GB2511421A

GB2511421A - Electrical machines

Info

Publication number: GB2511421A
Application number: GB201401206A
Authority: GB
Inventors: Michael Ulrich Lamperth; Mark Cordner
Original assignee: GKN EVO Edrive Systems Ltd
Current assignee: GKN EVO Edrive Systems Ltd
Priority date: 2013-01-24
Filing date: 2014-01-24
Publication date: 2014-09-03
Anticipated expiration: 2034-01-24
Also published as: WO2014114942A2; GB201401206D0; GB201301305D0; GB2511421B; WO2014114942A3

Abstract

The stator body of an axial flux machine stator assembly having concentrated windings, comprising a plurality of stator components 30, each of which provides a stator pole defining a portion of a coil slot 36 on opposite sides. Tongue 34 and groove 35 allow the wound components to be assembled to form the annular stator (fig 5 not shown). A groove 33b defines a coolant channel. A flexible film may provide a reinforcing member when applied to the first and second faces of the assembled stator. In an alternative arrangement, the stator comprises a toothed annular structure; the pole ends of the teeth 24 having recesses 27 which receive pole ends 28 which project over the interpole gap 25 to retain the wound coils. Individual fitting 28 may be replaced with a single fitting which bridges the poles teeth (fig 8 not shown) with its ends received in the recesses 27.The central area of the fitting may be formed of SMC material and be shaped to provide a saturation bridge. In a modification the bridging member may be in the form of a slot wedge/plate which engages slots formed in the facing sides of adjacent teeth.

Description

ELECTRICAL MACHINES

The present invention relates to electrical machines, and in particular to axial flux electrical machines.

BACKGROUND OF THE INVENTION

An axial flux electrical machine is shown schematically in Figure 1 of the accompanying drawings. The machine 1 comprises a shaft 10 mounted on bearings 11. The shaft is rotatable in the bearings 11. In the example shown in Figure 1, a pair of stators 12 and 14 are provided, such that the shaft 10 extends through a substantial central aperture of each of the stators 12 and 14. The stators 12 and 14 are arranged axially along the shaft 10, and a gap is defined between the stators 12 and 14. The shaft 10 is able to rotate with respect to the stators 12 and 14. The stators 12 and 14 are manufactured from magnetic material, such a steel or iron, and are shaped to hold respective stator coils (not shown for clarity), as is well known.

A rotor 15 is attached to the shaft 10, for rotation with the shaft 10 with respect to the stators 12 and 14. In the example shown, the rotor 15 carries a plurality of permanent magnets 16 which interact with the stator coils when the machine is in use. When such a machine is in use as an electric motor, electric current is supplied to the stator coils, in order to create a fluctuating magnetic flux in the gap between the stator and the rotor. This magnetic flux is oriented axially with respect to the shaft, with gives rise to such a machine being known as an axial flux electrical machine.

When the stator coils are energised is this way, the resulting magnetic flux interacts with the permanent magnets 16 mounted on the rotor 15, such that a force is induced in the magnets 16, and hence the rotor 15. This mechanism is well known and explained by Faraday's laws. The resultant force on the rotor results in the rotor 15 turning with respect to the stators 12 and 14. Since the rotor 15 is rigidly mounted on the shaft 10, rotation of the rotor causes rotation of the shaft 10.

The electrical machine 1 of Figure 1 can also be used as an electrical generator. In such a mode of operation, the shaft is caused to turn by external means. The rotor 15 and magnets 16 then rotate with respect to the stators 12 and 14 and the stator coils. Once again in accordance with Faraday's principles, this motion of the permanent magnets causes an electrical current to be induced in the stator coils.

Figure 2 of the accompanying drawings illustrates a stator 20 suitable for use in an axial flux electrical machine such as that illustrated in Figure 1. The stator 20 is shown without stator coils for the sake of clarity. The stator 20 comprises an annular body 20, in which are defined a plurality of coil slots 22. The coil slots extend from one face of the body 21 in an axial direction towards the opposite face of the body 21. The coil slots 22 also extend radially cross the annular body 21. The coil slots 22 provide locating features for the stator coils, and the body 21 provides a magnetic flux conducting body. The number of coil slots 21 is such that a distributed winding can be accommodated in aid of a low cogging torque. The slots themselves are semi closed which helps to reduce the torque ripple and consequently eddy current heating in the magnets by guiding the magnetic flux.

