupler according to the invention which comprises two rotating capacitors 1 , the fixed electrode 2 of each rotating capacitor 1 of the power coupler being fixed to the stator of the machine via an axis 6 and electrically connected to the AC current source via an electrical cable 8, the rotary electrode 3 of each rotating capacitor 1 being fixed to the rotor of the machine via an axis of rotation 7 and electrically connected to an AC-to-DC converter via an electrical cable 9. Finally, the AC-to-DC converter is connected to the electromagnetic rotor mounted on the axis of rotation 7.

[0027] In order to provide an efficient power transfer through the capacitive power coupler of the invention, the capacitance of each rotating capacitor 1 is around few to tens pF / cm2and the resistance Re - or real impedance Zreai - is below a few tenths of ohms. Indeed, the resistance should be at least 10% lower than the equivalent load resistance of the motor. For example, If the equivalent load resistance is 8 ohms, it is better that the electrolyte resistance is below 8x10% / 2 = 0.4 ohms (the factor 2 is because of the two capacitors). In addition, since capacitance tends to decrease when the frequency of the AC current is raising, said frequency is comprised between 10 kHz and 100 kHz. With electrolytes 4 respecting these conditions, a power transfer over 5kW can be expected.

[0028] In reference to figures 2 to 6, a choice of electrolytes 4 each composed of an organic solvent and an ionic salt, respecting the above conditions, will now be described.

[0029] In order to make a pre-selection of the electrolyte 4, a pre-selection of organic solvents has first been made. The first feature required for the solvent is to have the melting point and the boiling point lower than -30°C and higher than 150°C, respectively, in order to keep the liquid status when operating in such temperature interval. Table 1 below lists a table with solvents having such features.Table 1

[0030] Another requirement for the solvent is to dissolve a sufficient amount of some salt and to allow sufficient ion mobility to the ions of this salt, in order to decrease the electrolyte resistance Re. This feature cannot be predicted, and, hence, it has to be measured. However, three solvent properties can favor this requirement: the viscosity (the lower the better), the dielectric constant (the higher the better), the dipole moment (the higher the better).

[0031] The solvents of table 1 have, at the same time, a viscosity <3cP, a dielectric constant >35, and a dipole moment >3.5D. Finally, Table 1 indicates the chemical hazards of these solvents according to the European Reach convention. Chemical hazards should be considered in the case there is leakage of the electrolyte 4 from the capacitor 1. The solvents propylene carbonate (PC), gamma-valerolactone (GVL) and gamma-butyrolactone (GBL) have the best manageable hazards.

[0032] Table 2 below lists a summary of salts which can be dissolved, at least in some significant amount, in some organic solvents (but not necessarily on the solvents PC and GBL above mentioned). They are grouped depending on the cation, which are imidazolium-based type (1-R-3-R’ imidazolium or 1-R-2-R”-3-R’ imidazolium, R being a methyl, ethyl, propyl, isopropyl, butyl or tert-butyl group, R’ being a methyl, ethyl, propyl, isopropyl, butyl or tert-butyl group and R” being a methyl group), or Li. The list of table 2 remains not limitative.

[0033] Salts of small anions such as the known tetrafluoroborate, BF4, or hexafluorophosphate, PFe, can be dissolved in higher concentrations and have more mobility, but the anions react with humidity and decompose generating toxic hydrofluorhydric acid, HF. Other anions containing fluor, F, such as those with acronyms OTf (triflate or trifluoromethanesulfonate group), FSI (bis(fluorosulfonyl)imide), TFSI (bis(trifluoromethylsulfonyl)imide), or FTFSI ((fluorosulfonyl) (trifluoromethylsulfonyl)imide), are also anion candidates. Other anions with a wide electrochemical window and not containing F, such as methanesulfonate (or mesylate, acronym OMs), methylsulfate (MeOSO3), ethysulfate (EtOSO3), or perchlorate (CIO4), are also anion candidates."Table 2

[0034] Electrolytes 4 can be formulated by dissolving each salt in one of the three solvents above mentioned, propylene carbonate (PC), N,N-dimethylformamide (DMF), gamma-valerolactone (GVL) or gamma-butyrolactone (GBL). However, among this list, their electrolyte resistances and the capacitance have to be determined. In addition, both features are not predictable. Some salts of the imidazolium-based cation mentioned above are liquid at room temperature (22°C) and they are known as ionic liquids.

