GB2588789A - A segmented stator for an electrical machine - Google Patents

Abstract

An electrical machine for use in an aircraft, comprises a rotor 602 comprising plural rotor poles 606 and a stator comprising plural phases. Each respective phase occupies at least one elementary block 616, the elementary block of each phase comprising a set of conductors 614 of that phase wound around plural slots 612 of the block in a concentrated winding configuration. The stator comprises plural segments 630 that each comprise one or more elementary blocks 616 and adjacent segments have a gap 631 between them. The mechanical shift angle dph between end slots of adjacent segments is greater than a rotor pole pitch angle (fig 2,tp), the angle between adjacent rotor poles. Windings 614 can be of a single layer. Segments 630 cam comprise magnetisable material and the gap 631 between them non-magnetic material such as polyether ether ketone (PEEK). Rotor pole pitch and stator slot pitch can be the same and the stator gap angle can be the mechanical shift angle subtracted by the slot pitch. Segment laminations (fig 7C, 740A) can be cut from a sheet of lamination material and stacked together to form a segment 630.

Description

A Seomented Stator for an Electrical Machine The present invention relates to a segmented stator for an electrical machine for use in an aircraft. In particular, the invention relates to a stator winding arrangement that enables the stator core to be segmented to improve the ease of manufacture, whilst also improving the overall performance of the electrical machine.

Background to the Invention

Electric aircraft propulsion systems typically comprise a fan (propeller), which is connected to an electrical machine. The electrical machine is typically formed of an assembly of magnetic circuit components, comprising a rotor and a stator. As is well known, rotation of the rotor relative to the stator causes interaction of the magnetic field generated by the rotor with windings provided on the stator, generating an induced electromotive force (EMF) and/or electrical current. In a permanent magnet generator, the rotor's magnetic field is produced by permanent magnets, which induces an AC voltage in the stator windings as the stator windings pass through the moving magnetic field of the permanent magnet.

The stator may be provided with a number of different types of winding arrangements depending on the requirements of the aircraft propulsion system. One common type of windings are concentrated windings in which the conductors of each respective phase of the stator are wound around pairs of adjacent slots distributed around the circumference of the stator. However, in conventional concentrated winding arrangements, it is impossible to arrange the windings so as to obtain a unit winding factor as to do so would require the number of slots to equal the number of poles, which is not feasible. Such arrangement leads to an output torque comprising harmonics (noise and ripple) and also to a reduced efficiency.

Conventional stators are formed from stacking together annular laminations. The sheets from which such annular laminations are manufactured must have an area larger than the circular footprint of the stator, which depends upon the outer diameter of the stator. This leads to a large proportion of the lamination material being wasted. Furthermore, conventional stators with a large outer diameter require complex progression tooling of wide strips of lamination material; this increases the manufacturing lead time and cost. Furthermore, conventional stators require complex, manual stacking processes, as well as further processes such as the grinding of the outer diameter of the stator to allow for precision fitting into a housing. Furthermore, it is difficult to access the interior of the continuous ring of a conventional stator for the purposes of manually or automatically winding the conductor thereon, further increasing manufacturing lead time.

Therefore, there is a need for a concentrated winding arrangement that improves the overall performance of an electrical machine, and that enables the use of solid conductor bars if required. Furthermore, there is a need for a stator that is easier, cheaper and quicker to manufacture.

Summary of the Invention

A first aspect of the present invention provides an electrical machine for use in an aircraft, comprising a rotor, wherein the rotor comprises a plurality of rotor poles, and a stator comprising a plurality of phases, wherein each respective phase occupies at least one elementary block, the at least one elementary block of each phase comprising a set of conductors of the respective phase wound around a plurality of slots of the respective elementary block in a concentrated winding configuration, and a plurality of segments, each segment comprising one or more of the elementary blocks, wherein adjacent segments have a gap provided therebetween, and wherein a mechanical shift angle between end slots of adjacent segments is greater than a rotor pole pitch, the rotor pole pitch being an angle between adjacent poles of the rotor.

As such, the stator is comprised of a plurality of segments, instead of a continuous annulus. By grouping the concentrated windings for each phase into elementary blocks, the stator can be divided up into segments comprising one or more of the elementary blocks, the segments being spaced apart by an angle that is greater than the angle between adjacent magnets of the rotor. In doing so, the slots for each phase, that is, the slots within each elementary block, are such that they align with the rotor poles, that is, the rotor magnets. Consequently, the flux linkage between the rotor poles and the coils of each phase is maximised, thereby providing a unit winding factor, which results in a more efficient machine.

Each segment may comprise a plurality of stacked segment laminations. The segment laminations may be cut from a sheet of laminated material, such as electrical steel. Therefore, a sheet of the lamination material, that is, the material from which the laminations may be made, may comprise a sheet of electrical steel having an insulation layer on at least one side. By manufacturing the stator from a plurality of segments, the sheets from which the segment laminations are produced may be smaller than those from which conventional, continuous stator laminations are cut. This can dramatically reduce the area of lamination material wasted during the cutting process, thereby reducing the cost of manufacture. Indeed, it is very difficult to obtain sheet material having dimensions suitable for a continuous stator lamination, particularly for stators having an outer diameter larger than about 350mm. For stators with a high outer diameter, segmenting the stator can reduce the manufacturing lead time and cost by only requiring relatively simple progression tooling. Furthermore, the use of segments to manufacture the stator allows for a simpler stacking process to be adopted. In particular, the reduced size of a segment, compared to a continuous stator, allows for interlocking processes to be used, instead of conventional manual stacking processes; this further contributes to shorter manufacturing lead times for a segmented stator. Furthermore, manufacturing the elementary blocks having conductors of the respective phase wound around the slots is simpler for a segmented stator. In particular, the winding of the conductors may be performed before the segments are assembled, making the slots easier to access, thereby increasing the attainable copper fill factor. In addition to improved manufacturability, the presence of a gap, formed by segmenting the stator, improves the performance of the stator by increasing the torque. Furthermore, the density of the gap may be significantly less than the density of the lamination material, leading to an improved power to weight ratio.

The mechanical shift angle is defined as the angle between end slots of adjacent segments in the plane of the stator, that is, the angle between a first radius and a second radius, wherein the first radius extends from the centre of the electrical machine to a first end slot located at the end of one elementary block, and the second radius extends from the centre of the electrical machine to a second slot, the second end slot being the closest end slot in the next elementary block. In a non-segmented stator, the mechanical shift angle being greater than the rotor pole pitch may facilitate the provision of a large tooth. The gap may be provided in the region of the large tooth such that the performance of the electrical machine is not adversely affected by its introduction. Indeed, introducing a gap into the large tooth by segmenting the stator may result in increased torque.