Figure 3 of the accompanying drawings illustrates a portion of the stator 20 in more detail.

The body 21 comprises an annular portion 23, from which slot side portions 24 extend in an axial direction. A plurality of slot side portions 24 is arranged as a series around the body 23, such that a plurality of coil slots 22 are defined. Each slot side portion 24 carries a slot closure portion 26 which extends circumferentially towards adjacent slot side portions 24, so as to close partially the coil slots 22 in the axial direction.

In a known example stator construction, the body 21 is produced from a series of sheet portions which are laminated together in the axial direction to form the stator body. The slots are stamped out of the sheet material, in order to define the slot side portions 24 and the closure portions 26. Typically, the sheet material is sheet steel.

The current slot design makes packing of the coil wires difficult, as the space to insert the wires is very narrow. In practice, this means that stator coils need to be pre-formed in portions which are then inserted consecutively rather than one insertion step.

It is, therefore, desirable to provide a stator assembly that can overcome the drawbacks of the known designs.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, there is provided a stator assembly for an axial flux electrical machine, the stator assembly comprising a stator body which defines a series of stator poles, arranged such that adjacent poles in the series are separated by a coil slot for receiving portions of a stator coil therein, and a plurality of stator coils located around respective stator poles, and partially located in respective coil slots of the stator body, wherein the stator body comprises a plurality of stator components, each of which provides a stator pole and which defines a portion of a coil slot to opposite sides of the stator pole.

The stator component may be of a soft magnetic composite material, and the stator body may further comprise first and second reinforcing members to opposite planar faces of the stator assembly.

The stator component may provide, in a rear face thereof, at least one channel to allow fluid circulation around the stator for cooling purposes.

According to another aspect of the present invention, there is provided a stator assembly for an axial flux electrical machine, the stator assembly comprising a stator body which defines a series of stator poles, arranged such that adjacent poles in the series are separated by a coil slot for receiving portions of a stator coil therein, and a plurality of stator coils located around respective stator poles, and partially located in respective coil slots of the stator body, wherein the coil slots have an open region sized to receive a portion of such a coil, and wherein the stator body includes coil slot closure elements, located to close at least partially the open regions of the coil slots.

A coil slot may be closed, or partially closed, by a single closure element or by a pair of closure elements located at respective sides of the open region of the coil slot concerned.

According to another aspect of the present invention, there is provided a stator assembly for an axial flux electrical machine, the stator assembly comprising a stator body which defines a series of stator poles, arranged such that adjacent poles in the series are separated by a coil slot for receiving portions of a stator coil therein, and a plurality of stator coils located around respective stator poles, and partially located in respective coil slots of the stator body, wherein the coil slots have an open region sized to receive a portion of such a coil, wherein the stator body defines pairs of locating grooves for receiving respective slot closure elements, and wherein the stator assembly includes a plurality of slot closure elements located in the locating grooves, each such closure element serving to close or partially close a coil slot.

The stator body may be of a soft magnetic composite material. The closure element may be provided by a elongate planar strip of, for example, steel. The closure member may be provided with slots, or other features of shape, in order to provide the desired electrical, mechanical and magnetic chraracteristics.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic view of an axial flux electrical machine; Figure 2 is a schematic view of a stator suitable for use in the machine of Figure 1; Figure 3 is a schematic view of a portion of the stator of Figure 2 showing a section showing stator teeth and slot arrangement; Figure 4 is a schematic view of a portion of a stator embodying one aspect of the present invention; Figure 5 is a schematic view of a stator for an axial flux electrical machine embodying another aspect of the present invention; Figure 6 illustrates a portion of a previously-considered stator; Figures 7 and 8 illustrate portions of respective stator design embodying aspects of the present invention; Figure 9 illustrates a further portion of a stator design embodying an aspect of the present invention; and Figure 10 illustrates part of the portion of Figure 9.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As described above, Figure 1 illustrates schematically an axial flux electrical machine which includes a stator. Figure 4 illustrates a stator component 30 embodying one aspect of the present invention, and Figure 5 illustrates a stator 40 comprising a plurality of such stator components 30 embodying another aspect of the present invention.

The component of Figure 4 is manufactured from a soft magnetic composite (SMC) material.

An SMC is a metallic composite material which can be processed similar to powder metal.

