The present invention relates to magnetic gear arrangements, particularly magnetic gear arrangements having a variable gear ratio.

Gearboxes and gear arrangements are utilised in a wide range of situations in order to couple drive mechanisms. Traditionally, gearboxes have been formed from gear wheels having appropriate teeth numbers and sizes to provide a desired gear ratio. However, such gearboxes have a number of disadvantages. Firstly, they require the use of lubricating oils, which may act as contaminants or fire hazards and may prove ineffective in hot or cold environments, where the oil viscosity varies, or in a low pressure environment, where the oil may evaporate. Furthermore, gearboxes based on gear wheels may be noisy, making them unacceptable for low noise environments such as in hospitals, libraries and residential areas, or for clandestine military activities.

More recently, magnetic gearboxes have been provided which comprise respective gear rotors with interpoles between them. The rotors incorporate permanent magnets, and the interpoles act to modulate the magnetic flux transferred between the gear rotors. Such magnetic gearboxes enable a speed-changing mechanical device to be provided in which there is no mechanical contact between input and output shafts, thus avoiding many of the problems of noise and wear that arise in gearboxes having contacting moving parts.

Figure 1 shows a schematic cross-sectional view of a magnetic gear arrangement of the prior art. The magnetic gear arrangement 200 is an epicyclic gearbox and comprises an inner rotor 202 and an outer rotor 206. Permanent magnets providing respective pole pairs 204, 208 are fixed to the inner and outer rotors 202, 206, the opposing poles of each permanent magnet being respectively indicated by dark and light shading. The permanent magnets affixed to the inner rotor 202 have alternating polarity along the circumference of the rotor. Similarly, the permanent magnets affixed to the outer rotor 206 have alternating polarity along the circumference of that rotor. Typically, one rotor is mechanically coupled to a drive mechanism and the other rotor is mechanically coupled to a driven mechanism.

The inner and outer rotors 202, 206 have different numbers of pole pairs 204, 208. Typically, the number of pole pairs of the outer rotor 206 is greater than the number of pole pairs of the inner rotor 202.

Pole elements 210 are provided between the inner rotor

202 and the outer rotor 206 and form an array to provide a coupling element having a cylindrical shape.

Each pole element 210 forms one interpole for modulating the magnetic fields produced by the inner rotor 202 and the outer rotor 206, so as to couple the two fields and hence the motion of the rotors. The angular position of the interpoles is a factor in determining the gearing of the magnetic gearbox.

The motion of the rotors 202, 206 may be either co- rotational or counter-rotational, depending on the number of magnets affixed to each rotor and the angular position/number of interpoles.

WO 2007/135369 discusses a variety of magnetic gear arrangements .

It is desirable to provide gearboxes in which the number and/or spacing of interpoles, or the torque transmission capability of the coupling element is adj ustable . In general terms, the present invention provides a magnetic gear arrangement in which the number and/or torque carrying capacity of interpoles may be adjusted by varying the temperature of the elements that provide the interpoles.

In particular, a first aspect of the present invention provides a magnetic gear arrangement comprising: a first gear member for generating a first magnetic field,

a second gear member for generating a second magnetic field, and a coupling device which provides arrangements of interpoles between the first gear member and the second gear member, the interpoles coupling the first and second magnetic fields such that different arrangements of interpoles produce different gearings between the first and second gear members; wherein the coupling device comprises a plurality of superconducting pole elements which at least partly form the interpoles, each pole element having a critical temperature whereby the pole element is active in a respective interpole above its critical temperature and inactive below its critical temperature. By a "different gearing" is meant a different gear ratio between the first and second gear member and/or a different direction of relative movement between the first and second gear member. By a "different arrangement of interpoles" is meant a different number and/or distribution of interpoles in the coupling device, including the possibility that when the temperatures of the pole elements cross their respective critical temperatures, no interpoles are provided, such that the first and second gear members are decoupled.

Above the critical temperature, the pole elements are permeable to magnetic fields and are thus able to participate in coupling the magnetic flux of the first and second magnetic fields. Below the critical temperature, however, the Meissner effect causes the pole elements to repel surrounding magnetic fields. Thus simply varying the temperature of the pole elements can change the type and/or extent of coupling between the first and second gear members .

