Field of the Disclosure

The present disclosure relates generally to rotating electrical machines, and in particular to a rotating electrical machine comprising a plurality of rotors.

Background

Rotating electrical machines, such as motors and generators, generally comprise a rotor and a stator with an airgap between the two. Typically, a magnetic field produced by the rotor crosses the airgap and interacts with windings in the stator. In the case of a generator, when the rotor is rotated by a prime mover, the rotating magnetic field generates an electrical current in the stator windings. In the case of a motor, an alternating electrical current applied to the stator windings produces a rotating magnetic field which causes the rotor to rotate.

Rotating electrical machines are used in many applications, such as traction and power generation. In some applications it may be desirable to minimise the axial length of the machine, while maintaining output power capacity. For example, in some traction applications such as electric vehicles there may be limited axial space in which a motor/generator can be fitted.

One option for reducing the axial length of a machine is to increase its diameter. However, this may result in a rotor with a large rotational inertia. This may be undesirable, particularly in high speed or variable speed applications. Furthermore, increasing the diameter of the rotor may make it more difficult and expensive to manufacture.

It is known to provide a rotating electrical machine with a plurality of rotors. For example, a radial flux machine may have a rotor disposed radially inwards and radially outwards of a stator. Similarly, an axial flux machine may have a rotor disposed on each side of a stator axially. However, such arrangements may not satisfy space constraints and/or may be heavier, less powerful or have a higher rotational inertia than would be desirable, and may be difficult to manufacture. It would therefore be desirable to provide an electrical machine in which the axial length can be minimised for a given output power, preferably while avoiding excessive rotor inertia. It would also be desirable to provide an electrical machine which can satisfy space constraints while being relatively easy and cost effective to manufacture.

Summary

According to one aspect of the present disclosure there is provided an electrical machine comprising: a plurality of rotors, each of the rotors having an axis of rotation, wherein the axes of rotation are spaced apart; and a common stator core, the common stator core comprising a plurality of chambers, each chamber accommodating one of the plurality of rotors; wherein each of the chambers comprises a plurality of stator slots which accommodate stator windings, and each of the rotors is arranged to interact with the stator windings of the chamber within which the rotor is accommodated.

The present disclosure may provide the advantage that, by providing a plurality of rotors in a common stator core, it may be possible to reduce the axial length of the machine. Furthermore, it may be possible for this to be achieved without a corresponding increase in rotor inertia. In addition, the machine may be relatively easy and cost effective to manufacture, and/or provide good power density, when compared to arrangements with separate stator cores.

Preferably the stator slots of a chamber at least partially surround a corresponding rotor (that is, the rotor which is accommodated in the chamber). Thus, the stator windings of a chamber may at least partially surround the corresponding rotor. This may facilitate interaction between the magnetic field produced by the rotor and the corresponding stator windings, while helping to minimise interactions with other rotors or windings.

Preferably the stator windings of each of the chambers are armature windings, which may interact with a magnetic field produced by the corresponding rotor. Thus, in the case of motor operation, the stator windings may be used as an electrical input, while in the case of generator operation the stator windings may be used as an electrical output.

Preferably each of the rotors is arranged to produce a magnetic field which interacts with the stator windings of the chamber within which the rotor is accommodated. The magnetic field may be produced by magnets (such as permanent magnets or field coils) or it may be induced in the rotor (for example, in non-permanent magnetic poles or in bars in the rotor).

Preferably the magnetic field produced by a rotor crosses an airgap between the rotor and the chamber. The airgap is preferably small enough to allow magnetic flux to pass between the rotor and the common stator core, while large enough to allow the rotor to rotate inside the chamber.

Preferably each rotor and the corresponding part of the common stator core functions as an independent rotating electrical machine. By “corresponding part” it is preferably meant that part of the common stator core which at least partially surrounds the rotor, together with the stator windings in that part of the common stator core. This arrangement can allow the machine in effect to comprise a plurality of separate rotating electrical machines in a compact configuration.

Each of the independent rotating electrical machines may be any type of machine, such as: a permanent magnet machine; a machine with field coils; a switched reluctance machine; a synchronous reluctance machine; an induction machine; or any other type of machine; or any combination thereof. In some embodiments, it may be preferable to configure the machines as synchronous reluctance machines or induction machines, to minimise interactions between adjacent rotors. In other embodiments, a mix of topologies may be used. For example, a synchronous reluctance machine could include permanent magnets or induction bars so that it can function partially as a permanent magnet machine or an induction machine.

