GB2563941A

GB2563941A - Electric supercharger

Info

Publication number: GB2563941A
Application number: GB1710556.0A
Authority: GB
Inventors: Knight Stephen
Original assignee: Valeo Air Management UK Ltd
Current assignee: Valeo Air Management UK Ltd
Priority date: 2017-06-30
Filing date: 2017-06-30
Publication date: 2019-01-02
Also published as: GB201710556D0

Abstract

An electric motor suitable for use in a supercharger (1, figure 3) has a rotor (11a) and a stator (11). The stator comprises a plurality of pairs of opposing stator segments 3. Each stator segment comprises a housing element 29 and a stator core 27 that projects radially inwardly from the housing element, from a root 33 to a tip 34, with a stator wire 8 wound around the stator core. The root has a pair of side surfaces 40, 40' that each extend from a respective radially inner surface 31, 31' of the housing element and are oriented such that the side surfaces are inclined relative to one another. Each radially inner surface of the housing element is substantially perpendicular to the respective side surface. The relative inclination of the side surfaces may be between 1 and 30 degrees inclusively. Preferably the motor is a switched reluctance motor. An electric supercharger (9), an engine system and a vehicle are also claimed.

Description

ELECTRIC SUPERCHARGER

Technical Field

The present invention relates to electric superchargers. The present invention also relates to electric motors for electric superchargers.

Background of the Invention A typical electric supercharger comprises a compressor wheel driven by an electric motor such as a switched reluctance motor or permanent magnet motor. The compressor wheel is mounted on a drive shaft, which is rotated by the motor . A typical switched reluctance motor has a stator that comprises a plurality of stator segments arranged in a circle around the rotational axis of the rotor. The stator segments are divided into pairs that are diametrically opposite to each other, across the rotational axis of the rotor, and form opposite poles when energised.

Each stator segment comprises a housing element and a metallic stator core that projects radially inwardly from the housing element. Windings of wire are wrapped around the stator core.

The rotor is located radially inwardly of the stator segments and comprises a plurality of poles distributed in the circumferential direction. The rotor is formed of a stack of cross-shaped laminations, of low reluctance material. Therefore each rotor pole is connected to the diametrically opposite rotor pole by a low reluctance path.

The motor is driven by energising pairs of stator coils in turn to draw the poles of the rotor forward in the direction of rotation.

Switched reluctance motors can suffer from high noise levels caused by a relatively high degree of lateral movement of the stator cores when resonant frequencies occur within the working range of the motor.

Permanent magnet motors have a similar arrangement to the above described switched reluctance motor, but the rotor is made of a permanent magnet material. The stator coils are selectively energised to provide a rotating magnetic field that rotates the permanent magnet rotor.

The permanent magnet motor can also suffer from a corresponding problem, namely high noise levels caused by a high degree of lateral movement of the poles when resonant frequencies occur within the working range of the motor.

The present invention seeks to address or mitigate at least some of the above mentioned problems. Alternatively, or additionally, the present invention seeks to provide an improved electric supercharger. Alternatively, or additionally, the present invention seeks to provide an improved electric motor, in particular a switched reluctance motor. Alternatively, or additionally, the present invention seeks to provide an improved permanent magnet motor.

Summary of the Invention

According to a first aspect of the invention there is provided an electric supercharger comprising a compressor element and an electric motor configured to drive rotation of the compressor element, the electric motor comprising a rotor rotatable about an axis of rotation and a stator, the stator comprising a plurality of pairs of stator segments, each pair of stator segments comprising stator segments that are on opposite sides of the axis of rotation, each stator segment comprising a housing element and a stator core that projects radially inwardly from the housing element, from a root to a tip, with a stator wire wound around the stator core, wherein the root has a pair of side surfaces that each extend from a respective radially inner surface of the housing element and are oriented such that the side surfaces are inclined relative to each other, and wherein each radially inner surface of the housing element is oriented such that it is substantially perpendicular to the respective side surface of the root.

The feature that the side surfaces of the root are oriented such that they are inclined relative to each other is advantageous in that it may increase the stator core to housing element lateral stiffness, thereby reducing the lateral movement of the stator core, for example when resonant frequencies occur, and therefore reducing emitted noise and wear. In addition, the feature that each radially inner surface of the housing element is oriented such that it is substantially perpendicular to the respective side surface of the root, even though the side surfaces are inclined relative to each other, may facilitate winding of the stator wire on the stator core whilst still allowing for the advantage of increased lateral stiffness provided by orienting the side surfaces such that they are inclined relative to each other.

