GB2544712A - An improved electrical machine - Google Patents

Abstract

An electrical machine comprises at least two rotor discs 11 containing a magnetically hard material and at least two annular arrays of pole pieces 22 and an annularly wound coil 25. Each rotor disc can have an angular array of magnetic poles (fig 2, 10) that mirror the array of stator poles and face like poles on the next rotor disc. Current passing through coil 25 may cause each stator array pole piece to have an opposite polarity to an adjacent pole piece. Stator pole arrays can interleave and may magnetically interact with two rotor discs and each rotor disc can interact with two stator pole arrays. Stators may be separated axially and have a folded coil (fig 5) contained within a stack of two semi circular stators to allow assembly about a rotor assembly. The rotor discs may also have a coil (fig 15, 17) to vary the magnetic flux allowing control of the output of a generator or the speed of a motor. The machine may be an inner rotor type (fig 12) or an outer rotor type (fig 18) comprising half ring magnets (fig 19, 111) to allow assembly with interleaved stator pole piece arrays.

Description

An improved Electrical Machine

Field of the Invention

The present invention relates to a rotary electrical machine, used as an electric motor or generator, comprising a stack of hard magnetic discs as the rotating component of the machine. More specifically, the present invention relates to a type of motor commonly known in the art as a permanent magnet brushless or synchronous motor.

Background of the invention

Electric motors are an essential ingredient of modern life. Yet the basic design of the majority of motors sold today has change little in the past decades, despite much innovation in this field. Improvements in permanent magnets and the reducing cost of electronic motor control systems have resulted in the electronically commutated, permanent magnet motor gaining market share against traditional AC induction motors and brushed motors. The emergence of compression moulded soft magnetic compounds as a commercially viable alternative to stacks of iron laminations is also driving motor development. However, the vast majority of motors are of the cylindrical rotor and stator construction. The simple assembly of a traditional cylindrical rotor motor makes them very cost effective, although they can lack power density compared to some pancake style motors with disc rotors, including axial flux and some transverse flux motors.

Pancake motors typically have a greater power density as their disc rotors give a large surface area for rotor - stator field interaction, relative to the overall motor volume. Axial flux motors have the stator windings in the flux path of the rotor magnets and generally use rare earth magnets to compensate for the lack of iron in the magnetic circuit. The coil assembly is complicated compared to conventional motors and cooling of the coil can be problematic where the coil is sandwiched between the rotor discs. These factors tend to make axial flux motors expensive and limit their use to applications where a high power density or short axial length are the primary requirements. Transverse flux motors have a simpler bobbin wound coil construction and less air gap in the flux circuit but typically have a complicated stator iron construction. Another drawback of the disc rotor transverse flux motor is that all the stator poles facing a rotor disc are of the same polarity, hence there are half as many stator to rotor pole interactions as there are rotor poles and there are large voids in the iron circuit. Typically, a transverse flux disc rotor is sandwiched between two stators, with the stator coil wound annularly and on the same plane as the disc rotor.

In the field of electric motors, the rotor is typically located axially within the stator. It is possible to transpose the rotor and stator components so that the stator is located substantially within the rotor, regardless of the type of motor. Such motors are typically known as external rotor or outrunner motors. This patent describes a motor of an internal rotor design, however, the principles described hereinafter can be applied equally to an external rotor design. The external rotor configuration is well suited to brushless, permanent magnet type motors for two reasons;

Permanent magnets are typically of a thinner profile than an assembly comprised of copper coils and iron laminations. Locating the permanent magnets around the outside of the coils and iron increases the diameter of the rotor to stator interface and results in an increase of torque from the larger torque radius. The space at the centre of the motor is also better utilised by the coils and iron structure.

Summary of the Invention

In its simplest form, the present invention comprises a rotor, with a rotor shaft and a stator with a stator housing. The rotor shaft is aligned to the stator housing by a bearing set and there are at least two magnetically hard rotor discs axisymmetrically fixed to the shaft. The stator is comprised of at least two stator sections, each stator section having an axisymmetric array of magnetically soft pole pieces and a substantially annular electrical coil. The rotor shaft bearings align the rotor to the stator such that each rotor disc face maintains a clearance gap to each stator pole array. The plane of each annular coil is broadly aligned to the plane of each stator pole array. The rotor magnetic poles are on the flat faces of the rotor discs and there is at least one pair of poles on a disc face. Preferably, there are three, four or more pole pairs on each disc face. The discs are angularly aligned so that their like poles face each other and the discs would normally experience a repulsion force from each other if the stator poles were not located between the discs. Each stator section is segmented, with preferably as many segments as there are magnetic poles on a rotor face. The stator segments can be made of a suitable magnetically soft, compression moulded material, such as coated iron granules to reduce eddy current generation in the stator structure from the changing magnetic flux.

