Field of the Invention

The present invention relates to a magnetic gearbox, and in particular to the selection and control of gear ratios for a magnetic gearbox.

Rare earth metal magnets such as NdFeB have come to prominence due to their ability to increase the performance of electromagnetic and magnetic devices because of their high magnetic strength. This has spurred research interest into magnetic gears, as they do not require any lubrication, are free from frictional heat loss in the absence of physical contact, have high torque density and inherently provide overload protection, as an increase in load torque beyond the design limit causes the gear to slip.

It is known to provide magnetic gearboxes that have fixed gear ratios, which are determined by the number and arrangement of magnets on the gear rotors. An example of such a fixed gear ratio magnetic gearbox, with two shafts, has been described in GB 2439111 A. It is difficult to alter the gear ratios of such gearboxes, as that requires an alteration in the number or arrangement of the magnets, and that makes such gearboxes unsuitable for many potential applications.

WO 2007/107691 describes a magnetic gearbox system in which electromagnetic coils are provided on pole pieces positioned between inner and outer rotors. By applying different currents to the coils the magnetic flux modulation between the inner and outer rotors is altered and the gear ratio is changed. However, the system of WO 2007/107691 is relatively complex making construction and maintenance difficult.

It is an aim of the present invention to provide an improved, or at least alternative, magnetic gearbox.

Summary of the invention

In a first, independent aspect of the invention there is provided a magnetic gearbox comprising:- a first rotor; a second rotor; and a rotatable pole piece member, wherein the first rotor is a magnetic rotor and the second rotor is a further magnetic rotor or a further pole piece member, the pole piece member is magnetically coupled to at least one of the first rotor and the second rotor, and:- at least one of the first rotor, the second rotor and the rotatable pole piece member is arranged to operate as a control rotor, and each of the other two of the first rotor, the second rotor and the rotatable pole piece member are for coupling to a first, input shaft and a second, output shaft respectively; the control rotor is arranged so that in operation variation in the rate of rotation of the control rotor alters a gear ratio between the first shaft and the second shaft; and the gearbox further comprises gear control means for controlling the rate of rotation of the control rotor thereby to control the gear ratio between the first shaft and the second shaft.

The first, input shaft may be a shaft for connection to a driving device, for example, a prime mover such as a vehicle engine. In operation, the second, output shaft may be driven by rotation of the first, input shaft.

By providing a control rotor arranged so that the variation of the rate of rotation of the control rotor alters a gear ratio between a first shaft and a second shaft, a magnetic gearbox having particularly accurate and efficient control over the gear ratio can be provided whilst also providing for torque slip capability (for example, allowing the rotors to slip on overload without damaging the gearbox) due to the magnetic rather than mechanical coupling between the rotors.

The control means may be configured to control the rate of rotation to have any of a continuous range of values, thereby to provide a continuously variable gear ratio. Alternatively, the control means may be configured to select one of a plurality of predetermined rates of rotation of the control rotor (for example, one of 2, 3, 4, 5, 6, 7, or 8 predetermined rates of rotation), thereby to select one of a predetermined plurality of gear ratios.

The control means may be configured to control the rate of rotation to obtain a desired gear ratio or to obtain a desired speed of rotation of the output shaft. The control means may be configured to vary the rate of rotation of the control rotor over time to maintain a desired speed of rotation of the output shaft. Alternatively the control means may be configured to vary the rate of rotation of the control rotor over time to maintain a desired gear ratio. The pole piece member may be arranged to spatially modulate magnetic flux between the first rotor and the second rotor. The pole piece member may be arranged to spatially modulate in a circumferential direction magnetic flux between the first rotor and the second rotor.

The first rotor, the second rotor and the rotatable pole piece member may be arranged so that in operation movement of one of the first rotor, the second rotor and the rotatable pole piece member causes a change in magnetic flux at at least one other of the first rotor, the second rotor and the rotatable pole piece member. The change in magnetic flux may cause movement of at least one of the first rotor, the second rotor and the rotatable pole piece member.

At least two of the first rotor, the second rotor and the pole piece member may comprise a mechanical coupling for coupling to the first shaft or the second shaft respectively. The control rotor may comprise a mechanical coupling or magnetic coupling for coupling to a further shaft.

The gearbox may further comprise a transmission for coupling the control rotor and the first shaft or the second shaft thereby to drive rotation of the control rotor by rotation of the first shaft or the second shaft, and the control means comprises means for varying a gear ratio, for examplebetween the control rotor and the first shaft or the second shaft. The transmission may be configured to provide a plurality of pre-determined, selectable different gear ratios (for example, one of 2, 3, 4, 5, 6, 7, or 8 selectable different gear ratios), for example between the control rotor and the first shaft or second shaft.

The gearbox may optionally further comprise or be adapted for connection to a manual gear selection means for selecting one of the plurality of different gear ratios. The manual gear selection means may comprise a gear lever or gear stick. Thus the gearbox may be particularly easy to install in vehicles of existing design, and/or to retrofit to existing vehicles, that use manual gear levers or gear sticks.

The transmission may comprise a mechanical gear linkage for linking the control rotor and one of the first shaft and the second shaft. That can provide a particularly robust and reliable way of linking the control rotor and the first shaft or the second shaft, and can also be readily configured for use with a mechanical gear selection means for selecting gears. The mechanical gear linkage may comprise a plurality of mechanical gears, each gear being for providing a respective, different ratio between the control rotor and the first or second shaft. Each of the gears may be selectively engageable and disengageable by the control means.

The manual gear selection means may be mechanically coupled to the mechanical gear linkage, for selecting one of the plurality of mechanical gears by movement of the manual gear selection means.

The mechanical gear linkage may comprise at least one pinion gear. Pinion gears can handle higher torques than some other mechanical gears and can also be more easily sealed and/or lubricated than some other mechanical gears.