[0035] To make the selection, measurements of electrolyte resistance and capacitance have been conducted, with a commercial conductivity cell (CDC749 from Radiometer, made of two platinized platinum electrodes of a few mm2of area, separated by a few mm), which is immersed in a glass tube filled with several milliliters of the electrolyte 4. This system is considered close to the capacitor 1 of Figure 1 , although the electrolyte 4 is present not only between the electrodes but also behind and all around.

[0036] Electrolyte resistance Re and capacitance C was inferred by electrochemical impedance spectroscopy (EIS) using a Gamry Reference 600 impedance meter, using a sinusoidal voltage signal of 0.1 Vrms amplitude and a frequency, f, scan from 1 Hz to 1 MHz. The measurements are done at a constant temperature of 22°C, sinceboth electrolyte resistance and capacitance change with temperature. The exact value of the electrolyte resistance using the commercial conductivity cell can be extracted by calibration with a standard KCI solution and using either a conductivity meter (like, for example, the apparatus CDC 210 from Radiometer) or the Gamry Reference 600 impedance meter.

[0037] This allows to extract the cell constant of the commercial cell, which takes into account its geometry and edge effects. The values of electrolyte resistance obtained with the commercial conductivity cell can be applied to the device of Figure 1 with a good approximation. Concerning the values of capacitance obtained with the commercial conductivity cell, they can serve as a guide, but should be measured with the proper geometry of the device of Figure 1 and be tested for several areas of electrodes.

[0038] The measurements with the impedance meter directly give values of the impedances Z of the capacitor 1 as a function of the frequency, either as the impedance module Zmod and phase Ze, or as the real and imaginary parts of the impedance (Zreai and Zimag). The equivalent capacitance C of the commercial conductivity cell device is obtained assigning the Zimag to a capacitor 1 , that is, by doing C=1 / (2irf Zimag).

[0039] An example of values of the complex impedance Z is shown in Figures 2 and 3, in two representations. Figure 2 shows Z module and phase versus frequency of the AC voltage with a commercial conductivity cell in an electrolyte EMIM OMs -GBL (salt 1-ethyl-3-methylimidazolium methanesulfonate dissolved in gammabutyrolactone solvent) using a sinusoidal signal of 0.1 Vrms amplitude (Volts Root Mean Square). Figure 3 shows Z real and imaginary parts versus frequency of the AC voltage with a commercial conductivity cell in an electrolyte EMIM OMs - GBL using a sinusoidal signal of 0.1 Vrms amplitude.

[0040] As shown in Figure 2, the electrolytic capacitor 1 does not behave as an ideal capacitor, since the impedance phase is not constant at -90° and the real part of the impedance, which should have been equal to the electrolyte resistance, is not constant either. It is known from the electrochemistry field that electrolytic cells have no constant capacitance. It has been modelled as a circuit having in series a“Constant Phase Element” (CPE) and an equivalent serial resistance (ESR). The ESR is mainly due the electrolyte resistance. The CPE has a resistor and capacitor in series, where their values decrease with increasing frequency. The phase of a CPE element according to the model is constant. This model is an approximation, since the data of Figure 2 cannot be fitted to such model on the entire frequency range. Also, their fitting parameters cannot be predicted.

[0041] Figure 3 shows the same impedance data as Figure 2 but represented as real and imaginary parts of the impedance. It teaches us several things: First, the imaginary part (from which the equivalent capacitance can be extracted following the formula C=1 / (2rrf Zimag)) decreases with frequency, which means that the advantage of high capacitance of electrolyte capacitors 1 is lower at high frequencies. Second, above a certain frequency, the imaginary part Zimag is much smaller than the real part Zreai, as the latter is practically constant.

[0042] The electrolyte resistance Re is extracted from the value of the real part Zreai of the impedance at the highest frequencies, applying the cell constant of such conductivity cell using the calibration constant above mentioned.