In one embodiment, the stator windings of each of the chambers are arranged to transfer electrical power to or from the same electrical load or source. To achieve this, the stator windings may be connected together, either directly or via one or more power stages. For example, the stator windings may be connected to a battery or another DC power source via one or more inverters, which may be bidirectional inverters. This may allow each of the independent rotating electrical machines to transfer electrical power to or from the same electrical load or source.

The machine may comprise any appropriate number of rotors, such as 2, 3, 4, 5 or any other natural number above 1. Preferably the axes of rotation of the rotors are substantially parallel. This may facilitate a compact arrangement and suitable interconnection of the various rotors.

Each of the rotors may comprise a rotor shaft which may be supported by bearings. The bearings may be provided on each side of the machine, and may be supported by the machine housing, or in any other way.

Preferably the rotors are arranged to drive and/or be driven by a common shaft. This can allow a single shaft to provide the mechanical input to and/or output from the machine. The common shaft may pass through the centre of the machine, or elsewhere. Preferably, the common shaft has an axis of rotation which is parallel to (and non-coaxial with) the axes of the rotors.

The electrical machine may further comprise: a common shaft; and a torque transmission mechanism arranged to transmit torque between each of the rotors and the common shaft.

The torque transmission mechanism may comprise any suitable means for transferring torque, such as gears, chains, belts or any combination thereof. The torque transmission may be in either direction (to or from the common shaft).

Preferably, each of the rotors is disposed circumferentially about the common shaft. In this case, the rotors may be evenly spaced about the common shaft (for example, the angular separation between the axes of two adjacent rotors may be substantially equal to 360° divided by the total number of rotors). Preferably, each of the rotors has a rotor shaft which is parallel to (and non-coaxial with) the common shaft. These arrangements may help to provide a compact and efficient torque transmission mechanism.

In one embodiment, the torque transmission mechanism comprises a “planetary” gear system with a central “sun” gear and a plurality of “planet” gears. Thus, in one embodiment, the torque transmission mechanism comprises a central gear wheel mounted on the common shaft and a rotor gear wheel mounted on each of the rotors (for example, on a rotor shaft). In this case, each of the rotor gear wheels may engage with the central gear wheel. This may help to provide a compact and efficient torque transmission mechanism. The gearing may provide an increase in speed, or a reduction in speed, or no change in speed.

In some applications, such as some traction applications, it may be desirable for the machine to be decoupled when no power/torque generation is required. In order to achieve this, it may be possible to disengage the central gear wheel from the rotor gear wheels. This may be achieved, for example, by moving the central gearwheel axially, for example, using a dog clutch. Thus, the central gearwheel may be arranged to be disengaged from the rotor gear wheels.

Preferably each of the chambers has an inner surface which is at least partially cylindrical. This can allow an airgap between the chamber and the corresponding rotor to be substantially constant at least part of the way around the rotor. Preferably, the stator slots are disposed about the inner surface of the chamber.

In one embodiment, each of the chambers has an inner surface which is partially cylindrical, and the chambers are partially open to each other. This may help to provide a compact arrangement and minimise the amount of material used.

In another embodiment, each of the chambers has a cylindrical inner surface and the chambers are closed to each other. This may help to reduce potential interference between the various independent rotating electrical machines.

The stator of an electrical machine is usually formed from a plurality of stacked laminations. The laminations are usually punched from a sheet of electrical steel. However, this process may result in a certain amount of raw material being wasted. Therefore, it would be desirable to provide an electrical machine which can be manufactured in a cost-effective and/or less wasteful manner.

In an embodiment of the disclosure, the common stator core and each of the rotors are formed from laminations which are produced from the same sheet of raw material (such as electrical steel). In this case, a lamination for a rotor may be produced from a part of the raw material which is inside a part of the raw material which forms a stator lamination. For example, a lamination for a rotor may be produced from a part of the raw material which is inside a part of the raw material forming a chamber in a stator lamination. This may allow the machine to be manufactured in a cost-effective manner and help minimize waste.