The transmission may comprise a layshaft arrangement comprising a layshaft for driving rotation of the control rotor and a plurality of gears on the layshaft, and the control means may comprise a gear selector for engaging a selected one of the plurality of gears with the first shaft or second shaft. Thus, a particularly robust and reliable coupling between the control rotor and the first or second shaft may be provided.

The transmission may comprise a magnetic gear arrangement for coupling the control rotor and the first shaft or the second shaft.

The rotatable pole piece member may be arranged to operate as the control rotor.

The gear control means may be configured so that the rate of rotation of the control rotor varies in dependence on the rate of rotation of at least one of the first shaft and the second shaft. Thus, a magnetic gearbox in which precise control of the gear ratio linked directly to an input or output to the gearbox (for example the rate of operation of a drive motor or the speed of operation of an output device, for example a vehicle) may be obtained.

The first, input shaft and the control rotor may be arranged to rotate in opposite directions in operation. Alternatively, or additionally, the first rotor and the second rotor may be arranged to rotate in opposite directions. By providing for such contra- rotation, a particularly useful range of gear ratios can be provided. The gear ratio can be varied in a large, for example substantially infinitely variable, range to lower gear ratios.

Alternatively or additionally, the first rotor and the second rotor may be arranged to rotate in the same direction in operation. The control means may be configured to control the rate of rotation of the control rotor so that one of the first rotor and the second rotor is stationary or so that one of the first rotor and the second rotor switches from one direction of rotation to the other direction of rotation. Thus, a reverse gear may be provided.

The gear control means may be coupled to at least one of the first and second shafts, thereby to receive energy to drive rotation of the control rotor. By providing for rotation of the control rotor using energy obtained from at least one of the first and second shafts, a compact and efficient gearbox arrangement may be obtained, that may be particularly suitable for retrofitting to existing vehicles or other machines.

The magnetic gearbox may be for installation in a vehicle, one of the first shaft and the second shaft may be for coupling to a vehicle motor or engine, and the other of the first shaft and the second shaft may be for coupling to a drive shaft for driving rotation of the vehicle wheels.

The magnetic gearbox may be particularly suitable for installation in a vehicle, for example an electric vehicle, and may be retrofitted to existing vehicles.

The gearbox may be for installation in a vehicle, and the control means may be configured to control the rate of rotation of the control rotor in dependence on the speed of the vehicle.

The control means may be configured to control the rate of rotation in dependence on the speed of the vehicle offset by an actual or requested acceleration of the vehicle.

The control means may be configured to provide a plurality of selectable operational modes, each operational mode providing a different variation of gear ratio with vehicle speed and/or acceleration. The control means may comprise drive means having a control input and arranged to rotate the control rotor at a rate dependent on a signal applied to the control input, and the gearbox may further comprise means for providing a control signal that is dependent on the rate of rotation of one of the first shaft and the second shaft, and to apply the control signal to the control input. The drive means may comprise a motor, for example an electric motor.

The control means may comprise a gear control motor for driving rotation of the control rotor, and the control input may be a control input to control the rate of operation of the gear control motor.

The means for providing the control signal may comprise an electromagnetic generator coupled to one of the first shaft and the second shaft.

The magnetic gearbox may be for installation in a vehicle and the means for providing a control signal may be configured to provide a control signal that is dependent on the speed of the vehicle.

The means for providing a control signal may be configured to provide a control signal that is dependent on the speed of the vehicle offset by an amount dependent on an acceleration. The acceleration may be an actual acceleration of the vehicle or a requested acceleration.

The means for providing a control signal may be further arranged to apply an offset signal to the control input, such that the rate of rotation of the control rotor is dependent on a sum of or difference between the control signal and the offset signal.

The offset signal and the control signal may be of opposite polarities, and the offset signal may have a polarity such as to drive the control rotor in one direction and the control signal may have a polarity such as to drive the control rotor in the opposite direction.

The control means may be arranged so that the control rotor remains stationary unless the control signal exceeds the offset signal. The offset signal may be dependent on the acceleration. The magnetic gearbox may further comprise means for preventing rotation of the control rotor in one direction.

The second rotor may be a further magnetic rotor. Each of the first rotor and the second rotor may comprise a plurality of magnets, and the number of magnets included in the first rotor may be different from the number of magnets included in the second rotor.

The magnets may be electromagnets or may be permanent magnets, for example NdFeB magnets, SmCo magnets, or alnico magnets. By providing that all of the magnets are permanent magnets, demagnetisation of permanent magnets by high currents in electromagnet windings, which may occur when a mixture of permanent magnets and electromagnets is used, may be avoided.

For each of the first rotor and the second rotor, the magnets may be arranged in pole pairs, and the number of pole pairs of the first rotor may be different from the number of pole pairs of the second rotor.

Each magnet in a pole pair may be substantially oppositely aligned to the other magnet in the pole pair. For each of the first rotor and the second rotor the plurality of magnets may be evenly spaced apart from each other. Each pole pair may comprise magnets that are in diametrically opposite positions.

The pole piece member may comprise a plurality of pole pieces, and the number of pole pieces included in the pole piece member may be equal to the sum of the number of pole pairs included in the first rotor and the number of pole pairs included in the second rotor.

Each pole piece may comprise ferromagnetic material. The pole pieces may be spaced apart, and each pole piece may have a magnetic reluctance that is different to the magnetic reluctance at the spacing between the pole pieces. The pole pieces may be spaced apart in a circumferential direction.

Each of the first rotor, the second rotor and the pole piece member may be arranged to rotate around a common axis of rotation. The first rotor, the second rotor and the pole piece member may be arranged substantially concentrically. The pole piece member may be disposed between the first rotor and the second rotor.

The magnetic gearbox may be installed, for example, in a vehicle, a marine turbine or a wind turbine. The magnetic gearbox may include substantially no lubricating oil and/or transmission oil. Hence, for certain applications such as wind turbines or use in hazardous areas (for example, mining or offshore oil/gas operations), the fire/explosion risk associated with using conventional mechanical gears may be removed. The removal of transmission oil can also benefit marine renewable applications by removal of oil spill/environmental contamination concerns in subsea or offshore applications, for example of marine turbines.