[0043] The capacitance C as a function of frequency f is extracted from the values of the imaginary part Zimag of the impedance, using the above-mentioned formula. Figure 4 gives example of plots of C versus frequency for electrolyte capacitors (made with the conductivity cell) with two types of organic electrolyte 4, EMIM OMs - GBL and LiCIO4 - PC, and they are compared with the example of electrolyte capacitors with two types of aqueous electrolytes, Na2SC - H2O and LiCO4 - H2O. The capacitance is measured with a commercial conductivity cell with a sinusoidal voltage signal of 0.1 Vrms amplitude, at 22°C. Determining capacitance from Zimag facilitates comparison with capacitances of other capacitors, in particular air capacitors. This also makes possible to predict the value of capacitance C by changing geometry parameters such as electrode surface or gap.

[0044] Since too high electrolyte resistances or too low capacitances do not allow a good efficiency of power transfer, these features have been evaluated for different electrolyte 4 compositions. Since these features can change with the salt concentration, for each electrolyte type, we measured these features for differentsalt concentrations, expressed as salt mass percentage (%), that is, the mass of the salt divided by the total mass of salt plus solvent, multiplied by 100 to have a percentage.

[0045] An example of variation versus salt mass percentage for two organic electrolytes 4 is shown in Figures 5 and 6, respectively for the capacitance at a given frequency (chosen at 10kHz) and the electrolyte resistance. Capacitance and resistance are measured with a commercial conductivity cell with a sinusoidal voltage signal of 0.1 Vrms amplitude, at 22°C.

[0046] Figures 5 and 6 show that there is an optimum of salt mass percentage where the capacitance C is maximum and another where electrolyte resistance Re is minimum. This is the same for all measured electrolyte types. Moreover, the maximum of capacitance and the minimum of electrolyte resistance are smooth and allow an interval in which the variations of salt mass percentage from the optimum are small.

[0047] Besides, the intervals for the capacitance and for the electrolyte resistance around the optimum are quite close, which means that we can propose an interval and an approximate optimum value of salt mass percentage common to both features. The interval and the approximate optimum of the capacitance at a fixed frequency of 10kHz is very similar to those at other frequencies.

[0048] Table 3 below gives a table with the electrolyte resistivity, pe, obtained from the conductivity cell at 22°C, the calculated electrolyte resistance for a device like the one of Figure 1 with 31 ,5cm2electrode area and 1 mm gap using such electrolyte resistivity, and the capacitance C at 10kHz measured with the same conductivity cell, for several organic electrolytes 4 (salt - solvent) at the approximate optimum value of salt mass %. The interval of salt mass % around the optimum for which the values are close is also given between brackets.Table 3

[0049] According to the values of table 3, the electrolytes 4 having an electrolyte resistivity pebelow 150 Q.cm could be candidates. Capacitance measurement and estimated power transfer calculations were carried out with devices similar to those of Figure 1.Example

[0050] In reference to figures 7 and 8, an example will now be described.

[0051] In this example, an organic electrolyte 4 fills each capacitor gap of an “electrolyte” capacitive power coupler comprising two capacitors 1 each made of two stainless steel electrodes 2, 3 and separated by a Teflon spacer of 1mm thickness. Measurement of impedance of a single capacitor and estimation of power transfer onto a resistor load are made.

[0052] Two sizes of “electrolyte” capacitors 1 were measured, each comprising two rectangular stainless-steel plates 2, 3, of dimensions 35x35mm or 116x35mm. The pair of plates 2, 3 of each of these devices were separated by a Teflon spacer with U shape, of 1mm thickness and about 5mm of width. The gap of these devices 1 was filled with the organic electrolyte 4 coded “EMIM OMs - GBL”, with optimum of salt mass % at 36%, as described in table 3. Pressure was applied to the edges of the plates above the spacer to keep the liquid electrolyte 4 inside the gap of the devices 1 , avoiding electrolyte leakage.

[0053] The electrode area in contact with the liquid electrolyte 4 (also called active area) was about 7.5 cm2and about 31.5 cm2respectively for each of the two capacitors 1. Impedance spectroscopy was carried out using a Gamry Reference 600 impedance meter, using a sinusoidal voltage signal of 0.1 Vrms amplitude and a frequency, f, scan from 1 Hz to 1 MHz. The measurements were done at an ambient temperature of 22°C, and in static conditions, that is, without no speed of rotation. The plots of the real part Zreai of the impedance Z and the capacitance C calculated from the imaginary part Zimag of the impedance Z for both devices 1 are given in figures 7 and 8 respectively.