In one embodiment of the present invention, each stator pole array is segmented. Each stator segment is a pole piece that extends radially from the rotor magnets to the coil. In this way, the stator segments couple the flux from the rotor magnets with the flux generated by the coil current.

In contrast to disc rotor, transverse flux motors in the art, the plane of the coil is broadly coincident with the stator pole array and the gap between each pair of rotor discs. This allows the magnetic polarity of each stator pole piece to be determined by which side of the coil the pole piece iron extends to. When each stator coil is energised, the resultant stator coil flux forms alternating north and south poles on each associated stator pole array located between each pair of rotor discs, each pole in a stator pole array interacting with the magnetic flux from two rotor discs. The stator segments, toroid and coil are all on a common stator plane that is centred on the plane of the air gap between the rotor discs. The arrangement described above forms a single phase of a synchronous, permanent magnet motor.

In the embodiment of the invention described, each rotor disc has eight poles on each disc face.

Each stator section has eight similar iron segments, each segment being both a pole piece and a part of the coil magnetic circuit, each segment forming one eighth of a circle. The segments are arranged annularly so their pole pieces are aligned to both the magnetic pole pattern on the rotor discs and the gap between the rotor discs. Every other stator segment is inverted so that alternate pole pieces are coupled to alternate sides of the annular coil.

When two stator sections so assembled from eight segments are brought together, the stator segments of one stator section interlock with the segments of the other stator section so as to form a toroidal structure that encloses the annular coil. This method allows efficient stacking of stator sections and rotor discs while also providing twice as many stator poles to each rotor disc face than is possible with transverse flux designs known in the art.

One drawback with the above design and with disc rotor motors in general is that the rotor discs are interleaved with the stator sections, requiring the stator and rotor to be assembled in tandem. The assembly process is further complicated by the rotor discs being permanently magnetised. In a further preferred form of the invention, the stator and stator coils are assembled as two stator sub-assemblies, so allowing the rotor to be assembled separately to the stator. The stator would be assembled in two 'C' shaped parts. The two stator parts being brought together around the rotor at a late stage of the assembly process. This form of assembly requires the annular coils to also be of a 'C' shape and can be achieved by modifying some of the stator segments so as to allow an annularly wound coil to be folded in half, in the manner of folding an omelette and then be inserted within two stator half sections. This form of construction is described in more detail hereinafter.

Preferably the above configuration is repeated axially along the rotor shaft, with the stator sections being interleaved between the rotor discs. Further preferably the number of rotor discs exceeds the number of stator sections by one. Further preferably, there are two or three stator sections or a multiple of two or three stator sections, corresponding to a two or three phase machine. In the art. Permanent magnet rotor synchronous motors are normally configured as 2 or 3 phase machines, each phase having a coil or set of coils corresponding to a defined stator pole to rotor pole magnetic flux interaction and each interaction in a phase being angularly displaced 90 or 60 electrical degrees from the flux interaction in the next phase. This arrangement provides torque to the motor shaft when the coils are connected to a corresponding 2 or 3 phase AC power supply that is synchronised to the angular position of the rotor poles. The same principle of operation applies to the machine described in this patent, each electrical phase corresponding to a stator pole array or group of stator pole arrays.

Brief description of the drawings A preferred embodiment of the present invention is described below in more detail, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is an overview of the invention 5, showing a rotor shaft 15 with a magnetically hard disc rotor assembly 1 within a stator housing 3.

Figure 2 shows one of the magnetically hard discs 11 with the magnetic poles 10 and 10' on both flat faces of the disc.

Figure 3 shows four types of stator segment, 21, 22, 23 and 24. All the stator segments have a pole piece 20 and space for an electrical coil 25.

Figure 4 shows a coil winding 25 and two semi-circular coil formers 26 with a gap between the formers.

Figure 5 shows the coil assembly from figure 4, which has been folded to form two semi-circles. Stator segments 22 and 22' locate between the two semi-circular coil sections.