In a further independent aspect of the invention there is provided a method of controlling a gear ratio between a first shaft and a second shaft, wherein one of a first rotor, a second rotor and a rotatable pole piece member magnetically coupled to at least one of the first rotor and the second rotor is arranged to operate as a control rotor, and each of the other two of the first rotor, the second rotor and the rotatable pole piece member is coupled to the first shaft or the second shaft respectively, and the method comprises rotating the control rotor, and controlling the rate of rotation of the control rotor thereby to control the gear ratio between the first shaft and the second shaft.

The method may comprise operating the rotatable pole piece member as the control rotor.

The method may comprise controlling the rate of rotation of the control rotor in dependence on the rate of rotation of at least one of the first shaft and the second shaft.

The method may comprise obtaining energy from at least one of the first and second shafts to drive rotation of the control rotor.

The first shaft may be for coupling to a vehicle motor or engine. The second shaft may be for coupling to a drive shaft for driving rotation of the vehicle wheels. The method may comprise controlling the rate of rotation of the control rotor in dependence on the speed of the vehicle. The method may comprise controlling the rate of rotation of the control rotor in dependence on the speed of the vehicle offset by an actual or requested acceleration of the vehicle.

The method may comprise providing a plurality of selectable operational modes, each operational mode providing a different variation of gear ratio with vehicle speed and/or acceleration.

The method may comprise coupling the control rotor and the first shaft or the second shaft thereby to drive rotation of the control rotor by rotation of the first shaft or the second shaft. The method may further comprise varying a gear ratio between the control rotor and the first shaft or the second shaft.

In another independent aspect of the invention there is provided a gearbox controller for controlling operating of a magnetic gearbox, wherein:- the magnetic gearbox comprises a first rotor, a second rotor, and a rotatable pole piece member that is magnetically coupled to at least one of the first rotor and the second rotor; at least one of the first rotor, the second rotor and the rotatable pole piece member is arranged to operate as a control rotor, and each of the other two of the first rotor, the second rotor and the rotatable pole piece member are for coupling to a first, input shaft and a second, output shaft respectively; the control rotor is arranged so that in operation variation in the rate of rotation of the control rotor alters a gear ratio between the first shaft and the second shaft; and the gearbox controller comprises means for controlling the rate of rotation of the control rotor, thereby to control the gear ratio between the first shaft and the second shaft.

In another independent aspect of the invention there is provided a method of producing a magnetic gearbox comprising arranging a first rotor, a second rotor, and a rotatable pole piece member so that the rotatable pole piece member is magnetically coupled to at least one of the first rotor and the second rotor; arranging the first rotor, the second rotor, and the rotatable pole piece member so that at least one of the first rotor, the second rotor and the rotatable pole piece member is arranged to operate as a control rotor, and each of the other two of the first rotor, the second rotor and the rotatable pole piece member are arranged for coupling to a first, input shaft and a second, output shaft respectively, such that in operation variation in the rate of rotation of the control rotor alters a gear ratio between the first shaft and the second shaft; and providing gear control means for controlling the rate of rotation of the control rotor, thereby to control the gear ratio between the first shaft and the second shaft.

In another independent aspect of the invention there is provide a method of retrofitting a gearbox to a vehicle, comprising removing a gearbox from the vehicle and installing a magnetic gearbox as claimed or described herein in the vehicle.

In another independent aspect of the invention there is provided a computer program product comprising computer executable instructions for performing a method as claimed or described herein.

Any feature in one aspect of the invention may be applied to other aspects of the invention, in any appropriate combination. In particular, apparatus features may be applied to method features and vice versa.

Brief Description of the Drawings

Embodiment of the inventions will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a schematic cross-sectional diagram of a drive system for an electric vehicle that includes a magnetic gearbox;

Figure 2 is a schematic diagram of components of the magnetic gearbox of Figure 1 ;

Figure 3a is a schematic diagram of a mechanically controlled magnetic gearbox system;

Figure 3b is a schematic diagram of a further mechanically controlled magnetic gearbox system in a first, disengaged state;

Figure 3c is a schematic diagram of the mechanically controlled magnetic gearbox system in a second, engaged state;

Figure 4 is a schematic diagram of an automatically controlled magnetic gearbox system; Figure 5 is a graph of inner, outer and pole piece rotor speeds for contra-rotating inner and pole-pieces rotors; Figure 6 is a graph of inner, outer and pole piece rotor speeds for inner and pole pieces rotors rotating in the same direction;

Figures 7, 8, and 9 are graphs of the torque experienced by a rotor as a function of angular displacement, for each of the inner, outer, and pole piece rotors, whilst the other two rotors were held stationary;

Figures 10 to 13 are graphs of corresponding inner, rotor and pole piece rotor speeds measured on a test apparatus; and

Figures 14 and 15 are magnetic field plots for different magnetic gearbox arrangements.

Detailed Description of Embodiments

Embodiments of the invention will now be described, by way of example only.

A drive system for an electric vehicle that includes a magnetic gearbox 2 is shown in cross section in Figure 1. The magnetic gearbox comprises an outer rotor 4, a pole piece member in the form of a pole piece rotor 6 including a plurality of pole pieces 7, and an inner rotor 8.

An input, drive shaft 10 is connected to the inner rotor 8 of the magnetic gearbox 2. The input drive shaft 10 is connected to and driven by an electric motor 12 used to power an electric vehicle. An output shaft 14 is connected to the inner rotor 8 of the magnetic gearbox 2. The output shaft 14 is connected in turn to the drive mechanism (not shown) of the vehicle. Each of the inner rotor 8 and the outer rotor 4 includes a plurality of magnets 16, 17.

