WO2010108577A2

WO2010108577A2 - A magnetic harmonic gearbox

Info

Publication number: WO2010108577A2
Application number: PCT/EP2010/001084
Authority: WO
Inventors: John Edward Anthony
Original assignee: Rolls-Royce Pcl
Priority date: 2009-03-25
Filing date: 2010-02-22
Publication date: 2010-09-30
Also published as: WO2010108577A3; GB0905018D0

Abstract

A magnetic harmonic gearbox (110) according to the present invention comprises a first ferromagnetic member (112) and the first member (112) has a plurality of spaced teeth (114). A second member (116) is arranged within the first member (112) and the second member (116) has a plurality of spaced discrete ferromagnetic poles (118). The first and second members (112, 116) are relatively rotatable. A third member (120) is arranged within the second member (116) and the third member (120) is rotatable. The number of teeth (114) on the first member (112) is different to the number of poles (118) on the second member (116). The third member (120) comprises at least one electromagnet (122) and means (124) to modulate the current supplied to the at least one electromagnet (122) to produce a rotating magnetic field. The magnetic harmonic gearbox (110) may be operated to function as a clutch and may be used to drive an electrical generator from a wind turbine, a water turbine or a tidal turbine.

Description

A MAGNETIC HARMONIC GEARBOX

The present invention relates to a magnetic gearbox, in particular to a magnetic harmonic gearbox. A magnetic harmonic gearbox is disclosed in our International patent application PCT/GB2008/003136 filed 16 September 2008. In this magnetic harmonic gearbox 10, as shown in figure 1, a first ferromagnetic member 12 has a plurality of spaced teeth 14. A second member 16 is arranged within the first member 12 and the second member 16 has a plurality of spaced discrete ferromagnetic poles 18. The first and second members 12 and 16 are relatively rotatable. A third member 20 is arranged within the second member 16, the third member 20 is rotatable and the third member 20 has at least one permanent magnet 22. The number of teeth 14 on the first member 12 is different to the number of poles 18 on the second member 16 or the spacing between the teeth 14 on the first member 12 is different to the spacing between the poles 18 on the second member 16. The key to the operation of the magnetic harmonic gearbox 10 is the difference in number between the teeth 14 on the first member 12 and the poles 18 on the second member 16. The magnetic harmonic gearbox 10 operates with either the first member 12 or the second member 16 serving as a stator and either the second member 16 or the first member 12 serving as a low speed rotor and the third member 20 serving as a high speed rotor. The high speed rotor or the low speed rotor of the magnetic harmonic gearbox 10 may act as an input drive and the low speed rotor or high speed rotor of the magnetic harmonic gearbox 10 may act as an output drive. The gear ratio is a function of the number of teeth 14 and the number of poles 18 only.

A problem with the magnetic harmonic gearbox described above is that the gear ratio is fixed and it is difficult to disengage, or reengage, the input drive from the output drive in a controlled manner. Accordingly the present invention seeks to provide a novel magnetic harmonic gearbox which reduces, preferably overcomes, the above mentioned problem.

Accordingly the present invention provides a magnetic harmonic gearbox comprising a first ferromagnetic member having a plurality of spaced teeth, a second member arranged within the first member, the second member having a plurality of spaced discrete ferromagnetic poles, the first and second members being relatively rotatable, a third member being arranged within the second member, the third member being rotatable, the number of teeth on the first member being different to the number of poles on the second member or the spacing between the teeth on the first member being different to the spacing between the poles on the second member, the third member comprising at least one electromagnet and means to modulate the current supplied to the at least one electromagnet to produce a rotating magnetic field.

Preferably the first member comprises a stator having a plurality of circumferentially spaced teeth, the second member comprises a rotor arranged coaxially within the first member, the second member having a plurality of circumferentially spaced poles, the poles of the second member being arranged coaxially with the teeth of the first member and the third member comprises a rotor arranged coaxially within the second member.

Alternatively the first member comprises a rotor having a plurality of circumferentially spaced teeth, the second member comprises a stator arranged coaxially within the first member, the second member having a plurality of circumferentially spaced poles, the poles of the second member being arranged coaxially with the teeth of the first member and the third member comprises a rotor arranged coaxially within the second member. Preferably the first member and the teeth of the first member comprise steel. Preferably the first member and the teeth of the first member are laminated.