This aspect of the disclosure may be provided independently. Thus, according to another aspect of the disclosure, there is provided an electrical machine comprising: a plurality of rotors; and a common stator core, the common stator core comprising a plurality of chambers, each chamber accommodating one of the plurality of rotors; wherein the common stator core and each of the rotors are formed from laminations which are produced from a sheet of raw material, and a lamination for a rotor is produced from a part of the raw material which is inside a part of the raw material forming a chamber in a stator lamination.

Corresponding methods may also be provided. Thus, according to another aspect of the disclosure, there is provided a method of manufacturing an electrical machine, the method comprising: producing a plurality of rotors from a plurality of stacked laminations; and producing a common stator core from a plurality of stacked laminations, the common stator core comprising a plurality of chambers; wherein the laminations are produced from a sheet of raw material, and a lamination for a rotor is produced from a part of the raw material which is inside a part of the raw material forming a chamber in a stator lamination. Preferably each of the chambers comprises a plurality of stator slots for stator windings. The method may further comprise accommodating a rotor in each of the chambers.

Features of one aspect of the disclosure may be applied to any other aspect. Any of the method features may be provided as apparatus features and vice versa.

Brief Description of the Drawings

Preferred features of the disclosure will now be described, purely by way of example, with reference to the accompanying drawings, in which:

Figure 1 shows an axial cross section through part of a known rotating electrical machine;

Figure 2 shows parts of an electrical machine in an embodiment of the disclosure;

Figure 3 shows the machine of Figure 2 with parts of a transmission mechanism;

Figure 4 shows parts of an electrical machine in another embodiment of the disclosure;

Figure 5 shows another view of the electrical machine of Figure 4;

Figure 6 shows schematically parts of a control system for an electrical machine;

Figure 7 shows steps carried out by the system of Figure 6 when the machine is operating as a generator;

Figure 8 shows steps carried out by the system of Figure 6 when the machine is operating as a motor;

Figure 9 shows parts of an overall control system in one embodiment; Figure 10 shows parts of an overall control system in another embodiment; and

Figure 11 shows parts of an overall control system in a further embodiment.

Detailed Description of Embodiments of the Disclosure Figure 1 shows an axial cross section through part of a conventional rotating electrical machine. Referring to Figure 1, the machine comprises a rotor 2 which is located inside a stator 4, with an airgap 5 between the two. The stator 4 comprises a plurality of stator slots 6 disposed about its inner circumference. The stator slots 6 are used to hold stator windings. The rotor 2 comprises a plurality of permanent magnets 8. The permanent magnets produce a magnetic field which crosses the airgap 5 and interacts with the stator windings. When the machine is operated as a motor, an alternating electrical current is applied to the stator windings. This produces a rotating magnetic field which causes the rotor 2 to rotate. When the machine is operated as a generator, a prime mover applies torque to the rotor 2, causing it to rotate. The rotating magnetic field produced by the rotor crosses the airgap 5 and generates an electrical current in the stator windings.

Figure 2 shows parts of an electrical machine in an embodiment of the disclosure. Referring to Figure 2, the machine 10 in this embodiment comprises three separate rotors 12. Each of the rotors 12 has an axis of rotation which is separate from but parallel to those of the other rotors. The three rotors 12 are arranged in a “clover leaf configuration such that, when viewed in an axial direction, the axes are at the vertices of an equilateral triangle.

In this embodiment, rather than provide a separate stator for each rotor, the rotors 12 are disposed within a common stator core 14. The stator core 14 comprises three chambers 16, each of which accommodates a corresponding rotor 12. The chambers 16 each have an inner surface which is partially cylindrical. Each inner surface of a chamber faces an outer surface of the corresponding rotor, with an airgap 15 between the two. The chambers 16 are open to each other at the centre of the machine. A plurality of stator slots 18 are arranged about the inner surface of each of the chambers 16. The stator slots 18 are used to accommodate stator windings (not shown). The stator windings may be distributed windings, concentrated windings, or any other appropriate type of windings. Each of the rotors 12 is in the form of a cylinder which is arranged to rotate within the chamber 16 within which it is accommodated. A magnetic field produced by a rotor 12 crosses the airgap 15 between the rotor and the corresponding chamber 16 in the common stator core 14 and interacts with the stator windings in the stator slots 18.