Components of the magnetic gearbox 2 are shown in more detail in Figure 2. The magnetic gearbox 2 has three rotating parts:- the outer rotor 4, the pole pieces rotor 6 and the inner rotor 8. The outer rotor 4, the pole pieces rotor 6 and the inner rotor 8 are each annular in shape and are arranged concentrically around a common axis of rotation.

The outer rotor 4 and the inner rotor 8 are each magnetically polarised. The outer rotor 4 includes 22 sets of NDFeB magnets, each set making up a pole pair, and the inner rotor includes 8 sets of NdFeB magnets. The magnets are arranged, for each of the outer rotor 4 and the inner rotor 8, so that the magnetic polarisation of each successive magnet in a circumferential direction is in an opposite direction to the preceding magnet. The magnets of the outer rotor 4 are attached to the inner surface of that rotor, whereas the magnets of the inner rotor 8 are attached to the outer surface of that rotor.

Each set of magnets comprises a pole pair consisting of two magnets, of opposite polarisations. The magnets in each pole pair are positioned at diametrically opposite positions on the rotor.

The pole pieces rotor comprises 26 individual pole pieces 18. The pole pieces are evenly spaced apart in a circumferential direction around the pole pieces rotor 6. The pole pieces of the described embodiment are made of steel and are ferromagnetic, and are separated by non-ferromagnetic spacers 19. Each pole piece has a magnetic reluctance that is different to the magnetic reluctance of the spacers 19 between the pole pieces. The pole pieces modulate the magnetic flux density in the air gap between the outer rotor 4 and the inner rotor 8. In particular, the pole pieces spatially modulate in a circumferential direction the magnetic flux between the outer rotor 4 and the inner rotor 8. In one mode of operation, the outer rotor 4 and the pole pieces rotor 6 function as low speed rotors, whereas the inner rotor 8 functions as a high speed rotor.

Rotation of the drive rotor, in this case the inner rotor 8, causes rotation of the outer rotor 4 due to the magnetic coupling between the rotors. The gear ratio between the outer rotor 4 and the inner rotor 8, representing the number of rotations of the inner rotor 8 for a single rotation of the outer rotor 4, is determined by the number of pole pairs on each rotor, and the modulation of magnetic flux provided by the pole pieces on the pole pieces rotor. It has been found that the modulation of magnetic flux provided by the pole pieces rotor can be varied by rotating the pole pieces rotor at varying rates, which in turn varies the gear ratio between the inner rotor 8 and the outer rotor 4. Thus, by selecting the appropriate rate of rotation of the pole piece rotor 6, a desired gear ratio can be obtained. The embodiment of Figure 1 is a three rotating shaft design, which includes only permanent magnets and thus does not allow the outer rotor magnets to be de-magnetised by the effects of high currents in machine windings, which might occur if a mixture of electromagnets and permanent magnets were used. Table 1 provides some examples of rates of rotation of the outer rotor 4 provided by rotating the pole piece rotor 6 at different rates of rotation. Three specific examples of gear ratios between a primary input shaft (coupled to the inner rotor 8) and a main output shaft (for example, coupled to the wheels of a vehicle and to the outer rotor 4), obtained by means of controlling the speed and direction of rotation of the pole pieces rotor 6 are shown. In the examples, the inner rotor 8 is assumed to be driven at a constant speed (200rpm) in a clockwise sense, and the magnetic gear has 4 pole pairs on the inner rotor, 22 pole pairs on the outer rotor and 26 pole pieces.

Table 1

In the first example of Table 1 , the pole pieces rotor 6 is stationary i.e. fixed, and the outer rotor 4 rotates counter clockwise at a low speed (36.4rpm) and an effective gear ratio of 200:36.4 = 5.5:1 between the inner and outer shafts is provided.

In the second example of Table 1 , the pole pieces rotor 6 is driven counter-clockwise at a speed of 11.5rpm. The inner rotor 8 remains driven at 200rpm in a clockwise sense. The outer rotor 4 now rotates at a new speed of 50rpm in the counter clockwise sense resulting in an effective ratio between inner and outer shafts of 200:50 = 4:1.

In the third example of Table 1 , the pole pieces rotor 6 is driven counter clockwise at 53.9rpm, resulting in the outer rotor 4 being driven at IOOrpm in a counter clockwise sense (with the inner rotor remaining at 200rpm in the clockwise sense). This results in an effective gear ratio between the inner and outer rotor shafts of 200:100 = 2:1.

From Table 1 it can be seen that by suitable control of the rate of rotation of the pole pieces rotor 6 the effective gear ratio between the inner and outer shafts was varied from 5.5:1, to 4:1 , to 2:1. In other examples, any desired gear ratio can be provided by suitable selection of the rate of rotation of the pole pieces rotor 6. Three embodiments are now described in more detail in which different arrangements are used to control the magnetic gearbox by controlling the rate of rotation of the pole pieces rotor 6 in order to obtain desired gear ratios. In both embodiments, the rate of rotation of the pole pieces rotor (and hence the gear ratio) is dependent on the rate of rotation of one or other of the inner rotor 8 or outer rotor 4.

In the first and second embodiments a mechanical gear linkage is provided to effect gear ratio changes. In the third embodiment an automatic variable ratio control scheme is provided that employs a controlled motor driving the pole pieces rotor 6. The pole pieces rotor 6 can also be referred to as a control rotor.

The first embodiment is illustrated in Figure 3. The pole pieces rotor is coupled to a gear drive 20 that is connected to a layshaft 22. The layshaft includes two layshaft gears 24, 26. More than two layshaft gears can be provided in variants of the embodiment. Moveable gear collars 28, 30 are provided that in operation can be moved by a manual gear lever 32 and gear selector arrangement 34, comprising gear selector forks 35, to couple with the layshaft gears 24, 26 or with a fixed structure 36, for example a fixed metal structure.

