GB2588387A - A stator assembly for an electric motor - Google Patents

Abstract

A stator assembly 10 for an electric motor (100, fig 22) has a stator body 12, a plurality of slots 14 in the body, and a phase winding comprising a plurality of hairpin windings (24, fig 3) electrically connected together. The winding has first (50, fig 10) and second (52, fig 10) winding branches. Each winding has a plurality of arms (26, 28, fig 3). Each slot having first and second arms of a first cross-sectional area (CSA) (a1, a2, fig 13) located therein, and first and second arms of a second CSA (a3, a4, fig 13) located therein, where the first and second CSAs are different. The first arm of the first CSA is connected in series with the first arm of the second CSA in the first phase winding branch, the second arm of the first CSA is connected in series with the second arm of the second CSA in the second phase winding branch, and the first and second branches are connected in parallel with one another – fig 14. The first CSA may be bigger than the second CSA. The arms may be disposed in one of four different radial positions 16, 18, 20, 22, in the slots. The motor may be used in an electric vehicle (200, fig 22).

Description

A Stator Assembly for an Electric Motor

FIELD OF THE INVENTION

The present invention relates to a stator assembly for an electric motor, an electric motor comprising such a stator assembly, and an electric vehicle comprising such an electric motor.

BACKGROUND OF THE INVENTION

Electric motors having stator assemblies which utilise hairpin windings are known to provide benefits including increased slot fill factor. Conventional electric motors that utilise hairpin windings have, however, been found to suffer from numerous problems including, but not limited to, large values of back EMF generated in use, increased AC losses, and imbalanced back EMF, resistance and inductance paths which may lead to increased copper losses due to the circulating current.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is provided a stator assembly for an electric motor, the stator assembly comprising a stator body, a plurality of slots formed in the stator body, and a phase winding comprising a plurality of hairpin windings electrically connected together, the phase winding comprising first and second phase winding branches, each hairpin winding comprising a plurality of arms, each slot having first and second arms of a first cross-sectional area located therein, and first and second arms of a second cross-sectional area located therein, the first and second cross-sectional areas being different, wherein the first arm of the first cross-sectional area is electrically connected in series with the first arm of the second cross-sectional area in the first phase winding branch, the second arm of the first cross-sectional area is electrically connected in series with the second arm of the second cross-sectional area in the second phase winding branch, and the first and second phase winding branches are electrically connected in parallel with one another.

The stator assembly according to the first aspect of the present invention may be beneficial principally as each slot has first and second arms of a first cross-sectional area located therein, and first and second arms of a second cross-sectional area located therein, the first and second cross-sectional areas being different, wherein the first arm of the first cross-sectional area is electrically connected in series with the first arm of the second cross-sectional area in the first phase winding branch, the second arm of the first cross-sectional area is electrically connected in series with the second arm of the second cross-sectional area in the second phase winding branch, and the first and second phase winding branches are electrically connected in parallel with one another.

In particular, by splitting a phase winding into first and second phase winding branches electrically connected in parallel, the level of back [ME generated when the rotor rotates, and the magnitude of armature reaction volts induced when current flows through the phase winding during operation of the stator assembly may be reduced. Furthermore, by including arms of hairpin windings disposed in different slot positions in the first and second phase winding branches, the first and second phase winding branches may have more balanced resistance, inductance and induced volts, and hence induced circulating currents can be reduced and their associated losses may be minimised.

Locating arms of different cross-sectional areas within a single slot, and mixing the cross-sectional areas between each winding branch, may also lead to an arrangement having reduced levels of AC losses. In particular, use of arms having a different cross-sectional area may allow the choice of arm position within the slot to be tailored such that arms of smaller cross-sectional area are located in slot positions that typically experience greater levels of flux leakage across the slot, for example slot positions that are located closer to a rotor of a motor in use. Mixing the different cross-sectional areas between phase winding branches may balance their differing resistances and inductances between the parallel paths to minimise circulating currents and unequal current flows.

The first cross-sectional area may be bigger than the second cross-sectional area.

Each of the plurality of slots may comprise a plurality of radial slot positions, for example with a single hairpin arm located in each of the radial slot positions. The first and second arms of the first cross-sectional area may be disposed in first and second radial slot positions, and the first and second arms of the second cross-sectional area may be disposed in third and fourth radial slot positions. The first radial slot position may comprise a radially outermost slot position and the fourth radial slot position may comprise a radially innermost slot position.

This may be beneficial as arms in the third and fourth radial slot positions, which have the smaller cross-sectional area, may be located closer to a rotor of a motor in use. Locating arms of a smaller cross-sectional area in radially inner slot positions may therefore lead to a reduced AC loss in that arm. AC losses are proportional to the magnitude of cross slot leakage flux enclosed by the physical bounds of the arm, which is in turn proportional to its cross-sectional area, given the bounds of the slot. The AC loss in the arm may primarily be a reaction eddy current to that leakage flux. The intensity of cross slot leakage flux increases towards the rotor side of the stator and so the greatest loss minimisation may be achieved by placing the smaller cross-sectional area closer to the rotor. Thus AC loss may be minimised by careful choice of the relative cross-sectional areas. It further follows that the resistance and inductance of any hairpin is proportional to its cross-sectional area and position in the slot. The connection of these hairpins in parallel paths may cause additional circulating and unbalanced currents to flow between the paths, which may cause additional loss. To minimise this additional loss it may be beneficial to ensure each parallel path has an equal share of hairpins with each of the cross sectional areas and slot positions.

Half of the arms in the first phase winding branch may comprise the first cross-sectional area, and half of the arms in the first phase winding branch may comprise the second cross-sectional area. This may be beneficial as there may be an even split of the first and second cross-sectional areas within the first phase winding branch.

Half of the arms in the second phase winding branch may comprise the first cross-sectional area, and half of the arms in the second phase winding branch may comprise the second cross-sectional area. This may be beneficial as there may be an even split of the first and second cross-sectional areas within the second phase winding branch.