The at least one gap may comprise non-magnetic material. The gap may comprise air. The may comprise a ceramic material.

The non-magnetic material may comprise polyether ether ketone (PEEK).

Each one of the plurality of segments may comprise a non-permanent magnetisable material. The plurality of segments may comprise a magnetically soft material, that is, a material whose magnetic field is relatively easy to reverse.

The plurality of segments may be arranged such that the gap between adjacent segments is provided between the non-permanent magnetisable material of said segments.

Each segment may comprise one elementary block. Conversely, each segment may comprise more than one elementary block.

An angle between end slots of each pair of adjacent elementary blocks may be larger than the rotor pole pitch.

The rotor pole pitch may be equal to a stator slot pitch, the stator slot pitch being an angle between adjacent slots within each elementary block.

A stator gap angle may be defined as an angle between edges of adjacent segments. A gap width may be measured between edges of adjacent segments.

Preferably, the stator gap angle may be less than or equal to a first function. The mechanical shift angle and the stator slot pitch may be variables of the first function.

The first function may be the mechanical shift angle subtracted by the stator slot pitch. More specifically, the stator gap angle may be such that: aph -Os where: is the stator gap angle; 6ph is the mechanical shift angle; and os is the stator slot pitch.

The gap width may be equal to a second function. The stator gap angle and a radial distance may be variables of the second function.

The second function may comprise a first multiplier. The first multiplier may comprise a first trigonometric function of the stator gap angle. The first trigonometric function may be raised to the power of 0.5.

The second function may comprise a second multiplier. The second multiplier may be the radial distance, that is, the distance from the centre of the electrical machine to a point on the stator through which the stator gap angle is defined. More specifically, the gap may be defined as: e = r\12 -2 cos where: is the gap width; is the radial distance from the machine's centre to the measurement point; and cos ag is a cosine function that returns the cosine of the stator gap angle, Be.

An average gap width may be defined as the numerical mean of the gap widths. The gap widths may be measured between all opposing points on the edges of adjacent segments.

The average gap width may be less than or equal to a third function. The mechanical shift angle, the stator slot pitch and the radial distance may be variables of the third function.

The third function may comprise a third multiplier. The third multiplier may comprise a second trigonometric function of the mechanical shift angle and the stator slot pitch. The second trigonometric function may be raised to the power of 0.5. More specifically, the average gap width may be such that: er.12-2 cos(80 -0.0 where: is the gap width; is the radial distance from the machine's centre to the measurement point; and cos(60 -0,) is a cosine function that returns the cosine of the mechanical shift angle, Oph, subtracted by the stator slot pitch, Os.

A further aspect of the invention provides an aircraft propulsion system comprising an electrical machine as described above.

A further aspect of the invention provides a method of assembling an electrical machine for use in an aircraft, comprising the steps of providing a rotor having a plurality of rotor poles, providing a plurality of segments, each segment comprising at least one elementary block having a plurality of slots, winding a set of conductors of a respective phase around the plurality of slots of the respective elementary block in a concentrated winding configuration, and forming a stator by arranging the plurality of segments around the circumference of the rotor, such that a gap is provided between adjacent segments of the stator, wherein a mechanical shift angle between end slots of adjacent segments is greater than a rotor pole pitch, the rotor pole pitch being an angle between adjacent poles of the rotor.

The step of forming the stator may comprise mounting the segments in a housing.

The step of providing the plurality of segments may comprise cutting a plurality of segment laminations from at least one sheet of lamination material. The step of providing the plurality of segments may comprise stacking the plurality of segment laminations to form a segment. The step of cutting segment laminations from a sheet may be performed by stamping, wire electrical discharge machining (EDM), chemical etching, laser cutting or water jet cutting. Providing the plurality of segments by cutting segment laminations from a sheet allows for greater freedom in the choice of dimensions used for the sheet of lamination material. Indeed, for a conventional electrical machine comprising a continuous stator, the sheet of lamination material would be required to be at least as large as the footprint of the stator such that, when cutting a stator lamination from the sheet, the majority of the material does not contribute to the stator and is discarded. By segmenting the stator, the step of providing the sheet of lamination material is improved by reducing the volume of lamination material that is wasted. The lamination material may be any suitable non-permanent magnetisable material such as electrical steel.

Brief Description of the Drawings

Further features and advantages of the present invention will become apparent from the following description of embodiments thereof, presented by way of example only, and by reference to the drawings, wherein: Figure 1 is a diagram illustrating a prior art electrical machine with conventional concentrated windings; Figure 2 is a diagram illustrating an electrical machine; Figure 3 is a further diagram illustrating an electrical machine; Figure 4 is a further diagram illustrating an electrical machine; Figure 5 is a diagram illustrating an electrical machine; Figure 6A is a diagram illustrating an electrical machine according to the present invention; Figure 6B is a further diagram illustrating an electrical machine according to the present invention; Figure 6C is a graph of the performance of an electrical machine according to the present invention; Figure 7A is a flow diagram illustrating a manufacturing method for an electrical machine according to the present invention; Figure 7B is a further diagram illustrating a manufacturing method for an electrical machine according to the present invention; Figure 7C is a further diagram illustrating a manufacturing method for an electrical machine according to the present invention; Figure 8A is a flow diagram illustrating a method of calculating a winding arrangement according to the present invention; Figure 8B illustrates an example of a winding arrangement according to the present invention; Figure 9A is a flow diagram illustrating a method of calculating a winding arrangement according to the present invention.

Figure 9B illustrates an example of a winding arrangement according to the present invention; Figure 10 is a schematic of an aircraft propulsion system comprising an electrical machine according to the present invention.