Figure 6 shows a section view through the line X - X of the coil assembly in figure 5. Part of a rotor assembly is also shown, featuring a motor shaft 15, rotor discs 11 and 1Γ, and rotor disc clamps 14. A mirror image coil assembly 25' is also shown, being added to the assembly in the direction of the arrow.

Figures 7A and 7B show an assembly view of the invention. Figure 7A shows an assembly method whereby two stator sub-assemblies 2 and 2' are brought together about a rotor assembly 1. Figure 7B is a section view of figure 7A on the axis of the shaft.

Figures 8, 9 and 10 show three stator section views, broadly along planes A - A', B - B' and C-C respectively, referring to figure 7.

Figure 11 is a detail view of a rotor assembly 1, showing the main magnetic rings 11, the end magnetic rings 12, the flux linking plates 13, the magnet spacing clamps 14 and the rotor shaft 15.

Figure 12 is an assembly view of an alternative form of the invention showing a three phase configuration.

Figure 13 shows two section views of the invention along planes Y - Y' and Z - Z' referring to figure 14, to illustrate the magnetic circuit linking the rotor permanent magnets to the stator coils.

Figure 14 shows the additional magnetic circuits of the invention.

Figures 15 and 16 show two alternative forms of the rotor discs where a permanent magnet structure and an electrical coil are combined in a hybrid rotor arrangement.

Figure 17 relates to a second preferred form of the invention where the rotor and stator components are transposed.

Figure 18 shows the additional components required for the second preferred form of the invention.

Figures 19A and 19B show an assembly view of an alternative form of the invention, where the rotor and stator components are transposed. Figure 19A shows an assembly method whereby two rotor sub-assemblies 11 and 11' are brought together about a stator assembly 3. Figure 19B is a section view of figure 19A on the axis of the shaft.

Detailed description of the preferred embodiments of the invention

Figure 1 is a general overview of the invention 5, showing a rotor assembly 1 within a stator housing 3. The rotor assembly consists of a series of magnetically hard rotor discs or rings with a gap between the flat faces of each disc to accommodate the stator pole pieces. The discs or rings are attached axisymmetrically to a rotor shaft 15.

Figure 2 shows one of the magnetically hard rotor discs 11, having eight magnetic poles 10 and 10' on each of the flat faces of the disc and a broadly axial direction of magnetisation. The number of poles is chosen according to the required torque and speed characteristics and the size of the device. There must be at least two poles on any disc face, but preferably, there would be 4 to 16 poles.

There could be more than 16 poles on a large device. Any magnetically hard material may be used in the disc construction. The device of the invention provides a large surface area for rotor - stator flux interaction, relative to the overall device dimensions. This allows a high torque to be developed from low flux magnitudes and makes possible the design of a high performance machine, using low cost magnets made from ceramic or ferrite materials.

Figure 3 shows four types of stator segment, 21, 22, 23 and 24. All the stator segments have in common, a pole piece 20 and space for annular coil windings 25. The pole pieces are sized to maximise the flux interaction with the rotor while leaving sufficient clearance gap to the rotor to prevent physical contact with the stator in operation. There should also be sufficient gap between one pole piece and the next to limit the degree of magnetic short circuiting as the north and south poles are adjacent to each other.

Within a stator section, there are 8 pole pieces. These match the number and shape of the poles on a rotor disc face. The pole pieces on a stator section share a common plane that coincides with the plane of an annular coil. This is an important defining feature of the invention as it allows both north and south magnetic poles to be introduced to the plane of the rotor air gap. The transversal flux motors known in the art have a stator pole coincident with every other rotor pole, so that at any given moment the pole pieces on a common plane are all north or all south poles. This feature effectively halves the stator - rotor flux interaction on a disc rotor transversal flux machine compared to the present invention.

The stator of the present invention is designed to be assembled in two broadly semi-circular parts. This is a second important defining feature of the invention as it allows a stacked disc rotor to be assembled separately to the stator. In the art, disc rotor pancake motors typically have a higher power density than the more common cylindrical rotor motor as a disc generally has a higher surface area to volume ratio than a cylinder. A stacked disc rotor increases the power density further but this makes assembly of the machine problematic as the rotor stack has to be assembled within and in tandem to the stator, since the stator and rotor poles are interleaved. The stator of the present invention avoids this issue by forming the annularly wound coils as two semicircles. This gives at least two annular coil planes per electrical phase.