In embodiments of the invention the side surfaces are inclined relative to each other at an acute internal angle.

In embodiments of the invention the side surfaces are inclined relative to each other such that they extend towards each other as they extend radially inwardly. In this respect, in embodiments of the invention, the side surfaces converge towards each other as they extend radially inwardly.

The stator core tapers as it extends radially inwardly. In this respect, the width of the stator core (in the circumferential direction) reduces as the stator core extends radially inwardly.

Optionally the side surfaces are inclined relative to each other at an internal angle that is greater than or egual to 1° and less than or equal to 30°.

In embodiments of the invention the angle of inclination, between the side surfaces, is in a plane that is perpendicular to the rotational axis.

In embodiments of the invention the side surfaces are spaced in the circumferential direction and face opposite directions in the circumferential direction.

Optionally the side surfaces are oriented such that they are symmetrical about a radial plane that bisects the stator core. Optionally the stator core is symmetrical about a radial plane that bisects the stator core. Optionally the stator segment is symmetrical about a radial plane that bisects the stator segment.

In embodiments of the invention each side surface is substantially planar.

In embodiments of the invention each side surface extends radially from a first end to a second end, wherein the first ends are located at the same radial distance from the rotational axis and the second ends are located at the same radial distance from the rotational axis.

In embodiments of the invention each side surface supports the stator wire. In this respect, in embodiments of the invention, the stator wire is mounted on the side surfaces. An insulating element may be mounted on one, or both, side surfaces, between the side surface and the stator wire .

Optionally the tip of the core extends to a radially inner surface of the stator core. Optionally the radially inner surface extends in the circumferential direction and connects the side surfaces. The radially inner surface may be curved about the rotational axis, preferably concavely curved.

In embodiments of the invention each of the radially inner surfaces of the housing element is substantially planar .

In embodiments of the invention each of the radially inner surfaces of the housing element extend from a respective side surface of the root. Optionally one, or both, of the radially inner surfaces of the housing element extend to a respective circumferentially outer end of the housing element (outer relative to the stator core). Alternatively, one or both of the radially inner surfaces of the housing element may not extend all the way to a respective circumferentially outer end of the housing element (outer relative to the stator core).

Optionally each of the radially inner surfaces extends at least along the extent of the stator wire in the circumferential direction.

In embodiments of the invention the radially inner surfaces of the housing element are inclined relative to each other.

The housing element may be annular.

The housing element may have a radially outer surface that is curved about the rotational axis, preferably concavely curved. The radially outer surface may have a substantially constant radius, about the rotational axis.

Optionally the stator comprises three pairs of the stator segments.

In embodiments of the invention, for each stator segment, the stator core is integral with the housing element.

The stator core may have a substantially uniform cross-sectional shape (taken along a plane perpendicular to the rotational axis). The housing element may have a substantially uniform cross-sectional shape (taken along a plane perpendicular to the rotational axis).

The stator may form a path around the rotational axis. In this respect, the housing elements of the plurality of stator segments may together form a path around the rotational axis. The housing elements may form an annular ring around the rotational axis.

Optionally, circumferentially adjacent stator segments are attached to each other. This may be, for example, by suitable connectors.

On at least one side, and preferably both sides, of the stator core, a cover of electrically insulating material may be provided on the respective side surface of the root of the stator core and the respective radially inner surface of the housing element. The side surface of the root may be provided with a mounting formation, for mounting the cover on the side surface. The mounting formation may be a groove, channel, tongue, or lip, for example. The cover may be provided with a complimentary mounting formation, for engagement with the mounting formation, to retain the cover in place on the side surface. For example the mounting formation may be a groove and the complimentary mounting formation a protrusion for being received in the groove to retain the cover in place on the side surface.

The side surfaces may be substantially planar apart from the groove.

The side surfaces of the root may extend radially inwardly to the mounting formation. In this respect, the side surfaces of the root may be located radially outwardly of the mounting formation. The tip of the stator core may extend radially inwardly from the mounting formation to the radially inner end of the stator core. The tip may have side surfaces that are substantially planar. The side surfaces of the tip are preferably planar with the respective side surfaces of the root.