Detailed Description of Preferred Embodiments

Figure 1 shows an example of a prior art three phase electrical machine 100 with a concentrated winding arrangement, comprising a rotor 102 and a stator 108. The rotor 102 comprises a rotating component 104, preferably in the form of a ferromagnetic back iron, the rotating component 104 being surrounded by an array of permanent magnets 106 distributed around its circumference. The number of poles that the electrical machine 100 has is equal to the number of permanent magnets 106 on the rotor 102. The stator 108 comprises a magnetic stator core 110, and plurality of longitudinal slots 112 distributed evenly around the circumference of the stator 108 and that extend through the stator core 110 in the direction of the longitudinal axis. The slots 112 are separated by a plurality of stator teeth 116. The stator 108 further comprises a plurality of concentrated windings for each of the three phases (denoted A, B and C). For each phase, a plurality of conductors 114 are wound around six pairs of adjacent slots 112 distributed evenly around the circumference of the stator 108, and connected together by the end windings 118. As can be seen from Figure 1, this results in a complex network of end windings 118 in order to connect all of the coils of each phase (A, B and C). In the winding layout 120 of Figure 1, a solid dot 122 represents a conductor that is arranged such that an electric current travels along the conductor(s) in a first direction (i.e. out of the page as seen in Figure 1), whilst a cross 124 represents a conductor that is arranged such that an electric current travels along the conductor(s) in a second direction that is opposite the first direction (i.e. into the page as seen in Figure 1). It will be appreciated that the meaning of these symbols applies to all of the winding layouts described and illustrated herein.

Figure 1 shows a machine 100 having 36 slots and 42 poles, which is based on the basic winding structure of a machine having 12 slots and 14 poles, multiplied by a factor of 3. As such, in prior art concentrated windings, different winding arrangements can be determined by multiplying a basic slot and pole arrangement, for example, a 12 slot/14 pole or 9 slot/10 pole arrangement, by an integer number. This is not the case with the present invention, as will become apparent in the following description.

Figures 2 and 3 illustrate one example of an electrical machine 200 that forms the basis for the present invention, also comprising a rotor 202 and a stator 208. As before, the rotor 202 comprises a rotating component 204, preferably in the form of a ferromagnetic back iron, the rotating component 204 being surrounded by an array of permanent magnets 206 distributed around its circumference.

As described previously, the number of poles in the electrical machine is directly linked to the number of permanent magnets 206. The peripheral distance between the centres of two adjacent poles, that is, the angle between two adjacent rotor magnets 206, is referred to as the rotor pole pitch.

Specifically, the rotor pole pitch, Tp, is calculated by: 360 [1] "En =-2p Where: 2p is the total number of poles in the rotor.

Here, it will be appreciated that p refers to the number of pole pairs. In the example shown in Figure 2, the rotor 202 comprises 38 permanent magnets 206, that is to say, the total number of poles is 38.

As before, the stator 208 comprises a magnetic stator core 210, and plurality of longitudinal slots 212 that extend through the stator core 210 in the direction of the longitudinal axis. In the example shown in Figure 2, the stator 208 comprises 36 slots 212. The stator 208 further comprises a plurality of concentrated windings for each of the three phases (denoted A, B and C), as further shown in the winding layout 222 of Figure 2. For each phase, the windings are grouped such that a plurality of conductors 214 are wound around the slots 212 to form two elementary blocks 216A-216F of concentrated windings, with each slot 212 being configured to receive a single conductor 214, known as a single layer winding. In this example, the conductors 214 for phase A are wound around the slots 212 in the blocks denoted 216A and 216D, the conductors 214 for phase B are wound around the slots 212 in the blocks denoted 216C and 216F, and the conductors 214 for phase C are wound around the slots 212 in the blocks denoted 216B and 216E. As such, each phase has been divided into two diametrically opposing zones, which in turn has the effect of balancing the forces of the rotor 202, thereby reducing the amount of vibration and noise. Similarly, the two distinct zones of each phase may be used to provide separate power channels that act independently of one another. This is important for redundancy and achieving fault tolerant conditions whereby one power channel is able to continue operating at full power if the other power channel fails. Furthermore, this arrangement enables the use of solid bar conductors due to the fact that only a single jump is required between the end windings of the two elementary blocks 216A-216F for each phase.

Within each elementary block 216A-216F, the slots 212 are separated by a plurality of stator teeth 218 of a first size, such that the slot pitch, Ts, that is, the angle between adjacent slots 212, is equal to the rotor pole pitch, Tp, as illustrated in Figure 3. This ensures that the whole of the flux generated from the rotor 202 is linking with the coils formed by the windings in the stator 208, that is to say, the flux linkage is maximised, and consequently the winding factor is equal to one. This perfect alignment is impossible to obtain with the conventional concentrated winding, and consequently the winding factor will always be less than one. Indeed, the highest fundamental winding factor for the three phase single-layer conventional concentrated winding is 0.966. For example, an electrical machine having 12 slots and 14 poles and having a conventional concentrated winding arrangement, the basic winding structure upon which the example of Figure 1 is based, has a winding factor of 0.966 for single layer. In contrast, an electrical machine having 12 slots and 14 poles based on the new winding arrangement described herein has a unit winding factor.

The adjacent elementary blocks of the winding, 216A-216F, are mechanically shifted by an angle, Sph, as illustrated in Figure 3. The angle Sph is larger than the rotor pole pitch and the slot pitch within the elementary blocks 216A-216F in order to accommodate the 120° electrical shift between the three phases. In this example, this is achieved by the provision of a large tooth 220 between the end slots of adjacent elementary blocks 216A-216F. Consequently, in this arrangement, the number of slots is always lower than the number of poles.

In more detail, the mechanical shift angle, oph, is calculated by: 8p11 = 360-Tp(Wslot- [2] where: Noot is the total number of slots 212 in the stator 208; equal to 3 or 6; and is the number of elementary blocks 216A-216F in the stator 208, Tp is the rotor pole pitch.

The closer the number of slots 212, Nsk,t, is to the number of poles, 2p, the higher the machine performance. For example, a machine having 36 slots and 38 poles, such as that shown in Figure 2, will perform slightly better than a machine having 36 slots and 40 poles, as will be described in reference to Figure 4 below. Conversely, as the mechanical shift angle between the blocks 216A-216F, oph, increases, performance can decrease.

Therefore, the number of slots 212 and the number of poles 206 must be selected such that for a given (Nciab 2p), with Mirk being smaller than 2p, the following condition must be fulfilled: kph -f 51 I T7 2rp [3] = -Or - loor (-

TP

tp 3 3 where: Sph is the mechanical shift angle; Tp is the rotor pole pitch; and floor (S8) is a floor function that rounds the value of ( it) to the nearest integer less than or equal to that element.

Taking the electrical machine 200 shown in Figure 2 as an example, comprising 36 slots 212 and 38 permanent magnets 206, and having six elementary blocks, n=6.

