WO2014117350A1

WO2014117350A1 - Electrical machines

Info

Publication number: WO2014117350A1
Application number: PCT/CN2013/071175
Authority: WO
Inventors: Wee Keong Khoo
Original assignee: Tidal Harness Ltd.
Priority date: 2013-01-31
Filing date: 2013-01-31
Publication date: 2014-08-07
Also published as: GB2523974B; GB2523974A; GB201512637D0; CN104956573B; CN104956573A

Abstract

An electrical machine comprises a plurality of detachable modular stator parts (1, 50, 51, 200, 201, 210, 211, 220, 260, 390, 400, 410) configured to be able to be assembled to form a stator and a plurality of rotor parts (2, 52, 221, 261, 391, 401, 411) configured to be assembled to form a rotor. One of the stator parts and the rotor parts has a first radial component and a second radial component, and the other of the stator parts and the rotor parts has a third radial component. A separation gap is defined between the first radial component and the second radial component. The third radial component is located in the separation gap. The rotor is movable relative to the stator. The electrical machine may produce a sinusoidal air gap MMF distribution with minimal distortion.

Description

ELECTRICAL MACHINES

TECHNICAL FIELD

This invention relates to electrical machines and particularly, though not exclusively, relates to direct- drive electrical machines such as, for example, electrical generators having windings that allow the stator to be modularized so as to facilitate the production, test and assembly process.

BACKGROUND

Most wind and tidal turbine generators are driven through a mechanical gearbox which amplifies the rotational speed and thus increasing their power generation without increasing substantially their size. The transmission gearbox itself is of considerable weight and size. Despite a long history of technical development, present-day gearboxes still suffer reliability issues which add to the overall cost of energy. In response to gearbox weaknesses, it has been proposed to eliminate the gearbox by using direct- drive electrical generators in conjunction with solid-state power converters. In the absence of a gearbox, direct-drive generators are necessarily much larger machines than those employing a gearbox for the same power generation capacity because the former have lower rotational speeds. Direct-drive generators are not issue-free since the elimination of a gearbox shifts reliability to the generator. The outer diameter of direct-drive electrical generators poses a challenge in engineering development and also in manufacturing, production management and logistics.

Direct-drive tidal generators may have the air gap between the stator and rotor flooded with seawater. Mechanical rolling-element bearings are unsuitable for submarine applications and are prohibitively expensive for such large diameter machines. Instead, the stator and rotor of such "wet" generators are supported by marine grade, low friction journal bearings which operate on the basis of hydrodynamic lubrication. Wet generators with journal bearings typically have relatively large mechanical clearance between the stator and rotor compared to electrica machines supported by rolling-element bearings. Large direct-drive electrical generators are usually constructed by assembling together a plurality of small machine segments for ease of manufacturing and also for economic reasons. Several independent, parallel pairs of stator segments may be distributed around the periphery of the machine to form an axial flux electrical generator. Each stator segment may comprise an E-core, a concentrated coil and some electrical components encapsulated In a waterproof box. One shortcoming of such a topology is the limited power capability due to the substantial spacing between the stator segments which could otherwise be filled by active magnetic materials. A conventional three-phase double-layer winding would result in a higher power density machine, but since the phase windings overlap each other around the machine, the stator cannot be segmented without having overhanging coils.

Non-overlapping concentrated coils and/or fractional slot windings are very common in permanent magnet electrical machines. However, they are not used in induction machines as they induce opposing field harmonics in the rotor which results in very low torque. Squirrel cage and solid steel rotors are essentially a short circuited conductive structure without any predefined current paths and thus currents Induced will give rise to rich MMF harmonics. One remedy for this problem is to use a wound rotor having the same pole number as the fundamental stator MMF so that other harmonics are not induced. Whilst it is not impractical to use a wound rotor for wet induction generators, its construction could be somewhat complicated.

A linear induction motor has been proposed that has an opposing two-stator sandwiching a conductive rotor plate. The two stators are offset from each other so that the concentrated coils from the second stator are displaced by an odd multiple half a wavelength of the undesired pole number or by an odd multiple of a whole wavelength of the undesired pole number and with the current directions reversed. Such arrangements eliminate the even integer harmonic of the magnetic field while reinforcing the desired fundamental field. This harmonic cancellation requires equal and balanced contribution from both offset stators. If either stator fails to operate the even harmonics would not be cancelled, resulting in a large negative torque which counters the dominant positive torque. The air gap on both sides of the rotor should also be nominally 'ijqual. When there is a relatively large rotor excursion in the axial direction, the unwanted MMF harmonics may not be fully cancelled.

The construction of a wet electrical generator is somewhat more complicated than a dry generator because water ingression into critical electrical components must be prevented. Coils may be encapsulated with resin and the assembly further protected by a stainless steel casing. Alternatively, the stator segment and coils may be encapsulated in resin within a plastic box. For total removal of air voids in the resin and coil, the whole assembly would need to be impregnated under vacuum pressure which adds to the coil production cost. Should a failure is detected or the assembly is deemed unfit to be used, the whole coil assembly is discarded.

SUMMARY

According to one aspect there is provided an electrical machine that may be used for tidal energy harvesting applications where the only rotating part is the annular rotor. The absence of a gearbox, transmission shaft and mechanical rolling element bearings reduces cost and simplifies the overall system. The air gap of the machine may be filled with seawater.

There may be provided an axial-flux induction generator having a double-sided annular stator sandwiching an annular electrically conductive plate. The rotor plate may be ironless to reduce corrosion as a result of direct exposure to seawater. Another added advantage is that the rotor plate may weigh less than the permanent magnet mounted rotor and this results in a lighter, lower static friction and lower inertia rotor for the turbine. Utilizing an annular plate structure as the rotor simplifies considerably the manufacturing process compared to a wound or squirrel-cage rotor. Modular or segmented stators and rotors may be provided that can be assembled side-to-side to form respectively a continuous annular stator and a continuous annular rotor. The modular stators may be identical so that they can be tested independently during production. Any module in the generator may be able to be easily replaced. A subsea casing which houses the power conversion equipment may be integral to each modular stator. The modular stators may have winding schemes that produce substantially equivalent MMFs to those of conventional electrical machines with distributed and overlapping windings. The coils may not overlap between two adjacent modular stators and may not be in an overhung position. The MMF field may be characterised by a substantially sinusoidal shape. The fundamental harmonic component may be the most dominant.

The winding schemes may be such that the sinusoidal waveform of the induced MMF is substantially preserved even when there is large rotor excursion. The winding schemes may be configured for the generator to be a single-phase or multiple-phase machine.

The concentric or lap coils may be prefabricated before insertion into the stator slots. The coil number of turns may vary as they spread across a number of slots. The coils may be sufficiently insulated such that they do not need to be encapsulated in resin. The coils may also be directly exposed to seawater to facilitate heat removal.

There may be a generic axial-flux induction machine having a double-sided annular rotor sandwiching an annular stator. The stator may comprise discrete, ferromagnetic poles ^'that may be held and spaced apart by annular plates. Each rotor may comprise an electrically conductive plate and a ferrous plate (back-iron).