In operation, the position of the gear lever 32 is changed to engage the appropriate gear selector fork and move the required gear collar to assume drive from the inner rotor 8 to the layshaft 22 at the requisite speed of rotation to provide a desired gear ratio. In the following examples relating to the first embodiment the inner rotor 8 is rotated at a constant rate of 200rpm by the motor 12. If the rate of rotation of the inner rotor 8 varies then the gear ratios in the examples also vary.

For example, to obtain first gear, the left hand most gear collar 28 is moved left to engage with the fixed metal structure 36 causing the layshaft 22 (and hence the pole piece rotor 6) to be fixed i.e. stationary. This gives a first gear ratio of 5.5:1 as in Table t

To obtain second gear the left hand most gear collar 28 is disengaged, and the right most gear collar 30 is moved to the right to engage the right hand most layshaft gear 24. That results in the pole piece rotor 6 rotating at slow speed counter clockwise, to provide an effective gear ratio of 4:1. Finally, engaging third gear causes the right most gear collar 30 to disengage from the right hand most layshaft gear 24 and to engage the left hand layshaft gear 26, causing the pole piece rotor to rotate at a faster speed counter clockwise and hence providing the gear ratio of 2:1.

Although manually controlled, the drive for both the inner and pole piece rotors 8, 6 come from the same input shaft 10 driven by the traction motor 12 of Figure 1. The manual control method is simple to implement, efficient, and retains the natural torque slip capability of the main magnetic gear (i.e. will slip on overload hence can not damage the layshaft gears or other components), and can be easily integrated or retro-fitted to existing electric vehicles. It also enables the motor 12 to be operated at constant speed, and to drive operation of the magnetic gearbox whilst providing for variable gear ratio selection.

The manual control of the gear ratio in the first embodiment is achieved by controlling the pole pieces shaft manually, as an effective layshaft as in a conventional mechanical gearbox. However, although there is manual control, the variation in effective gear ratio is due to variation in magnetic coupling in the contra-rotating three shaft magnetic gearbox.

In variants of the embodiment of Figure 3a, magnetic layshaft gears are used in place of the mechanical layshaft gears as shown.

The second embodiment is illustrated in Figures 3b and 3c. The pole pieces rotor is again coupled to a layshaft 22. However, in this embodiment at least one set of pinion gears 70a, 70b is mounted on the layshaft 22. Only one set of pinion gears 70a, 70b is shown in Figures 3b and 3c for clarity.

A drive gear 72 is provided that is mounted to, and permanently driven by, the input rotor 10. The drive gear 72 is engaged to the pinion gears 70a, 70b.

A selector pin 74 is also provided that is mounted on the shaft 22, and coupled to the manual gear lever 32 via the gear selector forks 35. The selector pin 74 can be selectively engaged with or disengaged from the pinion gears by operation of the manual gear lever 32. In practice a separate selector pin 74 would be provided for each pinion gear 70a, 70b of each set but only a single selector pin is shown in Figures 3b and 3c for clarity. Furthermore, more than one set of pinion gears would usually be provided, each set of pinion gears providing a different gear ratio, and each set of pinion gears would have separate selector pins for selective engagement or disengagement by operation of the manual gear lever 32.

Figure 3b shows the situation in which the selector pin 74 is disengaged from the pinion gears. The pinion gears 70, 70b are thus free, and will rotate around their own axis driven by the input shaft 10 via the drive gear 72. The pinion gears will not transfer torque to the pole piece rotor 6 when the selector pin 74 is disengaged.

Figure 3c shows the situation in which the selector pins 74 have been engaged with the pinion gears 70a, 70b by manual operation of the gear lever 32 by a user, thus locking those pinion gears 70a, 70b. Once the pinion gears are locked, by use of the selector pins, they cannot rotate about their own axis but instead cause the pole piece rotor 6 to rotate, driven by the input rotor 10 at a rate determined by the rate of rotation of the input shaft 6 and the drive gear to pinion gear ratio for the pinion gear that has been engaged. The rotation of the pole piece rotor 6 causes a transfer of torque from the input shaft 10 to the output shaft 14. Different gear ratios for the torque transfer from the input shaft 10 to the output shaft 14 can be achieved by selectively engaging different selector pins with different pinion gears (not shown) by operation of the manual gear lever 32.

The embodiments of Figures 3a to 3c in particular are well suited for retro-fitting to existing vehicles (either petrol or electric powered). This is due to the fact that the gearboxes of Figure 3 require only manual driver input to effect the gear change and can be packaged to allow a direct swap with an existing manual mechanical gearbox. In the case of a petrol engine the magnetic gearbox may include a clutch to disengage the drive to the driven shaft, or alternatively the pole pieces rotor shaft can be driven at the same speed as the inner rotor and in the same sense, with a clutch being omitted.

The third embodiment is illustrated in Figure 4. An electric, gear control motor is provided that comprises a stator 40 and a rotor 42. The rotor 42 is coupled to the pole pieces rotor 6 and is used to drive rotation of the pole pieces rotor 6 at a desired speed.

The stator includes a control input 44. The electric motor is configured so that in operation, the rate of rotation of the rotor 42 of the gear control motor (and hence the rate of rotation of the pole pieces rotor 6) is dependent on the polarity and magnitude (or frequency if the gear control motor is an induction motor type and speed control is via frequency change) of the voltage applied to the control input 44.

A permanent magnet (PM) generator 46 is provided on the output rotor shaft 14, which drives the vehicle wheels, and an output from the generator 46 is connected to the control input 44. The generator 46 provides an output signal 50 that is linearly proportional to the vehicle speed.

An offset voltage signal 52 is also supplied to the control input 44 of the stator of the gear control motor from the electric vehicle traction motor, and is of opposite polarity to the output voltage from the generator 46.