An SMC comprises small separately electrically insulated powder grains, and so has a very high electrical resistance. This high electrical resistance reduces eddy currents and as such removes the need to laminate the material. Accordingly, an SMC can be formed using a sintering technique in which the powder grains are formed into a desired shape and then subjected to increased pressure and heat. The cooled material then retains the desired

S

form. The benefits of SMC over conventional electrical steel are most pronounced at high frequencies, where losses from eddy currents and hysteresis are significantly lower.

The component 30 shown in Figure 4 comprises a body portion 31 which extends axially from a first boundary portion 32 to a second boundary portion 33. The first and second boundary portions 32 and 33 are integral with the body portion 31 and extend outwardly from the body portion both radially and circumferentially. The first and second boundary portions 32 and 33 provide first and second faces of the stator 40, when a plurality of the components are arranged to form the stator 40. The component 30 includes a channel 33b that provides a flow path through which cooling fluid is able to flow when the stator is in place in an electrical machine.

The body portion 31 of the component 30, and the first and second boundary portions 32 and 33, have a tapered cross section in the axial direction, such that when a plurality of such components are arranged as a series next to one another, a substantially annular stator is formed. An example of such an annular stator is shown in Figure 5.

The first boundary portion 32 is substantially planar. The second boundary portion 33 defines at a first end region thereof a locating projection 34 which extends circumferentially from the portion 33. The locating projection also extends radially from an inner region to an outer region of the component 30. At a second end region of the second boundary portion 33, opposite to the first end region, a locating slot 35 is defined. The locating slot 35 extends circumferentially into the second end region of the second boundary portion 33, and radially from an inner region to an outer region of the component 30. The locating projection 34 of one component 30 is arranged to engage with the locating slot 35 of the next component 30 in the series forming the stator 40. The locating projection 34 and locating slot 35 form cooperating features of shape to locate adjacent stator components 30 in the stator 40. The locating projection 34 and locating slot 35 may also be described as a "tongue and groove" fitting.

The body portion 31 and first and second boundary portions 32 and 33 define a coil slot 36 therebetween. The coils slots 36 of adjacent stator components 30 on the stator 40 are contiguous to form complete coil slots for the reception of the stator coils (not shown for clarity).

Since SMC material can be more brittle than desirable, the stator 40 may be provided with respective reinforcing members that are annular in form, and which extend across the first and second faces of the stator 40. A reinforcing member may be provided by a flexible material, for example in the form of a film. The provision of the reinforcing members effectively makes the stator 40 of composite construction.

Figure 6 shows a known stator construction (manufactured from either an SMC material or laminated steel) in which the slot side portions 24 and closure portions 26 define a partially closed coil slot 25. This construction makes it difficult to insert the stator coils into the coil slots, but is necessary to provide the desired magnetic and electrical properties for the stator.

Figure 7 illustrates a first alternative construction of the stator, in which the closure portions 26 are replaced by receiving portions 27 and closure portions 28. The closure portions 28 are separate from the stator body and slot side portions 24, and are located on the receiving portions 27 following location of the stator coils in the coils slots 25. The closure portions may include interlocking features, and may be glued or pressed into place. Such a construction enables a larger opening for the coil slot to be provided, thereby aiding insertion of the coil into the coil slot. The closure portions 28 then provide the stator with the desired electrical and magnetic characteristics. The closure portions 28 may be manufactured from an SMC material, or from any other suitable material that provides the required characteristics.

Figure 8 illustrates a second alternative construction of the stator, in which the closure portions 26 are replaced by receiving portions 27 and a single closure portion 29. The closure portion 29 is separate from the stator body and slot side portions 24, and is located on the receiving portions 27 following location of the stator coils in the coils slots 25. The closure portion extends across the opening of the coils slot 25. Such a construction enables a larger opening for the coil slot to be provided, thereby aiding insertion of the coil into the coil slot. The closure portion 29 then provides the stator with the desired electrical and magnetic characteristics. The closure portion 29 may be manufactured from an SMC material, or from any other suitable material that provides the required characteristics. A part 29b of the closure portion 29 may provide a "saturation bridge" for the closure portion 29. A saturation bridge is a portion of magnetic material that reaches magnetic saturation, and so acts in a manner similar to an air gap, during operation of the electrical machine. As such, a saturation bridge can provide desirable magnetic characteristics, without unduly compromising the mechanical characterisitcs of the closure portion 29.