Each slot may comprise N radial slot positions, with N being an even integer. N 20 may comprise an even integer greater than or equal to 4. N may comprise a multiple of 4. Each slot may comprise N hairpin arms, with one arm located at each of the N radial slot positions.

N/2 of the arms in each slot may comprise the first cross-sectional area. N/2 of the arms in each slot may comprise the second cross-sectional area. The first phase winding branch may comprise N/2 of the arms of each slot electrically connection in series in the first phase winding branch, and N/2 of the arms of each slot electrically connected in series in the second phase winding branch. This may be beneficial as there may be an even number of arms from each slot in both the first and second phase winding branches.

The stator assembly may comprise a further plurality of slots. Each of the further plurality of slots may have third and fourth arms of the first cross-sectional area located therein. Each of the further plurality of slots may have third and fourth arms of the second cross-sectional area located therein. The third arm of the first cross-sectional area may be electrically connected in series with the third arm of the second cross-sectional area in the first phase winding branch. The fourth arm of the first cross-sectional may be electrically connected in series with the fourth arm of the second cross-sectional area in the second phase winding branch.

This may be beneficial as by including arms of hairpin windings disposed in both the plurality and further plurality of slots in each phase winding branch, any phase difference in back EMF generated by current flowing through the plurality and further plurality of slots may be reduced or eliminated.

The first arms of the first and second cross-sectional area may be electrically connected in series with the third arms of the first and second cross-sectional area in the first phase winding branch. The second arms of the first and second cross-sectional area may be electrically connected in series with the fourth arms of the first and second cross-sectional area in the second phase winding branch.

The stator assembly may comprise a plurality of slot pairs, each slot pair comprising one of the plurality of slots and one of the further plurality of slots. Slots of a slot pair may be adjacent to one another, for example with no intermediate slots located therebetween.

According to a second aspect of the invention there is provided an electric motor comprising a stator assembly according to the fourth aspect of the present invention.

According to a third aspect of the present invention there is provided an electric vehicle comprising an electric motor according to the fifth aspect of the present invention.

Preferential features of aspects of the present invention may be equally applied to other aspects of the present invention, where appropriate.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to better understand the present invention, and to show more clearly how the invention may be put into effect, the invention will now be described, by way of example, with reference to the following drawings: Figure 1 is a schematic perspective view illustrating a stator assembly according to the present invention; Figure 2a is a schematic plan view of the stator assembly of Figure 1; Figure 2b is an enlarged view of section X of Figure 2a; Figure 2c is a schematic plan view of the stator assembly of Figure 2 when wound; Figure 3 is a schematic perspective view of a hairpin winding used with the stator assembly of Figure 1, pre-bending, Figure 4 is a table illustrating the positioning of hairpin windings within slots of the stator assembly of Figure 1 pre-bending; Figure 5 is a schematic perspective view of a hairpin winding used with the stator assembly of Figure 1, post-bending; Figure 6a is a schematic view of the stator assembly of Figure 1 with hairpin windings inserted and bent to define radial rows; Figure 6b is an enlarged view of the section X of Figure 6a; Figure 7 is a table illustrating the positioning of connection portions of hairpin windings within radial rows post-bending; Figure 8 is a table illustrating the positioning of hairpin winding arms compared to their respective connection portions; Figure 9 is a first schematic view illustrating the electrical connection of phase windings of the stator assembly of Figure 1; Figure 10 is a second schematic view illustrating the electrical connection of phase windings of the stator assembly of Figure 1; Figure 11 is a first schematic view illustrating mechanical and electrical connection of windings used with the stator assembly of Figure 1; Figure 12 is a schematic view illustrating the radial positions of connection portions of hairpin winding arms relative to the slots of the stator assembly of Figure 1; Figure 13 is a schematic view of hairpin arms disposed in a slot pair of the stator assembly of Figure 1; Figure 14 is a schematic view illustrating the electrical connection of arms in a single slot of Figure 13; Figure 15a is a schematic view illustrating the electrical connection of arms of the slot pair of Figure 13; Figure 15b is a schematic view showing the physical position of hairpin arms in the electrical connection scheme of Figure 15a; Figure 16 is a schematic view illustrating a fully wound stator assembly according to the present invention; Figure 17 is a schematic view illustrating the full electrical connection of the stator assembly of Figure 16; Figure 18 is a schematic view illustrating a first further embodiment of hairpin arms disposed within a stator of the present invention; Figure 19 is a schematic view illustrating a second further embodiment of hairpin arms disposed within a stator of the present invention; Figure 20 is a schematic view illustrating a third further embodiment of hairpin arms disposed within a stator of the present invention; Figure 21 is a schematic view of an electric motor comprising the stator assembly of Figure; and Figure 22 is a schematic view of an electrical vehicle comprising the motor of Figure 17.

DETAILED DESCRIPTION

A stator assembly according to an embodiment of the present invention, generally designated 10, is shown in isolation, without any windings, in Figures 1 and 2a-c.

The stator assembly 10 has a stator body 12, which is generally cylindrical in form, and a plurality of slots 14 formed in the stator body 12. The plurality of slots 14 are generally rectangular in form, and each slot 14 has four radial slot positions 16,18,20,22, indicated schematically for a limited number of slots 14 in Figures 2a and 2b, arranged from a radially outer side of the stator body 12 to a radially inner side of the stator body 12. In the present embodiment there are 60 slots.

The stator assembly 10 is intended to be a stator assembly for a three-phase electric motor. Thus windings of the stator assembly 10 are electrically connected to form three phase windings, generally referred to as A, B, and C. The slots 14 are arranged in slot runs 13 around the stator body, with each slot run 13 corresponding to a particular phase. The slot runs 13 cycle circumferentially about the stator body 12, such that a first slot run 13 corresponds to phase A, a second slot run 13 adjacent to the first slot run 13 corresponds to phase B, a third slot run 13 adjacent to the second slot run 13 corresponds to phase C, a fourth slot run 13 adjacent to the third slot run 13 corresponds to phase A, and so on.