Advantageously, the coupling device can avoid using iron-based interpoles, and therefore avoid the consequent problems of magnetic saturation associated with ferromagnetic materials (typically of around 2T) . The coupling device may thus be compatible with higher flux density superconducting fields, which can lead to an improved torque carrying capacity in the arrangement.

The magnetic gear arrangement may have any one or, to the extent that they are compatible, any combination of the following optional features.

Typically, the first and the second gear members move relative to each during operation of the gear arrangement, and the coupling device is stationary. However, some arrangements may be configured so that the coupling device moves relative to one of the gear members during operation of the gear arrangement, and the other gear member is stationary. Alternatively both gear members and the coupling device could be free to move.

Preferably, the first gear member has superconducting magnets or coils for generating the first magnetic field and the second gear member has superconducting magnets or coils for generating the second magnetic field. Superconducting magnets or coils can produce higher flux densities than ferromagnets . In addition, gear members with superconducting magnets may be easier to manufacture than gear members with ferromagnets , since the superconducting magnets can be magnetised in situ, e.g. using flux pumping. However, the magnetic fields of the first and second gear members could be generated by ferromagnets or electrical windings.

Preferably, deactivating at least a portion of the pole elements provides a different arrangement of interpoles. In this way, a different gearingbetween the first and second gear members can be produced.

Deactivating at least a portion of the pole elements typically reduces the magnetic permeabilities of the respective interpoles to reduce the torque transmission capability of the coupling device. However, when pole elements are deactivated, the arrangement of interpoles, and thus the gearing, can remain unchanged. In this way, different torque limits can be placed on the same interpole arrangement.

Preferably a first portion of the pole elements has a first critical temperature, and another portion of the pole elements has a lower, second critical temperature. Providing a gear arrangement in which the coupling element comprises a plurality of superconducting pole elements having different critical temperatures can enable simpler temperature control strategies to provide different arrangements of interpoles.

For example, at temperatures above the first and second critical temperatures, all the pole elements of both portions can be permeable to magnetic flux. Therefore all the pole elements can be active and able to contribute to the coupling of the first and second magnetic fields. Thus, the coupling device provides a first, high- temperature interpole arrangement.

At temperatures between the temperatures of the first and second critical temperatures, the pole elements of the second portion are active as they have a lower critical temperature than the temperature of the coupling device. In contrast, the pole elements of the first portion are in a superconducting state, i.e. they are not able to contribute to the coupling of the first and second magnetic fields. Therefore, at temperatures between the first and second critical temperatures, the coupling device comprises some active pole elements and some inactive pole elements. Thus, the coupling device provides a second, intermediate- temperature interpole arrangement.

The change from the first to the second interpole arrangement, or vice versa, can be effected simply by lowering or raising the temperature of the entire coupling device .

The magnetic gear arrangement can be configured such that a change from a first, high-temperature interpole arrangement to a second, intermediate-temperature interpole arrangement results in a reversal of the direction of rotation of the second gear member relative to the first gear member. Thus, for example, the gear arrangement may function as a co-rotational gear arrangement at high temperatures (above the first and second critical temperatures) and as a counter-rotational gear arrangement at intermediate temperatures (between the first and second critical temperatures) . Alternatively, the gear arrangement may provide a counter-rotational gear at high temperatures and a co-rotational gear at intermediate temperatures. However, in other embodiments, both gears may be co-rotational, or both gears may be counter- rotational .

At temperatures below both the first and second critical temperatures, all the pole elements of the first and second portions are in a superconducting state and are therefore inactive. When no active pole elements are available to provide interpoles for coupling the first and second magnetic fields, the coupling device can no longer transfer power between the first and second gear members.

The magnetic gear arrangement may be configured such that at temperatures above the first and second critical temperatures, when all pole elements of the first and second portions in the coupling device are active, each pole element forms a single interpole. In this case, the number of interpoles will decrease as the temperature is reduced below the first critical temperature and certain pole elements become inactive. The gear arrangement will typically exhibit counter-rotating motion of the first and second gear members at high temperatures (i.e. above the first and second critical temperatures) and co-rotating motion of the first and second gear members at intermediate temperatures (i.e. between the first and second critical temperatures) .