The gear control motor is designed such that the offset signal 52 tends to drive the gear control motor in the same sense as the inner rotor shaft 10. The motor is physically prevented form turning in that direction by mechanical means (for example directional teeth on its shaft). If the offset signal 52 is greater than the output signal

50 from the generator 46 then the control motor rotor 42 (and hence the pole pieces rotor 6) is stationary, and provides a fixed gear ratio between the inner and outer shafts 10, 14 (corresponding to the first example of Table 1 ).

The system is configured so that above a certain road speed the output signal 50 from the generator 46 exceeds the offset signal 52 and the signal at the control input 44 (equal to the sum of the output signal 50 from the generator 46 and the offset signal 52) is of a sense to begin to drive the gear control motor in a counter clockwise direction. As the vehicle road speed increases, the signal at the control input 44 continues to increase and hence to drive the gear control motor faster in the counter clockwise sense. Thus, the rate of rotation of the pole pieces rotor 6 continuously increases, and the gear ratio continuously decreases as the vehicle speed increases above a certain level.

In a variant of the third embodiment, a signal 54 taken from the vehicle accelerator pedal is also provided to the control input 44 and added to the offset signal 52. If the accelerator pedal is pushed hard, implying strong acceleration demand, the resulting offset signal at the control input 44 is increased, meaning that the road speed necessary for the output signal 50 from the generator 46 to exceed the offset signal and drive a gear ratio change becomes higher, keeping the gearbox in a lower ratio for rapid acceleration.

The embodiment of Figure 4 enables different control modes to be provided, for example a sports mode as described in the previous paragraph, or an economy mode designed to ensure maximum electric vehicle drive efficiency. In the embodiment of Figure 4, the outputs 50, 52, 54 from the generator 46, the accelerator pedal 54 and the offset output 52 are connected directly to the control input 44, and the control of the gear ratio is determined by the configuration and properties of the generator 46, the accelerator pedal arrangement and the offset output arrangement. In a variant of the embodiment a controller, for example a control processor, is provided that takes inputs proportional to vehicle speed and/or requested acceleration and optionally subtracts an output from those inputs to provide the control signal, which is then provided to the control input 44. The controller is able to selectively apply or not any of the input signals to the control input, select a desired gain for any of the input signals, and to select a desired level for the offset, and to vary those gains or offset, in order to provide a desired gear ratio properties and variation of gear ratios with speed and/or acceleration. In one example, the controller is operable to select between a plurality of modes (for example sports and economy modes) by selecting different, predetermined gains and offsets.

In the embodiments described in relation to Figures 1 to 4, the principal drive is from the inner rotor 8 to the outer rotor 4. For the described design, that can provide the usual desired spread of gear ratios for automotive applications of around 3.5:1 to 0.8:1 in a straightforward manner. However, in alternative embodiments, the principal drive could act from the outer rotor 4 to the inner rotor 8, with the rate of rotation of the pole pieces rotor 6 again determining the gear ratio between the inner rotor 8 and the outer rotor 4. In other alternative embodiments, one of the inner rotor 8 and the outer rotor 4 operates as the control rotor to control the gear ratio between the pole pieces rotor 6 and the other of the inner rotor 8 and the outer rotor 4, which are each coupled to the input or output shaft respectively.

The gear ratios, the variation of those gear ratios with the speed of rotation of the pole pieces rotor, and operational torque limits are dependent on the size, spacing and construction of the inner, outer and pole pieces rotors, the number of pole pairs on each of the inner and outer rotors, the number of pole pieces, the strength of the magnetic field provided by the pole pairs, and the magnetic permeability of the pole pieces and the pole piece rotor. Each of those parameters can be selected to obtain a gearbox with desired characteristics, and the parameters are not limited to those described in relation to Figures 1 to 4.

The embodiment of Figure 4 may be retro-fitted to existing electric powered vehicles in particular, with suitable reconfiguration or reprogramming of the vehicle's existing drive system.

In the embodiments of Figures 1 to 4, the magnetic gearbox is installed in an electric vehicle. Each of the embodiments may also be installed in a petrol or diesel engine vehicle. The embodiment of Figure 3 is particularly suitable for installation in such petrol or diesel engine vehicles, as it does not require an electric power supply to drive gear ratio changes. In alternative embodiments, the magnetic gearbox is installed, or is for installation, in a variety of other machine or systems, for example a tidal generator, wind turbine or other renewable energy power system.

In the modes of operation described above in relation to Figures 1 to 4, the inner rotor 8 and the outer rotor 4 are rotated in opposite directions. Thus, the gear ratio can be varied from a fixed gear ratio defined by the ratio of the number of magnetic pole pairs of the outer rotor to pole pairs on the inner magnetic rotor in a potentially infinite range to lower gear ratios. For example for the outer rotor with 22 pole pairs, the pole pieces rotor with 26 pole pieces, and the inner rotor with 4 pole pairs, a fixed gear ratio of 5.5:1 with the pole pieces rotor held stationary is obtained. As the pole pieces rotor is rotated from stationary at an increasing speed in the opposite direction to the inner rotor, the effective gear ratio between the inner rotor and the outer rotor becomes less than 5.5:1 , and can reach 1 :1 or less than 1 :1. This is illustrated in Figure 5, which shows the inner rotor being run at constant speed, the pole pieces rotor being rotated from zero speed to ~120rpm in a contra-rotating sense (shown by the different sign used for the speed) from the inner rotor, and the resulting change in the outer rotor speed. Figure 5 illustrates a change in the effective gear ratio between the inner rotor and outer rotor across the range of 5:1 (500:100, at a speed of Orpm for the pole pieces rotor) to 2:1 (500:250, at a speed of ~125rpm for the pole pieces rotor).