Preferably the poles of the second member comprise steel . Preferably the poles of the second member are laminated.

Preferably the third member has a plurality of electromagnets .

Preferably the first member is cylindrical, the second member is cylindrical and the third member is cylindrical.

Preferably the magnetic harmonic gearbox is arranged to drive an electrical generator.

Preferably the third member of the magnetic harmonic gearbox is arranged to drive the electrical generator via a first shaft.

Preferably a wind turbine, a water turbine or a tidal turbine is arranged to drive a second shaft of the magnetic harmonic gearbox.

The second shaft may be connected to the first member or the second member of the harmonic magnetic gearbox.

Preferably the electrical generator is a multi-phase electrical generator. Preferably the electrical generator is a three phase electrical generator.

Preferably the electrical generator is a brushless electrical generator.

Preferably a matrix converter is mounted on the first shaft connecting the third member and the electrical generator. Alternatively other devices for converting one AC frequency to another AC frequency may be provided, for example a device to rectify an AC signal to an intermediate DC signal and a device to invert to an AC signal using pulse width modulation techniques .

The present invention also provides a novel method of operating a magnetic harmonic gearbox. Accordingly the present invention provides a method of operating a magnetic harmonic gearbox comprising a first ferromagnetic member having a plurality of spaced teeth, a second member arranged within the first member, the second member having a plurality of spaced discrete ferromagnetic poles, the first and second members being relatively rotatable, a third member being arranged within the second member, the third member being rotatable, the number of teeth on the first member being different to the number of poles on the second member or the spacing between the teeth on the first member being different to the spacing between the poles on the second member, the third member comprising at least one electromagnet and means to modulate the current the current supplied to the at least one electromagnet to produce a rotating magnetic field, the method comprising in a first mode of operation rotating the third member at a predetermined rotational speed in a first rotational direction and supplying electric current to the at least one electromagnet on the third member such that the rotating magnetic field rotates at the predetermined rotational speed in direction opposite to the first rotational direction such that substantially zero torque is transmitted between the third member and the second member or between the third member and the first member and in a second mode of operation gradually reducing the frequency of the supply of electric current supplied to the at least one electromagnet on the third member such that the speed of rotation of the rotating magnetic field reduces to zero such that the torque transmitted between the third member and the second member or between the third member and the first member gradually increases. Preferably in a third mode of operation gradually increasing the frequency of the supply of electric current supplied to the at least one electromagnet on the third member such that the speed of rotation of the rotating magnetic field increases from zero to the predetermined rotational speed in the direction opposite to the first rotational direction such that the torque transmitted between the third member and the second member or between the third member and the first member gradually decreases to zero.

Accordingly the present invention provides a method of operating a magnetic harmonic gearbox comprising a first ferromagnetic member having a plurality of spaced teeth, a second member arranged within the first member, the second member having a plurality of spaced discrete ferromagnetic poles, the first and second members being relatively rotatable, a third member being arranged within the second member, the third member being rotatable, the number of teeth on the first member being different to the number of poles on the second member or the spacing between the teeth on the first member being different to the spacing between the poles on the second member, the third member comprising at least one electromagnet and means to modulate the current supplied to the at least one electromagnet to produce a rotating magnetic field, the method comprising rotating the third member and varying the frequency of the supply of electric current supplied to the at least one electromagnet on the third member such that the speed of rotation of the rotating magnetic field varies such that the torque transmitted between the third member and the second member or between the third member and the first member varies .

The present invention also provides a method of operating a magnetic harmonic gearbox comprising a first ferromagnetic member having a plurality of spaced teeth, a second member arranged within the first member, the second member having a plurality of spaced discrete ferromagnetic poles, the first and second members being relatively rotatable, a third member being arranged within the second member, the third member being rotatable, the number of teeth on the first member being different to the number of poles on the second member or the spacing between the teeth on the first member being different to the spacing between the poles on the second member, the third member comprising at least one electromagnet and means to modulate the current supplied to the at least one electromagnet to produce a rotating magnetic field, the method comprising rotating the first or second member and varying the frequency of the supply of electric current supplied to the at least one electromagnet on the third member such that the speed of rotation of the rotating magnetic field varies such that the torque transmitted between the first member and the third member or between the second member and the third member varies .

The method may comprise varying the frequency of the supply of electrical current supplied to the at least one electromagnet on the third member such that the speed of rotation of the rotating magnetic field varies such that the speed of rotation of the third member remains substantially constant with variations in the speed of rotation of the first member or the second member.

The method may comprise providing a variable gear ratio and/or clutch between the first member and the third member or between the second member and the third member.

The present invention will be more fully described by way of example with reference to the accompanying drawings in which: - Figure 1 shows a cross-sectional view through a prior art magnetic harmonic gearbox.

Figure 2 shows a cross-sectional view through a magnetic harmonic gearbox according to the present invention. Figure 3 is a graph of power distribution against speed distribution for a magnetic harmonic gearbox according to the present invention.

Figure 4 shows a combination of a magnetic harmonic gearbox and electrical machine according to the present invention. A magnetic harmonic gearbox 110 according to the present invention, as shown in figure 2, comprises a first ferromagnetic member 112 and the first member 112 has a plurality of spaced teeth 114. A second member 116 is arranged within the first member 112 and the second member 116 has a plurality of spaced discrete ferromagnetic poles 118. The first and second members 112 and 116 are relatively rotatable. A third member 120 is arranged within the second member 116 and the third member 120 is rotatable. The number of teeth 114 on the first member 112 is different to the number of poles 118 on the second member 116 or the spacing between the teeth 114 on the first member 112 is different to the spacing between the poles 118 on the second member 116. The third member 120 comprises at least one electromagnet 122 and means 124 to modulate the current supplied to the at least one electromagnet 122 to produce a rotating magnetic field.

There are two embodiments of the present invention. In the first embodiment the first member 112 comprises a stator which has a plurality of circumferentially spaced teeth 114 and the second member 116 comprises a rotor arranged coaxially within the first member 112 and the second member 116 has a plurality of circumferentially spaced poles 118. The poles 118 of the second member 116 are arranged coaxially with the teeth 114 of the first member 112 and the third member 120 comprises a rotor arranged coaxially within the second member 116. In the second embodiment the first member 112 comprises a rotor which has a plurality of circumferentially spaced teeth 114 and the second member 116 comprises a stator arranged coaxially within the first member 112 and the second member 116 has a plurality of circumferentially spaced poles 118. The poles 118 of the second member 116 are arranged coaxially with the teeth 114 of the first member 112 and the third member 120 comprises a rotor arranged coaxially within the second member 116. The first member 112 and the teeth 114 of the first member 112 comprise steel and the first member 112 and the teeth 114 of the first member 112 are laminated. The poles 118 of the second member 116 comprise steel and the poles 118 of the second member 116 are laminated. The third member 120 has a plurality of electromagnets 122 arranged circumferentially around the third member 120. The first member 112, the second member 116 and the third member 120 are cylindrical. In the arrangement in figure 4 the magnetic harmonic gearbox 110 is arranged to drive an electrical generator 130. The third member 120 of the magnetic harmonic gearbox 110 is arranged to drive the electrical generator 130 via a first shaft 132. A wind turbine, a water turbine, a tidal turbine or other prime mover is arranged to drive a second shaft 134 of the magnetic harmonic gearbox 110. The second shaft 134 may be connected to the first member 112 or the second member 116 of the magnetic harmonic gearbox 110. The electrical generator 130 may be a three phase electrical generator. The electrical generator 130 may be a brushless electrical generator.

In addition a matrix converter 136 is mounted on the first shaft connecting 132 the third member 120 and the electrical generator 130. In the basic operation of the magnetic harmonic gearbox 110 the key is the difference between the number of teeth 114 on the first member 112 and the number of poles 118 on the second member 116. The difference between the number of teeth 114 on the first member 112 and the number of poles 118 on the second member 116 results in a difference between the angular spacing, or spacing, between the teeth 114 on the first member 112 and the angular spacing, or spacing, between the poles 118 on the second member 116. This difference produces a "vernier" effect, where a small angular movement, or relative rotation, between the first member 112 and the second member 116 result in a much larger angular movement, or rotation, of the minimum circuit reluctance, indicated by the position at which a pole 118 and tooth 114 are in perfect radial alignment in figure 2. A magnetised third member 120 will always tend to align itself with this orientation, or radial alignment of the tooth 114 and pole 118. With continuous rotation between the first member 112 and the second member 116, the effect is that the orientation of minimum reluctance rotates much faster, and hence the third member 120 rotates much faster and at the same speed as the orientation of minimum reluctance because it always aligns with the orientation of minimum reluctance. Thus this results in a gearing between the first member 112 and the third member 120, or between the second member 116 and the third member 120, depending on whether the second member 116 or the first member 112 respectively is a stator. The third member 120 is connected to the first shaft 132 and the first member 112 or the second member 116 is connected to the second shaft 134. The choice of the number of teeth 114 on the first member 112 and the number of poles 118 on the second member 116 has resulted in there being two circumferential positions at which a pole 118 on the second member 116 aligns with a tooth 114 on the first member 112 in figure 2. In figure 2 these two circumferential positions at which a tooth 114 on the first member 112 aligns with a pole 118 on the second member 116 are at diametrically opposite circumferential positions, ie 180² apart. However, the number of teeth 114 on the first member 112 and the number of poles 118 on the second member 116 may be chosen so that there is always an even number eg two, four, six, eight etc of circumferential positions at which a pole 118 on the second member 116 aligns with a tooth 114 on the first member 112. In the case of four circumferential positions at which a tooth 114 on the first member 112 aligns with a pole 118 on the second member 116, the four circumferential positions are 90 ^a apart. Increasing the number of circumferential positions at which a tooth 114 on the first member 112 aligns with a pole 118 on the second member 116 reduces the gear ratio but increases the torque transmission capability of the magnetic harmonic gearbox 110. The number of electromagnets 122, or number of poles of the electromagnet 122, must equal the number of circumferential positions at which a tooth 114 on the first member 112 aligns with a pole 118 on the second member 116. Aspects of the operation of a magnetic harmonic gearbox 110 according to the present invention will now be described.

Let ω_e be the speed of the rotating magnetic field relative to the third member 120. ω_e is determined by the means 124 to modulate the current supplied to the at least one electromagnet 122 to produce a rotating magnetic field. The means 124 may be an external solid-state power electronics, e.g. a VWF (Variable Voltage, Variable Frequency) motor drive unit. Electrical connections between the power electronics 124 and the coils of the electromagnets 122 are made via sliding contacts such as slip rings or brushless excitation techniques, possibly incorporating a three-phase to three-phase matrix converter, which are well known to those skilled in the art. Let ω_c be the rotational speed of the rotating third member 120.

From this, the apparent rotational speed of the rotating magnetic field when viewed from the stationary part of the magnetic harmonic gearbox 120 is given by ω_a = ω_{e +} ω_c. The operation is as follows :-

Suppose that the third member 120 is rotating at a speed ω_c, but that the external electrical excitation for the rotating magnetic field, the means 124, has set ω_e = - ω_c. Therefore, U)₃ = ω_{e +} ω_c = 0, and so the stationary part of the magnetic harmonic gearbox 110 simply sees a stationary magnetic field due to the rotating third member 120 and the rotating magnetic field rotating in opposite directions at the same speed. Therefore, the output drive, second shaft 134, whether the first member 112 or the second member 116 experiences zero torque, i.e. the input and output drives or the first and second shafts 132 and 134 respectively, are effectively disengaged from each other.

Now suppose that the means 124, the external electrical excitation for the rotating magnetic field, reduces the speed of the rotating magnetic field from - ω_c to 0. From the point of view of the stationary part of the magnetic harmonic gearbox 110, this sees this as the speed of the third member 120 increasing from 0 to ω_c, and therefore the output drive, second shaft 134, of the magnetic harmonic gearbox 110 gradually speeds up over the same time period.

This operation allows, for example, the smooth acceleration from stationary of a large load on the high torque output drive, as well as an ability to disengage and reengage the input and output drives, or first and second shafts 132 and 134 respectively, electrically.

Let the gearbox gearing ratio be C:l, where C= ω_high-speed/ωiow-s_Peed- This ratio is a constant, and is determined by the gearbox geometry. Note that C is slightly different depending on whether the first member 112 or the second member 116 is chosen for the output drive, second shaft 134, but this does not matter.

Rearranging: CUh_ow-_speed = ω_hi_gh-speed = ω_a = ω_c + ω_e, where coi_ow-spe_ed is the rotational speed of the output drive, either the first member 112 or the second member 116, and ω_a is the apparent rotational speed of the magnetic field when viewed from the stationary part of the magnetic harmonic gearbox 110. C may therefore be viewed as the "internal" gear ratio of the magnetic harmonic gearbox 110. The "external" gear ratio of the magnetic harmonic gearbox 110, i.e. the gear ratio between the third member 120 and the output drive, either the first member 112 or the second member 116, is given by ω_c/ωi_ov,-_spSed = ω_c/[(oj_c + ω_e)/C] = C ω_c/(ω_c + ω_e) . Therefore, the gearing ratio between the input and output drives, first and second shafts 132 and 134 respectively, may be varied by adjusting the value of ω_c/ (ω_c + ω_e) , which can be set by choice of ω_e, note that ω_e can be set both positive and negative. Note that for the third member 120 and its associated electromagnets 122, there is expected to be a linear relationship between the rotational speed (ω_c, ω_e) and high speed shaft mechanical power and electromagnets power. This is deduced from the principle of superposition. Figure 3 illustrates the power distribution as a function of speed distribution. This distribution of power is demonstrated by considering the cases where ω_e = 0 and ω_c = 0. In the first case where ω_e is zero the magnetic field is stationary relative to the third member, and therefore all the power passes from or to, depending on whether the third member 120 is driving or being driven by the magnetic harmonic gearbox 110, the mechanical high speed drive. In the opposite case, where ω_c is zero, all the power is flowing to or from the power electronics 124 via the electromagnets 122. Superposition suggests a linear relationship as shown in figure 3. This implies that operating the magnetic harmonic gearbox 110 away from its default gear ratio would mean significant power transfer either from or to the power electronics 124 that drive the electromagnets 122. This may not be a problem if the electrical power was generated using an electrical machine connected to the high speed drive, i.e. a brushless excitation arrangement, since this would effectively hide the magnetic harmonic gearboxes electrical operation and reduce it back to a pseudo-mechanical gearbox. Such an arrangement is shown in figure 4.

As mentioned before Figure 4 shows a magnetic harmonic gearbox 110 according to the present invention in combination with a brushless three phase electrical machine 130 and a three phase matrix converter 136. The brushless three phase electrical machine/electrical generator 130 is driven by the high speed drive from the magnetic harmonic gearbox 110 and the three phase matrix converter 136 is mounted on the shaft 132 of the high speed drive from the magnetic harmonic gearbox 110. Electrical power is transmitted along the shaft 132 of the high speed drive between the harmonic magnetic gearbox 110, the three phase matrix converter 136 and the three phase electrical machine 130. This arrangement reduces the magnetic harmonic gearbox 110 back to a pseudo-mechanical gearbox, where the electric power consumption/production is integrated into the mechanical power of the high .speed drive and is therefore irrelevant. This would then offer a truly variable gear ratio with all net power being transferred between the low speed drive and the high speed drive. The magnetic harmonic gearbox 110 shown in figure 4 operates as a variable ratio gearbox as long as the first shaft 132, and the third member 120 of the magnetic harmonic gearbox 110 are rotating such that the electrical generator 130 is capable of supplying electrical current to the electromagnets 122 on the third member 120. The electrical generator 130 cannot generate electrical current/electrical power when the first shaft 132 and third member 120 are stationary.

In a further magnetic harmonic gearbox, not shown, a wind turbine, a water turbine, a tidal turbine, or other prime mover is arranged to drive a second shaft of the magnetic harmonic gearbox. The second shaft may be connected to the first member or the second member of the magnetic harmonic gearbox. The second shaft is arranged to drive an electrical generator. The third member of the magnetic harmonic gearbox is arranged to drive a first shaft. However, in this arrangement the electrical generator is larger and heavier due to the lower rotational speed of the second shaft, but provides the advantage that the first shaft can be fully disengaged and reengaged by stopping and starting the supply of electrical current to the electromagnets on the third member of the magnetic harmonic gearbox. The magnetic harmonic gearbox according to the present invention has an advantage in that it enables operation as a clutch and provides the ability to disengage and/or reengage the input drive and output drive of the magnetic harmonic gearbox electrically and in a controlled manner. This is beneficial when accelerating a large torque load from stationary to running speed, decelerating a load to stationary or where minimum mechanical complexity or minimum moving parts are required for reliability and/or maintenance reasons . The magnetic harmonic gearbox according to the present invention may also be useful as a failsafe function, where loss of electrical power to the electromagnets on the third member causes the input drive and output drive of the magnetic harmonic gearbox to be disengaged. If a separate electrical power connection is used, then the variable gear ratio functionality is likely to only be of use where small deviations of gearing ratio are required so as to avoid drawing excessive power from, or injecting excessive power into, the power electronics. This may be of use in scenarios where the output drive is required to maintain a very steady rotational speed, but the input drive is subject to moderate speed fluctuations, e.g. a wind turbine. In this case, the power electronics could dynamically modulate ω_e in order to adjust the gear ratio to keep the rotational speed of the output drive constant. Although the present invention has been described with reference to a three phase electrical generator any- suitable multi-phase electrical generator may be used.

Although the present invention has mentioned a matrix converter any other suitable device for converting one AC frequency to another AC frequency may be provided. A suitable device comprises for example a device to rectify an AC signal to an intermediate DC signal and a device to invert to an AC signal using pulse width modulation techniques .

In the present invention the frequency of the electrical current supplied to the electromagnets is varied in order to vary the speed of rotation of the rotating magnetic field. A magnetic harmonic gearbox according to the present invention may be used in a variety of rotating equipment where a high gear ratio is required together with a clutch action. A magnetic harmonic gearbox according to the present invention is well suited for use with a wind turbine, a water turbine or a tidal turbine where a low speed turbine must be matched to a high speed electrical generator. The clutch action requires few moving parts, which is an advantage in offshore wind turbine installations or offshore tidal turbine installations, where maintenance is difficult.

In the present invention the ratio of power supplied mechanically to/by the third member, the high speed shaft, to the power supplied electrically by/to the electrical circuit driving the at least one electromagnet may be controlled by varying the excitation frequency of the at least one electromagnet. These two routes of power flow, e.g. mechanical power and electrical power, may have many applications, for example in hybrid vehicles, where mechanical power provided by an internal combustion engine or mechanical power source, is arranged to drive the third member, the high speed shaft, of the magnetic harmonic gearbox and a battery, a fuel cell or any other suitable electrical power source is arranged to drive the at least one electromagnet. The first member, or the second member, the slcv; speed shaft... of the magnetic harmonic gearbox is arranged to drive the wheels of the vehicle. By electrically controlling the frequency of excitation of the at least one electromagnet, the ratio of power supplied by the electrical power source to that supplied by the internal combustion engine may be controlled. This enables a fully variable and controllable transition from full mechanical power provided by the internal combustion engine to combined mechanical power and electrical power provided by the internal combustion engine and the electrical power source respectively to full electrical power provided by the electrical power source, or visa versa, all provided by the magnetic harmonic gearbox with no physical engaging and disengaging of shafts or gears etc. This would be useful for switching between mechanical and electrical power sources depending on the situation of the vehicle, for example by arranging for the electrical power source to drive the third member during urban driving when the vehicle is frequently accelerating and decelerating, when the battery or fuel cell provides the best efficiency, and arranging for the mechanical power source to drive the third member during constant speed driving, e.g. on a motorway, when the internal combustion engine provides the best efficiency.

Claims

Claims : -

1. A magnetic harmonic gearbox (110) comprising a first ferromagnetic member (112) having a plurality of spaced ceeth (114), a second member (Hfi) arranged within the first member (112), the second member (116) having a plurality of spaced discrete ferromagnetic poles (118) , the first and second members (112, 116) being relatively rotatable, a third member (120) being arranged within the second member (116) , the third member (120) being rotatable, the number of teeth (114) on the first member (112) being different to the number of poles (118) on the second member (116) or the spacing between the teeth (114) on the first member (112) being different to the spacing between the poles (118) on the second member (116), characterised in that the third member (120) comprising at least one electromagnet (122) and means (124) to modulate the current supplied to the at least one electromagnet (122) to produce a rotating magnetic field.

2. A magnetic harmonic gearbox as claimed in claim 1 wherein the first member (112) comprises a stator having a plurality of circumferentially spaced teeth (114) , the second member (116) comprises a rotor arranged coaxially within the first member (112), the second member (116) having a plurality of circumferentially spaced poles (118) , the poles (118) of the second member (116) being arranged coaxially with the teeth (114) of the first member (112) and the third member (120) comprises a rotor arranged coaxially within the second member (116) .

3. A magnetic harmonic gearbox as claimed in claim 1 wherein the first member (112) comprises a rotor having a plurality of circumferentially spaced teeth (114), the second member (116) comprises a stator arranged coaxially within the first member (112), the second member (116) having a plurality of circumferentially spaced poles (118) , the poles (118) of the second member (116) being arranged coaxially with the teeth (114) of the first member (112) and the third member (120) comprises a rotor arranged coaxially within the second member (116) .

4. A magnetic harmonic gearbox as claimed in claim 1, claim 2 or claim 3 wherein the first member (112) and the teeth (114) of the first member (112) comprise steel.

5. A magnetic harmonic gearbox as claimed in claim 1, claim 2, claim 3 or claim 4 wherein the first member (112) and the teeth (114) of the first member (116) are laminated.

6. A magnetic harmonic gearbox as claimed in any of claims 1 to 5 wherein the poles (118) of the second member (116) comprise steel.

7. A magnetic harmonic gearbox as claimed in any of claims 1 to 6 wherein the poles (118) of the second member (116) are laminated.

8. A magnetic harmonic gearbox as claimed in any of claims 1 to 7 wherein the third member (120) has a plurality of electromagnets (122) .

9. A magnetic harmonic gearbox as claimed in any of claims 1 to 8 wherein the first member (112) is cylindrical, the second member (116) is cylindrical and the third member (120) is cylindrical.

10. A magnetic harmonic gearbox as claimed in any of claims 1 to 9 wherein the magnetic harmonic gearbox (110) is arranged to drive an electrical generator (130) .

11. A magnetic harmonic gearbox as claimed in claim 10 wherein the third member (120) of the magnetic harmonic gearbox (110) is arranged to drive the electrical generator (130) via a first shaft (132).

12. A magnetic harmonic gearbox as claimed in claim 11 wherein a wind turbine, a water turbine or a tidal turbine is arranged to drive a second shaft (134) of the magnetic harmonic gearbox (110) .

13. A magnetic harmonic gearbox as claimed in claim 12 wherein the second shaft (134) is connected to the first member (112) or the second member (116) of the harmonic magnetic gearbox (110) .

14. A magnetic harmonic gearbox as claimed in any of claims 10 to 13 wherein the electrical generator (130) is a three phase electrical generator.

15. A magnetic harmonic gearbox as claimed in any of claims 10 to 14 wherein the electrical generator (130) is a brushless electrical generator.

16. A magnetic harmonic gearbox as claimed in any of claims 11 to 15 wherein a matrix converter (136) is mounted on the first shaft (132) connecting the third member (120) and the electrical generator (130) .

17. A method of operating a magnetic harmonic gearbox (110) comprising a first ferromagnetic member (112) having a plurality of spaced teeth (114) , a second member (116) arranged within the first member (112), the second member

(116) having a plurality of spaced discrete ferromagnetic poles (118), the first and second members (112, 116) being relatively rotatable, a third member (120) being arranged within the second member (116) , the third member (120) being rotatable, the number of teeth (114) on the first member (112) being different to the number of poles (118) on the second member (116) or the spacing between the teeth (114) on the first member (112) being different to the spacing between the poles (118) on the second member (116) , the third member (120) comprising at least one electromagnet (122) and means (124) to modulate the current supplied to the at least one electromagnet (122) to produce a rotating magnetic field, the method comprising in a first mode of operation rotating the third member (120) at a predetermined rotational speed in a first rotational direction and supplying electric current to the at least one electromagnet (122) on the third member (120) such that the rotating magnetic field rotates at the predetermined rotational speed in a direction opposite to the first rotational direction such that substantially zero torque is transmitted between the third member (120) and the second member (118) or between the third member (120) and the first member (112) and in a second mode of operation gradually reducing the frequency of the supply of electric current supplied to the at least one electromagnet (122) on the third member (120) such that the speed of rotation of the rotating magnetic field reduces to zero such that the torque transmitted between the third member (120) and the second member (116) or between the third member (120) and the first member (112) gradually increases.

18. A method as claimed in claim 17 wherein in a third mode of operation gradually increasing the frequency of the supply of electric current supplied to the at least one electromagnet (122) on the third member (120) such that the speed of rotation of the rotating magnetic field increases from zero to the first predetermined speed in the direction opposite to the first rotational direction such that the torque transmitted between the third member (120) and the second member (116) or between the third member (120) and the first member (112) gradually decreases to zero.

19. A method of operating a magnetic harmonic gearbox (110) comprising a first ferromagnetic member. (112) having a plurality of spaced teeth (114) , a second member (116) arranged within the first member (112), the second member (116) having a plurality of spaced discrete ferromagnetic poles (118), the first and second members (112, 116) being relatively rotatable, a third member (120) being arranged within the second member (116) , the third member (120) being rotatable, the number of teeth (114) on the first member (112) being different to the number of poles (118) on the second member (116) or the spacing between the teeth (114) on the first member (112) being different to the spacing between the poles (118) on the second member (116), the third member (120) comprising at least one electromagnet (122) and means (124) to modulate the current supplied to the at least one electromagnet (122) to produce a rotating magnetic field, the method comprising rotating the third member (120) and varying the frequency of the supply of electric current supplied to the at least one electromagnet (122) on the third member (120) such that the speed of rotation of the rotating magnetic field varies such that the torque transmitted between the third member

(120) and the second member (116) or between the third member (120) and the first member (112) varies.

20. A method of operating a magnetic harmonic gearbox (110) comprising a first ferromagnetic member (112) having a plurality of spaced teeth (114) , a second member (116) arranged within the first member (112), the second member

(116) having a plurality of spaced discrete ferromagnetic poles (118), the first and second members (112, 116) being relatively rotatable, a third member (120) being arranged within the second member (116) , the third member (120) being rotatable, the number of teeth (114) on the first member (112) being different to the number of poles (118) on the second member (116) or the spacing between the teeth (114) on the first member (112) being different to the spacing between the poles (118) on the second member (116) , the third member (120) comprising at least one electromagnet (122) and means (124) to modulate the current supplied to the at least one electromagnet (122) to produce a rotating magnetic field, the method comprising rotating the first member (112) or second member (116) and varying the frequency of the supply of electric current supplied to the at least one electromagnet (122) on the third member (120) such that the speed of rotation of the rotating magnetic field varies such that the torque transmitted between the first member (112) and the third member (120) or between the second member (116) and the third member (120) varies.

21. A method as claimed in claim 19 or claim 20 comprising varying the frequency of the supply of electrical current supplied to the at least one electromagnet (122) on the third member (120) such that the speed of rotation of the rotating magnetic field varies such that the speed of rotation of the third member (120) remains substantially constant with variations in the speed of rotation of the first member (112) or the second member (116) .

22. A method as claimed in claim 19 or claim 20 comprising providing a variable gear ratio and/or clutch between the first member (112) and the third member (120) or between the second member (116) and the third member (120) .