Alternatively, the magnetic gear arrangement may be configured such that at least one interpole in an arrangement of interpoles may be formed by a group of neighbouring active pole elements. That is, the coupling device may include a plurality of neighbouring pole elements that are sufficiently close together such that at temperatures above the first and second critical temperatures, i.e. when all the pole elements of the first and second portions are active, the plurality of neighbouring pole elements provides a single interpole for the purposes of determining the gear ratio between the first and second gear members.

Preferably, the interpoles are formed entirely from superconducting pole elements. In this way, reducing the temperature of the coupling device below the lowest critical temperature of the pole elements prevents the coupling device from transmitting torque between the first and the second gear members. That is, the gear arrangement can have a clutch functionality.

The coupling device may further comprise joining portions which join together neighbouring pole elements, the joining portions being formed of a superconducting material having a higher critical temperature than the highest critical temperature of the pole elements. To encourage magnetic flux to follow a path through active pole elements, regions between pole elements which do not form interpoles should have low magnetic permeabilities. The joining portions, being formed of a superconducting material having a higher critical temperature than the highest critical temperature of the pole elements, can be maintained in a superconducting state during normal operation of the gear arrangement, and thus can repel magnetic flux away from the joining portions and into the interpoles .

Indeed, the pole elements may be encased in a superconducting material having a higher critical temperature than the highest critical temperature of the pole elements, the encasing superconducting material having openings therein to allow magnetic flux to reach the interpoles from the first and second gear members. By encasing the pole elements in this way, all magnetic flux passing through the coupling device can be channelled through the interpoles.

The magnetic gear arrangement may further comprise a separator element between the first gear member and the second gear member, the separator element being formed of a superconducting material having a higher critical temperature than the highest critical temperature of the pole elements, wherein the separator element prevents or discourages magnetic flux extending between the first and second gear members but bypassing the coupling device. The magnetic gear arrangement may further comprise a housing for the first and second gear members and the coupling device, the housing being formed of a superconducting material having a higher critical temperature than the highest critical temperature of the pole elements. Such an arrangement may be particularly advantageous if the gear members have superconducting magnets or coils. The housing can then contain the very strong magnetic field which may be produced by the magnets.

Preferably, the magnetic gear arrangement further comprises a temperature controller for controlling the temperature of at least some of the superconducting pole elements such that, under the control of the temperature controller, the temperatures of pole elements can cross their respective critical temperatures and thereby change from active to inactive or the reverse.

For example, the temperature controller and coupling device may be configured such that the temperature controller varies the temperatures of all the superconducting pole elements of the coupling device, all the pole elements having substantially the same temperature .

Alternatively, the temperature controller and coupling device may be configured such that the temperature controller varies the temperatures of only a selected portion of the pole elements of the coupling device, and a temperature differential can be maintained between pole elements of the coupling device.

The temperature controller may include heating and/or cooling elements for actively controlling the temperature of the whole or portions of the coupling device, e.g. to speed up temperature changes. The temperature controller may comprises heating elements, such as resistance heating elements, for heating a portion of the pole elements, while other pole elements are unheated. The coupling device can be configured such that a temperature differential can be maintained between the heated and unheated pole elements. Even if both portions of pole elements have the same critical temperature, unheated pole elements can thus be cooled below their critical temperatures to make them inactive, while heated pole elements can be held above their critical temperatures and made active. By applying suitable temperature control, the gear arrangement can thus provide interpole arrangements similar to those discussed above in relation to a coupling device having a first portion of pole elements with a first critical temperature, and another portion of the pole elements with a lower, second critical temperature. That is, the unheated pole elements can correspond to the first portion and the heated pole elements can correspond to the second portion.

The action of the temperature controller may itself be determined by the operating condition of the first and/or second gear member. For example, the magnetic gear arrangement may further comprise a sensor for detecting the operating condition of the first and/or second gear member and communicating the operating condition to the temperature controller. The sensor may detect the speed of the first and/or second gear members, and/or the torque transmitted by them. The data gathered by the sensor can then be used to activate the temperature controller when a predetermined operational condition of the gear arrangement is attained.