In practice the pole pieces rotor does not have to contra-rotate with respect to the inner rotor, it can be controlled to rotate in the same direction as the inner rotor. In this case the effective gear ratio between the inner rotor and the outer rotor becomes greater than 5.5:1 , and indeed a speed can be found where the outer rotor speed becomes zero and then actually reverses. This is illustrated in Figure 6, which represents measurements in which the inner rotor is again run at constant speed, but now the pole pieces rotor is rotated in the same direction as the inner rotor (evidenced by the speeds having the same sign). This causes the outer rotor speed to decrease, for example from an effective inner rotor to outer rotor gear ratio of 5:1 (with the pole pieces rotor rotating at Orpm) to 10:1 (with the pole pieces rotor rotating at ~25rpm). Figure 6 also illustrates that the outer rotor can be brought to rest if the correct combination of pole pieces rotor speed and direction to inner rotor speed and direction is achieved i.e. there is zero torque transferred to the outer rotor hence it does not rotate and comes to rest. At pole pieces rotor speeds above this speed, the outer rotor changes direction of rotation and all three shafts rotate in the same direction.

By rotating the pole pieces rotor and the inner rotor in the same direction an effective reverse gear action can be obtained. In practice it may be more straightforward to implement reverse gear on a vehicle by controlling the main traction motor to reverse its direction of rotation. However, in some circumstances it may be chosen to obtain reverse gear by rotating the pole pieces rotor and the inner rotor in the same direction at the appropriate speeds.

In certain alternative embodiments, the magnetic gearbox comprises two pole pieces rotors, each comprising steel pole pieces, and a magnetic rotor disposed between the pole pieces rotors. Any one of the magnetic rotor and the two pole pieces rotor can be operated as the control rotor, but usually the magnetic rotor is operated as the control rotor in such alternative embodiments. Those alternative embodiments can provide high torque capability.

The embodiments described above in relation to Figures 3 and 4 are directed to the use of the magnetic gearbox in a vehicle. However the magnetic gearbox is not limited to use in a vehicle, and may be used in any suitable field or application. For example, the magnetic gearbox can be used for applications in the renewable energy field, for example for use with a contra-rotating marine current turbine or a wind turbine. In the case of a contra-rotating marine current turbine, the magnetic gear allows both the outer rotor and the pole pieces rotor to be driven in opposite directions by two low speed input shafts from the marine turbine, and via the gearing effect produce a single, high speed output shaft. The high speed output shaft can be coupled to a standard off-the-shelf 2 or 4 pole generator, rather than the more expensive multi- pole generators that must be used for a direct drive marine turbine that does not include a gearbox.

For wind turbine applications, where the primary input shaft is variable speed (given the stochastic nature of the winds), the use of the magnetic gearbox allows speed regulation of the output shaft at a constant speed, by control of the speed of rotation of the control rotor (for example, the pole pieces rotor). The speed of rotation of the control rotor can be constantly adjusted in dependence on the speed of the input shaft to maintain a constant speed of rotation of the output shaft. The output shaft can thus be coupled to a conventional synchronous generator, that can provide effective control of generator output voltage, power factor, synchronisation and fault level contribution.

Some aspects of operation of the magnetic gearbox, and some experimental measurements are now described.

Experimental measurements, and numerical simulations, were performed for a magnetic gearbox having 22 and 4 sets of NdFeB magnets (22 and 4 pole pairs) on the outer and inner rotors respectively, and a pole piece rotor having 26 pole pieces.

As the outer or inner rotors rotate, the steel (ferromagnetic) pole pieces modulate the air gap flux density between the rotors, creating a space harmonic flux density in an air gap between the rotors. The velocities of the space harmonics are different from the pole pairs of the driving rotor (either outer or inner rotor). The numbers of outer and inner pole pairs and ferromagnetic pole pieces are selected to generate specific space harmonics for the given rotational speed of output rotor. It has been found that the numbers of pole-pairs of the outer and inner rotor should be equal to the numbers of steel pole pieces to most effectively transmit torque at different speeds. The combination of steel pole pieces, inner and outer pole pairs give the effective gear ratio between the corresponding two rotors. This is mathematically expressed as:

where CD _O R, ωι _R and copp are the respective speeds of the outer rotor, inner rotor and pole piece rotor (negative values representing anti-clockwise rotation and positive values representing clockwise rotation, or vice versa), and P| _R , and P _PP are the number of magnetic pole pairs on the inner rotor and the number of pole pieces respectively, with the provision that:

where P _0R is the number of magnetic pole pairs on the outer rotor.

When the pole pieces are held stationary i.e. ω _PP =0, the gear ratio (G ₁ -) between the outer and inner rotors is given by:

When the inner rotor is held stationary i.e. COI _R =O, then the gear ratio between the ferromagnetic pole pieces and outer rotor becomes:

ω

G.. = OR PP _ 26

= -1.18 (4) ω PP ¹ P IR - ^λ P PP 4 - 26

Equations 3 and 4 can be combined to allow equation (1) to be defined when both the inner and pole piece rotors are rotating in opposite directions, for a magnetic gearbox having combination of 22 pairs of outer rotor poles and 4 pairs of inner rotor poles with 26 steel pole pieces, as:

ω _0R = -—∞ _IR - l Λ 8ω _PP (5)

A magneto-static analysis of the magnetic gearbox (taking it as having a 120 mm diameter and 15 mm stack length) has been performed. To predict the torque transmission capability all three rotors (the inner, outer and pole piece rotors) were rotated through 90° in increments of 3°, 0.54°, and 0.46° angular displacements individually, with the other rotors held stationary. The torque curves following a 90° rotation of each of the three rotors were found to be 1 complete cycle for the inner rotor, 5.5 complete cycles for the outer rotor and 6.5 complete cycles for the pole pieces rotor correspondingly as shown in Figures 7, 8, and 9, which are graphs of the torque experienced by the rotor as a function of angular displacement for each of the inner, outer, and pole piece rotors. The curve cycle ratio between the rotors gives the corresponding gear ratio.

The maximum torque produced on the three rotors was found to be 1.92 Nm, 10.5 Nm and 12.5 Nm. The rms torque transmission capability through all three rotors is therefore 1.35 Nm, 7.42 Nm and 8.83 Nm.