The stator assembly 10 is wound using hairpin windings 24, with the general form of a hairpin winding 24 being shown in Figure 3. Each hairpin winding 24 has first 26 and second 28 arms, with the first 26 and second 28 arms joined via a generally u-or v-shaped portion 30. Each hairpin winding 24 has first 32 and second 34 connection portions located at the end of respective first 26 and second 28 arms. The connection portions 32,34 are portions of the arms 26,28 that are bent to form the electrical connections between hairpin windings 24, with tips of the connection portions 32,34 having no electrically insulating coating. In the present embodiment 120 hairpin windings 24 are used to wind the stator assembly 10 in the manner described below.

Each hairpin winding 24 is inserted into the stator assembly 10 such that a first arm 26 of the hairpin winding 24 is located in a first slot 14, and a second arm 28 of the hairpin winding 24 is located in a second slot 14 spaced circumferentially around the stator body 10 from the first slot 14. In the embodiment shown, the slot pitch 14 of the hairpin winding is six, for example such that a hairpin winding 24 having a first arm 26 located in a nominal slot 14 labelled "1" has a corresponding second arm 28 located in a nominal slot 14 labelled "7". It will be recognised here that the numbered labelling of the slots is such that numbers increase in an anti-clockwise direction when viewed from a termination end of the stator assembly, as shown in Figure 2a.

The hairpin windings 24 are inserted into the stator assembly 10 such that the hairpin windings 24 form outer 36 and inner 38 winding layers, as shown schematically in Figure 2c. In the present embodiment, hairpin windings 24 of the outer layer 36 have arms 26,28 with a larger cross-sectional area than hairpin windings 24 of the inner layer 38, for reasons that will be discussed below. It will be appreciated, however, that hairpin windings having arms of the same cross-sectional area for both the outer 36 and inner 38 winding layers can also be utilised within the scope of the present invention.

In the outer winding layer 36, hairpin windings 24 are located in slots 14 such that first arms 26 are located in first radial slot positions 16, and second arms 28 are located in second radial slot positions 18. In the inner winding layer 38, hairpin windings 24 are located in slots 14 such that first arms 26 are located in third radial slot positions 20, and second arms 28 are located in fourth radial slot positions 22. Thus the outer winding layer 36 occupies the first 16 and second 18 radial slot positions, and the inner winding layer 38 occupies third 20 and fourth 22 radial slot positions. The positions of the arms 26,28 of the hairpin windings 24 within the slots 14 can be seen from Figure 4, with S referring to slot number, and L referring to radial slot position.

When the hairpin windings 24 are located in slots 14, the connection portions 32,34 of the hairpin windings extend axially outwardly from the stator body 12 of the stator assembly 10. To enable connection of the hairpin windings 24, the connection portions 32,34 of the hairpin windings are twisted/bent such that the connection portions 32,34 extend circumferentially about the stator body 12 relative to the corresponding arms 26,28, before being twisted such that the connection portions 32,34 extend in an axial direction of the stator assembly 10. Such a twisted hairpin winding is illustrated in isolation in Figure 5, with the positioning of the connection portions post-bending being shown in Figures 2b and 6 to 8. The first connection portions 32 and second connection portions 34 are twisted in opposing directions, ie with the first connection portions 32 being twisted in a clockwise direction when viewed from a termination end of the stator assembly 10, and the second connection portions 34 being twisted in an anti-clockwise direction when viewed from a termination end of the stator assembly 10.

Given that the first 26 and second 28 arms of each hairpin winding 24 are located at different radial slot positions of different slots, this results in connection portions 32,34 being sequentially twisted in different directions dependent on the radial slot position of their corresponding arms 26,28. Thus first connection portions 32 of the outer winding layer 36, which have first arms 26 located at the first radial slot position 16, are twisted in a clockwise direction. Second connection portions 34 of the outer winding layer 36, which have second arms 28 located at the second radial slot position 18, are twisted in an anti-clockwise direction. First connection portions 32 of the inner winding layer 38, which have first arms 26 located at the third radial slot position 20, are twisted in a clockwise direction. Second connection portions 34 of the inner winding layer 38, which have second arms 28 located at the fourth radial slot position 22, are twisted in an anti-clockwise direction.

The connection portions 32,34 of each hairpin winding 24 are twisted by the same degree. This gives an arrangement where connection portions 32,34 are aligned in radial rows 40, with each radial row 40 having four radial positions 42,44,46,48, as illustrated schematically in Figures 6a,b and 11. It will be appreciated that the radial positions for radial rows 44 where loop connections are formed may differ slightly from the radial positions of radial rows 44 where loop connections are not formed. For example, input and output connection portions for each phase winding may be at first radial positions 42 that are radially offset from first radial positions 42 of other radial rows 40.

In the present embodiment, the degree of twisting is such that each radial row 40 overlies a corresponding slot 14, although it will be appreciated by a person skilled in the art that arrangements where radial rows 40 are misaligned with slots 14 are also envisaged.

In the present embodiment the connection portions 32,34 are twisted with a pitch of 3 slots. This results in an arrangement where, for example, a first connection portion 32 of a hairpin winding 24, having a first arm 26 located in a first radial slot position 16 of a slot 14 labelled "1", is located in a first radial position 42 of a radial row 40 corresponding to a slot 14 labelled "58". A second connection portion 34 of the same hairpin winding 24, which has its second arm 28 located in a second radial slot position 18 of a slot 14 labelled "7", is located in a second radial position 44 of a radial row 40 corresponding to a slot labelled "10". I3

Similarly, a second connection portion 34 of a hairpin winding 24, having a second arm 28 located in a second radial slot position 18 of a slot 14 labelled "55", is located in a second radial position 44 of a radial row 40 corresponding to the slot 14 labelled "58". The radial position of the connection portions 32,34 relative to the radial slot position of the corresponding arms 26,28 can be seen from a comparison of Figure 5 with Figure 7, where R represents radial row number (corresponding to slot number S), and P represents radial position (corresponding to radial slot position L), as well as from Figure 8.