Stator segment 21 is designed to be fitted at the axial ends of the stator assembly. Four of segment 21 are used at each end plane of the stator.

Stator segments 22 and 23 are shaped to be used between the two halves of a semi-circular coil. The two parts are the same except segment 23 has a cut away to accommodate the coil end turns. Four of segment 22 and four of segment 23 are used in each centre plane of the stator. Every other segment is inverted about the rotor disc plane to match the alternating pole pattern of the rotor disc

Stator segment 24 is designed to fit on the boundary plane between two electrical phases. Eight pieces are used and every other piece is inverted about the rotor disc plane. This results in every other pole piece having a planar offset. The two pole face groups have an angular offset between them that is equal to the electrical phase angle of a two phase or three phase supply.

The arc of each segment is defined by angle A, which is equal to 360° divided by the number of poles. In this case, A = 360°/8, or 45°

Figure 4 shows a stator coil 25 that is wound on two semi-circular coil formers 26. There is a gap between the two formers to allow the coil to be folded in the manner of folding an omelette. The two formers may be placed on an ovoid winding mandrel to provide sufficient wire length between the formers to allow the coil to be folded and retain a semi-circular profile

Figure 5 shows a folded stator coil 25 and former 26 with stator blocks 22 and 22' located within the fold. Block 22' is identical to block 22 but is inverted. Blocks 23 and 23' have been omitted from the figure for clarity.

Figure 6 is a section view of figure 5 along the plane X - X'. This shows how the pole pieces of blocks 22 and 22' locate on opposite rotor disc faces and are angularly displaced by one pole position. The coils 25 and 25' are physically separate but are electrically connected. The rotor discs 11 and 11' are held in position by coil spacer and clamp 14 which slides over the rotor shaft 15. The spacer and shaft may be keyed, splined or similar to ensure good transmission of torque to the shaft.

Figure 7A shows two stator half assemblies 2 and 2'. Each stator half is comprised of two semicircular coils 25 and 4 each of stator blocks 21, 22, 23 and 24. In the view shown in figure 7A, stator blocks 21 and 24 are not shown. The figure shows the stator centre blocks 22 and 22', the coils 25 and 25' and the stator coil cut out blocks 23 and 23'. On the stator half assemblies 2 and 2', the stator blocks 22' and 23' are inverted blocks so that their pole pieces project behind the figure plane B - B', referring to figure 7B. Conversely, the pole pieces of blocks 22 and 23 project forward of the figure plane B - B'.

When the stator half assemblies 2 and 2' are brought together about the rotor assembly, the pole pieces of blocks 22' and 23' are positioned behind the rotor disc 11, so only the pole pieces of blocks 22 and 23 are in view. The spaces between the block pole pieces 22 and 23 are occupied by the pole pieces of stator end blocks 21 (not shown but on the plane of A - A', referring to figure 7B). Similarly, the spaces between the block pole pieces 22' and 23' are occupied by the pole pieces of stator end blocks 24 (on the plane of C - C', referring to figure 7B). In this way, every pole on every rotor disc face has a corresponding stator pole piece, thereby maximising the flux interaction between the rotor and the stator.

Figure 7B is a section view of figure 7A along the plane of the rotor shaft 15. The figure shows a two phase, four coil configuration of the invention with phase I and phase II being 90 electrical degrees displaced. For the 8 pole device shown here, this corresponds to 22.5 degrees of shaft rotation. The coil pair to the left of plane C - C', 25 and 25' are electrically connected so as to form a first electrical phase and the coil pair to the right of plane C - C' are electrically connected so as to form a second electrical phase. The two coil phases may be connected to a suitable two phase motor driver if the device is a motor. Alternatively, the coil phases may be connected to a diode rectifier if the device is to be a generator. Two shaft bearings 16 are also shown.

Figures 8, 9 and 10 show three stator section views, broadly along stator coil planes A - A', B - B' and C - C' respectively, referring to figure 7B. In Figure 8, each stator segment 21 conveys flux from the coil within to its pole piece 20. The four pole pieces 20' occupy the same plane as pole pieces 20 and are the pole pieces of four of the stator sections shown in figure 9. The same features are shaded in figures 8 and 9.