[0054] More precisely, figure 7 represents in dash line the real part Zreai 14 of the impedance Z for the device of 7.5 cm2active area, and in solid line the real part Zreai 15 of the impedance Z for the device of 31.5 cm2active area. Figure 8 represents in dash line the capacitance C 16 calculated from the imaginary part Zimag of the impedance Z for the device of 7.5 cm2active area, and in solid line the capacitance C 17 calculated from the imaginary part Zimag of the impedance Z for the device of 31.5 cm2active area.

[0055] The electrolyte resistance extracted from Zreai at high frequencies (above 10 kHz, where the impedance phase is close to zero degrees) were 1.530 and 0.410 for the two devices 1 of respectively 7.5 cm2and 31.5 cm2active area. These values are quite close to the calculated ones using the electrolyte resistivity pemeasured for this electrolyte 4, given in table 3, the active area, and the gap. The ratio of resistances (3.8) is quite close to the inverse ratio of active areas (~4.2).

[0056] The capacitances C were calculated from Zimag using the above-mentioned formula. The capacitance C decreases with increasing frequency, as other electrochemical devices. At the high frequencies (at some point close to 100 kHz), the capacitance increases with frequency, but this is due to an inductance component (likely due to the inductance of the plates), which makes the total Zimag to increase, and the impedance phase to change from negative (mainly capacitance) to positive (mainly inductance) angles. The values of capacitance at the frequency of 10 kHz were 19 pF and 52 pF for the devices of 7.5 cm2and 31.5 cm2active area respectively. The ratio of capacitances is a bit lower than the ratio of active areas (2.7 instead of ~4.2).

[0057] According to figure 9, the estimation of the electrical power transfer can be simplified by considering only the transfer of alternative current (AC) from a source 11 (i.e the alternative power source of the electrical machine) to the motor of the electrical machine, which is simulated as a load resistance 13, and the impedance 12 of two capacitors of the same size of the power coupler of the invention. For the impedance of the two similar capacitors, we take the experimental values of the real and imaginary parts Zreai, Zimag of the single capacitor 1 , multiplied by two since the power coupler of the invention consists of two similar capacitors 1 in series.

[0058] As the experimental values of impedance of the capacitor is taken in the estimation, this impedance will take into account the electrolyte resistance Re, the capacitance C, and the possible inductance from the plates 2, 3. The scheme 10 for this simple estimation of the power transfer from the source to the load is shown in Figure 9. The equations for calculation of the power are those used in the well-known domain of electric circuits. The power on electrical components under a sinusoidal electrical signal is averaged over a period. It is zero for the imaginary impedance parts Zimag and the product of the voltage by the current in the real impedance parts Zreai. This is what is called active power. These calculations also allow to estimate the voltage drop over the imaginary part of the capacitor components, to check that this voltage is under the limit of the electrochemical stability window of the system.

[0059] In general, the active power of a circuit component of impedance Z, is the product of the rms voltage across such component, multiplied by the rms currentmodule going through that component, and multiplied by the power factor, which is the ratio of the real part of the impedance over the module of the impedance of such component. In order to calculate the active power of the load resistance 13 of the schematic circuit 10 of Figure 9, one has to calculate the total impedance of the circuit, consisting of the power coupler 12 in series with the load resistance 13. The power coupler 12 impedance has real and imaginary parts (which can be named Zcp,reai and Zcp mag), and the load resistance 13 only a real part (named Ri_oad). The calculation of the real and imaginary parts of the total impedance (named Ztot eai, Ztot.imag) is done by addition of the respectively real and imaginary parts of 12 and 13. Then, this is converted into module (or amplitude) and phase of the total impedance of the circuit, with the well-known formulas of square root of the addition of the squares of the real and imaginary parts for the module, and arctangent of the imaginary part over the total module for the phase. This allows to get the total current amplitude or module (in rms) going through the circuit 10, as the voltage amplitude in rms divided by the module of the total impedance. The phase of the current would be the phase of the total impedance but with opposite sign. Once the current calculated, the voltage and the active power of each of the two components of the circuit 10, that is the load resistance 13 and the power coupler 12, can be calculated using the well-known formulas of the Ohm law (voltage = current x impedance) but using complex numbers or the module-phase notation.