The electrical connection of the hairpin windings 24 to form three phase windings, given the positioning of the connection portions 32,34, is discussed below, with reference to Figures 9 to 16.

As can be seen from Figures 9 and 10, each phase winding A,B,C has first 50 and second 52 phase winding branches electrically connected in parallel. The construction of phase winding A will be discussed below, although it will be appreciated by a person skilled in the art that phase windings B and C have a similar construction.

Each phase winding branch 50,52 is composed of a sequence of four loops electrically connected together in series. Each loop is constructed from a number of hairpin windings 24, and comprises hairpin windings 24 which when connected together create a loop about the stator body 12. Here a loop is considered to be formed where an electrical path can be traced from a first connection portion 32 at a first radial position 42 of a given radial row 40 to a second connection portion 34 at a second radial position 44 of the same radial row, or vice versa, and where an electrical path can be traced from a first connection portion 32 at a third radial position 46 of a given radial row 40 to a second connection portion 34 at a fourth radial position 48 of the same radial row, or vice versa. Loops are schematically indicated by dashed lines in Figure 12, which will be described later.

The loops 54,56,58,60 of the first phase winding branch 50 will be described below, although as will become clear later the loops of the second phase winding branch 52 have a similar construction.

The first loop 54 of the sequence of the first phase winding branch 50 utilises hairpin windings 24 of the outer winding layer 36. As mentioned briefly above a first connection portion 32 of a hairpin winding 24, having a first arm 26 located in a first radial slot position 16 of a slot 14 labelled "1", is located in a first radial position 42 of a radial row 40 corresponding to a slot 14 labelled "58". This first connection portion 32 is used as a start of the first loop 54, and in particular this start can be used as an input connection through which electrical current is inputted into the first loop 54 in use, for example from an inverter (not shown) of an electric motor. Arm positions for start and end of loops can be seen from Figure 10, whilst the connection portion radial position can be seen from Figures 11 and 12. Figure 12 in particular shows the radial position of connection portions 32,34 relative to the corresponding slot 14. Dashed sections of Figure 12 illustrate connection portions 32,34 having arms 26,28 in the same slot 14, whilst dotted sections illustrated connection portions 32,34 having arms 26,28 in a further slot 14.

A second connection portion 34 of the same hairpin winding 24, which has its second arm 28 located in a second radial slot position 18 of a slot 14 labelled "7"), is located in a second radial position 44 of a radial row 40 corresponding to a slot labelled "10". A first connection portion 32 of a next hairpin winding 24 in the first loop 54, which has its first arm 26 located in a first radial slot position 16 of a slot 14 labelled as "13", is located in a first radial position 42 of the radial row corresponding to the slot labelled "10".

This arrangement of hairpins is similarly repeated around the stator body 12 until the first loop 54 is ended by a second connection portion 34 of a hairpin winding 24, having a second arm 28 located in a second radial slot position 18 of a slot 14 labelled "55", with the second connection portion 34 being located in a second radial position 44 of a radial row 40 corresponding to the slot 14 labelled "58".

Thus the first loop 54 starts at the first radial position 42 of the radial row 40 corresponding to the slot 14 labelled "58", and ends at the second radial position 44 of the radial row 40 corresponding to the slot 14 labelled "58".

As will be appreciated, this gives an arrangement where the first and second connection portions 32,34 of sequential hairpin windings 24 that define the first loop 54 are adjacent to one another in the first 42 and second 44 radial positions of radial rows 40 about the stator body 12. This adjacent arrangement of connection portions 32,34 enables a direct weld to be made between the hairpin windings 24 within the first loop 54, thereby connecting the hairpin windings 24 within the first loop 54, both mechanically and electrically in series. Such a direct weld is a relatively straightforward connection compared to, for example, an arrangement where an intermediate electrical conductor is required.

The second loop 56 of the sequence utilises hairpin windings 24 of the inner winding layer 38. A first connection portion 32 of a hairpin winding 24, having a first arm 26 located in a third radial slot position 20 of a slot 14 labelled "1", is located in a third radial position 46 of the radial row 40 corresponding to the slot 14 labelled "58". This first connection portion 32 is used as a start of the second loop 56. The second loop 56 is then constructed in a similar manner to the first loop 54, until the second loop 56 is ended by a second connection portion 34 of a hairpin winding 24, having a second arm 28 located in a fourth radial slot position 22 of a slot 14 labelled "55", which is located in a fourth radial position 48 of the radial row 40 corresponding to the slot 14 labelled "58".

From the above, it can be seen that the end of the first loop 54 is located in a second radial position 44 of a radial row 40 corresponding to the slot 14 labelled "58, and the start of the second loop 56 is located in a third radial position 46 of a radial row 40 corresponding to the slot 14 labelled "58". As the end of the first loop 54 and the start of the second loop 56 are in adjacent radial positions of the same radial row 40, the first 54 and second 56 loops can be directly welded to one another, thereby connecting the first loop 54 and second loop 56, both mechanically and electrically in series. Such a direct weld is a relatively straightforward connection compared to, for example, an arrangement where an intermediate electrical conductor is required. The connection between loops can be seen with respect to connection portions 32,34 in Figures 11 and 12.