When the machine is in operation, each of the rotors 12 and the corresponding part of the stator core and stator windings on the other side of the airgap 15 operates as an independent rotating electrical machine. The rotor 12 produces the magnetic field, while the part of the stator core which partially surrounds the rotor acts as the armature. A rotating magnetic field produced by the rotor 12 interacts with the corresponding stator (armature) windings to produce an electrical output in the windings when the machine is operating as a generator, or to cause rotation of the rotor when the machine is operating as a motor. Thus, the electrical machine shown in Figure 2 comprises in effect three independent rotating electrical machines.

In this embodiment, each of the rotors 12 consists of a ferromagnetic material with a plurality of barrier slots 20 which run axially through the rotor. The barrier slots 20 create areas of high reluctance, with areas of low reluctance in between. The areas of low reluctance form rotor poles 21. In the arrangement shown, each rotor has eight poles, although it could be some other number such as 4 or 6. Each rotor 12 and the corresponding part of the stator core and stator windings on the other side of the airgap 15 function as a synchronous reluctance machine. A rotating magnetic field produced by the stator produces a magnetic flux path through two adjacent rotor poles 21. The magnetic field may be produced by the main stator windings, or by auxiliary excitation windings. The flux attempts to follow a minimum reluctance path through the rotor poles. Torque is produced by the tendency of the rotor to align its poles with the rotating magnetic field in order to maintain the path of minimum reluctance.

In the embodiment of Figure 2, the rotors 12 are arranged to drive or be driven by a common shaft. The common shaft passes through the centre of the machine, at a point equidistant from the axes of each of the rotors. A transmission mechanism is used to transfer torque between the common shaft and the rotors.

Figure 3 shows the electrical machine of Figure 2 with parts of a transmission mechanism in place. Referring to Figure 3, each of the rotors 12 comprises a rotor shaft 22 which is supported by suitable bearings (not shown). A rotor gearwheel 24 is located on each rotor shaft 22. A central gearwheel 26 is located on a common shaft (not shown) at the centre of the machine. Each of the rotor gearwheels 24 engages with the central gearwheel 26 in order to transfer torque between the common shaft and the rotor shafts 22. Thus, the transmission mechanism functions in a similar way to a planetary gearing system, with the central gearwheel 26 acting as a “sun” gear and the rotor gearwheels 24 acting as “planet” gears. The gear ratio is chosen to optimise performance considering system limitations (tip speed, bridge and rib stress etc).

In the arrangement shown the gearing provides an increase in speed from the common shaft to the rotor shafts. Alternatively, the gearing could provide a reduction in speed, in which case the planet gears would be larger than the sun gear, or no change in speed, in which case the gears would be the same size.

The electrical machine 10 of Figures 2 and 3 may be operated either as a motor by applying a suitable AC or commutated DC voltage across each of the stator windings, or as a generator by applying torque to the common shaft and generating a current in the stator windings. Depending on the configuration of the stator windings and the rotor poles, each independent electrical machine formed by a rotor and the corresponding part of the stator may function as a single phase or a three-phase machine. By appropriately indexing the magnetic fields and stator windings of each machine, the phases of the machines can be aligned. Thus, in this case, the three sets of stator windings can be connected together in series or in parallel. Alternatively, by appropriate indexing of the rotor poles and stator windings of each machine such that they are respectively 120° out of phase, it would be possible to produce a three-phase output, with each individual machine producing one of the phases. It will be appreciated that other configurations and numbers of phases are also possible.

If desired, a mix of topologies could be used, rather than configuring each machine as pure synchronous reluctance machine. For example, some or all of the slots 20 in the rotors may include permanent magnets. This may help with starting and low speed operation of the machines. Alternatively, or in addition, a plurality of conductors could be embedded in the rotors to allow the machines to function partially as induction machines.

The electrical machine described above with reference to Figures 2 and 3 comprises three rotors within a single stator core. Packaging three (or more) rotors within a single stator core may provide at least some of the following benefits:

• The total air gap circumference may be increased by approximately 10% in comparison to a single rotor/single stator design of a similar size. This may help to increase the power/weight ratio of the machine.

• Smaller diameter rotors may allow reduced rotor inertia. This may be advantageous in variable speed applications, and may help with the selection of materials used for the rotor.

• Smaller diameter rotors allow decreased tip speed whilst allowing increased rotational velocities.

• Single piece stator provides manufacturing/cost benefits when compared to a similar arrangement with three separate machines.