The first and second gear members typically have respective mechanical couplings. These couplings typically connect a drive mechanism and a driven mechanism.

The magnetic gear arrangement may be an inline gear arrangement, an epicyclic gear arrangement, or may have a different configuration. The gear members may be rotors or linear gear members.

A second aspect of the present invention provides a method of operating a magnetic gear arrangement, the method comprising:

providing a magnetic gear arrangement according to the first aspect; and controlling the temperature of at least some of the pole elements such that the temperatures of pole elements cross their respective critical temperatures, and thereby change from active to inactive or the reverse.

The magnetic gear arrangement of the method may have any one or, to the extent that they are compatible, any combination of the optional features of the magnetic gear arrangement of the first aspect.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which:

Figure 1 shows a schematic cross-sectional view of a magnetic gear arrangement of the prior art;

Figure 2 shows a schematic cross-sectional view of a magnetic gear arrangement according to an embodiment of the present invention;

Figure 3 shows schematically a coupling device for an epicyclic or in-line rotary gearbox in (a) a high temperature arrangement and (b) an intermediate temperature arrangement ;

Figure 4 shows a variant of the coupling device of Figure 3;

Figure 5 shows schematically a further coupling device for an epicyclic or in-line rotary gearbox in (a) a high temperature arrangement and (b) an intermediate temperature arrangement ; Figure 6 shows schematically a cylindrical coupling device for an epicyclic or in-line rotary gearbox;

Figure 7 shows schematically a linear gear arrangement; and

Figure 8 shows schematically a further linear gear arrangement .

Figure 2 shows a schematic cross-sectional view of a magnetic gear arrangement according to an embodiment of the present invention. The magnetic gear arrangement is in the form of an epicyclic gearbox 10 and comprises an inner rotor 12 (i.e. a first gear member) and an outer rotor 16 (i.e. a second gear member) . Superconducting permanent magnets fixed to the inner and outer rotors provide respective pole pairs 14, 18 , the opposing poles of each magnet being respectively indicated by dark and light shading. The magnets affixed to the inner rotor have alternating polarity along the circumference of the rotor. Similarly, the magnets affixed to the outer rotor have alternating polarity along the circumference of that rotor. Typically, one rotor is mechanically coupled to a drive mechanism and the other rotor is mechanically coupled to a driven mechanism.

The inner and outer rotors 12, 16 have different numbers of magnets. In the embodiment of Figure 2, the outer rotor is shown with 21 pole pairs, and inner rotor is shown with seven pole pairs.

Pole elements 20a, 20b formed of superconducting material are provided between the inner rotor 12 and the outer rotor 16, and form an array having a cylindrical shape. Active pole elements (i.e. pole elements whose temperature lies above their critical temperature) form interpoles that modulate the magnetic fields produced by the inner rotor 12 and the outer rotor 16, so as to couple the two fields and hence couple the motion of the rotors. The number and distribution of interpoles is a factor in controlling the gearing of the gearbox.

Effectively, the pole elements are part of a coupling device 20 that provides arrangements of interpoles for coupling the magnetic fields produced by the inner and outer rotors 12, 16. In the embodiment of Figure 2, each active pole element forms a single interpole of the coupling element.

A first set of the pole elements 20a has a higher critical temperature for superconductivity than a second set of the pole elements 20b. In the embodiment of Figure 2, there are 28 pole elements, half having the higher critical temperature and half having the lower critical temperature, in a circumferential arrangement with alternating critical temperature. At its critical temperature, the superconducting material of a pole element changes its behaviour from being permeable to magnetic fields and thus active in coupling the magnetic flux produced by the pole pairs 14, 18, to repelling magnetic fields due to the Meissner effect and thus inactive in coupling the magnetic flux. Because of the different critical temperatures, this change from active to inactive status occurs at different temperatures for the two sets of pole elements 20a, 20b.