The third loop 58 of the sequence utilises hairpin windings 24 of the inner winding layer 38. A second connection portion 34 of a hairpin winding 24, having a second arm 28 located in a fourth radial slot position 22 of a slot 14 labelled "2", is located in a fourth radial position 48 of a radial row 40 corresponding to a slot 14 labelled "5". This second connection portion 34 is used as a start of the third loop 58. The third loop 58 is then constructed in a similar manner to the first loop 54 and second loop 56, until the third loop 58 is ended by a first connection portion 32 of a hairpin winding 24, having a first arm 26 located in a third radial slot position 20 of a slot 14 labelled "8", which is located in a third radial position 46 of a radial row 40 corresponding to the slot 14 labelled "5".

From the above, it can be seen that the end of the second loop 56 is located in a fourth radial position 48 of a radial row 40 corresponding to the slot 14 labelled "58", and the start of the third loop 58 is located in a fourth radial position 48 of a radial row 40 corresponding to the slot 14 labelled "5". As the end of the second loop 56 and the start of the third loop 58 are in spaced apart radial rows 40, a direct weld cannot be made between the second 56 and third 58 loops. An electrical conductor 62, commonly referred to as a jumper, is used to bridge the gap, thereby providing an indirect connection between the second 56 and third 58 loops, connecting the second 56 and third 58 loops both mechanically and electrically in series.

Whilst it could be considered that an indirect connection is not as beneficial as a direct connection, it is nevertheless noted that the end of the second loop 56 and the start of the third loop 58 are located at the fourth radial positions 48 of their respective radial rows 40. This may enable a relatively straightforward indirect connection to be made between the second 56 and third 58 loops.

The fourth loop 60 of the sequence utilises hairpin windings 24 of the outer winding layer 36. A second connection portion 34 of a hairpin winding 24, having a second arm 28 located in a second radial slot position 18 of a slot 14 labelled "2", is located in a second radial position 44 of a radial row 40 corresponding to the slot 14 labelled "5". This second connection portion 34 is used as a start of the fourth loop 60. The fourth loop 60 is then constructed in a similar manner to the first loop 54, second loop 56 and third loop 58, until the fourth loop 60 is ended by a first connection portion 32 of a hairpin winding 24, having a first arm 26 located in a first radial slot position 16 of a slot 14 labelled "8", which is located in a first radial position 42 of a radial row 40 corresponding to the slot 14 labelled "5".

From the above, it can be seen that the end of the third loop 58 is located in a third radial position 46 of a radial row 40 corresponding to the slot 14 labelled "5", and the start of the fourth loop 60 is located in a second radial position 44 of a radial row 40 corresponding to the slot 14 labelled "5". As the end of the third loop 58 and the start of the fourth loop 60 are in adjacent radial positions of the same radial row 40, the third 58 and fourth 60 loops can be directly welded to one another, thereby connecting the third 58 and fourth 60 loops, both mechanically and electrically in series. Such a direct weld is a relatively straightforward connection compared to, for example, an arrangement where an intermediate electrical conductor is required.

Furthermore, as the start of the first loop 54 and the end of the fourth loop 60 are both located at first radial positions 42 of respective radial rows 40, both input and output connections for the sequence, ie input and output connections for the first phase winding branch 50, may be formed at the same radial position, which may enable a relatively simple connection arrangement. The start of the first loop 54 and the end of the fourth loop 60 are located in radial rows 40 spaced apart from one another, which may further contribute to the ease of connection.

As mentioned above, the second phase winding branch 52 is constructed in a manner generally similar to the construction of the first phase winding branch 50.

In particular, the second phase winding branch 52 is a sequence of four loops 64,66,68,70 electrically connected together in series. However, as will be appreciated, the loops 64,66,68,70 of the second phase winding branch have different start and end positions to the loops 54,56,58,60 of the first phase winding branch 50, as well as having arms 26,28 of hairpin windings 24 located in different slots 14.

The first loop 64 of the sequence of the second phase winding branch 52 utilises hairpin windings 24 of the outer winding layer 36. A first connection portion 32 of a hairpin winding 24, having a first arm 26 located in a first radial slot position 16 of a slot 14 labelled "2", is located in a first radial position 42 of the radial row 40 corresponding to the slot 14 labelled "59". This first connection portion 32 is used as a start of the first loop 64. The first loop 64 is then constructed in a similar manner to the first loop 54 of the sequence of the first phase winding branch 50, until the first loop 64 is ended by a second connection portion 34 of a hairpin winding 24, having a second arm 28 located in a second radial slot position 18 of a slot 14 labelled "56", which is located in a second radial position 44 of the radial row 40 corresponding to the slot 14 labelled "59".

It is therefore clear that the start of the first loop 54 of the sequence of the first phase winding branch 50 and the start of the first loop 64 of the sequence of the second phase winding branch 52 are adjacent to one another, at the first radial positions 42 of their respective adjacent radial rows 40. This enables a relatively straightforward input connection to be formed for the first 50 and second 52 phase winding branches.

The second loop 66 of the sequence of the second phase winding branch 52 utilises hairpin windings 24 of the inner winding layer 38. A first connection portion 32 of a hairpin winding 24, having a first arm 26 located in a third radial slot position 20 of a slot 14 labelled "2", is located in a third radial position 46 of the radial row 40 corresponding to the slot 14 labelled "59". This first connection portion 32 is used as a start of the second loop 66. The second loop 66 is then constructed in a similar manner to the first loop 64, until the second loop 66 is ended by a second connection portion 34 of a hairpin winding 24, having a second arm 28 located in a fourth radial slot position 22 of the slot 14 labelled "56", which is located in a fourth radial position 48 of the radial row 40 corresponding to the slot 14 labelled "59".

From the above, it can be seen that the end of the first loop 64 is located in a second radial position 44 of a radial row 40 corresponding to the slot 14 labelled "59", and the start of the second loop 66 is located in a third radial position 46 of a radial row 40 corresponding to the slot 14 labelled "59". As the end of the first loop 64 and the start of the second loop 66 are in adjacent radial positions of the same radial row 40, the first 64 and second 66 loops can be directly welded to one another, thereby connecting the first loop 64 and second loop 66, both mechanically and electrically in series. Such a direct weld is a relatively straightforward connection compared to, for example, an arrangement where an intermediate electrical conductor is required.

The third loop 68 of the sequence utilises hairpin windings 24 of the inner winding layer 38. A second connection portion 34 of a hairpin winding 24, having a second arm 28 located in a fourth radial slot position 22 of the slot 14 labelled "1", is located in a fourth radial position 48 of a radial row 40 corresponding to a slot 14 labelled "4". This second connection portion 34 is used as a start of the third loop 68. The third loop 68 is then constructed in a similar manner to the first loop 64 and second loop 66, until the third loop 68 is ended by a first connection portion 32 of a hairpin winding 24, having a first arm 26 located in a third radial slot position 20 of a slot 14 labelled "7", which is located in a third radial position 46 of a radial row 40 corresponding to the slot 14 labelled "4".

From the above, it can be seen that the end of the second loop 66 is located in a fourth radial position 48 of a radial row 40 corresponding to the slot 14 labelled "59", and the start of the third loop 68 is located in a fourth radial position 48 of a radial row 40 corresponding to the slot 14 labelled "4". As the end of the second loop 66 and the start of the third loop 68 are in spaced apart radial rows 40, a direct weld cannot be made between the second 66 and third 68 loops. An electrical conductor 72, commonly referred to as a jumper, is used to bridge the gap, thereby providing an indirect connection between the second 66 and third 68 loops, connecting the second 66 and third 68 loops both mechanically and electrically in series.

Whilst it could be considered that an indirect connection is not as beneficial as a direct connection, it is nevertheless noted that the end of the second loop 66 and the start of the third loop 68 are located at the fourth radial positions 48 of their respective radial rows 40. This may enable a relatively straightforward indirect connection to be made between the second 66 and third 68 loops.

It will also be noted here that end of the second loop 66 and the start of the third loop 68 of the sequence of the second phase winding branch 52 are located between the end of the second loop 56 and the start of the third loop 58 of the first phase winding branch 52. For example, it can be said that the radial rows having the starts and ends of the loops of the sequence of the second phase winding branch 52 are nested within the radial rows having the starts and ends of the loops of the sequence of the first phase winding branch 50. Such an arrangement may enable the electrical conductor 72 connecting the second 66 and third 68 loops of the second phase winding branch 52 to be nested within the electrical conductor 62 connecting the second 56 and third 58 loops of the first phase winding branch 50. This may enable a connection arrangement with a relatively small axial height.

The fourth loop 70 of the sequence utilises hairpin windings 24 of the outer winding layer 36. A second connection portion 34 of a hairpin winding 24, having a second arm 28 located in a second radial slot position 18 of the slot 14 labelled "1", is located in a second radial position 44 of a radial row 40 corresponding to the slot 14 labelled "4". This second connection portion 34 is used as a start of the fourth loop 70. The fourth loop 70 is then constructed in a similar manner to the first loop 64, second loop 66 and third loop 68, until the fourth loop 70 is ended by a first connection portion 32 of a hairpin winding 24, having a first arm 26 located in a first radial slot position 16 of a slot 14 labelled "7", which is located in a first radial position 42 of a radial row 40 corresponding to the slot 14 labelled "4".

From the above, it can be seen that the end of the third loop 68 is located in a third radial position 46 of a radial row 40 corresponding to the slot 14 labelled "4", and the start of the fourth loop 70 is located in a second radial position 44 of a radial row 40 corresponding to the slot 14 labelled "4". As the end of the third loop 68 and the start of the fourth loop 70 are in adjacent radial positions of the same radial row 40, the third 68 and fourth 70 loops can be directly welded to one another, thereby connecting the third 68 and fourth 70 loops, both mechanically and electrically in series. Such a direct weld is a relatively straightforward connection compared to, for example, an arrangement where an intermediate electrical conductor is required.

Furthermore, as the start of the first loop 64 and the end of the fourth loop 70 are both located at first radial positions 42 of respective radial rows 40, both input and output connections for the sequence, ie input and output connections for the second phase winding branch 52, may be formed at the same radial position, which may enable a relatively simple connection arrangement. The start of the first loop 64 and the end of the fourth loop 70 are located in radial rows 40 spaced apart from one another, which may further contribute to the ease of connection.

It can also be seen that the end of the fourth loop 60 of the sequence of the first phase winding branch 50 and the end of the fourth loop 70 of the sequence of the second phase winding branch 52 are adjacent to one another, at the first radial positions 42 of their respective adjacent radial rows 40. This enables a relatively straightforward output connection to be formed for the first 50 and second 52 phase winding branches.

From the discussion above, it can be seen that the first phase winding branch 50 and the second phase winding branch 52 are each formed from hairpin windings 24 having arms 26,28 disposed in both slots 14 of given slot pairs 19 about the stator body 12. Here the slot pairs 19 are considered to be slots spaced apart by a slot pitch of six, although, for the specific embodiment shown, it will be appreciated equally that a slot pair 19 may be considered to be a pair of adjacent slots 14 in a slot run 13, as seen from Figure 15b. If we consider the slots labelled "1" and "7", as in Figure 13, then the first phase winding branch 50 has first arms Li 26 disposed in the first 16 and third 20 radial slot positions of the slot 14 labelled "1", and second arms 28 disposed in the second 18 and fourth 22 radial slot positions of the slot 14 labelled "7". Similarly, the second phase winding branch 52 has first arms 26 disposed in the first 16 and third 20 radial slot positions of the slot 14 labelled "7", and second arms 28 disposed in the second 18 and fourth 22 radial slot positions of the slot 14 labelled "1". In Figures 13 to 15, arms 26,28 in slot "1" are labelled al -a4, whilst arms 26,28 in slot "7" are labelled b1-b4, for the sake of clarity This electrical connection can be more clearly seen in Figure 15a. It will be appreciated that the electrical connections shown in Figures 14 and 15a are not the full electrical connection for the phase winding, but that these are rather schematic figures intended to illustrate the electrical connections between arms disposed in slots of a slot pair 19.

It can therefore be said more generally that arms in the first 16 and third 20 radial slot positions of a first slot 15 of a slot pair 19 are electrically connected in series with arms in the second 18 and fourth 22 radial slot positions of a second slot 17 of the slot pair 19 to define a portion of a first phase winding branch 50, with a slot pair 19 being two slots 14 spaced circumferentially apart that carry the same phase current. Then arms in the first 16 and third 20 radial slot positions of the second slot 17 of the slot pair 19 are electrically connected in series with arms in the second 18 and fourth 22 radial slot positions of the first slot 15 of the slot pair 19 to define a portion of a second phase winding branch 52. This arrangement is repeated for all slot pairs 19 around the stator body 12 such that the three phase windings A,B,C are formed.

This electrical connection can be seen to provide an equal number of arms of hairpin windings 24 of the first 50 and second 52 phase winding branches distributed between the first 15 and second 17 slots of the slot pair 19, whilst the first 50 and second 52 phase winding branches each also have an equal number of hairpin arms distributed between corresponding radial slot positions 16,18,20,22 of the first 15 and second 17 slots of the slot pair 19. This may provide several benefits. In particular, by dividing each phase winding into two parallel branches, the level of induced voltage in the phase windings, due to both the back EMF and armature reactance, is reduced, to make operation compatible with the available supply voltage. Then forming each parallel branch of the phase winding from a mixture of arms from adjacent slots 14 of slot pairs 13 about the stator body 12 may reduce or eliminate any phase difference in the induced operating voltages between slots 14. Each parallel branch is not only formed from a mixture of arms from slots of each slot pair 19, but is also formed from a mixture of arms at each of the radial slot positions 16,18,20,22. This may reduce or eliminate any variance in inductance and/or resistance between adjacent slots of a slot pair 19, and hence may provide a reduction in circulating current and/or AC losses within the stator assembly 10 in use.

The reduction in circulating current and/or AC losses may further be enhanced by forming the outer 36 and inner 38 winding layers from hairpin windings having different arm thicknesses, as mentioned briefly above.

In an embodiment of the present invention, the outer winding layer 36 is formed from hairpin windings 24 having a larger arm cross-sectional area than that of hairpin windings 24 that form the inner winding layer 38. This leads to an arrangement for a given slot pair 19 as depicted in Figure 13. It will be appreciated that Figure 13 depicts the physical location of arms 26,28 within slots 14, and that the slots are depicted in increasing numerical order from left to right for the sake of clarity.

It can be seen that for a given slot 14 of a slot pair 19, the arms at the first 16 and second 18 radial slot positions have a larger cross-sectional area than arms at the third 20 and fourth 22 radial slot positions. The arms of a given slot are then electrically connected, as shown in Figures 14 and 15, such that one of the arms of a larger cross-sectional area is electrically connected in series with one of the arms of a smaller cross-sectional area to form a first slot sub-group 74 whilst the other arm of a larger cross-sectional area is electrically connected in series with the other arm of a smaller cross-sectional area to form a second slot sub-group 76. The first 74 and second 76 slot sub-groups are then electrically connected in parallel.

Repeating this structure for each slot of a given phase gives the arrangement previously described, where each phase winding branch is formed from a mixture of arms from adjacent slots of slot pairs, each phase winding branch is formed from a mixture of arms at each of the radial slot positions, and also each phase winding branch is formed from a mixture of arms of different cross-sectional area.

This arrangement may further reduce circulating currents, and hence the AC losses experienced, within the stator assembly 10 in use.

Locating arms of a smaller cross-sectional area in radially inner slot positions may lead to a reduced AC loss in that arm. AC losses are proportional to the magnitude of cross slot leakage flux enclosed by the physical bounds of the arm which is in turn proportional to its cross-sectional area, given the bounds of the slot. The AC loss in the arm may primarily be a reaction eddy current to that leakage flux. The intensity of cross slot leakage flux increases towards the rotor side of the stator and so the greatest loss minimisation may be achieved by placing the smaller cross-sectional area closer to the rotor. Thus AC loss may be minimised by careful choice of the relative cross-sectional areas. It further follows that the resistance and inductance of any hairpin is proportional to its cross-sectional area and position in the slot. The connection of these hairpins in parallel paths may cause additional circulating and unbalanced currents to flow between the paths, which may cause additional loss. To minimise this additional loss it may be beneficial to ensure each parallel path has an equal share of hairpins with each of the cross sectional areas and slot positions.

The electrical connection of the stator assembly 10 is depicted schematically in Figure 17, where isolated number illustrate hairpin numbers as per Figure 4, and SXLY denotes slot number and radial slot position for a given hairpin arm 26,28 of the corresponding hairpin winding 24. As can be seen, each loop is composed of five hairpin windings 24, such that each loop has five turns, giving 20 turns per phase A, B,C of the stator assembly 10.

Further examples of arrangements of hairpin arms that lead to an equal number of hairpin arms distributed between first and second slots of a slot pair, and the first and second phase winding branches each comprising an equal number of hairpin arms distributed between corresponding radial slot positions of the first and second slots, can be seen in Figures 18 to 20.

In the arrangement of Figure 18, the slots 14 are disposed in slot runs 300 about the stator body 12, with each slot run 300 having three slots. For example a first slot run 302 has slots "55", "56" and "57", whilst a second slot run 304 has slots "1", "2", and Here individual slots within the first 302 and second 304 slot runs can be considered to form slot pairs 19. For example, in the embodiment of Figure 18 there are three slot pairs 19 within the first 302 and second 304 slot runs, slots "55" and "1", slots "56" and "2", and slots "57" and "3". Taking each slot pair 19 into consideration, it can be seen that there are an equal number of hairpin arms spread between the slots of the slot pair, with hashed arms indicating the first phase winding branch, and dotted arms indicating the second phase winding branch. It can also be seen that within the slot pair, arms are distributed evenly between radial slot positions 16,18,20,22.