There may be provided winding arrangements applicable to linear electrical machines, radial-flux and axial-flux rotating electrical machines that may be induction, permanent magnet, wound-field synchronous, doubly-fed induction or synchronous reluctance electrical machines. BRIEF DESCRIPTION OF THE DRAWINGS

In order that the present invention may be easily understood and readily put into practical effect, there shall now be described by way of non-limitative example only exemplary embodiments, the description being with reference to the accompanying drawings. In the drawings:

Fig. 1 is an exploded perspective view of an axial-flux electrical machine with modular stators and modular rotors assembled to form respectively a full continuous stator and rotor; Fig. 2 illustrates one modular double-sided stator arrangement and one modular rotor of Fig. 1 ;

Fig. 3 shows a waterproof box accommodating a stack of stator laminations of Fig. 1 ;

Fig. 4 shows a prior art single-phase, induction motor with symmetrical windings distributed in a 16- slot staler where, (a) is a radial-flux machine, (b) is an axial-flux machine, and (c) is an equivalent representation of slots and coils;

Fig. 5 shows a single-phase, axial-flux induction machine where

(a) is a topology with concentric windings,

(b) is a topology with lap windings, and

(c) is an equivalent winding scheme comprising a plurality of base unit;

Fig.6 shows

(a) a single-phase, axial-flux induction machine comprising a double-sided stator with its auxiliary windings wound as consequent pole windings,

(b) the air gap field produced by the main windings, and

(c) the air gap field produced by the auxiliary windings;

Fig. 7 shows

(a) an equivalent full-pitched winding scheme of a single-phase, axial-flux induction machine, and

(b) its air gap field and harmonics;

Fig. 8 shows

(a) an equivalent short-pitched winding scheme of a single-phase, axial-flux induction machine, and

(b) its air gap field and harmonics;

Fig. 9 shows

(a) the winding scheme of a single-phase, axial-flux induction machine with sinusoidal windings, and

(b) its air gap field and harmonics;

Fig. 10 shows

(a) the winding scheme of a single-phase, axial-flux induction machine with sinusoidal windings and with a higher number of colls per base unit, and

(b) its air gap field and harmonics; Fig. 11 shows

(a) an equivalent full-pitched winding scheme of a three-phase, axial-flux induction machine, and

(b) the same winding scheme but with overlapped phase coils;

Fig. 12 shows

(a) an equivalent short-pitched winding scheme of a three-phase, axial-flux induction machine, and

(b) an equivalent short-pitched, double-layer winding;

Fig. 13 shows

(a) an equivalent 8/9 short-pitched winding scheme of a three-phase, axial-flux induction machine, and

(b) an equivalent 8/9 short-pitched, double-layer winding;

Fig. 14 shows

(a) an equivalent 7/9 short-pitched winding scheme of a three-phase, axial-flux induction machine, and

(b) an equivalent 7/9 short-pitched, double-layer winding;

Fig. 15 shows

(a) an equivalent short-pitched winding scheme of a three-phase, axial flux induction machine with better slot utilization,

(b) phase A coils with an average coil pitch of 4, and

(c) all phase coils have an average coil pitch of 6;

Fig. 16 shows the winding scheme of a three-phase, axial flux induction machine with an asymmetrical double-sided stator,

Fig. 17 shows the winding scheme of a three-phase, axial-flux induction machine with windings of one phase split between the symmetrical double-sided stator;

Fig. 18 shows the winding scheme of a three-phase, axial-flux induction machine with windings of one phase split between the symmetrical double-sided stator and the number of turns of all phase coils apportioned;

Fig. 19 shows the winding scheme of a five-phase, axial-flux induction machine; Fig. 20 shows the winding scheme of a five-phase, axial-flux induction machine having overlapped windings;

Fig. 21 shows

(a) the winding scheme of a six-phase, axial flux induction machine, and

(b) the power converters feeding the phase coils;

Fig. 22 is an exploded perspective view of an axial-flux electrical machine with 8 modular stators and 8 modular rotors assembled to form respectively a full continuous stator and rotor;

Fig.23 illustrates one modular stator and one modular double-sided rotor;

Fig.24 shows

(a) a stator pole comprising a stack of ferromagnetic laminations,

(b) a stator pole comprising a rectangular cross-section non-ferrous or ferrous block, and

(c) a stack of lamination housed in a waterproof box;

Fig. 25 illustrates the staggered ends of a modular stator;

Fig. 26 shows a single-phase, axial-flux induction machine where

(a) is a topology with concentric windings,

(b) is a topology with lap windings, and

(c) is an equivalent winding scheme comprising a plurality of base units;

Fig. 27 shows the winding scheme of a single-phase, axial-flux induction machine with its auxiliary windings wound as consequent pole windings;

Fig. 28 shows the winding scheme of a single-phase axial-flux induction machine having sinusoidal windings;

Fig. 29 shows the winding scheme of a single-phase axial-flux induction machine having sinusoidal windings and with a higher number of coils per base unit;

Fig. 30 shows the winding scheme of a three-phase, axial flux induction machine;

Fig. 31 shows the winding scheme of a three-phase, axial flux induction machine with a higher number of coils per base unit;

Fig. 32 shows the winding scheme of a three-phase, axial flux induction machine having overlapped windings;

Fig. 33 shows an equivalent 8/9 short-pitched winding scheme of a three-phase, axial flux induction machine; Fig. 34 is an equivalent short-pitched winding scheme of a three-phase, axial flux induction machine with better slot utilization;

Fig. 35 is the winding scheme of a three-phase, axial flux induction machine with its number of turns apportioned;

Fig. 36 is a variant winding scheme of a three-phase, axial flux induction machine with its number of turns apportioned;

Fig. 37 shows

(a) the winding scheme of a five-phase, axial flux induction machine, and

(b) with overlapped windings;

Fig.38 shows

(a) the winding scheme of a six-phase, axial flux induction machine, and

(b) the power converters feeding the phase coils;

in accordance with another embodiment of the present invention;

Fig.39 shows

(a) a radial-flux, rotating induction machine comprising a double-sided stator, and

(b) its modular stator and modular rotor;

Fig.40 shows

(a) a radial-flux, rotating induction machine comprising a double-sided rotor, and

(b) its modular stator and modular rotor;

Fig.41 shows

(a) a radial-flux, single-air gap rotating induction machine, and

(b) its modular stator and modular rotor;

Fig. 42 illustrates a permanent magnet electrical machine having a double-sided stator;

Fig. 43 illustrates a permanent magnet electrical machine having a double-sided rotor;

Fig. 44 illustrates a Halbach array permanent magnet electrical machine having a double-sided rotor; Fig. 45 is a wound-field synchronous machine having a double-sided stator;

Fig.46 is a wound-field synchronous machine having a double-sided rotor;

Fig.47 shows a synchronous reluctance machine having a double-sided stator; and

Fig. 48 shows a synchronous reluctance machine having a double-sided rotor. DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

The preferred embodiments disclosed herein relate mostly to large underwater turbines employing direct-drive electrical generators, but are equally applicable to wind turbines, wave powered machines and/or electric vehicles.

First Embodiment

Here is provided an axial-flux induction generator having an annular double-sided stator sandwiching a single annular rotor. Both the double-sided stator and rotor are modularized for production convenience and assembled end-to-end circumferentially to form a full electrical machine. FIG. 1 is an example of such an assembly where 12 modular stators 1 and 12 modular rotors 2, each spanning 30 mechanical degrees, constitute respectively the full annular stator and rotor. Any number of modular stators 1 or modular rotors 2 may be provided as practicable. The support structure 3 of the modular stator has mounting points for convenient assembly to adjacent modular stator. The assembled modular stators provide seamless current linkage distribution equivalent to that of an electrical machine built with a continuous annular stator. This results in a non-disrupted rotating magnetic field in the air gap that has substantially the same peak magnitude and spatial relationship throughout the annular machine.

FIG. 2 illustrates an example of one modular stator 1 and one modular rotor 2 according to the first embodiment of the present invention. An axial-flux machine with a double-sided stator 4 has two radially-extending and substantially parallel stator components defining an air gap therebetween where energy is derived. The rotor has a radially-extending rotor component 5 beatable between and rotatable relative to the coils 6 so that MMF components induced in the rotor 2 are contributed by both stators 4 which are most effective when they are sinusoidal. If appropriate current linkages can be provided through coils 6 such that magnetic flux from one side of the stator poles can traverse across the air gaps to reach the opposite side of the stator poles, by symmetry an ironless rotor can be used. Such a topology is advantageous in that corrosion of iron as a result of direct exposure to seawater is avoided and a simple electrically conductive plate S may be used. The rotor plate 5 may be made of metals having low resistivity including copper and aluminium. The modular rotor plates 5 may be connected or linked together circumferentially so that a continuous short circuit is created around the rotor.

FIG. 3 illustrates one side of the double-sided stator 4. The stator may comprise a stack of ferromagnetic materials 8, for example electrical steel or mild steel, appropriately laminated and conventionally insulated for dry axial-flux machine application. The whole stack of stator iron may be sprayed or painted with a plurality layer of waterproof protective coatings 9 suitable for marine applications. The coated stack of stator laminations is preferably housed inside a plastic box 10 serving to reinforce protection from accidental abrasion and water ingression, as illustrated in FIG. 3. The void 11 in the box may be filled with waterproof resin compound or replaced with ferrous material which results in slots having parallel sides. The box 10 has a toothed feature on its front face to match the physical protrusions 12 and slots 13 of the axial-flux stator. The flat surface at the back is hermetically sealed. In the example shown in FIG. 3 the box fully embraces the width of the stator poles including the pole tips 14. Although there may be thrust or journal bearings which prevent direct contact and rubbing between the stator and rotor, it is nevertheless preferable to cover and reinforce the stator pole faces of the box with a protection lid which serves as a secondary protection against abrasion. This protection lid may be made of substantially strong and non-metallic materials including fibreglass. The double-sided stator is fixed to a support structure 3. The coils 6 may be wound in-situ or prefabricated coils may be inserted into the stator slots 13 defined by the box 10 and external to the stator iron 8. Such an arrangement is advantageously simpler to manufacture compared to the process of encapsulating both stator iron and enamelled copper coils in resin. Direct exposure to seawater means that heat removal through the conductor's insulation sheath is more efficient. The conductors of the coils 6 are preferably insulated with high dielectric strength plastic sheaths that are impervious to seawater. Winding conductors insulated with pvc, PE2 + PA and HT4 sheaths typically found In submersible pumps may be used. The modular stator 1 may have staggered ends so that the coils 6 are fully contained within the assembly. The coils 6 may be concentric or lap coils. The number of turns may also vary in proportions across a number of slots 13 to obtain a more sinusoidal MMF. in FIG. 2, a waterproof subsea casing 7 may be made integral to the support structure 3. Coil terminals may enter the subsea casing 7 via appropriate compression glands where the interconnections with other coils 6 are made. Alternatively, the coil terminals may be spliced outside the subsea casing. The subsea casing 7 may also house the power conversion equipment responsible for converting power generated within the module. The integration of power conversion equipment to the stator simplifies the production and pre-deployment where each modular stator may be independently tested and configured as "plug and play". Failed modular stators may also be replaced with ease without affecting healthy modules. The winding schemes defining the phases of the axial-flux induction generator will now be described with reference to FIGS. 5 to 21. The term "base unit" will be frequently used to define the section having a base winding spanning the length of a pole-pair. Said modular stator 1 may comprise one base unit or multiple base units as appropriate for the application. FIGS. 4(a) and 4(b) show respectively the single-phase 2-pole winding scheme of prior art radial-flux and axial-flux induction motors. The term "single-phase" refers to windings that are connected to single-phase a.c. power grid or single-phase power converters although for practical reasons they usually have two-phase windings. The 16-slot cylindrical stators are hypothetical^ cut and unrolled (shown as dotted lines and arrows) so that their slots and coils are laid out flat as illustrated in FIG. 4(c). Single-phase induction machines have two-phase windings - one being the main winding and the other being the auxiliary winding denoted "M" and "A" respectively. The purpose of the auxiliary winding is to provide assistance to start the induction motor since energising only the main winding with sinusoidal voltage at zero rotor speed will give rise to a pulsating air gap MMF at the frequency of the source. This is due to the cancellation of the positive sequence torque (due to forward travelling MMF) and negative sequence torque (due to backward travelling MMF) which are both equal in magnitude but opposite in direction.

Referring now to the two-phase, single-layer winding of FIG. 4(c), the auxiliary winding is in space quadrature with respect to the main winding. The main winding and auxiliary winding each occupies 8 slots and the number of conductors per slot may or may not the same for both. The main and auxiliary windings have the same slot per pole per phase (slot/pole/phase) namely, q_M = q_A = 4. Even if the main and auxiliary windings have the same number of turns and same wire gage, their phase resistance is not the same since the end windings may be slightly different for the two phases. Because this machine has single-layer winding configuration, the end windings of the main and auxiliary coils overlap each other around the staior. There is not a location in the machine where the stator and coils can be physically split into equiangular segments without cutting at least some coils.

FIG. 5(a) illustrates how the two-phase winding may be split in modular fashion while retaining the conventional features of the main and auxiliary windings according to the first embodiment of the present invention. In this double-sided stator configuration the auxiliary windings are wound at one side of the stator 50 while the main windings are wound at the opposite side of the stator 51. The two- phases are in space quadrature to each other. A rotor plate 52 is sandwiched between the opposing stators 50 and 51. The arcs 53 represent the end windings linking the coil sides. For example, the coil side (M) at slot number 1 links to the coil side (M') at slot number 8. Both the main and auxiliary phases comprise concentric windings 53 with coil pitches of 5 and 7. This gives an average coil pitch of 6 which is the same as the prior art winding scheme of FIG.4. FIG. 5(b) shows that lap windings 54 with coil pitch of 6 may also be used which give the same current linkage distribution as that of FIG. 5(a). The winding schemes disclosed hereafter are shown as concentric windings but it will be appreciated by persons skilled in the art that lap windings may be used.

FIG. 5(c) is a winding schematic equivalent to FIG. 5(a) where a plurality of base units 55 are connected end-to-end forming a multi-pole machine. This schematic equivalent is shown without the stator iron and will be used hereafter to aid discussion. Each base unit 55 spans 16 slots as segregated by the bold lines. A modular stator 1 , as defined previously, may comprise one base unit 55 or multiple base units 55. The two opposite stators 50 & 51 are shifted by 4 slots with respect to each other so that they can be split without cutting or fouling the coils. The teeth and slots of the opposing stators are aligned across the axial air gap. It can be seen from FIG. 5(a) that not all the stator slots are filled with conductors. The stator back-iron may be extended to fill these slots since there is only one phase per stator side. For operation as a generator, the induction machine is connected to a power converter where reactive power is drawn from the energized a.c. line (in the opposite direction to the active power delivered to the a.c. line). Since the tidal flow causes the turbine/rotor to rotate, the auxiliary windings for starting may be omitted in the stator, leaving only the main windings in service. The disadvantage of this topology is that the induction machine cannot be operated as a motor to kick-start the turbine during periods of low tidal flow. It is preferred that both windings are present in the generator to assist the turbine in overcoming static friction between the bearings of the stator and rotor. Furthermore, the functions of the auxiliary and main windings may be interchanged similar to a bi-directional induction motor.

Kick starting of the turbine may be accomplished by injecting current in the auxiliary windings that shifts temporally ahead of the current of the main windings. This current injection sequence causes the rotor to rotate from the auxiliary windings to the main windings. Phase-advance in the current may be achieved by connecting a series capacitor between the auxiliary windings and the single-phase a.c. supply. Alternatively, the auxiliary windings may be connected directly to variable frequency converters such as a pulse-width modulated voltage-source inverter or a current-source inverter with a controlled rectifier. The voltage of the auxiliary winding may be varied to mitigate torque pulsations across all operating points. An alternative variant of the single-phase axial-flux induction generator is shown in FIG. 6(a). Such a winding scheme is derived from the fact that current linkages in the radial direction (inwards and outwards) are responsible for creating the fields in the air gaps regardless of how the end windings are organized around the circumference. The auxiliary windings are manipulated such that a consequent pole is obtained across the boundary between two adjacent base units. Such a winding arrangement enables the stators to be split into aligned modules as shown in FIG. 6(a). One modular stator may comprise one or multiple base units as defined by the bold lines. FIGS. 6(b) and 6(c) are respectively the air gap fields produced by the main and auxiliary windings where the latter Is phase advanced by 90 degrees. The variant scheme in FIG. 6(a) is equivalent to that of FIG. 5(c) in terms of current linkages and air gap magnetic fields. However, the end windings of the auxiliary will be correspondingly longer which makes the windings of the machine somewhat unsymmetrical. If required, the increase in resistance in the auxiliary phase may be adjusted by using conductors with larger cross-sectional area. Unsymmetrical windings are nevertheless common in practical single- phase induction machine to generate a high starting torque. The winding scheme of FIG. 6(a) may be extended to a double-layer winding configuration where each stator side comprises both main and auxiliary windings, as illustrated in FIG. 7(a). The auxiliary winding occupies the bottom layer of slots while the main winding resides at the upper layer of the slots. The stator teeth and coils of the main and auxiliary windings of one side of the modular stator are aligned to that of the opposing side and they are combined to produce an equivalent full-pitched winding. FIG. 7(b) shows the corresponding air gap flux density and harmonics produced by the main windings. The auxiliary windings would also produce the same results if the 'machine is symmetrical. If the main and auxiliary windings at the upper stator are connected to a power converter independent to that of the windings at the bottom stator, then there may be redundancy for fault tolerance. Any failure in the power converter or coils in either side of the modular stator will not result in a total f ilure since the other side is still functional (at a lower power rating). In FIG. 7(b) the fundamental component is the most dominant but some third and fifth harmonics also exist. The space harmonics content can be reduced by having current linkages which resemble that produced by short-pitched windings. It is common to find that conventional double-layer windings have short-pitched or chorded coils where each coil turn starts from the bottom layer of one slot and returns via the top layer of another slot. An equivalent short-pitching effect can be created by shifting the full-pitched coils of the top stator left or right with respect to the bottom stator. This configuration is shown in FIG. 6(a) where the bottom stator is shifted one slot to the right, creating a staggered base unit. One modular stator may comprise one base unit or multiple base units as defined by the bold line separations in FIG. 8(a). FIG. 8(b) compares the harmonics produced by the full-pitched and short-pitched winding schemes. The space harmonics content has been reduced by the latter and so is its fundamental component.

The current linkage distribution in a two-phase winding machine can be made to resemble a sinusoid by appropriately apportioning the number of turns in the slots. FIG. 9(a) shows how the machine may be adapted to incorporate approximate sinusoidal windings. The differences in text size designate the relative number of conductors in the slots where the bold font represents a higher number of conductors. FIG. 9(b) shows the resultant air gap flux density and harmonics which are similar to the scheme in FIG. 8(a). A better sinusoid air gap field can be obtained by increasing the number of coils per base unit. FIG. 10(a) is an example where six concentric coils per phase constitute a base unit as opposed to four concentric coils per phase in the winding scheme of FIG. 9(a). A larger font represents a higher number of conductors while a smaller font represents fewer conductors. The resultant two-pole field is closer to an ideal sinusoid as shown in FIG. 10(b). The unwanted harmonics which are prevalent to the machines in FIGS. 5(c), 6(a), 7(a), 8(a) and 9(a) have been reduced with the winding scheme of FIG. 10(a).

The concept of modularized stators arrangement is also applicable to three-phase machines. Compared to single-phase machines, three-phase electrical machines naturally produce more sinusoidal air gap MMFs without requiring sinusoidal windings. FIG. 11 (a) shows a three-phase axial- flux machine built with multiple base units stator. A modular stator may comprise one or multiple base units. The stator is subdivided into 3 layers and coils of one phase are confined within one layer without overlapping to other phases. Phase A, phase B and phase C coils are spaced at 120 electrical degrees apart and they occupy the top, middle and bottom layers of the slots respectively. Each base unit spans twelve slots with slot/pole/phase and an average coil pitch equal to two and six (for example, slots 1-8 and 2-7 for concentric coils and slots 1-7 and 2-8 for lap coils) respectively. Alternatively, the phase coils may be overlapped in a single-layer winding as shown in FIG. 11 (b). The teeth and slots of both opposing stators are aligned to each other. The current linkage distribution at one side of the stator is the mirror of the opposite side and they combine to produce the air gap MMF. Although the fundamental component is the highest in relative magnitude, the resultant air gap MMF also contains some non-triplen higher harmonics (5, 7, 11, 13, etc.) which is common for full-pitched coils.

Since the phase coils on one side of the stator are physically not overlapping with coils at the opposite side of the stator, it would be possible to shift one stator with respect to the other by one or higher number of slots thereby producing a chorded or short-pitched effect. In FIG. 12(a) the winding arrangement at the upper and bottom stators are identical to that of FIG. 11 (a) but the bottom stator is shifted left by one slot pitch with respect to the upper stator thereby creating an equivalent 5/6 short- pitched winding. The base units are staggered but the stator teeth and slots from both opposing stators are aligned. The staggered stators in FIG. 12(a) may be equivalents mapped as prior art double-layer winding as shown in FIG. 12(b). A conventional 5/6 short-pitched double-layer winding has coils spanning five slot pitches measuring from the inward current at a slot of the upper layer to the outward current at a slot of the bottom layer. For example, phase 'A' current flows inward via slot number 5 in the upper layer and outward via slot number 10 in the bottom layer. For clarity, only the chording of phase A coils are shown in FIG. 12(b). It is evident that the phase coils of prior art double- layer winding are overlapped all around the stator and there does not exist a point where the stator may be segmented without cutting the coils' end windings or having overhung coils. The present invention, however, can be segmented and yet the coils are fully contained within. Such staggered stators arrangement advantageously achieves better sinusoidal air gap MMF compared to the full- pitched arrangement.

Using the same winding philosophy, an 18-slot double-sided stator may be shifted one slot or two slot pitches to accomplish the 8/9 and 7/9 short-pitched arrangements respectively. FIGS. 13(a) and 13(b) respectively show the winding arrangement of an 8/9 short-pitched axial-flux induction machine and its equivalent prior art 8/9 short-pitched double-layer winding. FIGS. 14(a) and 14(b) respectively show the winding arrangement of a 7/9 short-pitched axial-flux induction machine and its equivalent prior art 7/9 short-pitched double-layer winding. The boundaries between identical base units are shown as bold lines. A modular stator may comprise one or multiple number of base units. With their slot/pole/phase equal to 3, the electrical machines in FIGS. 13(a) and 14(a) have even lower space harmonics contents compared to the machine of FIG. 12(a).

The coils may be made to occupy more slots so as to increase slot utilization of the machine. FIG. 15(a) shows a double-sided stator arrangement with triple-layer coils occupying 67% of the slots. Each phase winding has eight colls contributing to a pole-pair within the twelve slots. Since every slot of the stator has coil sides from two phases, the stator cannot be segregated without splitting the coil sets of one of the phases. The base unit defined by the bold lines in FIG. 15(a) encloses phase B and phase C coils but splits the phase A coil. Any attempt to shift phase A coils so that they are enveloped within the separation unit would distort the spatial relationship between the phases. However, the end windings of phase A may be manipulated so that the same current linkage distribution with respect to phases 8 and C can be achieved. FIG. 15(b) is the three-phase winding of one base unit. Phases B and C each has one set of winding comprising four concentric coils with an averaged coil pitch of six. Phase A coils may be split into two smaller sets of winding with each set comprising two concentric coils within the twelve-slot unit without changing the inward and outward current distribution.

With an averaged four coil pitch, the end windings of phase A are shorter than that of phases B and C which implies that phase A will have a lower phase resistance. The number of turns in phase A coils is preferably unchanged to keep the current linkage of the machine balanced. But the length of coil interconnections within phase A or its conductor size may be adjusted to obtain the same resistance as that of phases B and C. Alternatively, the concentric coils of phase A may be made to span the same coil pitch as phases B and C as shown in FIG. 15(c). Spanning six coil pitches imply that the end winding of phase A are necessarily overlapped. Nevertheless, the end windings of phase A do not overlap that of phases B and C and this represents a distinct advantage.

There may be provided yet another winding arrangement where the coil pitch is equal among the phases and none of the phase coils or end windings are overlapped. Since the air gap MMF is independently contributed by each of the opposing stators, it would be possible to have an asymmetrical stators arrangement which enables the three-phase windings to be conveniently segmented. FIG. 16 shows an asymmetrical double sided stator where one side of the stator is wound with phase A coils and the other side is wound with phases B and C coils. Each base unit which spans twelve slots is defined by the bold lines. A modular stator may comprise one or multiple base units. Every set of windings regardless of the phase has the same coil pitch (τ » 6 slot pitches). Such an arrangement is phase-balanced and has the advantage of maximizing the slot utilization while retaining the non-overlapping windings. If geometrically symmetrical stators are required, one of the stators may be wound with the full set of coils of the first phase and half the number of turns of the second phase coils, and the other stator may be wound with the full set of coils of the third phase and half the number of turns of the second phase coils. FIG. 17 illustrates such a winding arrangement according to the first embodiment of the present invention. Tne upper stator has all the phase C coils and half the number of turns of phase B coils whereas the bottom stator has the all the phase A coils and half the number of turns of phase B coils. In each side of the stator the winding layers of phases A and C are twice the size of phase B but when the both sides are combined the total slot area of phase B is equal to that of phases A and C. The axes of phase B coils from both sides of the stator match each other in space and direction and the coils are connected in series so as to obtain the same number of turns as that of phases A and C. The coils in FIGS. 16 and 17 may have different number of turns within their respective layer to produce the short-pitched effect. By way of example, FIG. 18 shows an equivalent short-pitched winding scheme where phase coils designated with bold font has twice the number of conductors than those with normal font. Modularized stators for axial-flux machines having higher number of phases (4, 5, 6, 7 phases, etc.) may be derived using the winding arrangement principles disclosed above. There are many permutations of coil arrangement. For example, each side of the double-sided stator may have all the phases or half of the phases. Asymmetrical split of phase coils may also be accomplished where the phases are split unequally between the two opposing stators. Additionally; the phases may all be tied at one neutral point or each phase may be connected to a power converter as an independent circuit.

FIG. 19 is an example of a five-phase machine having an asymmetrical double-sided stator where the upper stator has three phases while the bottom stator has the remaining two phases. The spatial angle between each two phases is 72 electrical degrees apart. The base unit is defined by a segregation (in bold lines) enclosing the 10-slot stator segment. One modular stator may comprise one base unit or multiple base units. The five phases may be tied to a neutral point and the terminals are connected to a five-phase power converter. For fault tolerance, the phases may be connected as independent circuits with five separate power converters feeding phase coils "A to E. The phase coils may also be compacted into a single layer and seated on both sides of the stator. As shown in FIG. 20, the phase coils of the upper stator 200 mirror that of the bottom stator 201 and they may be connected together in series or parallel to a power converter. Alternatively, the phase coils of the upper stator 200 and bottom stator 201 may each be connected to an independent power converter.

A multi-phase axial-flux machine may also be constructed with two or more groups of symmetrical phase windings. FIG. 21 (a) is an example of a six-phase machine constructed with two groups of symmetrical three-phase windings A1 , B1, C1 and A2, B2, C2 which are shifted by 30 electrical degrees. The first group of windings is wound at the upper stator 210 while the second group is wound at the bottom stator 211. The winding arrangement at the upper stator 210 mirrors that of the bottom stator 211 , but the latter is shifted right by one slot thereby creating a 30 electrical degree phase shift. A base unit spans twelve slots as defined by the bold lines in FIG. 21 (a). A modular stator may comprise one base unit or multiple base units. Since the coil sides occupy odd numbered slots at the upper stator and even numbered slots at the bottom stator, a double-sided stator having six slots per base unit may be used. The hatched areas in FIG. 21(a) may be empty slots for a twelve-slot base unit or ferromagnetic iron for a six-slot base unit. The latter has non-aligned stator teeth between the opposing stators.

FIG. 21 (b) shows the first group of the three-phase windings is fed by a three-phase power converter 212 while the second group is fed by a separate three-phase power converter 213 whose phases are 30 electrical degrees delayed with respect to power converter 212. This six-phase winding configuration has the advantage of eliminating lower order harmonics and may be more fault tolerant due to its dual-power converter topology. The windings in FIGS. 19, 20 and 21 may be extended to schemes where coil sides occupy a plurality of slots for improvement of MMF distribution.

Features common to the winding schemes described above will now be described. Each side of the stator produces approximate sinusoidal current distribution independently of the other. When both stators are arranged to sandwich a non-magnetic and electrically conductive rotor plate, the total air gap MMF is the superposition of air gap MMFs generated by the respective stators. For an axial-flux wet generator with hydrodynamic journal bearings, the air gap separation between the rotor and stators is all but equal throughout the whole machine. Any rotor excursion in the axial direction within the permissible mechanical clearance would still result in predominantly a sinusoid-shape induced air gap MMF. This is a distinct advantage of the present invention.

Although the axia!-flux induction generators disclosed in the present invention utilize slotted stators, the winding schemes may also be applicable to slotless electrical machines. The term "slotless" refers to a smooth magnetic air gap separating the stator and rotor. Lorentz forces exerting on the conductors during operation may dislodge the coils in a slotless machine so the coils must be secured in position mechanically by means of non-magnetic fixtures including protrusions that resemble the shape of actual stator teeth. Alternatively, the coils may be resin impregnated within a suitable mounting casing. Coils encapsulated in resin may be made wider to fill the space that would otherwise be occupied by resin. Slotless induction machines require very high magnetizing current because of their large air gap. Without the presence of the stator teeth the coil number of turns may be increased to compensate for the reduction in the magnetizing MMF. Second Embodiment

According to a second embodiment, there is provided an axial-flux machine having a pair of radially- extending and substantially parallel annular rotors components 221 sandwiching a radially-extending, annular stator component 220. The continuous annular stator is formed by assembling end-to-end circumferentially a plurality of modular stators. FIG. 22 illustrates an example of such an assembly where eight modular stators 220 and eight modular rotors 221 , each spanning 45 mechanical degrees, constitute the full annular stator. Similar to the machines disclosed in the first embodiment, any number of modular stators 220 and modular rotors 221 may be provided. The rotor 221 may or may not have the same number of modules as the stator 220. When assembled, the modular stators 220 provide seamless current linkage distribution equivalent to that of an electrical machine built with a continuous annular stator. The air gap magnetic field has substantially the same peak magnitude and spatial relationship throughout the annular machine.

FIG. 23 exemplifies what a modular stator 220 and rotor segment 221 may comprise. In this single stator configuration, magnetic flux predominantly traverses axially along the stator poles 233 and returns circumferentially via the rotor plates 237. Since there is nominally zero flux traversing circumferentially in the stator, the stator iron yoke may be eliminated leaving only the stator poles 233. This is particularly advantageous as laminated stators for axial-flux machines are difficult to manufacture. Without the yoke the stator poles 233 can be made of discrete stacks of laminations having the same cross-sectional profile across the machine's axial depth. These discrete stator poles are spaced apart by inserting them through the slots of the rigid annular plates 235 and appropriately clamped together. The stator assembly is fixed to a support structure 222.

The discrete stator poles for 3 different axial-flux machines are shown in FIG. 24. FIG.24(a) illustrates a stator pole which comprises a stack of ferromagnetic laminations 240 including electrical steel and mild steel which are conventionally insulated for dry environment applications such as wind turbines and electric vehicles. A coil 234 is wound on or around the laminated face of the stator pole. FIG. 24(b) shows the discrete stator pole may be a simple rectangular cross-section block 241 made of non-ferromagnetic material. Such a stator pole whose only purpose is to support the coil is particularly suited to air-cored slotless machine. This simple rectangular cross-section block may also be ferrous to concentrate and guide flux with minimal effective air gap. FIG. 24(c) illustrates an example of a stator pole for marine applications where the lamination stack 240 is housed in a waterproof box 242. The hermetically sealed top cover has been removed to reveal the laminated face 240. The whole stack of ferromagnetic laminations 240 may be sprayed or painted with a plurality layer of waterproof protective coatings. The waterproof box 242 reinforces protection from accidental abrasion and water ingression. The void 243, if any, in the box may be filled with resin compound or ferrous core to give structural strength to the assembly.

The coils 234 may be wound in-situ or prefabricated coils are inserted into the slots between the stator poles 233. For marine applications the conductors for the coils 234 are preferably insulated with high dielectric strength plastic sheaths that are impervious to seawater. Winding wires having insulations including PVC, PE2 + PA and HT4 sheaths typically found in submersible pumps may be used. Heat removal is more efficient when the coils 234 are exposed directly to seawater. The coils 234 may be concentric or lap coils and are fully contained within the modular stator 220. The coils 234 may span across a number of slots and the number of turns may also vary in proportions across a number of slots to obtain a more sinusoidal MMF. The support structure 222 may be staggered so that the coils 234 can be fully contained within the assembly. FIG. 25 illustrates an example where the upper part 250 of the support structure is shifted by one stator pole to the left with respect to the bottom part 251. The first stator pole 252 on the left extends axially from the top to the centre while the last stator pole 253 on the right extends axially from the bottom to the centre. When a first modular stator is assembled to a second modular stator, the last stator pole 253 of the first modular stator combines with the first stator pole 252 of the second modular stator to form a full length stator pole. Such a construction enables the modular stators to be assembled and removed with ease. The upper 250 and lower parts 251 of the support structure may be shifted by a number of stator poles depending on the winding schemes.

For marine applications a waterproof subsea casing 236 may be made integral to the support structure 222 (FIG. 23). Coil terminals may enter the subsea casing via appropriate compression glands and the interconnections with other coils within the modular stator may be made in the casing 236 itself. The subsea casing 236 may also house the power conversion equipment responsible for converting power generated within the module. Such modular stators 220 simplify the production process where each module can be independently built and tested prior to full assembly. They may be configured as "plug and play" to facilitate swapping and replacement of failed modules. The modular rotor assembly 221 in FIG. 23 comprises two opposing rotors 237. Each opposing rotor 237 further comprises an electrically conductive plate 238 for carrying induced currents and a ferrous plate (back-iron) 239 for magnetic flux to return to the stator. The electrically conductive rotor plate 238 may be made of metals having low resistivity including copper and aluminium. When assembled, each modular rotor plate 237 may be connected to its neighbour circumferentially to form a continuous short circuit path. The ferrous plate 238 may be formed by discrete stacks of ferromagnetic laminations. For applications with exposure to corrosive environment, the laminations may be painted with a plurality layer of waterproof protective coatings and may be housed in a waterproof box. The void may be filled with resin compound. The winding schemes defining the phases of the electrical machine will now be described with reference to FIGS. 26 to 38. The term "base unit" will be frequently used to define the section having a base winding spanning the length of a pole-pair. Said modular stator 220 may comprise one or multiple of base units as appropriate for the application.

FIG. 26(a) shows a single-phase axial-flux induction machine having opposing rotors 261 sandwiching a stator 260 according to the second embodiment of the invention. In this axial-flux, single stator configuration the auxiliary windings are wound at the first layer of the stator 260 while the main windings are wound at the second layer. The two-phases are in space quadrature to each other. The shaded areas represent stator iron poles. The arcs 262 represent the end winding linking the coil sides. For example, the coil side (M) at slot number 1 links to the coil side (M') at slot number 8. This machine has non-overlapping concentric windings 262 with an average coil pitch of 6, similar to the prior art machine in FIG. 4. The top and the bottom layers of the stator 260 are shifted by four slots so as to avoid splitting or fouling the coils. FIG. 26(b) shows that lap windings 263 with coil pitch of 6 may also be used which give the same current linkage distribution as that of FIG. 26(a). The winding schemes disclosed hereafter are concentric windings but it will be appreciated by persons skilled in the art that lap windings may also be used. FIG. 26(c) is a winding schematic equivalent to FIG. 26(a) where a plurality of base units 264 are connected end-to-end forming a multi-pole machine. The stator iron and rotor are omitted for clarity. Each base unit 264 of the stator 260 spans 16 slots as shown by the bold lines. One modular stator may comprise one or multiple base units.

The end windings of one side of the stator may be manipulated without changing the current linkage distribution. FIG. 27 shows an alternative variant of the single-phase axial-flux induction machine where a consequent pole is obtained across the boundary between two adjacent base units. Such a winding arrangement enables the stators to be split into aligned modules. One modular stator may comprise one or multiple base units as defined by the bold lines in FIG. 27. The corresponding air gap fields generated by the main and auxiliary windings are similar in shape to that of FIGS. 6(b) and 6(c). Because the auxiliary windings in FIG. 27 have a coil pitch of eight as opposite opposed to six in FIG. 26, the end windings of the former will be correspondingly longer which makes the windings of the machine somewhat unsymmetrical. If required, the increase in resistance in the auxiliary phase may be adjusted by using conductors with a larger cross-sectional area. Unsymmetrical windings are nevertheless common in practical single-phase induction machine to generate a high starting torque.

The number of conductors in the slots may be appropriately apportioned to resemble a sinusoid. FIG. 28 is a winding schematic of a single-phase axial-flux machine having approximate sinusoidal windings. The bold font represents a higher number of conductors. The air gap flux density and harmonics for this winding scheme are similar to those of FIG. 9(b). To further reduce the unwanted harmonics, the number of concentric coils per phase per base unit may be increased to 6 as illustrated in FIG. 29. Such a winding scheme increases the slot utilisation and creates a more sinusoidal magnetic field in the air gap similar to that of FIG. 10(a). The fundamental component of the air gap field is the highest and all other higher order harmonics are reduced.

The winding philosophy disclosed in the second embodiment can be extended to three-phase machines. FIG. 30 shows a three-phase axial-flux machine built with multiple base units stator. The stator is subdivided into three layers and coils of one phase are confined within one layer without overlapping to other phases. Coils of phases A, B and C are spaced at 120 electrical degrees apart and they occupy the top, middle and bottom layers of the slots respectively. Each base unit spans twelve slots with slot/pole/phase and an average coil pitch equal to two and six (for example, slots 1-8 and 2-7) respectively. Another example winding scheme with slot/pole/phase and an average coil pitch equal to three and nine respectively is shown in FIG. 31.

The winding schemes of FIGS. 30 and 31 are equivalent to a full-pitched winding where their phase coils occupy only a third of the total stator slots. The phase coils may be compacted into one layer by having overlapping coils as shown in FIG. 32. If a second layer having identical coil pitch to that of the first layer is wound and shifted by a plurality of slot pitches with respect to the first layer, an equivalent short-pitched or chorded winding is obtained. FIG. 33 shows an example where the upper layer is shifted right with respect to the lower layer resulting in an equivalent 8/9 short-pitched double-layer winding. One base unit for producing a two pole-pair field is defined by the bold lines. A modular stator may be built to comprise one or more base units. The windings are fully contained and bounded within the modular stators. Such a short-pitched winding produces even better sinusoidal air gap MMF compared to the full-pitched scheme of FIG. 32.

The slot utilization of the three-phase axial flux machine may be increased to 67% with the winding scheme shown in FIQ. 34. Each phase has four concentric coils per base unit (slots 1-10, 2-9, 3-8 and 4-7) with an average coil pitch of six. It lap coils are used, they occupy slots 1-7, 2-8, 3-9 and 4- 10. This winding scheme requires the stator to have staggered ends to accommodate coil sides of one of the phases. Another variant winding scheme having the same slot utilization as that of FIG. 34 but with shorter end windings is shown in FIG. 35. Each phase of this winding scheme also occupies one layer and has eight coil sides contributing to a pole-pair field within twelve stator slots. Within one base unit, the eight coil sides are split into two sets of concentric coils (slots 1-4 & 2-3 and slots 7-10 & 8-9) which effectively shortens the end windings. This variant winding scheme may have its number of turns apportioned to obtain a better sinusoidal air gap magnetic field. In FIG. 35 the bold text represents slots having higher number of conductors, for example, phase A coil spanning slots 1 and 4 has a higher number of turns compared to its concentric counterpart spanning slots 2 and 3. An alternative variant of the three-phase axial-flux induction machine is shown in FIG. 36. There are 2 sets of concentric coils where each set spans an average three slots (for example, slots 1-5 & 2-4 and slots 7-11 & 8-10). The concentric coils may have a different number of turns to produce a more sinusoidal MMF than with the same number of turns used in each coil. Phase coils designated with bold font indicate a higher number of turns.

The concept of modularized stators arrangement is also applicable to axial-flux machines having higher number of phases (4, 5, 6, 7 phases, etc.) according to the second embodiment of the invention. The phases may all be tied to one neutral point or may be made independent to each other. FIG. 37(a) is an example of a five-phase machine where the phase coils are arranged in five layers (one phase per layer). FIG. 37(b) is yet another example of a five-phase machine where all the phase coils are compacted into a single layer. The base unit encloses 10 stator slots as shown by the bold lines and one modular stator may comprise one or multiple base units. The spatial angle between each two phases is 72 electrical degrees apart. Such machines may have a neutral point and may be fed by a single power converter. Alternatively, the phase coils may also be independent circuits with each phase connecting to a separate power converter. A multi-phase machine may also comprise two or more groups of symmetrical phase windings. FIG. 38(a) is an example of a six-phase machine constructed with two groups of symmetrical three-phase windings A1 , B1 , CI and A2, B2, C2. The first group of windings is wound at the upper layer of the stator while the second group is wound at the bottom layer. The winding arrangement at the upper layer mirrors that of the bottom layer, but the latter is shifted right by one slot thereby creating a 30 electrical degree phase shift. In this example, a base unit spans twelve slots as defined by the bold lines. A modular stator may comprise one or multiple integrated base units. In FIG. 38(b) the first group of the three-phase windings is fed by a three-phase power converter 380 while the second group is fed by a separate three-phase power converter 381 whose phases are 30 electrical degrees delayed with respect to power converter 380. This six-phase winding configuration has the advantage of eliminating lower order harmonics and may be more fault tolerant due to its dual-power converter topology. To further improve the MMF distribution, the windings in FIGS. 37 and 38 may be extended to schemes where coil sides occupy a plurality of slots. Axial-flux electrical machines having higher number of phases may be derived using the winding arrangement principles disclosed in the first and second embodiments of the present invention.

Third Embodiment

The attributes of the modularized axial-flux induction machines described in the first and second embodiments above may be generalized to other types of electrical machines.

The modularized winding schemes may also be generalized to linear induction machines. It is intuitive that the winding arrangements of FIGS. 5-21 and 26-38 resemble that of linear induction machines. The stators and rotors can be manufactured as modular units and assembled together to form a linear machine. If a linear machine is hypothetically rolled so that one end meets the other, a cylindrical radial-flux electrical machine results. FIGS. 39(a) and 40(a) respectively show radial-flux electrical machines having a double-sided stator 390 sandwiching a rotor 391 and a double-sided rotor 401 sandwiching a stator 400. Such machines are built from modular stators and modular rotors of FIGS. 39(b) and 40(b). A cylindrical single air gap radial flux machine may also be derived where one side of the double-sided stator is omitted and an appropriate flux return path is provided. FIG. 41 illustrates a single air gap, radial-flux machine built from modular stators 410 and modular rotors 411.

While there are disclosed features specific to axial-flux induction machines, they are nevertheless applicable to axial-flux or radial-flux permanent magnet machines. It is preferred to have sinusoidal MMF distribution by using greater than, or equal to, one slot per pole per phase. For high performance, induction machines require the current linkage distribution to be almost sinusoidal. The winding schemes described in the previous embodiments are predominantly integral slot windings (the number of slot/pole/phase is an integer). Fractional slot windings (the number of slots per pole per phase is a fraction) offer more freedom in selecting the number of slots and poles and are more straightforward to be segmented compared to integral slot windings. However, fractional slot windings are not common in induction machines because of the high unwanted harmonics associated with non- sinusoidal current linkage distribution. These harmonics are not an issue in permanent magnet machines as the air gap flux density is mainly created by the permanent magnet rotor. As far as current linkage distribution is concerned, any stator winding schemes that meet the demand of high performance induction machines, including those disclosed above, would more often than not exceed the requirement of permanent magnet machines.

FIGS. 42 and 43 respectively show a permanent magnet machine having a double-sided stator and a permanent magnet machine having a double-sided rotor. The rotor iron of the double-sided rotor may be eliminated by using a Halbach array permanent magnet as shown in FIG. 44. The same modular stators and windings may also be used in wound-field synchronous machines. The air gap field of the machine is generated by the rotor field winding supplied with direct current via slip rings or supplied via a brushless excitation system. FIGS. 45 and 46 respectively illustrate schematically a wound-field synchronous machine having a double-sided stator and a wound-field synchronous machine having a double-sided rotor. The wound rotor may also be a multiphase winding set connected to a power converter via, for example, a multiphase slip ring. Such a machine topology is known as a doubly fed induction machine where both the stator and rotor windings participate in the energy conversion process. Practical synchronous reluctance machines have stators and windings identical to that of induction machines but the former have a rotor comprising a stack of ferrous laminations shaped so that it tends to aligned itself with the stator field. FIGS. 47 and 48 respectively show a synchronous reluctance machine having a double-sided stator and a synchronous reluctance machine having a double-sided rotor.

Direct-drive tidal generators may employ rare-earth magnets to increase the power density of the electrical generators. Typically, Neodymium Iron Boron magnets are sought due to their high remanence, high coercivity and high energy product. However, the steep price hike of rare-earth elements in recent years has prompted many industries to contemplate for rare-earth-free solution in order to remain competitive on price. Beside cost, assembling the stator and the permanent magnet rotor together would require a special fixture because of the unbalanced magnetic pull that exists when the magnets are brought close to the stator iron. Another disadvantage from safety point of view is the generator electromotive force (EMF) that exists at the terminals whenever there is relative movement between the stator and rotor. To avoid electric shock hazard, the rotor must come to a complete stop and held stationary before any service or maintenance may be performed.

The present embodiments may provide an electrical machine, in particular a direct-drive electrical machine, which does not involve the use of gearbox, transmission shaft and mechanical rolling element bearings. The absence of these components may simplify the system as well as reduces the cost. Also, the electrical machine does not involve the use of rare earth magnets which may otherwise increase cost and cause electric shocks. The electrical machine in the present invention can be assembled from a plurality of modular stators and modular rotors. This significantly increases the ease of manufacturing and maintenance and further reduces cost. More Importantly, the electrical machine in the present invention may produce a sinusoidal air gap MMF distribution with minimal distortion. It will be appreciated by persons skilled in the art that the above embodiments have been described by way of example only, not in any limitative sense, and that various alterations and modifications in details of design, construction or operation are possible without the departure from the scope of the invention as defined by the claims.

Claims

CLAIMS 1. An electrical machine comprising:

a plurality of detachable modular stator parts configured to be able to be assembled to form a stator;

a plurality of rotor parts configured to be assembled to form a rotor;

one of: said stator parts and said rotor parts, having a first radial component and a second radial component; and the other of: said stator parts and said rotor parts, having a third radial component;

a separation gap between and defined by said first radial component and said second radial component,

said third radial component being beatable in said separation gap and between said first radial component and said second radial component;

said rotor being movable relative to said stator.

An electrical machine as claimed in claim 1, where

said first radial component comprises a first stator component and said second radial component comprises a second stator component spaced from, parallel to and opposing said first stator component;

said first stator component and said second stator component each being configured to receive therein a plurality of windings for producing a magnetic field that is substantially sinusoidal; and wherein

said third radial component comprises a rotor component configured to be located in said separation gap and between said first stator component and said second stator component, and being further configured to move relative to said first stator component and said second stator component.

An electrical machine according to claim 2, wherein said first stator component and said second stator component each comprises a ferrous core having a plurality of slots. An electrical machine according to claim 2 or 3, wherein said first stator component and said second stator component each has a single-phase winding; and that said single-phase winding of said first stator component is in space quadrature to said single-phase winding of said second stator component. An electrical machine according to claim 2 or 3, wherein said first stator component has at least one phase winding and said second stator component has at least one phase winding, said windings of said first stator component and said second stator component being substantially the geometric mirror of each other. An electrical machine according to claim 2 or 3, wherein said first stator component has at least one phase winding and said second stator component has at least one phase winding, said windings of said first stator component and said second stator component being shifted by at least one slot pitch with respect to each other to create an MMF equivalent to that produced by short-pitched windings.

An electrical machine as claimed in claim 1 , wherein

said third radial component comprises a stator component;

each of said modular stator parts comprises a plurality of stator teeth and stator slots; each of said modular stator parts being configured to receive therein a plurality of windings for producing a magnetic field that is substantially sinusoidal; and wherein

said first radial component comprises a first rotor component and said second radial component comprises a second rotor component, said first rotor component being configured to be locatable on a first side of said stator component and said second rotor component being configured to be locatable on a second, opposite side of said stator component,

said first rotor component and said second rotor component defining therebetween said separation gap adapted to receive therein said stator component for motion of said first rotor component and said second rotor component relative to said stator component. An electrical machine according to claim 7, wherein said stator teeth and stator slots are formed by a plurality of discrete ferrous or non-ferrous cores. An electrical machine according to any one of claims 2, 7 or 8, wherein said magnetic field has substantially the same peak magnitude and spatial relationship throughout the whole machine. An electrical machine according to claim 3 or 8, wherein said ferrous core or non-ferrous core is waterproof. An electrical machine according to any one of claims 2, 3 or 7-10, wherein said windings are fully contained and bounded within said modular stator. An electrical machine according to any one of claims 7 to 11 , wherein said windings are connected as two-phase windings. An electrical machine according to any one of claims 7 to 11, wherein said windings are connected as multi-phase windings. An electrical machine according to any one of claims 4-6 or 12-13, wherein at least one of said phase windings has a consequent pole winding topology. An electrical machine according to any one of the claims 2 to 14, wherein said windings are coils spanning a plurality of stator slots. An electrical machine according to claim 15, wherein the number of turns of said coils varies in proportion across a plurality of said stator slots.

An electrical machine according to any one of claims 2 to 16, wherein said windings are insulated and waterproofed. An electrical machine according to any one ol claims 1 to 17, wherein an enclosure lor housing power conversion equipment is integral to said modular stator. An electrical machine according to any one of claims 1 to 18, wherein each said rotor part comprises at least one selected from a group consisting of: a conductive component, a permanent magnet, a field winding, a multiphase winding or a ferrous core. An electrical machine according to any one of claims 1 to 19, wherein said stator and said rotor are linear. An electrical machine according to any one of claims 1 to 19, wherein said stator and said rotor are cylindrical. An electrical machine comprising:- a plurality of detachable modular stator parts configured to be able to be assembled to form a stator,

each of said modular stator parts comprising a first stator component spaced from, parallel to and opposing a second stator component,

a separation gap between and defined by said first stator component and said second stator component,

each of said first stator component and said second stator component each being configured to receive therein a plurality of windings for producing a magnetic field that is substantially sinusoidal; and

a plurality of rotor parts configured to be assembled to form a rotor, and wherein each of said rotor parts is configured to be located in said separation gap between said first stator component and said second stator component, and being further configured to move relative to said first stator component and said second stator component. An electrical machine according to claim 22, wherein said first stator component and said second stator component each comprises a ferrous core having a plurality of slots. An electrical machine according to claim 22 or 23, wherein said first stator component and said second stator component each has a single-phase winding and that said single-phase winding of said first stator component is in space quadrature to said single-phase winding of said second stator component.

An electrical machine according to claim 22 or 23, wherein said first stator component has at least one phase winding and said second stator component has at least one phase winding, said windings of said first stator component and said second stator component being substantially the geometric mirror of each other.

An electrical machine according to claim 22 or 23, wherein said first stator component has at least one phase winding and said second stator component has at least one phase winding, said windings of said first stator component and said second stator component being shifted by at least one slot pitch with respect to each other to create an MMF equivalent to that produced by short-pitched windings.

An electrical machine comprising:- a plurality of detachable modular stator parts configured to be able to be assembled to form a stator,

each of said modular stator parts comprising a plurality of stator teeth and stator slots, each of said modular stator parts being configured to receive therein a plurality of windings for producing a magnetic field that is substantially sinusoidal; and

a plurality of rotor parts configured to be assembled to form a rotor,

each of said plurality of rotor parts comprising a first rotor component configured to be beatable on a first side of said modular stator parts and a second rotor component configured to be locatable on a second, opposite side of said modular stator parts, said first rotor component and said second rotor component defining therebetween a separation gap adapted to receive therein said modular stator parts for motion of said first rotor component and said second rotor component relative to said modular stator parts. 28. An electrical machine according to claim 27, wherein said stator teeth and stator slots are formed by a plurality of discrete ferrous or non-ferrous core. 29. An electrical machine according to any one of claims 22, 27 or 28, wherein said magnetic field has substantially the same peak magnitude and spatial relationship throughout the whole machine.

30. An electrical machine according to claim 23 or 28, wherein said ferrous core or non-ferrous core is waterproof.

31. An electrical machine according to any one of claims 22, 23 or 27-30, wherein said windings are fully contained and bounded within said modular stator. 32. An electrical machine according to any one of claims 27 to 31 , wherein said windings are connected as two-phase windings.

33. An electrical machine according to any one of claims 27 to 31 , wherein said windings are connected as multi-phase windings.

34. An electrical machine according to any one of claims 24-26 or 32-33, wherein one of the said phase windings has a consequent pole winding topology.

35. An electrical machine according to any one of claims 22 to 34, wherein said windings are coils spanning a plurality of stator slots.

36. An electrical machine according to claim 35, wherein the number of turns of said coils varies in proportion across a plurality of said stator slots. An electrical machine according to any one of claims 22 to 36, wherein said windings are insulated and waterproofed. An electrical machine according to any one of claims 22 to 37, wherein an enclosure for housing power conversion equipment is integral to said modular stator. An electrical machine according to any one of claims 22 to 38, wherein each said rotor part comprises at least one selected from a group consisting of: a conductive component, a permanent magnet, a field winding, a multiphase winding or a ferrous core. An electrical machine according to any one of claims 22 to 39, wherein said stator and said rotor are linear. An electrical machine according to any one of claims 22 to 39, wherein said stator and said rotor are cylindrical.