= -= 9.47° 360-3c"1(36-6) Oph = = 12.63° [5] 6 h 12.63 [5ph -floor (-P)Tpi = [12.63 -floor H7)9.471 = 3.16° [6] [4] Tp = 3.16° As such, since in the above example Tp = 9.47'; and Tp / 3 (rounded to 2 decimal places) gives the same result as equation [6], this number of poles and slots is feasible.

Having determined that a given number of rotor magnets and slots is feasible according to the above method of calculation, the specific winding layout can then be determined. In order to determine the winding layout of an electrical machine according to the present method, a series of calculations are used to evaluate the winding arrangement required for a given number of slots and poles in order to accommodate the 120° electrical shift between the three phases, as will now be described. Figure 8A illustrates the method 800 of determining the winding layout in the case of six elementary blocks (n = 6). Here, it can be seen that the method 800 comprises a series of calculating steps 802, 806, 810, 814 using the rotor pole pitch, Tp, of the given number of rotor magnets to determine the required winding layout 804, 808, 812, 816 based on the number of slots. In one example, for a machine having six elementary blocks, n=6, the calculating steps 802, 806, 810, 814 are based on the condition that the 120° electrical phase shift between the three phases is maintained, with each block spanning over a mechanical angle of 60°. Taking the example of Figure 2, wherein the electrical machine 200 comprises 36 slots 212 and 38 rotor magnets 206, it can be seen that the rotor pole pitch fulfils the first calculation 802 as follows: = -360= 9.47° 38 [8] (360) [60 -floor (- 60 -floor 60)" = 0 = 3.16° [9] 2(3)) k. 38 =3.16° [10] The number of slots 212 is then used to determine the winding layout 804, which ensures that the correct electrical shift between the three phases A, B and C is obtained for the given number of slots and poles. In the case of Figure 2, comprising 36 slots 212, the resulting configuration requires that each elementary block comprises 6 slots 212, as further illustrated by the winding layout 818 shown in Figure 8B.

In this arrangement, the concentrated windings for each phase of the stator are grouped together in at least one block, spaced apart by an angle that is greater than the angle between adjacent magnets of the rotor. In doing so, the slots for each phase, that is, the slots within each elementary block, are such that they align with the rotor poles, that is, [7] the rotor magnets. Consequently, the flux linkage between the rotor poles and the coils of each phase is maximised, thereby providing a unit winding factor, which results in a more efficient machine.

Furthermore, another benefit of this arrangement is that it enables the use of solid conductors, if desired. In conventional concentrated winding arrangements, it is very difficult to use solid bar conductors due to the complex network of jumps between the end windings of each phase, and so loose wound wires are used instead. For loose wound wires, the fill factor within each slot is usually less than 50%, whereas solid bar conductors enable more copper to fit within each slot, enabling more current to flow and thus achieve higher torque. Furthermore, because of a smaller amount of air, and thus less insulation in place, the thermal behaviour of the machine is improved. Therefore, the ability to use solid bars further helps to improve the performance of the machine by increasing the fill factor within each slot. This enables more current to flow within each slot, thereby achieving a higher torque, whilst also improving the thermal behaviour of the machine.

Figure 4 illustrates a further example of an electrical machine 400 that forms the basis for the present invention, also comprising a rotor 402 and a stator 408. As before, the rotor 402 comprises a rotating component 404, preferably in the form of a ferromagnetic back iron, the rotating component 404 being surrounded by an array of permanent magnets 406 distributed around the circumference of the rotor.

As before, the stator 408 comprises a magnetic stator core 410, and plurality of longitudinal slots 412 that extend through the stator core 410 in the direction of the longitudinal axis. The stator 408 further comprises a plurality of concentrated windings for each of the three phases (denoted A, B and C), as further shown in the winding layout 422. For each phase, the windings are grouped such that a plurality of conductors 414 are wound around the slots 412 to form two elementary blocks 416A-416F of concentrated windings, with each slot 412 being configured to receive a single conductor 414, known as a single layer winding. In this example, the conductors 414 for phase A are wound around the slots 412 in the blocks denoted 416A and 416D, the conductors 414 for phase B are wound around the slots 412 in the blocks denoted 416B and 416E, and the conductors 414 for phase C are wound around the slots 412 in the blocks denoted 416C and 416F. As with the example of Figure 2, each phase has been divided into two diametrically opposing zones, which in turn has the effect of balancing the forces of the rotor 402 and thereby reduce the amount of vibration and noise. In this example, the machine 400 comprises 36 slots 412 and 40 permanent magnets 406.

As before, the feasibility of the slot number and pole number can be verified using equations [1] to [3] above. The electrical machine 400 of Figure 4 comprises 36 slots 412, 40 permanent magnets 406, and six elementary blocks, n=6.

r" = -= 9° [11] r 40 360-9(36-6) [12] Sph -15° kph -floor (6M r I= 115 -floor P9)9] = 6° Tp P [13] 2-rp = 6c [14] As such, since in the above example Tp = 90; and 2Tp / 3 gives the same result as equation [13], this number of poles and slots is feasible.

As before, having determined that the given number of rotor magnets and slots is feasible, the specific winding arrangement can then be determined using the method 800 illustrated by Figure SA. In the example of Figure 4, wherein the electrical machine 400 comprises 36 slots 412 and 40 rotor magnets 406, it can be seen that the rotor pole pitch fulfils the second calculation 806 as follows: = 34D60 = 9c [15]

P \

[60-floor (26°p) 27:pi = [60 -floor 60]))2(9)1] = 6° [16] 3 = 6. [17] The total number of slots 412 is then used to determine the winding layout 808, ensuring that the correct electrical shift between the three phases A, B and C is obtained for the given number of slots and poles. In the case of Figure 4, comprising 36 slots 412, the resulting configuration requires that each elementary block comprises 6 slots 412, as illustrated by winding layout 422.

Figure 5 illustrates a further example of an electrical machine 500 that forms the basis for the present invention, also comprising a rotor 502 and a stator 508. As before, the rotor 502 comprises a rotating component 504, preferably in the form of a ferromagnetic back iron, the rotating component 504 being surrounded by an array of permanent magnets 506 distributed around its circumference. In this example, rotor 502 comprises 16 permanent magnets 506.