[0060] For the device of active area of about 31.5 cm2these calculations give a high power transfer. For a source voltage signal of 250 Vrms, a frequency of the AC signal in the range of 20 kHz to 40 kHz and a load of 8 ohms, the power transferred to the load is higher than 6kW, with an AC-AC efficiency (AC power to the load divided by AC power of the source) larger than 90%, and a voltage drop across the capacitive part of each capacitor of less than 1 ,8V. The optimization of the source voltage, the load, the frequency, and the design of the capacitor (area and gap) allows to achieve values of power transfer and efficiency of similar order.

Claims

CLAIMS1. Power coupler for rotating capacitive power transfer, comprising two rotating capacitors 1 , each one comprising:- A fixed electrode 2 provided to be electrically connected to an AC power source;- A rotating electrode 3 facing the fixed electrode 2 and provided to be electrically connected to a rotor of an electrical machine;- An electrolyte 4 located in a gap between the two electrodes 2, 3;- Said electrolyte 4 comprising a solvent and a dissolved salt, characterized in that the solvent is organic.

2. The coupler according to claim 1 , characterized in that the electrolyte 4 remains in a liquid state between -30°C and 150°C.

3. The coupler according to claim 1 or claim 2, characterized in that the electrolyte resistivity peis at most 150 Q.cm4. The coupler according to any of claims 1 to 3, characterized in that the viscosity of the solvent is at most 3 mPa.s, in that the relative dielectric constant of the solvent is at least 35, and in that the dipole moment of the solvent is at least 3.5 Debye.

5. The coupler according to any of claims 1 to 4, characterized in that the solvent is a member of the following families: carbonate ester, lactone, amide or a mixture thereof.

6. The coupler according to the preceding claim, characterized in that the solvent is propylene carbonate, N,N-dimethylformamide, gamma-butyrolactone, gammavalerolactone or a mixture thereof.

7. The coupler according to any of claims 1 to 6, characterized in that the salt is a combination of an anion and a cation, wherein the cation is a member of the following cation families or a mixture thereof: 1 R-3R’-imidazolium, 1 R-2R”-3R’- imidazolium, lithium or a mixture thereof, wherein R is a methyl, ethyl, propyl, isopropyl, butyl or tert-butyl group, wherein R’ is a methyl, ethyl, propyl, isopropyl, butyl or tert-butyl group, wherein R” is a methyl group, and wherein the anion is a member of the following anion families or a mixture thereof: non-oxidizable inorganic anions, notably CIO4-, alkylsulfates R-O-SO3, wherein R is a methyl or ethyl group, methanesulfonates R3C-SO3, wherein R is H or the methyl group, fluorinated methanesulfonates, notably F3C-SO3, and fluorinated sulfonylimides (R-SO2)-N-(SO2-R’), wherein R and R’ are chosen among F or the trifluoromethyl group -CF3.

8. The coupler according to claim 6 or claim 7, characterized in that the salt is one of the following compounds: 1-ethyl-3-methylimidazolium methanesulfonate, 1- ethyl-3-methylimidazolium bis(fluorosulfonyl)imide, 1 -ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, 1 ,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide, 1 -butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, 1-ethyl-2,3-dimethylimidazolium trifluoromethanesulfonate, lithium perchlorate, lithium bis(trifluoromethylsulfonyl)imide or a mixture thereof.

9. The coupler according to any of claims 1 to 8, characterized in that the electrolyte 4 is maintained in the gap between the electrodes of each capacitor with at least a sealing ring 5.

10. An electrical machine comprising a stator, a DC power source, a DC-to-AC converter electrically connected to the power source, a power coupler according to any of claims 1 to 10, the fixed electrode 2 of each rotating capacitor 1 of the power coupler being fixed to the stator of the electrical machine and electrically connected to the DC-to-AC converter, and the rotating electrode 3 of each rotating capacitor 1 being electrically connected to an AC-to-DC converter which is electrically connected to an electromagnetic rotor of the machine which ismounted on a rotational axis 7, the rotating electrode 3 of each rotating capacitor 1 being mounted on the rotational axis 7.

Images