In the arrangement shown in Figures 2 and 3, the rotors 12 are each located in a partial chamber 16, with the chambers open to each other at the centre in a “clover leaf configuration. In this case, the slots 18 for the stator windings extend partially around the corresponding rotor, rather than the whole way around the windings as would be the case in a conventional machine. The distances between the rotors 12, and the amounts by which the chambers 16 surround the rotors, are chosen to be large enough to minimize any potential interactions between the machines, while being small enough to satisfy overall space requirements. in this arrangement, each of the three independent machines is configured as a synchronous reluctance machine. As a consequence, a magnetic flux path is only produced through those rotor poles which are opposed to the stator windings at any one time. This may help to minimise interactions between adjacent rotors, since their rotor poles will be substantially unmagnetized as they pass each other. However, if desired, some or all of the rotors could also include permanent magnets or a rotor bars (squirrel cage), to allow operation as a permanent magnet machine or an induction machine.

Figure 4 shows parts of an electrical machine in another embodiment of the disclosure. Referring to Figure 4, the machine 30 in this embodiment comprises three separate rotors 32, which may be similar to or the same as those described above. Each of the rotors 32 has an axis of rotation which is separate from but parallel to those of the other rotors. As in the embodiment of Figures 2 and 3, the rotors 32 are disposed within a common stator core 34. However, in this embodiment, each of the rotors is located in a separate chamber 36 which is not open to the other chambers. Each chamber 36 has a cylindrical inner surface with a plurality of stator slots 38 which extend around the whole circumference of the rotor. The stator slots 38 are arranged to accommodate stator windings (not shown).

In the arrangement of Figure 4, each of the rotors 32 and the corresponding part of the stator core 34 and the corresponding stator windings act as an independent rotating electrical machine, in a similar way to the arrangement of Figures 2 and 3. However, in the arrangement of Figure 4, rotors are in chambers which are not open to each other, and the stator (armature) windings extend the whole way around the circumference of the corresponding rotor. This may help to reduce any interference between the magnetic circuits of the various rotating electrical machines, although at the cost of a slightly increased diameter.

In the arrangement of Figure 4, each of the rotors 32 comprises a rotor shaft 40 which is supported by first rotor bearings 42. The bearings 42 are supported by the machine housing (not shown). A common shaft 44 is located at the centre of the machine. The common shaft 44 passes through the centre of the machine in a direction which is parallel to that of the rotor shafts 40. The common shaft may be supported by bearings, which may be supported by the machine housing, or it may be at least partially supported by another component for example in the drive train of a vehicle. A central gearwheel 46 is located on a common shaft.

Although not visible in Figure 4, the central gearwheel 46 engages with rotor gearwheels on the rotor shafts, in a similar way to the transmission mechanism of Figure 3. Figure 5 shows the electrical machine of Figure 4 from the opposite side.

Referring to Figure 5, each rotor shaft 40 is further supported by second rotor bearings 43 on the opposite side of the machine to the first rotor bearings 42.

The bearings 43 are supported by the machine housing (not shown). A rotor gearwheel 48 is located on each rotor shaft 40. Each of the rotor gearwheels 48 engages with the central gearwheel 46 in order to transfer torque between the common shaft 44 and the rotor shafts 40.

If desired, with the addition of a clutch mechanism, the central gearwheel 46 can be decoupled from the rotor gearwheels 48. This may be achieved by moving the central gearwheel 46 in an axial direction away from the machine, for example using a dog clutch. This can allow the machine to be decoupled when no power/torque generation is required. In certain applications, such as electric and hybrid vehicles, this may help to increase overall drive cycle efficiency.

It will be appreciated that, rather than the planetary-type gearing system described above, the transmission mechanism may be arranged to transfer torque between the common shaft and the rotors in other ways. For example, a different arrangement of gears could be used, the gears could be either toothed or toothless (engaged by friction) or a belt or a chain could be used, or any combination thereof. Furthermore, the common shaft need not be located at the centre of the machine, and it could be located to one side.

In a typical rotating electrical machine, the stator and the rotor may be formed from stacks of steel laminations. Typically, such laminations are punched from sheets of electrical steel. A steel with good ferromagnetic properties is usually chosen for the stator. However, such steel tends to be less strong than some other types of steel, which may make it less suitable for the rotor. Thus, a stronger (but less ferromagnetic) steel may be used for the rotor. However, this may add to the cost and complexity of manufacture.

In an embodiment of the disclosure, laminations for the common stator core and the rotor cores are all punched from the same sheet of electrical steel. This may be possible due to the fact that the rotors have a relatively low inertia, and thus it may be possible for them to use less strong (but more magnetic) steel than would otherwise be the case. Furthermore, the rotor laminations may be formed from the parts of the raw material which are inside the laminations for the common stator core. This may reduce the amount of electrical steel consumed, leading to reduced manufacturing costs, as well as improving the magnetic properties of the rotor. In addition, the “clover leaf design of the common stator core may allow successive laminations to be partially nested inside each other in the raw material, allowing more efficient use of the raw material.

In the arrangements described above, each of the rotors and the corresponding part of the stator is configured as a synchronous reluctance machine. In a synchronous reluctance machine, internal flux barriers, direct the magnetic flux produced by the stator through the rotor poles.

In an alternative embodiment, each of the three machines may be an induction machine, with the electric current needed to produce the rotor’s magnetic fields being obtained by electromagnetic induction from the magnetic field of the stator windings. In this case, each of the rotors 12, 32 may be of a squirrel-cage type, with a number of conductors embedded in its surface and connected at both ends by shorting rings. A rotating magnetic field produced by the stator induces currents in the conductors, which in turn create the rotor’s magnetic fields. The magnetic field produced by a rotor 12 crosses the airgap 15 between the rotor and the corresponding chamber 16 in the common stator core 14 and interacts with the stator windings in the stator slots 18.

In another embodiment, each of the three machines may be a permanent magnet machine. In this case, the rotors may comprise a plurality of permanents magnets which produce the magnetic fields. The magnets may be interior permanent magnets or surface permanent magnets, or a combination of the two. Permanent magnet machines typically provide high efficiency and high torque density. However, they may suffer from certain disadvantages, including the risk that the magnets from adjacent rotors could interfere with each other.

In further embodiment, each of the rotors and the corresponding part of the stator is configured as a switched reluctance machine. In this case, each of the rotors has a number of poles which are formed from a magnetisable material. The corresponding part of the stator comprises a number of poles each of which is wound with a coil. The number of stator poles is different from the number of rotor poles, so that at any one time not all of the rotor poles are aligned with the stator poles. In the case of motor operation, power is applied to the stator windings, and the rotor's magnetic reluctance creates a force that attempts to align the rotor poles with the nearest stator pole. In order to maintain rotation, a drive system switches current through the windings of successive stator poles in sequence so that the magnetic field of the stator "leads" the rotor pole, pulling it forward. In the case of generator operation, the rotor is driven mechanically by a prime mover, and the load is switched to the coils in sequence to synchronize the current flow with the rotation.

Furthermore, embodiments of the disclosure are not limited to a single machine topology, and a mix of different topologies could be used. For example, each of the three machines could have the same topology or a different topology, and a mix of topologies could be used within each machine.

Figure 6 shows schematically parts of a control system for one of the independent rotating electrical machines described above. In Figure 6, the machine comprises a rotor 12 and the corresponding part of a common stator 14 as described above with reference to Figures 2 and 3. However, it will be appreciated that a similar arrangement could be used with a machine formed by a rotor 32 and the corresponding part of a common stator 34 as shown in Figures 4 and 5.

Referring to Figure 6, the stator 14 includes stator windings 28 which extend all or part way around the rotor. The stator windings 28 are connected to a bi directional three-phase inverter 50. The inverter 50 is controlled by a control unit 52. The other side of the inverter 50 is connected to a battery 54 via a DC link. The system also includes a current sensor which senses the current in the stator windings, and a position sensor which senses the angular position of the rotor.

In operation, the current in the stator windings is sensed by the current sensor and input to the control unit 52. The angular position of the rotor is sensed by the position sensor and input to the control unit 52. The control unit 52 controls the inverter 50 based on the sensed current and the rotor position. The control unit may use techniques such as pulse width modulation control which are known in the art.

Figure 7 shows steps carried out by the system of Figure 6 when the machine is operating as a generator. Referring to Figure 7, in step 100 the common shaft receives torque from the prime mover. In step 102 the gears distribute the torque equally to each of the rotors. In the case of generator operation, the stator windings initially need to receive a magnetizing current from the inverter power stage. Thus, in step 104 the control unit controls the inverter to deliver the required magnetizing current based on the rotor position as measured by the position sensor. In step 106 a reference power or voltage signal is generated, the reference power or voltage signal representing a desired electrical output of the machine. In step 108, the control unit calculates the control signals which are required to drive the inverter so as to convert the battery DC voltage to the required AC voltage between the stator windings. In step 110 the control unit delivers the control signals to the inverter. In step 112 the control unit measures the induced currents in the stator windings and recalculates the required control signals in order to meet the required power or voltage demand. Processing then loops through steps 110 and 112.

Figure 8 shows steps carried out by the system of Figure 6 when the machine is operating as a motor. Referring to Figure 8, in step 120 the battery delivers DC voltage to the inverter. In step 122 the control unit controls the inverter to deliver the required magnetizing current based on the rotor position as measured by the position sensor. In step 124 a reference torque or speed signal is generated, the reference torque or speed signal representing a desired mechanical output of the machine. In step 126, the control unit calculates the control signals which are required to drive the inverter so as to convert the battery DC voltage to the required AC voltage between the stator windings. In step 128, the control unit delivers the control signals to the inverter. In step 130 the control unit measures the induced currents in the stator windings and recalculates the required control signals in order to meet the required torque or speed demand. Processing then loops through steps 128 and 130. Figure 9 shows parts of an overall control system in one embodiment. Referring to Figure 9, three independent rotating electrical machines are shown, each comprising a rotor 12 in a common stator core 14. A planetary gearing system is used to transfer torque between the rotors and a common shaft. Each part of the stator core which opposes a rotor is provided with stator windings 28. The stator windings 28 of each of the machines is connected to power stage 56 which comprises an inverter and a control unit in the form described above. The other side of each power stage is connected to the battery 52. The current in each of the stator windings 28 is sensed by a current sensor and the sensed current is input to the corresponding power stage 56. A rotor position sensor is connected to each rotor shaft, and the sensed rotor position is input to the corresponding power stage 56. In this embodiment, control of each independent machine is performed independently by the corresponding power stage 56.

Figure 10 shows parts of an overall control system in another embodiment. In the arrangement of Figure 10, the stator windings 28 of each machine is connected to a power stage 56 in a similar way to that shown in Figure 9. In addition, overall system control is performed by a master control unit 58. The master control unit 58 receives the sensed current from each of the stator windings and the sensed rotor position from each of the rotor position sensors. The master control unit 58 outputs commands to each power stage 56 to ensure that the overall system operates within desired parameters. Other parts of the system may be the same or similar to those described above with reference to Figure 9.

Figure 11 shows parts of an overall control system in a further embodiment. In this embodiment, the stator windings of each machine are connected together, with each phase of one machine connected to the corresponding phase of the other machines. The stator windings 28 are all connected to a single power stage and master control unit 60. The combined current in the stator windings 28 is sensed by a current sensor and the sensed current is input to the power stage and master control unit 60. The rotor position of one of the rotors 12 is sensed by a rotor position sensor and input to the power stage and master control unit 60.

In addition, the speed of the central shaft is sensed by a speed sensor and input to the power stage and master control unit 60. In the arrangement of Figure 11, a single bidirectional three-phase inverter within the power stage and master control unit 60 is connected on one side to the stator windings 28 and on the other side to the battery 52. The inverter is controlled by an inverter controller using for example pulse width modulation, based on the sensed current and the rotor position. In addition, the inverter controller is issued with commands to ensure that the system operates within desired parameters, based on amongst other things the sensed speed. Thus, in this embodiment a single control strategy is applied to each of the individual machines. This may allow a reduction in the number of components and a simpler overall design, but potentially at the cost of less control of each individual machine.

The electrical machines disclosed herein could be mounted within existing voids/spaces and coupled to an existing electrical machine. For example, the machine could be mounted within the inner diameter of a larger motor in an area that is typically a void (e.g. within a traditional rotor of larger diameter or within the stator core of an out-runner type motor).

In the arrangements described above, three rotors are provided in a common stator core. However, a different number of rotors could be used instead. For example, the electrical machine could comprise two side-by-side rotors, or four rotors arranged in a square pattern, or five rotors arranged in a pentagonal shape, or any other appropriate number of rotors arranged in any appropriate configuration. Furthermore, the outer perimeter of the stator core could be any appropriate shape when viewed in axial cross-section, such as round, oval, square or clover leaf.

It will therefore be appreciated that embodiments of the present disclosure have been described above by way of example only, and further modifications in detail will be apparent to the skilled person.