As before, the stator 508 comprises a magnetic stator core 510, and plurality of longitudinal slots 512 that extend through the stator core 510 in the direction of the longitudinal axis. In this example, the stator 508 comprises 15 slots 512, and each slot 512 is configured to receive a single conductor 514. The stator 508 further comprises a plurality of concentrated windings for each of the three phases (denoted A, B and C), as further shown in the winding layout 522. For each phase, the windings have again been grouped, however, in this example the conductors 514 for each phase are wound around the slots 512 to form a single elementary block 516A-516C of concentrated windings. In more detail, the conductors 514 for phase A are wound around the slots 512 in the block denoted 516A, the conductors 514 for phase B are wound around the slots 512 in the block denoted 516C, and the conductors 514 for phase C are wound around the slots 512 in the block denoted 516B. This arrangement, with n=3, is particularly advantageous for the use of solid bar conductors since there are no jumps between the parts of each phase since they are all grouped in a single elementary block 516A-516C.

Furthermore, for an odd number of slots 512, the neutral connection could be moved to the back of the machine (as seen in Figure 5) and the front side (as seen in Figure 5) will have only the winding terminals (power supply bars). This is different from prior art arrangements, because in any conventional winding, the neutral connection and the winding terminals are located at the same longitudinal end (e.g. the 'front side' as shown in Figure 5) of the core 510, which leads to bulky and unbalanced end windings.

Within each elementary block 516A-516C, the slots 512 are separated by a plurality of stator teeth 518 of a first size, wherein the slot pitch, Ts, is equal to the rotor pole pitch, I. As with the examples shown in Figures 2 to 4, in order to group the concentrated windings of each phase in this way, the mechanical shift angle, Sph, between the concentrated windings of adjacent elementary blocks 516A-516C is larger than the rotor pole pitch and the slot pitch within the elementary blocks 516A-516C. In this example, this is again achieved by the provision of a large tooth 520 between the end slots of adjacent elementary blocks 516A-516C.

As before, the feasibility of the slot number and pole number can be verified using equations [1] to [3] above. The electrical machine 500 of Figure 5 comprises 15 slots 512, 16 permanent magnets 506, and three elementary blocks, n=3.

= -360 = 22.5° [18] 360-22.5(15-3) Sph -300 [19] [do -f loor (6±) r = [30 -floor 22.51 = 7.5° [20]

TP 22c

11" = 7.5° 3 [21] As such, since in the above example Tp = 22.5'; and Tp / 3 gives the same result as equation [20], this number of poles and slots is feasible.

Having determined that the given number of rotor magnets and slots is feasible, the specific winding arrangement, required for a given number of slots and poles in order to accommodate the 120° electrical shift between the three phases, can then be determined using the method 900 illustrated by Figure 9A. Figure 9A illustrates the method 900 of determining the winding layout in the case of three elementary blocks, n = 3. Here, it can be seen that the method 900 comprises a series of calculating steps 902, 906 using the rotor pole pitch, Tp, of the given number of rotor magnets to determine the required winding layout 904, 908 based on the number of slots. For a machine having three elementary blocks, n=3, the calculating steps 902, 906 are again based on the condition that the 120° electrical phase shift between the three phases is maintained, with each block spanning over a mechanical angle of 120°. In the example of Figure 5, wherein the electrical machine 500 comprises 15 slots 512 and 16 rotor magnets 506, it can be seen that the rotor pole pitch fulfils the second calculation 906 as follows: 360 [21] TP =6 = 22.5° [1 20 -floor V) = [120 -f loor (2)22 5)) 2(22.5)1 = 300 [22] Tp = 30o [23] The total number of slots 512 can then be used to determine the winding layout 908, ensuring that the correct electrical shift between the three phases A, B and C is obtained for the given number of slots and poles. In the case of Figure 5, comprising 15 slots 412, the resulting configuration requires that each elementary block comprises 5 slots 412, as illustrated by winding layout 522. A further example of a winding arrangement 910 having three elementary blocks, n=3, and determined using the method of Figure 9A is shown by Figure 9B.

In the arrangements described above, where the number of slots, Nsiab is smaller than the total number of rotor poles, 2p, the required electric shift is achieved through the provision a large tooth between the end slots of elementary blocks. Such arrangements are advantageous in that it offers more flexibility in terms of the possible combinations of (Nsior, 2p). That is to say, for a given number of slots, there are many solutions with different numbers of poles, and vice versa. For example, a machine having Nsiot = 36, 2p = 38,40,44,46 and so on. Although, as discussed previously, the closer the number of slots, Nsiot, is to the number of poles, 2p, the higher the machine performance. To achieve the best performance in terms of torque density (Nm/kg), the optimal arrangement is a machine having N.5lot =2p -1 for n=3 and Moe = 2p -2 for n= 6.

The present invention seeks to optimise the manufacturing method of the electrical machine by taking advantage of the large tooth between adjacent elementary blocks. As described hereinafter, a gap may be provided in the region of a large tooth which allows the stator to be manufactured from a plurality of segments. The provision of such a gap does not affect the angular spacing of the slots; therefore, the slots within each elementary block are still arranged to align with the rotor poles, thereby maintaining a unit winding factor, as described above. As will be described later, the gap may increase the torque of the electrical machine, thereby improving its performance.

Figures 6A and 6B illustrate one example of an electrical machine 600 according to the present invention, also comprising a rotor 602 and a stator 608. As before, the rotor 602 comprises a rotating component 604, preferably in the form of a ferromagnetic back iron, the rotating component 604 being surrounded by an array of permanent magnets 606 distributed around its circumference.

The stator 608 comprises six segments 630A-630F to define a stator core 610 formed from a non-permanent magnetic material. Each segment 630A-F comprises a plurality of longitudinal slots 612 that extend through the stator core 610 in the direction of the longitudinal axis. The stator 608 further comprises a plurality of concentrated windings for each of the three phases (denoted A, B and C). For each phase, the windings are grouped such that a plurality of conductors 614 are wound around the slots 612 to form six elementary blocks 616A-616F of concentrated windings, with each slot 612 being configured to receive at least a single conductor 614. In this example, the conductors 614 for phase A are wound around the slots 612 in the blocks denoted 616A and 616D, the conductors 614 for phase B are wound around the slots 612 in the blocks denoted 616B and 616E, and the conductors 614 for phase C are wound around the slots 612 in the blocks denoted 616C and 616F. As with the example of Figure 2, each phase has been divided into two diametrically opposing zones, which in turn has the effect of balancing the forces of the rotor 602 and thereby reducing the amount of vibration and noise. In this example, the machine 600 comprises 36 slots 612 and 38 permanent magnets 606.