For example, considering slot pair 19 of slots "55" and "1", it can be seen that in slot "55" there is an arm in the first phase winding branch 50 at the fourth radial slot position 22, and an arm in the second phase winding branch 52 at the third radial slot position 20. Then in slot "1" there is an arm in the first phase winding branch 50 at the third radial slot position 20, and an arm in the second phase winding branch 52 at the fourth radial slot position 22.

Considering slot pair 19 of slots "56" and "2", it can be seen that in slot "56" there are arms in the first phase winding branch 50 at the second 18 and third 20 radial slot positions, and arms in the second phase winding branch 52 at the first 16 and fourth 22 radial slot positions. In slot "2" there are arms in the first phase winding branch 50 at the first 16 and fourth 22 radial slot positions, and arms in the second phase winding branch 52 at the second 18 and third 20 radial slot positions.

Considering slot pair 19 of slots "57" and "3", it can be seen that in slot "57" there is an arm in the first phase winding branch 50 at the first radial slot position 16, and an arm in the second phase winding branch 52 at the second radial slot position 18. Then in slot "3" there is an arm in the first phase winding branch 50 at the second radial slot position 18, and an arm in the second phase winding branch 52 at the first radial slot position 16.

Thus each slot 14 of the slot pair 19 is effectively the reverse pattern of the other slot 14 of the slot pair 19. This is repeated for consecutive slot runs 300 about the main body, as can be seen from the arrangement of a third slot run 306 of slots "7", "8", and "9" indicated in Figure 18, which is the same as that of slots "55", "56", and "57".

Figure 18 also briefly schematically indicates how the hairpin arms show are disposed within loops that make-up the phase winding branches, as per the previous embodiment.

It will be appreciated by a person skilled in the art that an arrangement as shown in Figure 18 may require the use of hairpin windings 24 having different length arms in practice. For example, the hairpin arm in the second radial slot position 18 of slot "56" may need to be connected to the hairpin arm in the third radial slot position 20 of slot "1". Such a connection would typically be made above slot "58", requiring a different, ie shorter, connection extension from slot "56" when compared to the longer connection extension required from slot "1". This arrangement may be similar to what is referred to as "short-pitching" in the art.

Similar alternative embodiments that have three slots 14 per slot run can be seen in Figures 19 and 20. It will be appreciated by a person skilled in the art that an arrangement as shown in Figure 19 may require the use of hairpin windings 24 having the same length arms in practice.

A motor 100 comprising the stator assembly 10 is shown schematically in Figure 21, and comprises a rotor 102 which rotates relative to the stator assembly 10 when current flows through the phase windings A,B,C in use. Further details of the motor 100 are not pertinent to the present invention, and so will not be described here for the sake of brevity, but will be well known and appreciated by a person skilled in the art.

An electric vehicle 200 comprising the motor 100 is shown schematically in Figure 22. Again, further details of the vehicle 200 are not pertinent to the present invention, and so will not be described here for the sake of brevity, but will be well known and appreciated by a person skilled in the art.

Claims (9)

1. CLAIMS1. A stator assembly for an electric motor, the stator assembly comprising a stator body, a plurality of slots formed in the stator body, and a phase winding comprising a plurality of hairpin windings electrically connected together, the phase winding comprising first and second phase winding branches, each hairpin winding comprising a plurality of arms, each slot having first and second arms of a first cross-sectional area located therein, and first and second arms of a second cross-sectional area located therein, the first and second cross-sectional areas being different, wherein the first arm of the first cross-sectional area is electrically connected in series with the first arm of the second cross-sectional area in the first phase winding branch, the second arm of the first cross-sectional area is electrically connected in series with the second arm of the second cross-sectional area in the second phase winding branch, and the first and second phase winding branches are electrically connected in parallel with one another.
2. 2. A stator assembly as claimed in Claim 1, wherein the first cross-sectional area is bigger than the second cross-sectional area.
3. 3. A stator assembly as claimed in Claim 1 or Claim 2, wherein the first and second arms of the first cross-sectional area are disposed in first and second radial slot positions, the first and second arms of the second cross-sectional area are disposed in third and fourth radial slot positions, the first radial slot position comprises a radially outermost slot position and the fourth radial slot position comprises a radially innermost slot position.
4. 4. A stator assembly as claimed in any preceding claim, wherein half of the arms in the first phase winding branch comprise the first cross-sectional area, and half of the arms in the first phase winding branch comprise the second cross-sectional area.
5. 5. A stator assembly as claimed in any preceding claim, wherein half of the arms in the second phase winding branch comprise the first cross-sectional area, and half of the arms in the second phase winding branch comprise the second cross-sectional area.
6. 6. A stator assembly as claimed in any preceding claim, wherein the stator assembly comprises a further plurality of slots, each of the further plurality of slots having third and fourth arms of the first cross-sectional area located therein, each of the further plurality of slots having third and fourth arms of the second cross-sectional area located therein, the third arm of the first cross-sectional area being electrically connected in series with the third arm of the second cross-sectional area in the first phase winding branch, and the fourth arm of the first cross-sectional being electrically connected in series with the fourth arm of the second cross-sectional area in the second phase winding branch.
7. 7. A stator assembly as claimed in Claim 6, wherein the first arms of the first and second cross-sectional area are electrically connected in series with the third arms of the first and second cross-sectional area in the first phase winding branch, and the second arms of the first and second cross-sectional area are electrically connected in series with the fourth arms of the first and second cross-sectional area in the second phase winding branch.
8. 8. An electric motor comprising a stator assembly according to any preceding claim.
9. 9. An electric vehicle comprising an electric motor according to Claim 8.