When the gearbox 10 is operated at temperatures above the critical temperatures of both sets of pole elements 20a, 20b, all the pole elements are active and form respective interpoles for the modulation of the magnetic fields produced by the inner and outer rotors 12, 16. The number of interpoles is thus equal to the sum of the number of pole pairs 14, 18, and if symmetrically placed provide a counter-rotational gearbox with a gear ratio of 3:1 (see K. Atallah, S.D. Calverley and D. Howe, Design, analysis and realisation of a high-performance magnetic gear, IEE Proc- Electr. Power Appl . , Vol. 151, No. 2, March 2004 for details of how magnetic gear ratios are determined) .

When the gearbox 10 is operated at temperatures lower than the critical temperature of the pole elements 20a, but higher than the critical temperature of the pole elements 20b, only the pole elements 20b form interpoles for the modulation of the magnetic fields produced by the inner and outer rotors 12, 16. In the embodiment of Figure 2, therefore, the number of active pole elements is halved, and so is the number of interpoles. The number of interpoles is now equal to the difference of the number of the pole pairs 14, 18, and if symmetrically placed provide a co-rotational gearbox with a gear ratio of 3:1. Further, the torque carrying capability of the gearbox is reduced, in line with the reduction in the number of active pole elements .

When the gearbox 10 is operated at temperatures lower than the critical temperatures of both sets of pole elements 20a, 20b, all the pole elements are deactivated. That is, no interpoles are formed for modulating the magnetic fields produced by the inner and outer rotors 12, 16. The gear arrangement is therefore no longer able to couple the motion of the inner and outer rotors 12, 16, producing a clutch-like effect.

Figures 3 (a) and (b) show schematically a coupling device 30 for a further epicyclic or in-line gearbox having one rotor with magnets or coils (superconducting or otherwise) producing four pole pairs and another rotor with magnets or coils producing eight pole pairs. The coupling device is required to provide 12 evenly spaced interpoles for a counter-rotating gear with a 2:1 gear ratio and four evenly spaced interpoles for a co-rotational gear with a 2:1 gear ratio, but here the higher temperature interpole arrangement is the co-rotational one. Eight pole elements 30a have a higher critical temperature for superconductivity, and 12 pole elements 30b of have a lower critical temperature for superconductivity. The pole elements are all of the same width, and are positioned in four groupings with two pole elements 30a and three pole elements 30b in each grouping. The gap 32 between each grouping has the same width as the pole elements 30a that are to be deactivated. As shown in Figure 3 (a) , at temperatures above the higher critical temperature, the grouped pole elements form four equally- spaced interpoles, producing a co-rotational gear. As shown in Figure 3 (b) , at temperatures between the critical temperatures, the pole elements 30a become inactive, and the remaining pole elements 30b form 12 equally-spaced interpoles producing a counter-rotational gear. At a temperature below the critical temperatures of pole elements 30a, 30b the gearbox stops transferring power from one rotor to the other.

To avoid magnetic flux extending through the gaps 32 rather than through the active pole elements 30a, 30b, the gaps can contain joining portions formed of material that is relatively impermeable to magnetic fields. As shown in Figure 4, that material can be a further superconducting material 34 that has a higher critical temperature than highest critical temperature of the pole elements 30a, 30b, the gearbox normally operating at a temperature below the critical temperature of the joining portions so that magnetic field is repelled from the gaps 32 by the Meissner effect. Indeed, in Figure 4, the pole elements 30a, 30b are encased in the further superconducting material 34. The encasing material, which can have openings to allow the magnetic field to reach the pole elements 30a, 30b from the rotors, effectively guides magnetic flux through the interpoles. There are four openings above the plane of the drawing in Figure 4 and four openings below the plane, each opening delineating a window around a respective end of one of the groupings of pole elements 30a, 30b, the coupling device being in this case for an in-line, rather than an epicyclic, gearbox.

However, if the magnetic permeability of the coolant used to cool the coupling device is low relative to that of the pole elements 30a, 30b when above their critical temperatures, the joining portions or encasing material may not be needed, as the pole elements can be surrounded by the coolant.