In the example shown in Figure 2, a large tooth 220 is provided between the end slots of adjacent elementary blocks 216A-216F. In the present invention, as shown in Figure 6A, a gap 631 in the stator core 610 is provided between the end slots of adjacent elementary blocks 616A-616F. The six gaps 631 are distributed evenly around the stator 608 such that the stator 608 is comprised of six separate segments 630A-630F. Therefore, each pair of adjacent segments 630A-630F has a gap 631 provided therebetween. In this example, each elementary block 616A-616F is provided in a respective segment 630A-630F. Therefore, each pair of adjacent elementary blocks 616A-616F has a gap 631 provided therebetween. In other words, the following pairs of blocks: block 616A and block 616B; block 616B and block 616C; block 616C and block 616D; block 616D and block 616E; block 616E and block 616F; and block 616F and block 616A, each have a gap 631 provided therebetween. However, it will be appreciated that the segments of the stator may comprise more than one elementary block. For example, in the case of 6 elementary blocks, the stator may comprise three segments, each comprising two elementary blocks, with a gap provided between each of the three segments.

The gap 631 between each segment 630A-630F extends through the stator core 610 in the direction of the longitudinal axis. Although not shown, the gap 631 may be at least partially filled with a spacer material. It should be noted that the gap 631 is intended to denote that which exists between the magnetic material of each adjacent pair of segments 630A-630F. Therefore, the gap 631 is not limited to a gap comprising air, and even in such cases where the gap 631 is filled with a non-magnetic spacer material, it will be understood that a gap 631 still exists between the magnetic material of each adjacent pair of segments 630A-630F.

Assembling the stator 608 from a plurality of segments 630A-630F, instead of a continuous annulus, brings numerous advantages. Indeed, the stator 608 comprises a plurality of segment laminations, formed from separate laminations that are cut from a sheet of the lamination material and stacked on top of one another. For a continuous stator, a larger area of sheet material would be required to form each annulus. By segmenting the stator 608 in the way described above, the sheets from which the laminations are produced may be smaller than that for the continuous stator. Furthermore, for a continuous stator, only one lamination can be cut from each sheet, with a large amount of waste material resulting from the material cut from the centre of the annulus. In the present invention, multiple segments can be cut from each sheet of lamination material to thereby minimise the amount of waste from each sheet. This makes the stator easier and quicker to manufacture, requiring simpler tooling, resulting in lower costs and reducing waste. Furthermore, winding the phases around the slots 612 is more easily achieved when performed on each individual segment, rather than an entire annulus. Accordingly, this leads to a higher copper fill factor and higher performance. In addition to improved manufacturability, the presence of a gap 631, formed by segmenting the stator, improves the performance of the stator by increasing the torque. Furthermore, since the density of the gap may be significantly less than the density of the lamination material, the total weight of the stator can be reduced by the introduction of gaps.

Given that slot number and pole number for the electrical machine 600 in Figure 6A is the same as that for the electrical machine 200 in Figure 2, the feasibility thereof can be similarly verified using equations [6] and [7]. Indeed, the presence of segments 630A-630F with gaps 631 provided therebetween does not change the variables of equations [6] and [7]. As such, the number of poles and slots is still feasible, and provides the same effect on performance. Furthermore, the winding arrangement will be similar to that of the electrical machine 200 in Figure 2.

Referring to Figure 65, a gap 631 is shown between a first segment 630A and a second segment 630B, such that the segments 630A, 630B are not in contact at any point along the longitudinal axis of the stator core 610. The stator slot pitch is denoted by es, which is the angle between two adjacent slots 612A, 613A within a single elementary block 616A. The stator gap angle is denoted by eg, which is the angle between the edges 632A, 6325 of respective adjacent segments 630A, 630B.

As outlined earlier, the mechanical shift angle, Oph, is the angle between adjacent elementary blocks 616A, 6165. In Figure 65, the mechanical shift angle, oph, is the angle between the two end slots 613A, 613B of elementary blocks 616A, 616B. The first end slot 613A is the slot in segment 630A adjacent to the gap 631. The second end slot 613B is the slot in segment 6305 adjacent to the gap 631. That is to say, the end slots 613A, 6135 are disposed on opposite sides of the gap 631 to one another. The edge slot angle is denoted by Des, which is the angle between the centre of an end slot 613A, 613B and its corresponding segment edge 632A, 6325.

An increase in the gap angle results in a decrease in the volume of magnetic material between the end slot 613A, 6135 and the segment edge 632A, 6325. Below a certain volume of magnetic material, magnetic flux may become saturated therein, leading to flux leakage and an associated decrease in performance of the electrical machine. In order to guarantee that the performance of the electrical machine 600 is not impaired by the introduction of a gap 631 between adjacent pairs of segments 630A-630F, a minimum preferable value for the edge slot angle ees has been identified. As such, the values of the edge slot angle Des that provide sufficient torque can be described by the following inequality: [24] where: 0" is the edge slot angle and Os is the stator slot pitch.

The minimum preferable value for the edge slot angle Eles corresponds to a maximum preferable value for the stator gap angle (3,. Therefore, an equivalent inequality representing the values of the stator gap angle that provide sufficient torque may be described by following inequality: < -0, [25] where: 0 is the stator gap angle; 6ph is the mechanical shift angle; and 0" is the stator slot pitch.

The optimal values of the stator gap angle Og are therefore dependent upon the mechanical shift angle Oph and the stator slot pitch Os of a particular machine. When the stator gap angle Og is equal to the expression on the right hand side of inequality [25], the torque can be maximised. When the stator gap angle Og is greater than the expression on the right hand side of inequality [25], thereby not satisfying the inequality, the torque decreases as the stator gap angle eg increases. Such behaviour is described later in reference to the graph in Figure 6C.

The calculation of the required width of the gap 631 based on the above gap angle Eig is dependent upon the points along the edges 632A, 632B between which it is measured. A variable for the radial distance, r, can be defined as the distance from the centre 650 of the electrical machine 600 to a point lying on the axis labelled 'ry in Figure 6A. Inner point 633A lies on the edge 632A of segment 630A at a position that is a minimal radial distance, ra, away from the centre 650 of the electrical machine 600. Therefore, rs represents the bore radius. A corresponding inner point 633B is positioned opposite to inner point 633A and on the edge 632B of segment 630B. The distance between the inner points 633A, 633B is represented by an arrow labelled eo in Figure 6B. Since the distance eo is measured between inner points 633A, 633B which are disposed on the edges at the minimum radial distance (i.e. the bore radius) from the centre 650 of the bore of the electrical machine 600, it will be understood that the distance eo represents the minimum gap width.

Similarly, outer points 635A, 635B lie on the respective edges 632A, 632B of the respective segments 630A, 6306 at a position that is a maximal radial distance away from the centre 650 of the electrical machine 600. The distance between the points 635A, 635B is represented by the arrow labelled eN. It will be similarly understood that the distance eN represents the maximum gap width. The average gap width, represented by the arrow labelled edv, is measured between median points 634A, 634B that lie on the respective edges 632A, 6326 of the respective segments 630A, 6306 at a position that is midway between the inner points 633A, 633B and the outer points 635B, 635B.

The minimum eo, maximum eN and average eav gap widths, as well as gap widths between any other opposing points on the edges of adjacent segments, may be calculated using known values of the stator gap angle øg and the radial distance, r. The gap width, e, between any two opposing points on the edges of adjacent segments may be calculated as follows: e = rip -2 cos 09 [26] where: is the gap width; is the radial distance from the machine's centre to the measurement point; and Og is the stator gap angle.

An isosceles triangle (not shown) is formed from inner points 633A, 6336 and the centre 650 of the electrical machine. The two equal sides of the triangle have a length, 1-0, which represents the inner radial distance, i.e. the bore radius. By substituting the variable, r, for the bore radius, ro, into equation [26], the minimum gap width, eo, may be calculated. The gap width as measured between other pairs of opposing points along the edges of adjacent segments may similarly be calculated by substituting the relevant radial distance into equation [26].

Using equation [26], inequality [25] may be similarly represented in terms of the gap width as follows: e 2 cos (aph Os) [27] where: is the gap width; is the radial distance from the machine's centre to the measurement point; Oph is the mechanical shift angle; and es is the stator slot pitch.

The preferable values of the gap width are dependent upon the mechanical shift angle oph and the stator slot pitch 0,, and also depend upon the radial distance, r, at which the gap width measurement is taken. For example, preferable values for the average gap width may be found by substituting r in inequality [27] for the radial distance from the centre 650 to a median point 634A, 634B. When the gap width e is equal to the expression on the right hand side of inequality [27], which occurs when the gap angle 0, is equal to the expression on the right hand side of inequality [25], the torque can be maximised. When the gap width e is greater than the expression on the right hand side of inequality [27], thereby not satisfying the inequality, the torque decreases as the gap width increases.

Figure 6C illustrates a graph with average torque on the y-axis and average gap width on the x-axis for an electrical machine with 36 slots and 38 poles. The graph illustrates how the torque, and therefore the performance, changes with gap width, specifically the average gap width eav. The average gap width edv may be related to the gap angle 0, using equation [26] where the radial distance, r, corresponds to the distance from the centre 650 to a median point 634A, 634B. As shown, an electrical machine with an average gap width of 0 mm (labelled "A"), which is equivalent to having a continuous, non-segmented stator such as that in the electrical machine 200 described in relation to Figure 2, demonstrates an average torque of 1 pu. An electrical machine with an average gap width of approximately 3.5 mm (labelled "B"), which in this example is equivalent to having the maximum preferable gap angle as defined by inequality [25] and the maximum preferable gap width as defined by inequality [27], demonstrates an average torque of approximately 1.035 pu; an increase of 3.5% in torque compared to the electrical machine with a continuous, non-segmented stator. The range of gap widths between points A and B is labelled 'Zone 1' in Figure 6C, being to the left of the vertical dashed line; this defines a 'preferable zone'. The graph also shows that when the gap width, and therefore gap angle, is increased further, such that neither inequality [25] nor inequality [27] is satisfied, the performance begins to decrease. This decrease in performance occurs in the range of gap widths labelled 'Zone 2' in Figure 6C.

Figure 7A illustrates a manufacturing method of an electrical machine according to the present invention. The method has a first step 7.2 of providing a plurality of segments, each segment having a plurality of slots, as described with reference to Figures 6A-B. A segment may be provided by: cutting segment laminations, with slots, from a sheet of lamination material and stacking the segment laminations to form a segment. These steps are repeated until the required number of segments have been made. For the electrical machine 600 described in relation to Figure 6A, this process would be performed six times to provide the six individual segments 630A-F.

At step 7.4, conductors will be wound around the slots of segments. In this respect, each segment will comprise at least one group of slots, such that, the set of conductors wound around each group of slots define an elementary block corresponding to a phase of the electrical machine. As such, each segment comprises at least one elementary block of conductor windings. For each elementary block, the set of conductors are wound in a concentrated winding configuration. For the electrical machine 600, each segment 630A-F comprises one elementary block 616A-F, such that each segment 630A-F comprises the conductors 614 of one phase A, B, and C, to thereby provide six elementary blocks, with two of the resulting elementary blocks being of phase A, two being of phase B, and two being of phase C. In the next step 7.6, the segments are assembled together to form a segmented stator. To do this, the segments are arranged in a circular array in the required phase order to form the stator, such that a gap is provided between adjacent segments, as described above. The circular array may be formed by mounting the segments within an external housing, but it will be appreciated that any other suitable method of securing the circular array may be adopted. For the electrical machine 600 described above, each of the segments 630A-F are arranged in the circular array such that the phases of the elementary blocks are ordered as shown in Figure 6A.

In the final step 7.8, the segmented stator will be arranged around a rotor such as those described previously. With the segmented stator mounted in the housing, for example, the step of arranging the segmented stator around the rotor may be performed by sliding at least a portion of the rotor into the centre of the stator. Alternatively, the rotor may be first positioned in the housing before the segments are mounted to the housing around the rotor.

Figure 7B illustrates one arrangement in which segment laminations could be cut from a sheet of lamination material during step 7.2. A sheet 760A is cut to form six segment laminations 740A with slots 712 provided therein. The segment laminations 740A are arranged in the same orientation as one another, and dispersed in a single row.