The stationary stable position (i.e. rest) of the inner rotor is 22.5° from the z axis (defined as being orthogonal to the axis of rotation of the rotors) when the outer rotor magnet edge and the pole piece edges were aligned with the z-axis. At this position the torque experienced on all the three rotors is zero, as the maximum numbers of outer rotor poles attempt to stay aligned with the inner rotor poles to reduce the reluctance path through the steel pole pieces.

When the outer and inner rotor magnet edges are aligned with the z-axis, the pole pieces move 3.46° in a clockwise direction to achieve the same previous stable position, where the torque on all the rotors becomes zero. Thus with every 22.5° rotation of inner rotor in a clockwise direction, the pole piece rotates 3.46° in a clockwise direction, which gives a gear ratio of 1 :6.5. In the same way when the pole pieces are maintained stationary, then with every 22.5° clockwise rotation of the inner rotor, the outer rotor rotates 4.09° in an anticlockwise direction to achieve the previous stable zero torque position. This gives a gear ratio between the outer and inner rotor of 1 :5.5.

When the outer and inner magnet edges and pole piece edges lie along the z-axis, maximum torque is experienced on all three rotors. This is due to the increase of air gap energy. However the total flux linkage between the outer rotor and inner rotors is assumed constant but is concentrated across the gap; therefore the tractive force on the rotor is at a maximum. A practical test rig was developed, in which a torque transducer was connected to the outer rotor shaft via a spur gear. The remaining shafts (inner rotor shaft and pole pieces shaft) were fitted directly to respective torque transducers. Pull-out torque tests were carried out on both input shafts. From the high speed to low speed side: the low speed (pole piece and outer rotor) shafts were locked in turn and by driving the high-speed shaft (inner rotor) a maximum torque exerted on the respective free low speed shaft was measured, as 10.17 Nm on the pole piece shaft and 8.47 Nm on the outer rotor shaft, until slip occurred. These pull out torques were found to be 20 % less than the simulation predictions, due to deviation of magnetic material, air gap, and pole piece characteristics, and accuracy of measurement. Another reason for the discrepancy between experiment and simulation is that the geometry of pole pieces was manufactured as square segments rather than arc segments, which tends to create a non-uniform air gap, unlike in the simulation model.

The following four sets of experiments were carried out

1. The pole piece rotor was kept stationary and the outer rotor was rotated to test the speed ratio. The same test was done from pole piece rotor to inner rotor with a stationary output rotor;

2. The pole piece rotor was rotated at 40 rpm and the outer rotor was rotated with a variable speed to examine speed variation;

3. Both input (outer and pole piece) rotors were rotated at different speeds to validate variable output rotor speed; and

4. A constant input rotor speed was achieved for different speed combinations of outer and pole piece rotors.

The practical gear ratio tests show good correlation between the simulation and practical results. The results of the gear ratio test with stationary pole piece are shown in Figure 10, which is a plot of inner rotor speed and corresponding outer rotor speed for nine different measured speeds. It is noticed that for a stationary pole pieces rotor, the output (inner rotor) speed varied from 145 to 567 rpm with an input (outer rotor) speed variation of 25 to 105 rpm, maintaining a constant speed ratio of 1 :5.5.

A further experiment was carried out in which the outer rotor speed was varied from 35rpm to 125rpm and the pole pieces rotor speed was kept constant at 40 rpm. The results are plotted in Figure 11 , in which the inner rotor speed is scaled by a quarter. The results of a further experiment in which both the input outer and pole pieces rotors were allowed to rotate at different speeds with an initial gear ratio from the outer to inner rotors of 1 :23, are shown in Figure 12 (again the inner rotor speed is scaled by a quarter). With decreased pole piece rotor speed, the gear ratio is decreased because of the change in relative speed between the outer and pole piece rotors, and finally reaches 1 :9.5.

From Figure 12 it can be concluded that, a constant output can be achieved by the speed regulation of any single input shaft or both input shafts controlled together.

In a further experiment, both input shafts (outer and pole piece) were regulated and a constant output speed of inner rotor shaft (560 rpm) was achieved as shown in Figure 13 (in which the inner rotor speed is again scaled by a quarter).

It has been demonstrated that with a stationary pole piece rotor, a constant gear ratio can be achieved. For a given outer rotor speed, when the pole piece rotor speed is increased, the relative speeds of the outer rotor and the inner rotor increase resulting in increased gear ratio and output speed. Consequently, a constant magnetic gear output speed can be achieved by the speed regulation of any single input shaft or both input shafts controlled together.

In embodiments described herein, the sum of the number of magnetic pole pairs of the inner and outer rotors is equal to the number of ferromagnetic pole pieces of the pole pieces rotor, as this has been found to be a particularly suitable arrangement, as is now described with reference to Figures 14 and 15.

Figure 14 is a magnetic flux plot for a magnetic gearbox arrangement with an outer rotor having twenty two magnetic pole pairs 60a, 60b, a pole piece rotor with twenty six pole pieces 62, and an inner rotor with four pole pairs (not shown). This arrangement naturally modulates the flux to give a balanced strength eight pole (four pole pair) magnetic field distribution (indicated by contour lines) that can directly and strongly couple to the four pole pairs of the inner rotor, providing an effective gearing arrangement.

Figure 15 is a magnetic flux plot for a magnetic gearbox arrangement with an outer rotor having twenty two pole pairs, and a pole pieces rotor having twenty seven pole pieces. It can be seen that this gives an imbalanced ten pole (i.e. five pole pair) magnetic field distribution (indicated by contour lines) in the central region. Coupling of that ten pole modulated magnetic field to a four pole pair inner rotor is sub optimal (the gearbox of Figure 15 will still function, but not as well as the 22:26:4 arrangement of Figure 14).

It will be understood that the invention has been described above purely by way of example, and modifications of detail can be made within the scope of the invention.

Each feature disclosed in the description, claims and (where appropriate) the drawings may be provided independently or in any appropriate combination.