Preferably the electric motor is a switched reluctance motor .

Alternatively, the electric motor may be a permanent magnet motor.

According to a second aspect of the invention there is provided an electric motor, for use as the electric motor in the first aspect of the invention, wherein the electric motor comprises a rotor rotatable about an axis of rotation and a stator, the stator comprising a plurality of pairs of stator segments, each pair of stator segments comprising stator segments that are on opposite sides of the axis of rotation, each stator segment comprising a housing element and a stator core that projects radially inwardly from the housing element, from a root to a tip, with a stator wire wound around the stator core, wherein the root has a pair of side surfaces that each extend from a respective radially inner surface of the housing element and are oriented such that the side surfaces are inclined relative to each other, and wherein each radially inner surface of the housing element is oriented such that it is substantially perpendicular to the respective side surface of the root.

Preferably the electric motor is a switched reluctance motor .

Alternatively, the electric motor may be a permanent magnet motor.

The electric motor of the second aspect of the invention may have any of the features of the electric motor in the first aspect of the invention.

According to a third aspect of the invention there is provided an engine system comprising an engine and an electric supercharger according to the first aspect of the invention, arranged to supply compressed air to the engine.

According to a fourth aspect of the invention there is provided a vehicle comprising an engine system according to the third aspect of the invention.

It will of course be appreciated that features described in relation to one aspect of the present invention may be incorporated into other aspects of the present invention.

Other preferred and advantageous features of the invention will be apparent from the following description.

Description of the Drawings

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying schematic drawings of which:

Figure 1 is a sectional perspective view of a known supercharger;

Figure 2a is a perspective view of a stator of a Switched Reluctance Motor (SRM) for a known electric supercharger;

Figures 2b and 2c are perspective views showing how the individual stator segments of Figure 2a are constructed;

Figure 3 is a schematic cross-sectional view of a supercharger according to a first embodiment of the invention;

Figure 4 is a cross-sectional view of two circumferentially adjacent segments of a stator of the supercharger of Figure 3, taken along a plane corresponding to that of plane B in Figure 3;

Figure 5 is a perspective view of a stator segment of a stator of the supercharger of Figure 3, with the stator wire omitted for illustrative purposes;

Figure 6 is a cross-sectional view of a stator segment of a stator of the supercharger of Figure 3, with a stator wire wound around the stator segment, and

Figure 7 is a schematic view of an engine system comprising the supercharger of the first embodiment of the invention.

Detailed Description

Figure 1 is a sectional perspective view of a known electric supercharger 109, disclosed in UK patent publication GB2508647. The electric supercharger 109 includes an electric drive assembly having an electric motor 101 (comprising a stator 111 with stator segments 103 (described in more detail below with reference to Figures 2a-2c) , and a rotor 111a) , and a control unit 112 in the form of a Printed circuit Board (PCB) located to the rear of the motor 101. DC Power to the electric motor 101 and control unit 112 is supplied by a lead/acid battery (not shown) charged by an alternator (not shown) associated with an engine. In another embodiment (not shown) alternative power sources may be used such as a Lithium battery, or an Ultra/Super capacitor.

The drive assembly is arranged to drive a compressor element 114, in the form of a compressor wheel, via a drive shaft 113.

In this respect, the rotor 111a is arranged to rotate about a rotational axis 111c. The rotor 111a is rotationally fixed to the drive shaft 113 such that rotation of the rotor 111a rotates the drive shaft 113 about the rotational axis 111c, which rotates the compressor element 114 about the rotational axis 111c.

It will be appreciated that references to radial, axial and circumferential directions, are relative to the rotational axis 111c.

The drive shaft 113 is supported by a front bearing assembly 116a and a rear bearing assembly 116b.

In common with known superchargers, the supercharger 109 receives air through an inlet 117. The compressor element 114 then compresses the inlet air and expels it into a radial chamber 119 and through an outlet (not shown in Figure 1).

Figure 2a shows a perspective view of an example of the stator 111 of the switched reluctance motor 101, suitable for use in the prior art arrangement of Figure 1. The stator 111 of Figures 2a (and 2b-2c) is disclosed in GB2510382.