Figure 9 illustrates a third alternative construction of the stator component 30 which is provided with a means of closing the coil slot 36. In the example shown in Figure 9, the body portion 31, first boundary portion 32 and second boundary portion 33 define reception grooves 37 arranged to receive a closure member 38 which is inserted into a pair of opposing grooves 37 provided by adjacent stator components 30. Each groove extends into the body portion 31, first boundary portion 32 and second boundary portion 33 in a circumferential direction, and has a desired height in the radial direction. Each groove extends from a front face of the stator component 30 to a rear face of the stator component 30. That is, the groove extends axially through the stator component 30, and defines respective openings in the first and second faces of the stator component 30.

In another example, the groove 37 extends from one face of the component 30 in the direction of the other face of the component 30. In this second example, the groove 37 does defines an opening on only one of the faces of the stator 40.

The closure member is inserted into the grooves 37 following insertion of the stator coils into the coil slots 36. In such a manner, the closure member 38 closes the coil slot 36 defined between adjacent stator components. Closing the coil slot 36 following the insertion of the coils allows for more straightforward manufacturing of the stator.

Figure 10 illustrates a plan view of one example of the closure member 38 of Figure 9. The closure member is elongate and includes a series of perforations 39 therethrough. Such perforations can be arranged to provide the stator with the required electrical and magnetic properties. The closure member is preferably of a magnetic material, such as sheet steel, or of an SMC material. In another example, the slot closure member 38 may be provided by a plurality of individual components. Such an arrangement serves to reduce magnetic eddy currents in the closure member.

Use of the stator component 30 shown in Figure 4, together with a separate slot closure member or members, such as that shown in Figures 7 to 10, enables more straightforward manufacturing of a stator having desirable electrical and magnetic characteristics.

Claims

CLAIMS: 1. A stator assembly for an axial flux electrical machine, the stator assembly comprising: a. a stator body which defines a series of stator poles, arranged such that adjacent poles in the series are separated by a coil slot for receiving portions of a stator coil therein, and b. a plurality of stator coils located around respective stator poles, and partially located in respective coil slots of the stator body, wherein the stator body comprises a plurality of stator components, each of which provides a stator pole and which defines a portion of a coil slot to opposite sides of the stator pole.
2. An assembly as claimed in claim 1, wherein each stator component is of a soft magnetic composite material, and the stator body further comprises first and second reinforcing members to opposite planar faces of the stator assembly.
3. An assembly as claimed in claim 1 or 2, wherein stator component defines, in a rear face thereof, at least one channel to allow fluid circulation around the stator for cooling purposes.
4. A stator assembly for an axial flux electrical machine, the stator assembly comprising: a. a stator body which defines a series of stator poles, arranged such that adjacent poles in the series are separated by a coil slot for receiving portions of a stator coil therein, and b. a plurality of stator coils located around respective stator poles, and partially located in respective coil slots of the stator body, wherein the coil slots have an open region sized to receive a portion of such a coil, and wherein the stator body includes coil slot closure elements, located to close at least partially the open regions of the coil slots.
5. An assembly as claimed in claim 4, wherein each coil slot is closed, or partially closed, by a single closure element or by a pair of closure elements located at respective sides of the open region of the coil slot concerned.
6. A stator assembly for an axial flux electrical machine, the stator assembly comprising: a. a stator body which defines a series of stator poles, arranged such that adjacent poles in the series are separated by a coil slot for receiving portions of a stator coil therein, and b. a plurality of stator coils located around respective stator poles, and partially located in respective coil slots of the stator body, wherein the coil slots have an open region sized to receive a portion of such a coil, wherein the stator body defines pairs of locating grooves for receiving respective slot closure elements, and wherein the stator assembly includes a plurality of slot closure elements located in the locating grooves, each such closure element serving to close or partially close a coil slot.
7. An assembly as claimed in claim 6, wherein, the stator body is of a soft magnetic composite material.
8. An assembly as claimed in claim 6 or 7, wherein the closure element is provided by a elongate planar strip of material
9. An assembly as claimed in claim 6, 7 or 8, wherein the closure member is provided with slots, or other features of shape, so as to provide desired electrical, mechanical and magnetic characteristics.