In figure 8, the pole pieces 20 and 20' are opposite in magnetic polarity and are positioned between a first pair of rotor discs. The stator segments are on a first stator plane A - A'

In figure 9, the pole pieces are the same magnetic polarity due to the stator segments 22, 22', 23 and 23' being on a second stator plane B - B' in the annular coil stack. The four unshaded poles are on a different pole plane to the unshaded poles and are positioned between a second pair of rotor discs along with the shaded pole pieces shown in figure 10. This method of interleaving common coil planes with different pole planes allows the flux from different coil planes to interact with the same pair of magnetic rotor disks. Thus each pair of rotor discs has eight stator poles of alternating magnetic polarity between them.

In figure 10, there are eight identical stator segments 24 and 24' on a third stator plane C - C'. Every other stator segment is inverted so that their pole pieces are displaced by 90 electrical degrees and are also on a different pole plane. The four unshaded pole pieces of stator segments 24' are positioned between a third pair of rotor discs, along with four poles from the fourth stator plane. The fourth stator coil plane is the same configuration of stator segments shown in figure 9 but rotated by 90 electrical degrees, so forming the second phase of a two phase motor. The fifth and final stator plane utilises the same stator segment formation shown in figure 8 but inverted and rotated by 90 electrical degrees. There are a total of 5 stator planes, 5 rotor discs, 4 rotor disc pairs, 4 pole planes and 4 coils in the two phase motor described here. As each rotor disc has 8 poles on each face, this would be described as a two phase, 8 pole motor. However, there are 16 pole faces present on each pair of rotor discs, giving a total of 64 stator to rotor pole interactions.

Figure 11 shows a detail view of a rotor assembly 1, showing the main magnetically hard discs 11, the end magnetically hard discs 12, the disc spacing clamps 14 and the rotor shaft 15. There is also a pair of magnetically soft plates 13 to complete the magnetic circuit on the two end discs 12. The discs are oriented so that the like poles face each other and would repel each other if there was not an array of magnetically soft pole pieces between each pair of discs.

Figure 12 shows an alternative configuration of the invention, where coils I, II, I' and II' are connected together, forming a first phase of the invention. Similarly. Coils III, IV, III' and IV are connected together to form a second phase and coils V, VI, V' and VI' form a third phase. The rotor bearings 16 are also shown

Figure 13 shows two section views of the invention along planes Y - Y' and Z - Z' referring to figure 14, to illustrate the magnetic circuit linking the rotor permanent magnets to the stator coils.

Between the three middle rotor discs 11, there are flux paths that are both radial and axial in direction. The end rotor discs 12 have a soft iron plate 13 attached to the unused face of the disc to complete the magnetic circuit and reduce stray flux from the rotor. This allows the end disc magnets 12 to be of a thinner profile than the other disc magnets. The direction of current in each coil 25 is denoted by the + and - signs.

Figure 14 shows the additional magnetic circuits of the invention. The flux paths follow both helical and arcuate paths. This is the flux path predominant in the end rotor discs and stator sections.

Figure 15 shows an alternative form of rotor disc construction that incorporates an electrical coil 17 that is wound around and between hard magnetic blocks 10 and 10'. The coil and blocks are axisymmetrically aligned to the rotor shaft 15 by a backing disc 11. Any or all of the rotor discs in a rotor assembly may take this form of construction, with each rotor disc coil being connected to a common rotor circuit that is supplied with direct current from a slip ring assembly.

The coil is wound so as to allow the flux magnitude from the permanent magnet structure to be varied. When the machine is operating as a generator, this arrangement allows the use of a low current electronic regulator to provide stability of the generated output voltage, irrespective of speed or load variations. In this way, the efficiency of a permanent magnet generator can be combined with the controllability of a separately excited generator.

When the machine is operating as a motor, this arrangement allows the motor speed and torque characteristics to be modified by the injection of a small rotor current. This can enhance performance where the motor operates over a wide range of speed and torque conditions, as in, for example an electric vehicle.

Figure 16 shows an alternative form of rotor disc, incorporating an electrical coil. In this case there are two annularly wound rotor coils 17 and 17' connected to a common rotor circuit and an external electrical supply via slip rings (not shown) on the rotor shaft 15. The outer coil 17 has an associated magnetically soft structure 19 to form an alternating pattern of poles on both faces of the rotor disc. The soft magnetic poles align with the hard magnetic south poles on both faces of the disc. The inner coil 17' similarly has an associated arrangement of magnetically soft poles 18 that align with the hard magnetic north poles. The hard magnetic poles may be separate blocks or a solid ring. The coil connections and soft magnetic poles are arranged so that a current flow to the rotor in one direction augments the flux induced in the stator from the permanent magnets and reversing the rotor current reduces the flux induced in the stator.

Figure 17 relates to a second preferred form of the invention where the rotor and stator components are transposed. In this configuration, the stator 102 is within the rotor outside diameter and is comprised of a stack of at least two coils 125, each coil being sandwiched between two magnetically soft stator parts 121, each part having at least one pole piece. There are four pole pieces in this figure. For clarity, only one coil and stator part is shown in the figure.

Figure 18 shows the additional components required for the second preferred form of the invention. The rotor assembly 101 has an annular magnetically hard assembly 111 fixed to it. The rotor magnets maintain a clearance gap to the stator pole pieces 120 and 120' since the rotor radial and axial position is fixed by the rotor shaft 115 and bearings 116. There are magnetically soft rings 113 attached to each end of the magnet assembly to complete the magnetic circuit. There are two annular electrical coils 25, each coil located between a pair of magnetically soft pole plates 121. Each pair of pole plates forms an annular plane of pole pieces 120 and 120', each pole piece being of nominal opposing magnetic polarity to the adjacent pole pieces. Each coil is driven by a phase of a two phase electrical supply. There may be two or three coils or a multiple of two or three coils as previously described.

Figure 19A shows an assembly method for the external rotor form of the invention. This figure shows a section view of the plane X - X' in figure 19B. The stator pole plates are divided into segments 121 and these are located within a rotor shell 101. The permanent magnet assembly on the rotor takes the form of a stack of split ring permanent magnets. Each stack is comprised of a series of 'C' shaped, half ring magnets with the magnetic poles being on the flat faces of each half ring. For ease of assembly, a stack of split rings may be formed into two 'C' shaped rotor sub-assemblies, 111 and 111'. The magnet sub-assemblies can then be brought together as indicated by the arrows so the rotor magnets and stator pole pieces are interleaved. The rotor shell 1 can then be fitted over the two magnet sub-assemblies 111 and 111' and the sub-assemblies can then be secured to the rotor shell by various means, including glue, screws or by pressing the two sub-assemblies against the inside of the rotor shell by inserting retaining pins in the gaps between the two sub-assemblies, indicated by the arrows B. The rotor shell may also have locating features to assist in the correct positioning and spacing of the magnets in relation to the stator poles.

Figure 19B is a section view along axis Y - Y' in figure 19A, showing the position of the annular coils 125 and the rotor magnets 111 within the stator pole segments 121. The rotor magnets 111 maintain a clearance gap to the stator poles 121 through the alignment of the rotor shell 101 and shaft 115 to the stator assembly 103 by two bearings 116. The stator assembly incorporates a mounting flange for motor installation. As the coil windings are of an annular form, only two coils and two stator and rotor sections are required for a two phase electrical machine, resulting in a device that is reduced in length axially, compared to the drawing. Equally, an axially longer device could be made by increasing the number of coils, stator and rotor sections. The size of the radial gap between the rotor and stator is not critical and the magnetic forces are predominantly in the axial direction. This eases the constraints on the length of an external rotor motor, compared to a conventional radial flux, external rotor motor.

The invention described in this patent has a number of benefits over disc rotor motors known in the art and over permanent magnet, synchronous motors generally. Compared to transverse flux disc rotor motors, the motor detailed in this patent has the following advantages;

The assembly of multiple disc rotor motors is greatly eased due to the split stator and split rotor design and assembly methods as it allows the rotor and stator to be assembled independently of each other. The two outer assembly 'C' stacks are fitted around the inner assembly stack at a latter assembly stage. This makes it an easy matter to make a 6, 9 or 12 stack, three phase motor of the same profile as a standard induction motor. This facilitates the use of this design as a high torque, energy saving replacement for a standard motor.

The stator sections stack together and interlock in a space efficient manner. This makes for a compact motor with high specific power. This also facilitates the use of compression moulded, Soft magnetic compound (SMC) stator segments in the motors magnetic circuit. The stator segments may have self-aligning features to facilitate quick assembly of the stator split assemblies.

On a disc rotor transverse flux motor, the stator poles are all north or all south poles at any given moment. The device of the invention has stator pole pieces that have an alternating magnetic polarity. This allows for the stator to have the same number of stator poles as rotor poles, twice as many as in a disc rotor, transverse flux motor. This increases the area of magnetic interaction between the rotor and stator for a given rotor size and so increases the torque from a given magnet grade. The increased number of stator poles also reduces cogging and gives more uniform torque at low revolutions.

When compared to a multiple disc rotor axial flux motor, the motor detailed in this patent provides better heat dissipation from the copper coils. The presence of a stator iron to effectively couple the rotor and coil flux means the device of the patent does not require Neodymium magnets for good performance. The coil construction is simple and the end turns are very short, so reducing the copper losses.

When compared to a standard, cylindrical rotor, brushless permanent magnet motor, the motor detailed in this patent provides a lower rotor inertia for a given rotor diameter and length due to the gaps between the rotor discs. The motor also has a higher torque and power density due to the surface area for rotor to stator flux interaction being higher than on a solid cylindrical motor.

The rotor of a conventional brushless permanent magnet motor has the rotor magnets either glued externally to the surface of an iron cylinder or has the magnets located internally to the rotor surface. The former rotor design limits the maximum speed of the rotor to prevent the centripetal force on the magnets exceeding the strength of the glue. The latter design provides greater mechanical strength but also increases the separation of the rotor magnet and the stator poles. A permanent magnet disc rotor can have an outer diameter made from a material of high tensile strength while still minimising the gap between the rotor magnets and stator poles.

The hybrid rotor design, combines an electrical coil and a hard magnetic structure in the rotor assembly. The coil form allows the permanent magnet flux to be augmented or weakened by the injection of current to the coil via slip rings. This combines the efficiency of a permanent magnet motor or generator with the versatility of a field coil wound rotor.

The drawings and descriptions in this patent show a segmented external stator design with each stator component having one stator pole piece. Also described is an external rotor design with each stator component having multiple stator pole pieces. There is great flexibility in the scope of this patent for variation in the size, form and number of stator components and how they interlock, while still providing an alternating stator pole pattern between a pair of rotor discs.

The concepts described in this patent allows for a high degree of flexibility in the choice of materials, the stator configuration, the number of poles and the number of electrical phases on the device of the invention. The principle of operation can be applied to both internal and external rotor type machines. It will be appreciated by someone knowledgeable in the art that the present invention may take many forms without deviating from the scope of this patent.

Claims (9)

An improved Electrical Machine Claims: -

1. An electrodynamic machine comprised of rotor and stator components, the rotor having at least two axisymmetric discs or rings containing a magnetically hard material; the stator having at least two axisymmetric annular arrays of magnetically soft stator pole pieces, each array of stator poles having a substantially annularly wound electrical coil. Further: - Each rotor disc has an annular array of magnetic poles on the flat faces of the disc that substantially mirrors the array of magnetically soft stator pole pieces; Each rotor disc is angularly aligned to the next rotor disc so as to have their like magnetic poles facing each other; Each stator pole array may magnetically interact with two rotor discs and each rotor disc may magnetically interact with two stator pole arrays; A current passing through the annular coil of a stator section causes each pole piece in the associated stator pole array to have the opposite magnetic polarity to an adjacent pole piece; The stator pole array and rotor discs are axially interleaved and radially overlap such that the void between each pair of rotor discs is majority occupied by the array of stator pole pieces.

2. A machine as described in claim 1, where the plane of the substantially annular electrical coil broadly coincides with the plane of the stator pole array.

3. A machine as described in claims 1 or 2, where each stator section or a stack of stator sections is axially separable into two broadly equal parts or stator sub-assemblies.

4. A machine as described in claim 3, where an annular electrical coil is folded and contained within a stack of two stator sub-assemblies.

5. A method of assembly where a rotor comprising a stack of discs may be assembled between two stacks of stator sub-assemblies described in claims 3 or 4.

6. A machine as described in claims 1 or 2, where each rotor disc or a stack of rotor discs is axially separable into two broadly equal parts or rotor sub-assemblies.

7. A method of assembly where a stator comprising a stack of stator pole arrays may be assembled between two stacks of rotor sub-assemblies described in claim 6.

8. A machine as described in any preceding claim where at least one of the rotor discs incorporates an electrical coil or coils.

9. A device as substantially described hereinbefore, with reference to figures 1 to 16.