Figure 7C illustrates another arrangement in which segment laminations could be cut from a sheet of lamination material during step 7.2. A sheet 760B is cut to form six segment laminations 740B with slots 712 provided therein. The segment laminations 740B are arranged in two rows. The first three segment laminations 740B are disposed in a first row 741 while the remaining three segment laminations 740B are disposed in a second row 742. The segment laminations 740B in the first row 741 are oriented in a first orientation. The segment laminations 740B in the second row 742 are oriented in a second orientation, the second orientation being a 1800 rotation of the first orientation in the plane of the sheet 760B. As shown in Figure 7C, the segment laminations may be offset vertically such that the rows of segment laminations 7405 overlap, thereby reducing the height of the sheet 7605.

Providing the segment laminations in the arrangements described above allows for greater freedom in the choice of dimensions used for the sheet of lamination material. Indeed, for a conventional electrical machine comprising a continuous stator, the sheet of lamination material would be required to be at least as large as the footprint of the stator such that, when cutting a stator lamination from the sheet, the majority of the material does not contribute to the stator and is discarded. Indeed, it can be very difficult to obtain sheet material having dimensions that are suitable for a continuous stator lamination, particularly for stators having an outer diameter larger than 350mm. By segmenting the stator, the step of providing the sheet of lamination material is improved by reducing the volume of lamination material that is wasted. As shown in Figure 7B and 7C, the segment laminations can be tiled on the sheet of lamination area so as to maximise the amount of lamination material that is actually used, and thereby minimise the amount discarded. It will of course be appreciated that any suitable tiling pattern may be used depending on both the size and the number of segments.

All of the arrangements described above relate to a machine having three phases, however, it will be appreciated that these arrangements may be extended to a machine having a number of phases greater than three. For example, a six phase machine may comprise six elementary blocks, that is, one block for each phase, with each corresponding to a 60° mechanical angle. Alternatively, a six phase machine may comprise twelve elementary blocks, that is, two blocks for each phase, with each corresponding to a 30° mechanical angle. As such, it follows that whilst the arrangements described herein relate to a machine having three or six elementary blocks, it will be appreciated that these arrangements may also be extended to a machine that comprises a number of elementary blocks, n, greater than 6.

Although the arrangement of the segmented stator described above in relation to Figure 6A has six segments, each having one elementary block, it will be appreciated that the present invention of segmenting the stator may be extended to a machine having a different number of elementary blocks per segment. For example, a three phase machine comprising six elementary blocks may have a stator formed from three segments, that is, two elementary blocks per segment. Alternatively, a three phase machine comprising six elementary blocks may have a stator formed from two segments, that is, three elementary blocks per segment.

Figure 10 illustrates an example of a full-electric or hybrid-electric aircraft propulsion system 1000 comprising an electrical machine 1002 as described herein. The electrical machine 1002 comprises a rotor 1004 and a stator 1006 according to the arrangements described above. In this respect, the stator 1006 has a concentrated winding configuration, wherein the coils corresponding to each phase are arranged into elementary blocks that are separated by a mechanical shift angle that is greater than the rotor pole pitch. The electrical machine 1002 is connected to an aircraft propeller 1008 by means of a rotating shaft 1010, wherein the electric motor 1002 drives the shaft 1010 to thereby drive the propeller 1008.

In the context of a full electric or hybrid-electric aircraft, the electrical machine described herein may be used in a propulsive system, wherein the electric motors driving the propellers of the aircraft by converting the electrical power, supplied by electrical generators driven by a turboshaft or given by the battery, to a mechanical power (torque).

Various modifications, whether by way of addition, deletion and/or substitution, may be made to all of the above described embodiments to provide further embodiments, any and/or all of which are intended to be encompassed by the appended claims.

Claims (15)

1. CLAIMS1. An electrical machine for use in an aircraft, comprising: a rotor, wherein the rotor comprises a plurality of rotor poles; and a stator comprising: a plurality of phases, wherein each respective phase occupies at least one elementary block, the at least one elementary block of each phase comprising a set of conductors of the respective phase wound around a plurality of slots of the respective elementary block in a concentrated winding configuration; and a plurality of segments, each segment comprising one or more of the elementary blocks, wherein adjacent segments have a gap provided therebetween, and wherein a mechanical shift angle between end slots of adjacent segments is greater than a rotor pole pitch, the rotor pole pitch being an angle between adjacent poles of the rotor.
2. 2. An electrical machine according to claim 1, wherein the at least one gap comprises nonmagnetic material.
3. 3. An electrical machine according to claim 2, wherein the non-magnetic material comprises polyether ether ketone (PEEK).
4. 4. An electrical machine according to any of the preceding claims, wherein the plurality of segments comprises a non-permanent magnetisable material.
5. 5. An electrical machine according to claim 4, wherein the plurality of segments are arranged such that the gap between adjacent segments is provided between the nonpermanent magnetisable material of said segments.
6. 6. An electrical machine according to any of the preceding claims, wherein each segment comprises one elementary block.
7. 7. An electrical machine according to any of the preceding claims, wherein an angle between end slots of each pair of adjacent elementary blocks is larger than the rotor pole pitch.
8. 8. An electrical machine according to any of the preceding claims, wherein the rotor pole pitch is equal to a stator slot pitch, the stator slot pitch being an angle between adjacent slots within each elementary block.
9. 9. An electrical machine according to any preceding claim, wherein a stator gap angle is defined as an angle between edges of adjacent segments.
10. 10. An electrical machine according to claim 9, wherein the stator gap angle is less than or equal to a first function, wherein the mechanical shift angle and the stator slot pitch are variables of the first function.
11. 11. An electrical machine according to claim 10, wherein the first function is the mechanical shift angle subtracted by the stator slot pitch.
12. 12. An aircraft propulsion system comprising an electrical machine according to any of the preceding claims.
13. 13. A method of assembling an electrical machine for use in an aircraft, comprising the steps of: providing a rotor having a plurality of rotor poles; providing a plurality of segments, each segment comprising at least one elementary block having a plurality of slots; winding a set of conductors of a respective phase around the plurality of slots of the respective elementary block in a concentrated winding configuration; and forming a stator by arranging the plurality of segments around the circumference of the rotor, such that a gap is provided between adjacent segments of the stator, wherein a mechanical shift angle between end slots of adjacent segments is greater than a rotor pole pitch, the rotor pole pitch being an angle between adjacent poles of the rotor.
14. 14. A method according to claim 13, wherein the step of forming the stator comprises mounting the segments in a housing.
15. 15. A method according to claim 13 or claim 14, wherein the step of providing the plurality of segments further comprises: cutting a plurality of segment laminations from at least one sheet of lamination material; and stacking the plurality of segment laminations to form a segment.