A sheet of further superconducting material having a higher critical temperature than highest critical temperature of the pole elements 30a, 30b may be positioned between the in-line rotors to prevent magnetic flux from travelling directly from one rotor to the other (bypassing the interpoles) .

The positioning of the rotors may be chosen to ensure that the flux paths are relatively simple i.e. with few changes in direction. For example, slots in which the rotors can run may be formed in the further superconducting material 34 encasing the pole elements 30a, 30b. Indeed, if the cylinder of further superconducting material 34 in Figure 4 is extended beyond the rotors of the gearbox, the ends of the cylinder can be closed off around the rotors (with holes in the end walls to allow shafts to attach to the rotors) to form an enclosing housing for the gearbox. This can provide shielding (by the Meissner effect) to contain the high magnetic fields which may be formed in the gearbox, limiting the interference they may cause to other systems. This might be beneficial e.g. in submarine applications .

Figures 2 to 4 illustrate just two possible configurations for the pole elements of the coupling device. Many other configurations are possible for different rotor configurations. For example, providing three or more sets of pole elements, each set being formed from a respective superconducting material having a different critical temperature from the other sets, can increase the number of interpole arrangements possible, allowing harmonic gears to be achieved as well as counter and co-rotating gears.

Further, in Figures 2 and 4, the high and intermediate temperature interpole arrangements are rotationally symmetrical. However, the deactivation of pole elements reduces the capacity of the coupling device to transmit torque in the intermediate temperature interpole arrangement. That capacity can be increased if non- rotationally symmetric interpole arrangements are adopted. Figure 5(a) shows a coupling device 40 having 16 pole elements 40a with a higher critical temperature, and 56 pole elements 40b with a lower critical temperature. The device is for two rotors having respectively six and 14 magnetic pole pairs, and thus needs to provide 20 interpoles for a contra-rotating gear and eight interpoles for a co-rotating gear. Eight equally-spaced gaps 42, having the same width as the pole elements 40a, divide the pole elements into eight groupings, and hence provide eight equally-spaced interpoles in the high temperature arrangement. Figure 5(b) shows the intermediate temperature arrangement, in which the 16 pole elements 40a are inactive. There are now 20 equally-spaced interpoles, as required for a contra-rotating gear, but four of those interpoles (indicated by dotted lines) are incomplete, lacking one central pole element relative to the other 16 interpoles. Nonetheless, the four incomplete interpoles, although causing a departure from an ideally symmetric interpole arrangement, do not change the coupling of the magnetic fields sufficiently to prevent the coupling device producing the contra-rotating gear.

For torque limitation applications (e.g. in precision machinery devices, or where a variable torque is required) , the interpoles can be made up of two or more pole elements. At high temperatures, all the pole elements are active, giving the interpoles a large cross-sectional area, and hence a large pull out torque. To reduce the torque capability, the outer pole element (s) of each interpole are taken below their critical temperature, reducing the cross- sectional areas of the interpole, but maintaining the angular positions of the interpoles.

For mechanical strength, the coupling device for rotary gearbox applications preferably has the interpoles in the form of a hollow cylinder, as illustrated in Figure 6 which shows schematically a coupling device 50 with interpoles formed of two types of pole element 50a and 50b (each having a different critical temperature) and joining portions formed of inactive material filling the gaps 52 between interpoles.

As well as rotary gear arrangements, the present invention can also be applied to e.g. linear gear arrangements. Figure 7 shows schematically a linear gear arrangement 60 having a low speed member 62 and a high speed member 66. Magnetic poles 64, 68 of alternating polarity are provided by magnets or coils on the members. Between the low and high speed members is a coupling device 70 having interpoles formed of superconducting pole elements 70a. Figure 8 shows schematically a further linear gear arrangement 80 having a low speed tubular member 82 and a coaxial high speed central shaft member 86. Magnetic poles 84, 88 of alternating polarity are provided by magnets or coils on the members. Between the low and high speed tubular members is a coaxial tubular coupling device 90 having interpoles formed of superconducting pole elements 90a.

Advantageously, the superconducting pole elements of the coupling devices described above are compatible with a superconducting magnetic field. For example, the superconducting field could be between 10 and 17T - leading to very dense gearboxes.

While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention .