GB2496435A - Poly-phase stator having coils switchable between series and parallel connection - Google Patents

Abstract

A poly-phase stator 110 for an electrical machine 101 comprises a plurality of coils 170. The coils are mounted on a stator body to form poles of the stator. These coils are discrete coils having terminals that are electrically connected to end-connection apparatus. The end-connection apparatus provides end connections between the coils and is arranged substantially to provide electrical balance between the phases (U, V, W). The switching means is operable to switch coils in the same phase between series and parallel connection to a power supply. The switching means may be arranged to switch at least one coil in one of the phases between a series connection with at least one other coil in that phase and a parallel connection with that at least one other coil. The electrical machine may comprise an external permanent magnet rotor 100 and internal stator 110. The electrical machine may be housed in a wheel hub and/or a crank housing of a vehicle.

Description

A STATOR FOR AN ELECTRICAL MACHINE

FIELD

This invention relates to a stator for an electrical machine. In embodiments, it relates to an internal stator for a radial-flux electrical machine. In other embodiments, it relates to an axial-flux machine. Applications include use of the electrical machine as a motor to provide automotive drive.

BACKGROUND

The demand for efficient electric traction motors for vehicles has increased in recent years and is likely to continue growing as fuel costs increase and regulations aimed at reducing vehicle emissions become stricter.

Attempts to supply the demand for such motors have led to proposals for new types of compact electric machines which are characterised by low power losses in the permanent magnet cores, in the conducting wires which make up the coils, in the bearings and in the cooling fan as well as low aerodynamic losses and losses due to circulating currents flowing around the windings of parallel coils. In addition, these motors have accurate position and velocity control in both rotor directions and during both acceleration and deceleration. These motors also run smoothly throughout their speed and torque bands, have low noise emissions and have the potential to reach the requirement for traction motors of high specific torque at low speed and high specific power at high speed.

However, without relatively high voltage supplies, the band in which these machines operate efficiently is fairly narrow. Supply voltages are limited by safety considerations since in general, the higher the supply voltage, the higher the possibility of short circuits and the attendant danger of a fire due to overheating of parts of the electric circuit. Higher supply voltages can also increase the danger from electric shock. In addition to the limit due to safety considerations, the maximum supply voltage of the electric machines of many electric vehicles is limited by law.

Thus, at present, if an electric motor is to operate within legal requirements, it must be accepted that its good efficiency range will be small.

High supply voltages are also required to achieve high motor speed ranges and high power, but if the supply voltage is limited then with conventional speed controllers (which vary individual coil voltage input by varying the three phase supply voltage by electronic chopping) then the maximum motor speed and power are also limited.

Because of conventional winding techniques, the coils in each phase for most multi-pole motors are generally wound in series. However a speed optimised motor wound in such a way cannot also have a high torque output at constant field flux. This is because for high motor speeds, the number of turns on each coil must be low and the flux through the coil must be minimised to reduce the motor's back electromotive force (EMF). Since the torque produced by the motor is proportional to the current through the coils multiplied by the number of coil turns per stator pole, a motor with a low number of turns on each coil can only run at low flux and torque levels.

\Vinding each coil within a phase in parallel with the other coils in that phase might therefore appear to be a solution when supply voltages are limited. In this way, high motor speed at low current and high motor torque at low speeds (or a compromise between the two) can be achieved. For example, in a six pole motor with a three phase supply and therefore two coils per phase, each coil has only half of the full supply voltage when the coils within a phase arc wound in series. If these two coils were wired in parallel, the voltage on each coil would be equivalent to the full supply voltage and the motor could run at twice the original speed. Alternatively twice as many turns could be used on the coils, meaning that the supply current per coil could be reduced to half of the original.

There are complications, however, if the voltage magnitude across each coil is not the same. This can arise, for example, if the conductors which are wound to form the coils are of different lengths. Circulating currents will flow around the windings of parallel coils and produce 12R conductor losses. If the impedance of coils connected in parallel is not equal, they will not share the load current equally. This increases the sum of the conductor losses.

A further obstacle to the wide acceptance of electric machines to be used in traction drives in general is their high manufacturing cost, due in part to the difficulty of winding motor coils.

An object of at least certain embodiments is to address one or more of these problems.

SUMMARY

According to an aspect of this invention, there is provided a poly-pliase stator for an electrical machine, the stator comprising a plurality of coils, end-connection apparatus and switching means, wherein: the coils are mounted on a stator body to form poles of the stator, the coils being discrete coils having terminals that are electrically connected to the end-connection apparatus; the end-connection apparatus providing end connections between the coils and arranged substantially to provide electrical balance between the phases; the switching means operable to switch coils in the same phase between series and parallel connection to a power supply.

The switching means may be arranged to switch a plurality of coils in one of the phases between a series connection with another plurality of coils in that phase and a parallel connection with that other plurality of coils. The switching means may be arrangcd to switch a single coil in one of the phases between a series connection with another coil in that phase and a parallel connection with that coil. The switching means maybe arranged to switch the coils of a phase between a first configuration in which all of the coils are in series, a second configuration in which a plurality of the coils are in series with each other and in parallel with another plurality of coils also in series with each other, and a third configuration in which all of the coils are in parallel with each other.

The switching means may be operable to switch coils in each of the phases in the manner defined above in relation to the one phase.

The switching means may comprise switching modules which comprise a plurality of switches. Each switching module may comprise a plurality of "on/off' switches. Each switching module may comprise a plurality of changeover switches. Each switching module may comprise at least one "onloff' switch and at least one changeover switch.

The switching means may comprise solid state switching means so that the switches are solid state switches, such as, for example, MOSFETs.

The switching means may be arranged substantially to control the voltage across the coils to which they are connected. The switching means may be operable to chop the supply to the coils, for example using pulse width modulation, to control the voltage thereacross. In this way, the switching means may control both the series and parallel switching of the coils and also the voltage across those coils when connected in parallel and/or series. Thus, the need for a separate speed controller is, in embodiments, avoided.

There may be control means for controlling operation of the switching means.

There may be a speed controller mounted on the substrate. The speed controller may comprise a phased voltage input controller mounted on the substrate. There maybe at least one rotor position sensor mounted on the substrate. There may be an overload cut out for the electrical machine mounted on the substrate. There may be a temperature cut out for the electrical machine mounted on the substrate.

Each coil may be substantially the same as each other coil. Each coil may be arranged to have substantially the same resistance andlor impedance as each other coil. Each coil maybe wound from a single length of conductor. The length of the conductors from which the coils are wound may be substantially the same. A plurality of the coils may be wound from a single length of conductor.

The end-connection apparatus may comprise a substrate to which terminals of the coils may be physically connected. The substrate may support thereon conductive tracks that form end connections between at least some of the coils. The substrate may be a circuit board. The terminals may be inserted into the circuit board. They may be soldered to the circuit board. They may be welded to the circuit board. They may be glued to the circuit board. They may be crimped to the circuit board.

The circuit board may be integrally formed. The circuit board may be made up of a plurality of separate segments that are electrically connected to each other-The conductive tracks of the circuit board may be copper. The conductive tracks may be aluminium. The conductive tracks may be produced by etching. The conductive tracks may be produced by computer numerical control (CNC) machining. They may be produced by laser cutting. They may be produced by stamping. Some of the conductive tracks may be on one face of the substrate; others of the tracks may be on the other face of the substrate, At least some of the tracks may be on the same face of the substrate, with one or more tracks being in a first layer and one or more tracks being in a second layer electrically insulated from the or each track in the first layer.

One or more of the tracks may be formed from an electrically conducting material that is high-strength, such as, for example, carbon fibre, copper alloy or aluminium alloy.

This may strengthen the substrate. One or more of the tracks may be formed from an electrically conducting material that is a good thermal conductor, thereby acting as a heat sink.

The tracks may be arranged substantially to provide electrical balance between the phases. Tracks that provide end connections between coils in a first phase may be of substantially the same resistance as corresponding tracks in the or each other phase.

Corresponding tracks may be of substantially the same length and cross-sectional area. Corresponding tracks may be of differing length and cross-sectional area but with the length and cross-sectional area selected to provide the corresponding tracks with substantially the same resistance.

The substrate may be positioned adjacent the coils. The substrate may he axially juxtaposed with the stator body to minimise distance between the windings of the coils and the tracks on the substrate to minimise the length of conductors electrically connecting the coils to the tracks. The substrate may be mounted to the stator body.

According to a second aspect of this invention, there is provided an electrical machine comprising a stator according to the first aspect.

The electrical machine may be a radial-flux electrical machine. It may be an axial-flux electrical machine. The electrical machine may be arranged to operate as a motor and/or a generator.

According to a third aspect of this invention, there is provided a method of operating an electrical machine according to the second aspect, the method comprising the step of control means controlling the switching means to switch coils in the same phase between series and parallel connection to the power supply.

The method may comprise the step of the control means controlling the switching means to switch coils in the same phase from series to parallel configuration in response to the control means receiving an input indicative of an increased desired speed output of the electrical machine. The method may comprise the step of the control means controlling the switching means to switch coils in the same phase from parallel to series configuration in response to the control means receiving an input indicative of a decreased desired speed output of the electrical machine.

The method may comprise the steps of the control means controlling the switching means to switch coils in a first phase between series and parallel connection to the power supply and then subsequently switching coils in a second phase between series and parallel connection to the power supply.

The method may comprise switching the coils in the first phase from series to parallel and then subsequently switching the coils in the second phase from series to parallel.

This staggered switching avoids a rapid increase in the speed of a motor of which the stator may form part. The method may comprise switching the coils in the first phase from parallel to series and then subsequently switching the coils in the second phase from parallel to series. This staggered switching avoids a rapid decrease in the speed of a motor of which the stator may form part.

The method may comprise the control means operating the switching means to chop the voltage across the coils, for example by pulse width modulation. The method may comprise, when the control means operates the switching means to switch coils between series and parallel, substantially simultaneously chopping the voltage such that the voltage across each coil does not substantially change as the coils are switched between series and parallel. This avoids any rapid acceleration or deceleration when the coils are switched.

The method may comprise an automatic mode of operation in which the control means senses an input indicative of a desired speed, and responsive at least to the desired speed being greater than a speed threshold, the control means operating the switching means to switch coils from series to parallel, substantially simultaneously chopping the voltage across those coils such that the voltage increase thereacross as a result of the switching is less than it would otherwise be. In the automatic mode, responsive at least to the desired speed being less than a speed threshold, the control means operating the switching means to switch coils from parallel to series, substantially simultaneously chopping the voltage across those coils such that the voltage decrease thereacross as a result of the switching is less that it would otherwise be.

The method may comprise a semi-automatic mode of operation in which the control means switches in response at least to an input from a user indicative of switching the speed range of the electrical machine, the method further comprising substantially simultaneously chopping the voltage to lessen the change in voltage across the coils in the manner previously defined.

The method may comprise a manual mode of operation in which the control means switches in response at least to an input from a user indicative of switching the speed range of the electrical machine, and in which the method does not substantially simultaneously chop the voltage to lessen the change in voltage across the coils in the manner previously defined, leaving instead the user to vary the input indicative of a desired speed in order to lessen the change in voltage across the coils.

The control means may be arranged to control operation of the electrical machine in accordance with the method of the third aspect. The control means may be programmed and operable to control operation in this way. The control means may comprise processing means and may comprise storage means. The processing means may comprise a microprocessor. The storage means may comprise solid-state storage means andlor optical and/or magnetic storage means.

According to a fourth aspect, there is provided a record carrier comprising computer-readable instructions that are executable by computer processing means to cause those means to carry out the steps of the method. Record carrier may comprise solid-state storage means andlor optical andlor magnetic storage means.

According to a fifth aspect of this invention, there is provided a vehicle comprising an electrical machine as defined hereinabove.

The electrical machine may be housed in a wheel hub and/or a crank housing of the vehicle.

The vehicle may be, for example, an aircraft, boat, car, bicycle or other vehicle. In other embodiments, the electrical machine may find use in other applications, for example, a turbocharger or wind turbine.

According to a sixth aspect of this invention, there is provided a rotor for an electrical machine, the rotor comprising a plurality of coils arranged to form poles of the rotor, and further comprising switching means to switch coils in the same phase between series and parallel configuration.

Optional features of each aspect are also optional features each other aspect, with changes of terminology being inferred by the skilled addressee where necessary for these to make sense. For example, the word "stator" would be interpreted as "rotor" when applying optional features of other aspects to this sixth aspect.

BRIEF DESCRIPTION OF THE DRAWINGS

Specific embodiments will be described below by way of example only and with reference to the accompanying drawings, in which: Figure 1 shows an axial view of components of an electrical machine, including a stator in accordance with a first embodiment; Figure 2 shows a schematic sectional view of the electrical machine, the section being taken through an axial plane; Figure 3 shows a schematic sectional view of an axial-flux electrical machine in accordance with a ninth embodiment, the section being taken through an axial plane; Figure 4 is a schematic illustration of a circuit board for stator in accordance with the first embodiment; Figure 5 shows the actual physical and spatial layout of tracks on a circuit board for the stator; Figure 6 shows in schematic form the wiring and switching arrangement provided by the circuit board; Figure 7a shows a switching relay module providing a switching arrangement in accordance with the first embodiment; Figure 7b shows an alternative switching relay module; Figure 8 shows an axial view of components of an electrical machine, including a stator in accordance with a second embodiment; Figure 9 shows in schematic form the wiring and switching arrangement provided by a circuit board for the stator in accordance with the second embodiment; Figure 10 shows in schematic form an alternative wiring and switching arrangement provided by a circuit board for the stator; Figure 11 is a schematic illustration of the stator in accordance with the first embodiment as part of an electric motor used in a vehicle; Figure 12 shows the torque and power output through the process of acceleration for a motor in accordance with a fourth embodiment; and Figure 13 shows an axial view of components of an electrical machine, including a stator in accordance with an eighth embodiment.

SPECIFIC DESCRIPTiON OF CERTAIN EXAMPLE EMBODIMENTS Figure 1 shows an electrical machine 101 comprising an external rotor 100 and an internal stator 110. The rotor 100 is annular and the stator 110 takes the form of an annulus with a plurality of radially-projecting pole-pieces 120 projecting outwardly therefrom. The stator 110 is mounted coaxially with the rotor 100 so that the rotor forms a ring around the stator 110. The rotor 100 is a permanent-magnet rotor with magnets 140 arranged around the radially innermost surface to form a number of poles and with an electrical steel backing 150 for these magnets 140 radially outside them. The backing 150 is arranged, in conjunction with rotor segment location pins to keep the magnets 140 in position. An example of the particular arrangement of the external rotor 100 is given in international application PCT/GB2OI 1/001410, the contents of which in so far as they describe a rotor suitable for use with the present stator 110 are hereby incorporated by reference and will be described no further here, The stator 110 described herein comprises cassette-type coils 170 slotted on to pole-pieces 120, and a circuit board 210 comprising tracks arranged to connect the coils 170. Corresponding tracks have substantially the same resistance. The stator 110 forms part of a motor 1110, and there are switching relay modules 230 mounted on the circuit board 210 which are arranged to switch the coils 170 between series and parallel configuration in order to extend the range of the motor 1110. The arrangement of the stator 110 will be described in more detail below with reference to Figures 1 to 10 and 13. The method in which the coils are switched will also be described in more detail with reference to Figures 1,6, la, 8, 10, 11 and 12.

In this embodiment, the stator 110 is a twelve-pole, twelve-slot single-layer stator.

This means that the stator 110 has twelve pole-pieces 120. These are of uniform cross section projecting radially outwards. The twelve slots of the stator 110 are the gaps between these pole-pieces 120. There is a coil 170 of conducting material around every second pole-piece 120. The arrangement is called single-layer because, since there is a coil 170 around every other pole-piece 120, each slot carries a part of only one coil 170. Each length of conducting material is substantially the same in thickness, resistance and in length when uncoiled and straight. The coils 170 are individual, identical, cassette-type coils 170 which can be slotted onto the pole pieces of the stator 110. The two ends of each coil 170 project in the same axial direction with respect to the electrical machine 101 so that they can be electrically connected to a circuit board 210 of the sort described below.

The coils 170 described above are wound by machine with a separate length of conducting wire for each coil 170. The coils 170 are not wound by the machine around their respective pole-pieces 120 but are instead first wound and then slotted onto the pole-pieces 120. The machine winding ensures that the coils 170 closely t0 resemble one another in shape as well as in conductor length and that their ends project axially with respect to the electrical machine 101 at predetermined positions.

Once the machine-wound coils 170 have been slotted on to the pole-pieces 120, their ends are connected to the circuit board 120 through holes in the circuit board also at predetermined positions. Slotting the coils 170 on to the pole-pieces 120 is made easier since in this embodiment the pole-pieces 120 have no pole shoes. Without pole shoes, those iron losses in the electrical machine which are due to eddy currents in the pole shoes are eliminated. Machine winding of individual coils 170 can reduce the cost of manufacture since it reduces the cost of labour compared to hand-winding and is also quicker than hand-winding. The coils 170 may be wound differently in other embodiments.

Figure 2 shows a schematic sectional view of the radially outer part of a radial-flux electrical machine 101 comprising the stator 110. The coils 170 are all electrically connected to a circuit board 210 which comprises conducting tracks arranged to connect the coils together as will be described in more detail with reference to Figure 4. In this embodiment, the circuit board 210 is positioned axially adjacent to the coils so that the length of conductor between the body of each coil 170 and its respective coil terminal 220 on the circuit board 210 is minimised. This has the effect of reducing power losses due to conductor resistance. The circuit board 210 provides a base for switching relay modules 230 (described in more detail below with reference to Figure 7a). In this embodiment, a position sensor 240 for the rotor 100 is also mounted on the circuit board 210 for reasons which will be explained below with reference to Figure 11.

Figure 4 is a schematic illustration of the circuit board 210 for this twelve-pole, six-coil stator 110. In this Figure, the position of connections and switches is presented to aid clarity of description and does not necessarily reflect their actual position in a working embodiment. Although the switches are shown as mechanical switches, in this embodiment they are actually solid state switches as will be made clear below.

The switches are shown as mechanical switches in this Figure and in Figures 6, 9 and purely to aid understanding of their various switching states. It

The coils 170 of the stator 110 are electrically connected to a power supply with three phases U, V and W. Two coils, coil Cl and coil C4, are electrically connected to phase U of the power supply; another two coils, coil C2 and coil CS, are connected to phase V of the power supply; and a final pair of coils, coil C3 and coil C6, are connected to phase W of the power supply. The coils 170 are also connected back to the power supply via star terminals 410 on a star ring 420. Ia this embodiment, the star ring is made of copper. In other embodiments, the star ring 420 is made from a high strength electrically conducting material and therefore strengthens the circuit board 120, for example, copper and aluminium alloys may be used. In this embodiment the star ring 420 is made from copper. Since copper is also a good thermal conductor, the star ring 420 can act as a heat sink for the circuit board 120.

The coils 170 are electrically connected to switches. Each phase of the power supply is considered as being made up of two branches: UA and 118; VA and VB; WA and WB. Depending on the state of the switches, either both coils 170 in the same phase are connected in series with each other or one coil in a phase is connected in parallel with the other coil in that phase.

The end of coil Cl which projects axially with respect to the electrical machine 101 from the "top" of the coil (that is, the end of coil Cl which is radially outermost) is electrically connected via connection 11 and terminal Ui to branch UA, which is one of the two branches of phase U of the power supply to the stator 110. The other end of the coil (which is the "bottom" of coil Cl, or the end of coil Cl which is radially innermost) is arranged to be electrically connected via connection lB and switch SUA to the star terminal 410 of the power supply.

As can be more easily seen in Figure 1, coil C4 is located radially opposite coil Cl, around pole-piece 120 number 4. Returning to Figure 4, coil Cl is arranged to be electrically connected via switch SUA to the radially innermost end of coil C4 at connection 48, while the radially outermost end of coil C4 is connected via connection 4T to the star terminal 410.

Finally, the radially innermost end of coil C4 is arranged to be electrically connected via a switch SUB and terminal U2 to UB, the other branch of phase U of the power supply.

The switches are arranged such that in a first switching state coil Cl is electrically connected at terminal U 1 to branch UA of the power supply and (via connection I B, switch SUA and connection 4B) to coil C4. which is in turn connected to the star terminal 410. In a second switching state, coil Cl is electrically connected at terminal UI to branch UA of the power supply and, via connection lB and switch SUA, to the star terminal 410. In this second arrangement, coil C4 is electrically connected at one end (via connection 4B, switch SUB and terminal U2) to branch UB of the power supply, and at the other end (via connection 4T) to the star terminal. In the fIrst switching state, coils Cl and C4 are electrically connected to phase U in series. In the second switching state, coils Cl and C4 are electrically connected to phase U in parallel.

The coil layout and wiring has been described for coils 170 in phase U alone, since the wiring for coils 170 in phase W is the same as this wiring and the wiring for coils in phase V is the same as for phases U and W except that the windings of coils C2 and CS with respect to their respective pole-pieces 120 are in the opposite direction to the windings of the coils 170 in both phase U and phase W. This is because the coils 170 in phase V must be of the opposite polarity to the corresponding coils 170 in phases U and W. The correct polarity for coils 170 in phase V could be achieved by connecting terminal 2B to the power supply at V 1, arranging terminal 2T to be connected to either coil CS or to the star terminal 410, connecting terminal SB to the star terminal 410 and arranging terminal ST to be connected to either coil C2 or to the power supply at V2. However in the present embodiment, in order to ensure the correct coil polarities for coils 170 in phase V, while ensuring that each circuit board 210 track in phase V is as practically as possible arranged in the same way as a corresponding track in each other phase, the windings of coils C2 and C5 with respect to their respective pole-pieces 120 are in the opposite direction to the windings of the coils 170 in phases U and phase W. Further information on the polarity of coils 170 in single-layer electrical machines such as the electrical machine 101 of the present embodiment can be found in Mosebach, 1-]. Systematik dreistrangiger symmetriseher PM-erregi!er PPSM, Jahresbericht des IMAB, TU Braunschweig, 2005. The particular equivalence between switches, coils and terminals will be specified below with reference to Figure 6.

Figure 5 shows the actual physical and spatial layout of the tracks on the circuit board 210 for the stator 110. It therefore differs from Figure 4 in not being schematic. It does not therefore show the individual switches but instead the outline and terminals of switching relay modules 230 that provide the function of the previously described switches. Each switching relay module 230 is a single electronic chip, for example consisting of metal-oxide-semiconductor field-effect transistors (MOSFETs). The switching relay modules 230 are described in more detail below with reference to Figure 7a.

The layout of the tracks is designed such that the tracks which form electrical connections between the power supply and the coils 170, between the power supply and the switches and between the coils 170 and the switches are as short as possible consistent with the arrangement of the coils 170 (as shown in Figure 1) and the requirement that the tracks do not make electrical contact with one another along their lengths. This is in order to reduce power losses due to conductor resistance. As a consequence of this track layout designed to minimise the lengths of the tracks, the tracks between coils 170 in phases V and W are arranged in the present circuit board 210 embodiment such that, when shown in Figure 5 they appear to cross.

Conventional techniques would be used to electrically separate the tracks. For example, there can be two conductive layers of the circuit board 210, one layer on one face of the disc, the other layer on the other face. The track between connection 5B and its respective switching module 230 could be on a first conductive layer of the circuit board 210 while the track between connection 6B and its respective switching module 230 could be on a second conductive layer of the circuit board 210. That arrangement is used in the present embodiment.

In order to provide phases of substantially the same resistance, each track in each phase is as practically as possible (consistent with the arrangement of the coils 170 as shown in Figure 1 and the requirement that the tracks do not make electrical contact with one another along their lengths) the same length as a corresponding track in each other phase. Specifically, the track between connection SB and its respective switching module 230 and the track between connection 6B and its respective switching module 230 are substantially the same length. This requirement on track lengths is to ensure that the tracks and connections of each phase have similar electrical resistance to the tracks and connections of each other phase. By providing corresponding tracks of substantially the same length, circulating currents flowing around the windings of parallel coils can be minimised and 12R conductor losses can thus be reduced.

In certain embodiments, it may not be possible or it may be difficult for the tracks of each phase to be of the same length as the tracks of each other phase due to coil 170 arrangement and the requirement that the tracks do not make electrical contact with one another along their lengths. In this case, the resistance of each track in a phase can still be ensured to be approximately the same as the resistance of the corresponding tracks in other phases by making an appropriate choice of the cross-sectional area or resistivity for the conductors making up the conducting tracks. For example, in the present embodiment, the track between connection 4B and its respective switching module 230 is longer than the corresponding tracks in phases V and W. To ensure that its resistance is the as close as possible to the resistance of each the other corresponding tracks, the cross-sectional area of the conductor making up this track is greater than the cross-sectional area of the conductors making up the corresponding tracks in phases V and W. Figure 6 shows in schematic form the wiring and switching arrangement provided by the circuit board 210 in this embodiment. It shows the wiring and switching for two phases (U and V) of the three phases into which the supply current is divided.

Depending on the state of the switches shown, either both coils 170 in the same phase are connected in series with each other or one coil in a phase is connected in parallel with the other coil in that phase.

Only the wiring arid swicliing for phase U will be described in dctail here, since the wiring for phase W is the same as wiring for phase U and the wiring for phase V is also the same as the wiring for phases U and W. except that the windings of coils C2 and CS with respect to their respective pole-pieces 120 are in the opposite direction to the windings of the coils 170 in both phase U and phase W. This is to improve symmetry in the arrangement of tracks on the circuit board 210 without reversing the polarities of coils in phase V, as explained above with reference to Figure 4.

Connection lB is connected to a solid state changeover switch SUA, which forms part of a switching module 230 described in more detail below with reference to Figure 7a.

The switch SUA is arranged to switch coil Cl between (a) being electrically connected at terminal Ui to branch UA of the power supply and at the other end to the star terminal 410 of the power supply and (b) being electrically connected at terminal UI to branch UA of the power supply and at the other end to connection 4B (which, as will be remembered, is electrically connected to the radially innermost end of coil C4) and from there through coil C4 and to the star terminal 410, as will be explained below.

From connection 4B, there extends a track which branches into two terminals, T1UB and T2UB. The first terminal TIUB is a terminal of the switch, SUB. When switch SUA is in a state that connects Cl to the star terminal 410, as in switching state (a) above, switch SUB is closed and electrically connects terminal T1UB via terminal U2 to branch UB of the power supply. When switches SUB and SUA are in these positions, Cl is electrically connected in parallel to C7.

The second terminal T2UB is arranged to be electrically connected via switch SUA through terminal lB to coil Cl when switch SUB is in the position described in switching state (b) above. This has the effect of connecting coils Cl and C2 in series with one another, from branch UA of the power supply at one end back to the star tenninal 410 at the other. In this switching state, switch SUB does not connect terminal T1UB to branch UB of the power supply, but is instead in the "open" position.

The arrangement described in detail here for terminals Ul and U2, switches SUB and SUA, terminals T1UB and T2UB and coils Cl and C4 is the same firstly as for terminals Vl and V2 and for coils C2 and CS and secondly as for terminals Wl and W2 and coils C3 and C6. Terminal Ul is the same as terminals Vi and Wl and terminal U2 is the same as V2 and W2. Switch SUB is the same as switches SVB and SWB and switch SUA is the same as switches SVA and SWA. Terminal flUB is the same as terminals T1VB and T1WB and terminal T2UB is the same as terminals T2VB and T2WB. Coil Cl is the same as coils C2 and 0 and coil C4 is the same as coils C5 and C6.

Switches SUB and SUA are provided by a switching relay module 230 for phase U of the power supply as shown in Figure 7a. The switching relay module 230 consists of one changeover switch SUA and one "on/off' switch SUB. There is a switch relay control input 530 into the switching relay module 230 arranged to cause switch SUA to switch between connecting coil Cl (a) to branch UA of the power supply at one end and to the star terminal 410 of the power supply at the other end and (b) to branch UA of the power supply at one end and to coil C4 at the other end. The switch relay control input 530 is also arranged to cause switch SUB to switch between being open and closed. When switch SUB is open, and switch SUA connects the coils as described in switching state (b) above, coils Cl and C4 are electrically connected in series with each other. When switch SUB is closed, coil C4 is electrically connected to the branch UB of the power supply.

Figure 7b shows an alternative switching relay module 230 comprising three "on/off' switches SUB, SUA1 and SUA2 rather than one changeover switch (switch SIJA) and one "on/off' switch (switch SUB). In this embodiment, therefore, there are two switches SUAI and SUA2 on a track extending from 4B. The terminal for switch SUA2 is electrically connected to the star terminal 410. The terminal for switch SUAI is electrically connected to 4B. These two switches together are arranged to perform the same function as changeover switch SUA described above.

In a first switching state, switch SUA2 is closed and switch SUA1 is open. In a second switching state, switch SUA2 is open and switch SUA1 is closed. In the first switching state, coil Cl is electrically connected at terminal Ul to branch UA of the power supply and electrically connected at the other end to the star terminal 410. In the second switching state, coil Cl is electrically connected at terminal Ul to branch UA of the power supply and electrically connected at the other end to coil C4 and through that to the star terminal 410.

The third switch, SUB, is electrically connected to branch UB of the power supply.

The contact for switch SUB, terminal T1UB, is electrically connected to terminal 4B.

When switches SUA1 and SUA2 are in the first switching state, switch SUB is closed.

Coil C4 is thus electrically connected to branch UB of the power supply at one end and to the star terminal 410 at the other. When switches SUAI and SUA2 are in the second switching state, switch SUB is open. In this switching state, coil C4 is not connected to phase U2 of the power supply.

Although described here in detail fbr phase U, there are switching modules 230 for phases V and W which are the same as the switching module 230 for phase U. In the second embodiment, shown in Figure 8, the stator 110 is a twelve-pole, twelve-slot double-layer stator. This means that the stator 110 has twelve pole-pieces 120.

Each of these is of uniform cross section and projects radially outwards. Twelve slots are therefore provided which are the gaps between these pole-pieces. There is a coil of conducting material around each pole piece 120. The arrangement is called double-layer because, since there is a coil 170 around each pole piece, each slot carries a part of two different coils 170.

Figure 9 shows in schematic form the wiring and switching arrangement provided by the circuit board 210 in this second embodiment, which is similar to thc wiring for the first embodiment described with reference to Figure 6 but with four instead of two coils 170 per phase. It shows the wiring for each of the three phases U, V and W into which the supply current is divided. Depending on the state of the switches shown, either all four coils 170 in the same phase are connected in series or two coils 170 are connected in parallel with the other two. Only the switching and wiring for phase TI will be described in detail here, since the switching for phases V and W is the same as the switching and wiring for phase U. Connection 2B is connected to a solid state changeover switch SUA, which forms part of a switching module 230 similar to that described with referencJo Figure 7a. The switch SUA is arranged to switch coils Cl and C2 between (a) being electrically connected at terminal UI to branch UA of the power supply and at the other end to the star terminal 410 of the power supply and (b) being electrically connected at terminal UI to branch UA of the power supply and at the other end to 71 and from there through coils C7 and C8 to the star terminal 410, as will be explained below.

From 71, there extends a track which branches into two terminals, TIUB and T2UB.

The first terminal T1UB is the contact of a switch, SUB. When switch SUA is in a state that connects coil C2 to the star terminal 410, as in switching state (a) above, switch SUB is closed and connects terminal Ti UB to branch UB of the power supply.

When switches SUB and SUA are in these positions, coils Cl and C2 are electrically connected in series with each other but in parallel to coils C? and C8, which are also in series with each other.

The second terminal T2UB is arranged to be electrically connected via switch SUA to coil C2 when switch SUB is in the position described in switching state (b) above.

This has the effect of connecting coils Cl, C2, C? and C8 in series with one another, from branch UA of the power supply at one end back to the star terminal 410 at the other. In this switching state, switch SUB does not connect the first terminal to branch UB of the power supply, but is instead in the "open" position.

The wiring for phase V differs from the wiring for phase U in that the windings of coils 170 in phase V (coils CS, C6, Cli and C12) with respect to their respective pole-pieces 120 are in the opposite direction to the windings of the coils 170 in both phase U and phase W. This is to improve circuit board symmetry while maintaining the appropriate polarity for the coils 170 in phase V. In all other respects, the arrangement described in detail here for terminals UI and U2 and coils Cl and C4 is the same firstly as for terminals VI and V2 and for coils C2 and C5 and secondly as for terminals WI and W2 and coils C3 and C6. Terminal Ul is the same as terminals Vi and Wl and terminal U2 is the same as terminals V2 and W2. Switch SUB is the same as switches SVB and SWB and switch SUA is the same as switches SVA and SWA. Terminal T1UB is the same as terminals TIVB and TIWB and terminal T2UB is the same as terminals T2VB and T2WB. Coil Cl is the same as coils C2 and C3 and coil C4 is the same as coils CS and C6.

Figure 10 shows in schematic form an alternative wiring and switching arrangement provided by a circuit board 210 for a twelve-pole, twelve-slot double-layer stator 110 as shown in Figure 8. It shows the wiring and switching for only phase U of the three phase supply, however the switching and connections for the other two phases, V and W are the same as for phase U, In this third embodiment, each phase is considered as being made up of four branches: UA, UB, UC and UD; VA, VB, VC and VP; WA, WB, WC, and WD. There are three possible connection configurations for the coils in each of the three phases, depending on the state of the switches shown. Either all four coils 170 in the same phase are connected in series or two coils 170 are connected in parallel with the other two or each coil is connected in parallel with each other coil.

Coil Cl is electrically connected to branch UA of the power supply via terminal U 1. It is also arranged to be connected to the star terminal 410 via switch SI VA and terminal T1UA. Coil C2 is arranged to be electrically connected to branch UC of the power supply via connection CIUC, switch S1UC and terminal U3 and to the star terminal 410 via switch S2UC and terminal TIUC. Coil C7 is arranged to be electrically connected to branch UB of the power supply via terminal T1UB, switch SIIJB and terminal U2 and to the star terminal 410 via switch S2UB and terminal T1UB. Coil CS is arranged to be electrically connected to branch UD of the power supply via terminal TI UD, switch S1UD and terminal U4 and is also connected to the star terminal 410.

Coil Cl is also arranged to be electrically connected to coil C2 via switch S1UA and terminal TI UC. In this switching state, switch SI IJC (to branch UC of the power supply) is open. Coil C2 is also arranged to be electrically connected to coil C7 via switch S2UC and terminal T2UB. In this switching state, switch S1UB is open. Coil C7 is also arranged to be electrically connected to coil CS via switch S2UB and connection CIU4. In this switching state, switch S1UD is open.

In the present (first) embodiment, the stator 110 forms part of an electric motor 1110 used in a vehicle. This arrangement is shown in Figure 11. In this embodiment, there is a vehicle power supply 1120 connected to provide direct current (DC) power to a motor controller 1130, which is arranged to convert this direct current into alternating current (AC) and to split the power supplied into three phases with a voltage pattern determined by the inputs from a position sensor 240 and a microprocessor 1140. The microprocessor 1140 is arranged to send signals to the motor controller 1130 and the switching relay modules 230 according to driver inputs and inputs from the position sensor 240. In Figure Il, all switching relay modules 230 are shown in a single block for ease of description. En other embodiments, the switching relay modules 230 are arranged to perform the function of the motor controller 1130.

The operation of the twelve-pole single-layer stator 110 (see Figure 1) with switching as shown in Figure 6 in combination with a rotor 100 of the type shown in Figure 1 will now be described for the case when they operate as a motor 1110.

In this embodiment, there is a three-phase alternating current with each phase divided into two branches (UA and UB; VA and VB; WA and WB). The switches are arranged so that coils 170 of the same phase are connected in series. Branch UA of the power supply provides current to both coils 170 in phase U, branch VA of the power supply provides current to both coils 170 in phase V and branch WA of the power supply provides current to both coils 170 in phase W. The voltage is the same in all branches and is constant in time. Each coil therefore has one half of the supply voltage across it. The interaction of the rotor poles with the stator coils 170 causes the rotor 100 to spin.

The switches are now switched to a configuration as described above wherein one coil in each phase is connected in parallel with the other coil 170 in each phase.

Branch UA of the power supply provides current to coil Cl, while branch UB of the power supply provides current to coil C4. The same holds for the corresponding coils and the branches of the other phases of the power supply. Each coil 170 therefore has the full supply voltage across it; that is, the voltage across each coil 170 doubles upon switching from series to parallel configuration. The rotor 100 therefore turns at up to twice the speed at which it would turn when the coils 170 are connected in series.

In the fourth embodiment, the motor 1110 comprises the twelve-pole double-layer stator 110 of Figure 8 with switching as shown in Figure 10. Tn this embodiment, the coils 170 can be switched twice. At first, coils 170 in the same phase are all in series with one another, so that each coil 170 has one quarter of the supply voltage across it.

Then, they are switched so that two coils 170 are in parallel with the other two coils in the same phase. Finally, the coils 170 are switched so that each coil 170 in a phase is connected in parallel with each other coil 170 in that phase. This has the effect of first doubling the speed of the rotor 100 from its initial speed and then doubling it again.

Returning now to the single-layer embodiment of Figure 1, and with reference to Figure 11, the operation of the twelve-pole single-layer stator 110 with switching as shown in Figure 6 in combination with a rotor 100 of the type shown in Figure 1 will be described for the case where they are operating as a motor 1110 in a vehicle.

In an example of the operation of the electric motor 1110, the vehicle powered by the motor 1110 is accelerating from rest. The driver provides an input to the microprocessor 1140, for example by depressing the accelerator, indicating an increase in the desired speed of the rotor 100. The microprocessor 1140 then sends a signal to the motor controller 1130, which draws DC power from the vehicle power supply 1120, converts it into three-phase alternating current and supplies it to each of the switching relay modules 230 described above with reference to Figure 7a. The microprocessor 1140 also sends a signal to the switching relay modules 230 to switch both coils 170 in the same phase to being connected in series configuration. The current through the coils 170 of the stator 110 causes the rotor 100 to start spinning.

The position sensor 240 sends signals to the microprocessor 1140. The microprocessor 1140 sends a signal to the motor controller 1130 when the rotor 100 has reached a predetermined speed threshold, that threshold being slightly below the speed that corresponds to the maximum effective coil voltage when both coils in the same phase are connected in series configuration. This signal causes the motor controller 1130 to reduce the input voltage into the switches to one half of the previous input voltage. The microprocessor 1140 simultaneously sends a signal to the switching relay module 230, causing it to switch coils 170 in the same phase from being connected in series to being connected in pairs in the parallel configuration described above with reference to Figure 6. As previously described, the voltage across each coil 170 doubles, although since the voltage supplied to the coils 170 was also halved by the motor controller 1130, the voltage across each coil 170 will therefore remain the same. The reduction of the supply voltage to the switching relay module 230 is done to prevent rapid acceleration of the motor 1110 when the coils are switched from series to parallel configuration.

Thereafter, while continued acceleration is desired, (as indicated by the driver input to the microprocessor 1140), the microprocessor 1140 sends signals to the motor controller 1130 causing it to increase the voltage supplied to the switching relay module 230 up to the voltage supplied by the vehicle power supply 1120.

The embodiment described here is a self-synchronous AC motor, so a signal is transmitted from the position sensor 240 to the motor controller 1130 which adjusts the frequency and timing of the alternating current it supplies to the switching relay module 230 according to the frequency of rotation of the rotor 100. This part of the operation is conventional.

It will be understood by the skilled reader that the motor 1110 of the fourth embodiment comprising a twelve-pole double-layer stator 110 (see Figure 8) with switching as shown in Figure 10 may also be used to power a vehicle. The control apparatus will be similar to that described for a motor 1110 comprising a twelve-pole single-layer stator 110 (see Figure 1) with switching as shown in Figure 6, but the switching will be carried out twice and the motor speed will therefore double twice.

Figure 12 shows the torque and power output through the process of acceleration for a motor 1110 in accordance with the fourth embodiment wherein the stator coils 170 are switched as shown in Figure 10. At rest, the hill supply voltage is applied across the coils 170 connected in series. There is therefore the maximum current through each of the coils 170 and the maximum possible torque output front the motor 1110, but only a quarter of the supply voltage across each of the coils 170. The rotor 100 begins to accelerate until it has reached the maximum speed it can at this effective coil voltage.

At this point, the voltage supplied to the coils 170 is reduced as described above to prevent jerking, the coils 170 are switched so that two of the coils 170 in each phase are in parallel with the other two coils 170 and the supply voltage is gradually increased to its maximum again. Switching in this way halves the current through each of the coils 170, thereby reducing the torque generated by half, but also doubles the effective coil voltage for constant suppty voltage, causing the rotor 100 to accelerate again.

When the rotor 100 has reached the maximum speed that it can at this effective coil voltage (one quarter of the supply voltage to the switching relay 230), the voltage supplied to the coils 170 is reduced to prcvent jerking, the coils 170 are switched so that each coil 170 in each phase is in parallel with each other coil 170 in the same phase and the supply voltage is gradually increased to its maximum again. The current through the coils 170 is thereby halved again, to a quarter of the initial current and the voltage across each of the coils 170 is doubled again to the full supply voltage. The rotor 100 now accelerates to its maximum possible speed. For a given supply voltage, switching as described above gives high motor torque at low speeds and high motor speed at low currents.

Without switching, the same coil voltage is needed to achieve maximum speed, but since the coils 170 are all in series, the power supply voltage must be up to four times the supply voltage with switching. This may be a problem where the supply voltage is limited by law and safety considerations. The current through the coils 170 connected in series and without switching is shown by the dashed line. A comparison of this line with the maximum current curve (the solid line) reveals that the power for a given speed (above quarter speed) is much higher when the coils 170 are connected without switching than it needs to be.

In a fifth embodiment a "semi-automatic" arrangement is provided which works similarly to the arrangement described above with reference to Figure 11. In the semi-automatic case, however, switching is not initiated automatically based on inputs to the microprocessor 1140 from the position sensor 240 and on driver inputs such as accelerator position, but instead is initiated by driver input alone, for example by the driver moving a selector similar to a gear selector. In the "semi-automatic" embodiment, the reduction in input voltage to the switches 230 prior to switching is still initiated automatically.

In a sixth embodiment, a "manual" arrangement is provided which again works similarly to the arrangement described above with reference to Figure 11. Switching and the accompanying input voltage reduction are both initiated by driver inputs, for example by the driver moving a selector similar to a gear selector and lifting the accelerator pedal of the vehicle.

In a seventh embodiment, the motor 1110 comprising the twelve-pole single-layer stator 110 (see Figure 1) with the switching arrangement as shown in Figure 6 is switched according to a different method to that described above with reference to Figure 11. In this seventh embodiment, the switching of coils 170 between series and parallel configuration is "staggered" between phases. The switches are initially arranged so that coils 170 of the same phase are connected in series, as described above. The rotor 100 begins to spin. When the rotor 100 has reached a predetermined speed threshold, that threshold being slightly below the speed that corresponds to the maximum effective coil voltage when both coils in the same phase are connected in series configuration, the switches are switched to a configuration wherein one coil in a first phase is connected in parallel with the other coil 170 in that phase. The motor controller 1130 does not reduce the voltage input to the switching modules 230.

Each coil 170 in each of the other two phases is still conncctcd in parallel with the other coil 170 in its respective phase. The speed of the rotor 100 will again increase.

V/hen the rotor 100 has reached a second predetermined speed threshold, the switches are switched to a configuration wherein one coil 170 in a second phase is connected in parallel with the other coil 170 in that phase. Finally, at a third predetermined speed threshold, the switches are switched to a configuration wherein one coil 170 in the third phase is connected in parallel with the other coil 170 in that phase. Thus each coil 170 in each phase is connected in parallel with each other coil 170 in that phase.

The speed increase of the rotor 100 when the coils 170 of one phase are switched from series to parallel configuration at constant input voltage to the switching modules 230 is smaller than the speed increase of the rotor 100 when the coils 170 of all phases are switched from series to parallel configuration at constant input voltage to the switching modules. The switching method of this seventh embodiment can therefore remove the need for the motor controller 1130 to reduce the input voltage into the switching modules 230 upon switching since switching the coils 170 in one phase does not produce undesirably rapid acceleration. This staggered switching without voltage reduction by the motor controller is suitable for small motors 1110, for example for use with bicycles.

In an alternative embodiment, the switches provided by the switching relay module 230 as shown in either Figure 7a or 7b may be soil switches. Thus instead of switching with a step function, the switches switch with a ramp. This removes the need for the voltage reduction by the motor controller 1130 since the rotor 100 speed will increase more gradually upon switching with soft switches than switches which switch with a step ftmction.

In a further alternative embodiment, the circuit board 210 may have mounted on it overload and temperature cut out and/or current limitation devices.

In addition to the twelve-pole single-layer stator 110 of the first embodiment and the twelve-pole double-layer stator of the second embodiment described above, there is a plurality of different possibilities for the number of coils 170 or poles in the stator 110. For example, Figure 13 shows a section of an electrical machine in accordance with the eighth embodiment wherein the stator 110 is a twelve-pole, 24-slot single-layer stator. There are also different possibilities for the number of phases of the power supply to the coils 170. The minimum arrangement for an electrical machine comprising a stator 110 as defined in claim 1 is a two-phase, four-coil, eight-pole, eight-slot single-layer stator with a six-pole rotor. As the number of poles increases, the number of different parallel configurations for the coils 170 increases. Thus, the highest possible torque/speed conversion factor also increases, as shown in Table 1 below.

Table 1: relationship of number of coils per phase to achievable maximum torque -speed conversion factors.

Total no. coils Coils per Maximum number per Highest possible (stator poles) phase of switching relay phase torque / speed modules ________ conversion factor 6 2 3 ______ 1 2 __ 12 4 9 _______ 3 4 __ 18 6 9 __________ 3 4 241 8 21 _________ 7 8 36 12 33 11 12 ____________ 16 45 15 16 96+etc 32 93 31 32 Each of the combinations set out in Table 1 may be realised in alternative embodiments.

Stators 110 with very large pole numbers, for example stators for use in wind turbines, may require different construction techniques. To make it easier to align coil ends with their terminals 220 on the circuit board 210, the circuit board 210 can be constructed from of a plurality of separate segments that are then electrically connected together.

In the ninth embodiment, the stator 110 may form part of an axial-rather than a radial-flux electrical machine. Figure 3 shows a schematic sectional view of the radially outer part of an axial-flux electrical machine 400 comprising the suitor 110.

In this embodiment, there are two stators 110 which are arranged on the same axis as the rotor 100, one on each side of it. There are therefore two circuit boards 210 axially adjacent to the side of each stator 110 which is furthest from the rotor 100.

In a tenth embodiment, any of the electrical machines described with reference to Figures 1, 2, 3, R and 13 may be used as a generator, for example in a traction vehicle during regenerative braking. Based on the foregoing description of the structure and operation of the electrical machine when used as a motor 1110, the skilled reader will understand how the electrical machine would be arranged and controlled for operation as a generator.

Claims (1)

1. <claim-text>CLAIMSI. A poly-phase stator for an electrical machine, the stator comprising a plurality of coils, end-connection apparatus and switching means, wherein: the coils are mounted on a stator body to form poles of the stator, the coils being discrete coils having terminals that are electrically connected to the end-connection apparatus; the end-connection apparatus providing end connections between the coils and arranged substantially to provide electrical balance between the phases; the switching means operable to switch coils in the same phase between series and parallel connection to a power supply.</claim-text> <claim-text>2. A stator according to claim 1, wherein the switching means are arranged to switch at least one coil in one of the phases between a series connection with at least one other coil in that phase and a parallel connection with that at least one other coil.</claim-text> <claim-text>3. A stator according to claim I or claim 2, wherein the switching means is arranged to switch the coils of a phase between a first configuration in which all of the coils are in series, a second configuration in which a plurality of the coils are in series with each other and in parallel with another plurality of coils also in series with each other, and a third configuration in which all of the coils are in parallel with each other.</claim-text> <claim-text>4. A stator according to any preceding claim, wherein the switching means comprise a plurality of switches which are optionally solid-state switches.</claim-text> <claim-text>5. A stator according to any preceding claim, wherein the switching means are arranged substantially to control the voltage across the coils by chopping the supply to the coils, 6. A stator according to any preceding claim, wherein each coil is substantially the same as each other coil.7. A stator according to any preceding claim, wherein each coil is arranged to have substantially the same resistance as each other coil.8. A stator according to any preceding claim, wherein each coil is a preformed coil arranged for slotting onto the stator body.9. A stator according to any preceding claim, wherein the end-connection apparatus comprises a substrate to which terminals of the coils are connected.10. A stator according to claim 9, wherein the substrate supports thereon conductive tracks that form end connections between at least some of the coils; optionally the substrate is a circuit board.11. A stator according to claim 9, wherein the terminals are inserted into holes in the substrate for physical and electrical connection thereto.12. A stator according to any of claim 9 to claim 10, wherein the substrate is made up of a plurality of separate segments that are electrically connected to each other.13. A stator according to any of claim 9 to claim 12, wherein some of the conductive tracks are on one face of the substrate and others of the tracks are on the other face of the substrate.14. A stator according to any of claim 9 to claim 13, wherein at least some of the tracks are on the same face of the substrate, with one or more tracks being in a first layer and one or more tracks being in a second layer electrically insulated from the or each track in the first layer.15. A stator according to any one of claim 9 to claim 14, wherein one or more of the tracks is formed from an electrically conducting material that is high-strength, such as, for example, carbon fibre, copper alloy or aluminium alloy; andlor one or more of the tracks is formed from an electrically conducting material that is a good thermal conductor, thereby acting as a heat sink.16. A stator according to any one of claim 9 to claim 15, wherein the tracks are arranged substantially to provide electrical balance between the phases.17. A stator according to claim 16, wherein tracks that provide end connections between coils in a first phase are of substantially the same resistance as corresponding tracks in the or each other phase; optionally, corresponding tracks are of substantially the same length and cross-sectional area; optionally, corresponding tracks may be of differing length and cross-sectional area but with the length and cross-sectional area selected to provide the corresponding tracks with substantially the same resistance.18. A stator according to any preceding claim, wherein the substrate is positioned adjacent the coils, for example by being axially juxtaposed with the stator body to minimise distance between the windings of the coils and the tracks on the substrate to minimise the length of conductors electrically connecting the coils to the tracks.19. An electrical machine comprising a stator according to any preceding claim.20. A method of operating an electrical machine according to claim 19, the method comprising the step of control means controlling the switching means to switch coils in the same phase between series and parallel connection to the power supply.21. A method according to claim 20 and comprising the step of the control means controlling the switching means to switch coils in the same phase from series to parallel configuration in response to the control means receiving an input indicative of an increased desired speed output of the electrical machine.22. A method according to claim 20 or claim 21, wherein the method comprises the step of the control means controlling the switching means to switch coils in the same phase from parallel to series configuration in response to the control means receiving an input indicative of a decreased desired speed output of the electrical machine.23. A method according to any of claim 20 to claim 23 and comprising the steps of the control means controlling the switching means to switch coils in a first phase between series and parallel connection to the power supply and then subsequently switching coils in a second phase between series arid parallel connection to the power supply.24. A method according to any of claim 20 to claim 23 and comprising switching the coils in the first phase from series to parallel and then subsequently switching the coils in the second phase from series to parallel; and optionally comprising switching the coils in the first phase from parallel to series and then subsequently switching the coils in the second phase from parallel to series.25. A method according to any one of claim 20 to claim 24 and comprising the control means operating the switching means to chop the voltage across the coils, for example by pulse width modulation.26. A method according to claim 25, wherein, when the control means operates the switching means to switch coils between series and parallel, the control means also operates the switching means to substantially simultaneously chop the voltage such that the voltage across each coil does not substantially change as the coils are switched between series and parallel, thereby avoiding rapid acceleration or deceleration when the coils are switched.27. A method according to any of claim 20 to claim 26 and comprising an automatic mode of operation in which the control means senses an input indicative of a desired speed, and responsive at least to the desired speed being greater than a speed threshold, the control means operating the switching means to switch coils from series to parallel, substantially simultaneously chopping the voltage across those coils such that the voltage increase thereacross as a result of the switching is less than it would otherwise be; and, optionally, responsive at least to the desired speed being less than a speed threshold, the control means operating the switching means to switch coils from parallel to series, substantially simultaneously chopping the voltage across those coils such that the voltage decrease thereacross as a result of the switching is less than it 28. A method according to any of claim 20 to claim 27 and comprising a semi-automatic mode of operation in which the control means operates the switches to switch the coils between series and parallel in response at least to an input from a user indicative of switching the speed range of the electrical machine, the method further comprising substantially simultaneously chopping the voltage such that the voltage change across the coils as a result of the switching is less than it would otherwise be.29. A method according to any of claim 20 to claim 28 and comprising a manual mode of operation in which the control means operates to control the switches to switch the coils between series and parallel in response at least to an input from a user indicative of switching the speed range of the electrical machine, and in which the method does not substantially simultaneously chop the voltage to lessen the change in voltage across the coils, leaving instead the user to vary the input indicative of a desired speed in order to lessen the change in voltage across the coils.30. Control means programmed and operable to control operation of an electrical machine according the method of any of claim 20 to claim 29.31. A record carrier comprising computer-readable instructions that are executable by computer processing means to cause those means to carry out the steps of a method according to any of claim 20 to claim 29.32. A vehicle comprising an electrical machine as defined an any preceding claim.33. A vehicle according to claim 32, wherein the electrical machine is housed in a wheel hub andlor a crank housing of the vehicle.</claim-text>