The stator 111 comprises six stator segments 103 arranged in a circle. Each stator segment 103 comprises a housing element 129 and a metallic stator core 127 (not visible in Figure 2b) that projects radially inwardly from the housing element 129. Windings of wire 108 (only the ends of which tend to be visible in Figures 2a-2c) are wrapped around the stator core 127.

The segments 103 can be divided into pairs of diametrically opposite segments 103A-C, each segment 103 in the pair being arranged to form an opposite pole to the other when energised by a control module (not shown). Within each pair of segments, the segment for forming one pole is indicated by the light-coloured wire of the respective windings, and the segment for forming the opposing pole is indicated by the dark-coloured wires of the respective windings.

Figures 2b and 2c show perspective views showing a part of the stator 111 of Figure 2a in different stages of construction. Figure 2b shows two adjacent segments (a first segment from pair 103A, and a second segment from pair 103C).

For the sake of clarity, the housing element 129 is only labelled for one of the segments in Figure 2b.

The housing elements 129, attached to the cores 127 around which the respective windings of wire are wrapped, are connected together using a tongue and groove arrangement 123. Figure 2b shows the two segments 103A/C separated, and Figure 2c shows them assembled.

To form the stator 111, two additional sets of segments are connected together in a similar fashion and then arranged to form a circle of six segments (see Figure 2a) . The segments diametrically opposite one another each define a pair of segments 103A-C, and are often known as pairs of stator poles.

The spaces between the housing element 129 and the stator core 127 of one segment, and the housing element and core of the neighbouring segment defines a set of circumferentially-spaced pockets. The windings of stator wire 108 are located within the volume of these pockets. It will be appreciated that one side of the windings on neighbouring segments 103A/103C occupies half the volume of each pocket.

The switched reluctance motor 101 can suffer from high noise levels caused by a relatively high degree of lateral movement of the stator cores 127 (and windings 108) when resonant frequencies occur within the working range of the motor .

Referring to Figure 3 there is a schematic cross-sectional view of an electric supercharger 9 according to a first embodiment of the invention. Figures 4 to 6 show segments 3 of a stator 11 of a switched reluctance motor 1 of the supercharger 9 (as will be described in more detail below).

The electric supercharger 9, of the first embodiment of the invention, is substantially identical to the known supercharger 109 described above with reference to Figures 1 to 2c, except for the differences described below. Thus, unless described otherwise, it can be assumed that the supercharger 9 of the first embodiment of the invention has the features of the known supercharger 109 described above. Features in the first embodiment of the invention that correspond to similar features in the prior-art described above, are shown with the same reference numerals as in the prior-art described above, but without the prefix '1' (or '10' where appropriate).

As with the known supercharger 109 described above, the stator 11 of the switched reluctance motor 1, of the supercharger 9 of the first embodiment of the invention, comprises six stator segments 3 arranged in a circle. Each stator segment 3 comprises a metallic housing element 29 and a metallic stator core 27 that projects radially inwardly from the housing element 29. The stator core 27 is integral with the housing element 29. Both the stator core 27 and housing element 29 are made of iron and comprise a stack of laminations .

Windings of wire 8 are wrapped around the stator core 27 .

The stator segments 3 can be divided into three pairs of diametrically opposite segments, with each segment 3 in the pair being arranged to form an opposite pole to the other when energised by a control module (not shown).

For each stator segment 3, the stator core 27 has a substantially uniform cross-sectional shape (taken along a plane perpendicular to the rotational axis), along its axial length. Similarly, the housing element 29 has a substantially uniform cross-sectional shape (taken along a plane perpendicular to the rotational axis) along its axial length.

The housing elements 29 of the stator segments 3 together form a path around the rotational axis 11c. In this respect, the housing elements 29 together form an annular ring around the rotational axis 11c.

The differences of the supercharger 9 of the first embodiment of the invention, from the known supercharger 109 described above, will now be described.

In the supercharger 9 of the first embodiment of the invention, each stator segment 3 is substantially identical and therefore only one of the stator segments 3 will be described. However, it will be appreciated that the description of the stator segment 3 below applies to each stator segment 3 of the stator 11.

It will also be appreciated that Figure 3 is a schematic view, so the shape of the stator segments 3 are shown schematically and it is not to be taken that this Figure necessarily shows the exact shape of the segments 3. The actual shape of the segments 3 is shown in Figures 4 to 6.

Referring to Figures 4 to 6, for each segment 3 the housing element 29 is an annular wall that extends from a radially outer surface 30 to radially inner surfaces 31, 31'. The radially inner surfaces 31, 31' are located on opposite circumferential sides of the stator core 27 (described in more detail below).

The housing element 29 extends in a circumferential direction between a pair of sides 32, 32' . Each side 32, 32' connects the radially outer surface 30 to a respective radially inner surface 31, 31' . The sides 32, 32' are each oriented such that as they extend radially inwardly, from the radially outer surface 30 to the respective radially inner surface 31, 31', they extend in the circumferential direction towards the stator core 27. The sides 32, 32' are generally planar, but one of the sides 32 is provided with a tongue 23 and the other of the sides 32' is provided with a groove 23' . The tongue 23 and groove 23' are for connection to a corresponding groove and tongue respectively, of adjacent stator segments 3.

The radially outer surface 30 is concavely curved about the rotational axis 11c of the rotor 11a and has a substantially constant radius of curvature. In this respect, the radially outer surface 30 forms a sector of a cylindrical surface. Similarly, the housing element 29 forms a sector of a cylindrical wall.

Each radially inner surface 31, 31' of the housing element 29 is substantially planar. The plane that the radially inner surface 31 extends in, is shown as plane C in Figure 6. For clarity the corresponding plane of the other radially inner surface 31' has not been shown in Figure 6.

The radially inner surfaces 31, 31' are inclined relative to each other.

The radially inner surfaces 31, 31' extend outwardly from a respective side surface 40, 40' of a root 33 of the stator core 27 (described in more detail below) to a respective circumferentially outer end of a radially inner side of the housing element 29 (outer relative to the stator core 27).

Each of the radially inner surfaces 31, 31' acts as part of a housing of the stator wire 8 and extends along the extent of the stator wire 8 in the circumferential direction.

The stator core 27 projects radially inwardly from the housing element 29, from a root 33 to a tip 34.

The stator core 27 extends in the circumferential direction between a pair of sides 35, 35'. The sides 35, 35' are generally planar, apart from a semi-circular axially extending groove 36, 36' provided in each side 35, 35'.

Each groove 36, 36' is for mounting a respective cover 60, 60' of electrical insulation (discussed in more detail below) on the stator core 27.

The root 33 is the region of the stator core 27 where the stator core 27 is attached to the housing element 29. The tip 34 is the region of the stator core 27 that extends from the root 33 (from a radially inner end of the root 33) to a radially inner surface 37 of the stator core 27.

The radially inner surface 37 connects the pair of sides 35, 35' and is concavely curved about the rotational axis 11c with a substantially constant radius.

The root 33 extends in the circumferential direction between a pair of side surfaces 40, 40', which are formed by radially extending regions of the sides 35, 35' of the stator core 27 (and which extend axially across the entire axial length of the stator core 27).

The side surfaces 40, 40' of the root 33 each extend from a respective radially inner surface 31, 31' of the housing element 29.

Each side surface 40, 40' of the root 33 extends radially inwardly from a first end to a second end, wherein the first ends are located at the same radial distance from the rotational axis and the second ends are located at the same radial distance from the rotational axis.

In the currently described embodiment, the side surfaces 40, 40' of the root 33 extend to the radially outer sides of the grooves 36, 36'. Each side surface 40, 40' of the root 33 is substantially planar, In this respect, each side surface 40, 40' is substantially parallel to, and contained within, a respective plane F, F' .

In the currently described embodiment, respective fillets are provided between the radially outer ends of the side surfaces 40, 40' of the root 33 and the radially inner surfaces 31, 31' of the housing element 29, to form rounded corners. The fillets each have a radius of curvature that is substantially the same as the radius of curvature of the stator wire 8. Alternatively, the fillets may have a radius that is smaller than that of the wire 8.

However, it will be appreciated that references to the side surfaces 40, 40' of the root 33 and to the radially inner surfaces 31, 31' of the housing element 29 are to those respective surfaces, which are planar, and not to the curved fillet between them. Similarly it will be appreciated that, even though there are fillets between these surfaces, the side surfaces 40, 40' of the root 33 still, under a conventional interpretation, 'extend from' the radially inner surfaces 31, 31' of the housing element 29.

Alternatively, there may be no fillet provided between the side surfaces 40, 40' of the root 33 and the radially inner surfaces 31, 31' of the housing element 29.

The tip 34 extends in the circumferential direction between a pair of side surfaces 41, 41', which are formed by radially extending regions of the sides 35, 35' of the stator core 27 (and which extend axially across the entire axial length of the stator core 27)..

The side surfaces 41, 41' of the tip 34 extend from radially inner ends of the side surfaces 40, 40' of the root 33 (i.e. from the radially outer side of the respective groove 36, 36' ) to the radially inner surface 37 of the stator core 27.

The region of each side surface 41, 41' of the tip 34, that extends from the radially inner side of the respective groove 36, 36', to the radially inner surface 37, is substantially planar and is coplanar with the plane F, F' of the respective side surface 40, 40' of the root 33.

Radially inner flanges 61, 61' project outwardly, in the circumferential direction, away from the side surfaces 41, 41' at the radially inner end of the tip 34.

Each of the planes C, F, F' of the radially inner surfaces 31, 31' of the housing element 29 and of the side surfaces 40, 40' of the root 33 are substantially parallel to the rotational axis 11c.

The stator segment 3 is symmetrical about a radial plane R that bisects the stator segment (i.e. a plane R that is parallel to the local radial direction and passes through the mid-point along the stator segment 3 in the circumferential direction).

In this respect, the stator core 27 is symmetrical about a radial plane R that bisects the stator core 27. The side surfaces 40, 40' of the root 33 are oriented such that they are symmetrical about the radial plane R that bisects the stator core 27. A cover 60, 60' of electrically insulating material which, in the currently described embodiment is plastic but may be of any suitable electrically insulating material, is provided on each side of the stator core 27. Each cover 60, 60' has a generally U-shaped cross-sectional shape and extends along the respective radially inner surface 31, 31' of the housing element 23, 23', side surface 40, 40' of the root 33, part way along the radial extent of the side surface 41, 41' of the tip 37 and along a radially outer surface of the radially inner flange 61, 61'.

Each cover 60, 60' is provided with a semi-circular protrusion that is mounted in the respective groove 36, 36' so as to retain the cover 60, 60' in place. In this respect, each groove 36, 36' forms a mounting formation and each protrusion forms a complimentary mounting formation.

The cover 60, 60' acts to insulate the wire 8 from the stator core 27 and to retain the wire 8 on the stator core 27 and away from the rotor 11a.

Alternatively, the covers 60, 60' may be omitted.

The side surfaces 40, 40' of the root 33 are oriented such that they are inclined relative to each other. The side surfaces 40, 40' of the root 33 are oriented such that they extend towards each other as they extend radially inwardly. In this respect, the side surfaces 40, 40' converge towards each other as they extend radially inwardly.

The stator core 27 tapers as it extends radially inwardly. In this respect, the width of the stator core 27 (in the circumferential direction) reduces as the stator core 27 extends radially inwardly.

In this respect, the side surfaces 40, 40' of the root 33 are inclined relative to each other at an internal angle (0) of 19° (see Figure 6) . The internal angle (0) is taken in a plane that is perpendicular to the rotational axis 11c.

It will be appreciated that the internal angle (0) is the internal angle that the planes F, F' are inclined relative to each other. It is the angle that is located internally of the closed shape formed by the intersecting planes F, F' and the radially inner surface 37 of the stator core 27 (taken in a plane that is perpendicular to the rotational axis 11c).

The internal angle (0) is preferably greater than or egual to 1° and less than or equal to 30°.

Each radially inner surface 31, 31' of the housing element 29 is oriented such that it is substantially perpendicular to the respective side surface 40, 40' of the root 33, i.e. to the side surface 40, 40' that extends from that radially inner surface 31, 31'. In this respect, the plane (plane C for the surface 31) of each radially inner surface 31, 31' of the housing element 29 is substantially perpendicular to the plane F, F' of the respective side surface 40, 40' of the root 33 (shown, for one of the sides, by the right angle (Ψ) in Figure 6).

The feature that the side surfaces 40, 40' of the root 33 are oriented such that they are inclined relative to each other, and particularly at an internal angle that is greater than or equal to 1° and less than or equal to 30°, is advantageous in that it may increase the stator core 27 to housing element 29 lateral stiffness, thereby reducing lateral movement of the stator core 27 during use, for example when resonant frequencies occur, therefore reducing emitted noise and wear. In addition, the feature that each radially inner surface 31, 31' of the housing element 29 is oriented such that it is substantially perpendicular to the respective side surface 40, 40' of the root 33, even though the side surfaces 40, 40' are inclined relative to each other, may facilitate winding of the stator wire 8 on the stator core 27 whilst still allowing for the advantage of increased lateral stiffness provided by orienting the side surfaces 40, 40' such that they are inclined relative to each other.

Referring to Figure 7, there is schematically shown a vehicle 92 comprising an engine system 91 that forms the power-plant of the vehicle 92. The engine system 91 comprises the supercharger 9 of the first embodiment of the invention and an internal combustion engine 90 that drives the vehicle 92. The supercharger 9 is arranged to supply compressed air, from its outlet, to an air intake 93 of the engine 90, so as to boost the power from the engine 90.

It will be appreciated that features described in relation to one embodiment of the present invention may be incorporated into other embodiments of the present invention and vice-versa.

Whilst the present invention has been described and illustrated with reference to particular embodiments, it will be appreciated by those of ordinary skill in the art that the invention lends itself to many different variations not specifically illustrated herein. By way of example only, certain possible variations will now be described.

For example, in the currently described embodiment the planar radially inner surfaces 31, 31' of the housing element 29 each extend to a respective circumferentially outer end of a radially inner side of the housing element 29. Alternatively, the planar radially inner sides 31, 31' may not extend all the way to a respective circumferentially outer end of the radially inner side of the housing element 29. In this respect, the radially inner sides may comprise non-planar surfaces located outwardly (in the circumferential direction) of the planar radially inner sides 31, 31' .

In the currently described embodiment of the invention the electric motor is a switched reluctance motor. Alternatively, the motor may be a permanent magnet motor.

Where in the foregoing description, integers or elements are mentioned which have known, obvious or foreseeable equivalents, then such equivalents are herein incorporated as if individually set forth. Reference should be made to the claims for determining the true scope of the present invention, which should be construed so as to encompass any such equivalents. It will also be appreciated by the reader that integers or features of the invention that are described as preferable, advantageous, convenient or the like are optional and do not limit the scope of the independent claims.

Claims

1. An electric supercharger comprising a compressor element and an electric motor configured to drive rotation of the compressor element; the electric motor comprising: a rotor rotatable about an axis of rotation; and a stator, the stator comprising: a plurality of pairs of stator segments, each pair of stator segments comprising stator segments that are on opposite sides of the axis of rotation; each stator segment comprising a housing element and a stator core that projects radially inwardly from the housing element, from a root to a tip, with a stator wire wound around the stator core; wherein the root has a pair of side surfaces that each extend from a respective radially inner surface of the housing element and are oriented such that the side surfaces are inclined relative to each other, and wherein each radially inner surface of the housing element is oriented such that it is substantially perpendicular to the respective side surface of the root.

2. An electric supercharger according to claim 1 wherein the side surfaces are inclined relative to each other at an internal angle that is greater than or equal to 1° and less than or equal to 30°.

3. An electric supercharger according to either of claims 1 or 2 wherein the electric motor is a switched reluctance motor .

4. An electric motor, for use as the electric motor in any preceding claim, wherein the electric motor comprises: a rotor rotatable about an axis of rotation; and a stator, the stator comprising: a plurality of pairs of stator segments, each pair of stator segments comprising stator segments that are on opposite sides of the axis of rotation; each stator segment comprising a housing element and a stator core that projects radially inwardly from the housing element, from a root to a tip, with a stator wire wound around the stator core; wherein the root has a pair of side surfaces that each extend from a respective radially inner surface of the housing element and are oriented such that the side surfaces are inclined relative to each other, and wherein each radially inner surface of the housing element is oriented such that it is substantially perpendicular to the respective side surface of the root.

5. An electric motor according to claim 4, wherein the electric motor is a switched reluctance motor.

6. An engine system comprising an engine and an electric supercharger according to any of claims 1 to 3, arranged to supply compressed air to the engine.

7. A vehicle comprising an engine system according to claim 6.