GB2619931A

GB2619931A - Stator

Info

Publication number: GB2619931A
Application number: GB2209086.4A
Authority: GB
Inventors: David Flower Paul; Varsani Dipesh
Original assignee: Safran Electrical and Power SAS
Current assignee: Safran Electrical and Power SAS
Priority date: 2022-06-21
Filing date: 2022-06-21
Publication date: 2023-12-27
Also published as: GB202209086D0

Abstract

A stator for an electric machine comprising: a stator core 210 with back iron, teeth 211, 212 arranged in a circular array defining a plurality of stator slots 213; a winding 221 wound around a first stator tooth 211; and between the winding 221 and a second stator tooth 212, or perhaps a second winding 222 on the second tooth, a separator member 230 which may include a thermally-activated expandable material 232. When activated, the material expands and exerts a force on the winding towards the first stator tooth, and perhaps on the second winding towards the second tooth. The separator may have the expanding foam in a layer between a carrier or support 231, and an insulating layer 233, and may also include a slot closer or plug 234. The expandable material may be a foamable composite such as a syntactic foam. The coil may be wound in place before the separator is fitted and expanded. Potting compound may be applied. The pressure applied on the winding by the foam may help thermal conduction from the winding into the stator tooth.

Description

STATOR

Technical Field

The invention relates to a stator for an electric machine for use in aircraft.

Background of the Invention

Electric machines can be used for electrical power generation or to provide a drive source, for engine starting, or to provide motive power in aircraft. An electric machine is typically formed of an assembly of magnetic circuit components, comprising a rotor and a stator. Rotation of the rotor relative to the stator causes interaction of the magnetic field generated by the rotor with windings provided on the stator, generating an induced electromotive force (EMF) and/or electric current. In a generator, the rotor's magnetic field is produced by magnetic elements such as permanent magnets, or rotor windings and induces an AC voltage in stator windings as the stator windings pass through the moving magnetic field of the permanent magnet. In a motor, a rotating magnetic field generated in windings of a stator can induce torque in a rotor.

The performance of conventional stators can be hindered by the inefficiency with which heat generated in the windings is transferred away from the stator teeth.

There exists a need for an improved stator and related method of manufacture.

Summary of the Invention

According to an embodiment, there is provided a stator comprising any or all of the following features: a stator core comprising: a back iron; and a plurality of stator teeth arranged in a circular array about a longitudinal axis of the stator core and defining a plurality of stator slots therebetween; a first winding wound around a first stator tooth of the plurality of stator teeth; and a separator member arranged between the first winding and a second stator tooth of the plurality of stator teeth located adjacent the first stator tooth, the separator member comprising a thermally-activated expandable material.

The thermally-activated expandable material may be arranged to expand upon activation with sufficient thermal energy. The thermally-activated expandable material includes the material when in a pre-expanded state or when in an expanded state. In other words, the term is used to describe a class of materials capable of expansion upon activation with heat. Such 'activation' is not intended to encompass expansion due merely to a material's coefficient of thermal expansion.

By providing a separator member comprising an expandable material between the first winding and the second stator tooth, the first winding can be forced in the opposite direction to the second stator tooth upon activation of the expandable material, thereby forcing the first winding against the first winding. This improves the thermal contact between the first winding and the stator core, which increases performance of the stator. By adopting an expandable material that is thermally-activated, use of a mechanical wedge arrangement is avoided, thereby reducing the likelihood of damaging the windings or distorting the stator core.

The separator member may be arranged adjacent to the first winding. In this way, when activated, the thermally-activated expandable material may exert a force on the first winding towards the first stator tooth. The stator may further comprise a second winding wound around the second stator tooth. The separator member may be arranged between the first winding and the second winding. The separator member may be arranged such that, when activated, the thermally-activated expandable material exerts a force on the first winding towards the first stator tooth and exerts a force on the second winding towards the second stator tooth. This has the advantage of providing a double winding arrangement in which the first and second windings may be separated by the separator member and may be pressed against their respective stator teeth when the expandable material is activated, thereby improving the thermal contact of each winding with its respective stator tooth.

The separator member may comprise a carrier. This has the advantage of providing structural support to the expandable material. The separator member may comprise an insulating layer. The thermally-activated expandable material may be arranged between the carrier and the insulating layer. This has the advantage of providing a separator member wherein the expandable material can be sealed onto the carrier member by the insulation. In particular, this arrangement may allow the expansion of the expandable material to be controlled by the insulating layer. Furthermore, this allows the expandable material to push the winding, via the insulating later, against or towards the tooth around which it is wound, without expanding into the gaps between the wires of the winding.

The separator member may further comprise a slot closer. The slot closer may be configured to connect the separator member to the first stator tooth and/or the second stator tooth. By integrating a slot closer with the separator member, an arrangement which is easier to assemble is provided, because the separator member may be located by the connection of the slot closer to one or both stator teeth.

The thermally-activated expandable material may be a foamable composite. The thermally-activated expandable material may be in its pre-expanded state or its expanded state. The separator member may be configured such that the thermally-activated expandable material exerts a force on the first winding to retain the first winding against or towards the first stator tooth.

According to an embodiment, there is provided an electric machine comprising the stator as described hereinabove.

According to an embodiment, there is provided a method for assembling a stator comprising any or all of the following steps: providing a stator core comprising: a back iron; and a plurality of stator teeth arranged in a circular array about a longitudinal axis of the stator core and defining a plurality of stator slots therebetween; winding a first winding around a first stator tooth of the plurality of stator teeth; providing a separator member between the first winding and a second stator tooth of the plurality of stator teeth located adjacent the first stator tooth, the separator member comprising a thermally-activated expandable material.

The method may further comprise activating the thermally-activated expandable material by applying heat thereto. The method may further comprise providing a potting compound to the first winding.

Brief Description of the Drawings

Further features and advantages of the present invention will become apparent from the following description of embodiments thereof, presented by way of example only, and by reference to the drawings, in which: Figure 1 is a schematic view of a stator according to a prior art arrangement; Figure 2 is a schematic view of a stator slot according to an embodiment; Figure 3 is a flow diagram of a method according to an embodiment; Figure 4 is a schematic diagram of an aircraft incorporating an embodiment. Detailed Description A stator according to a prior art arrangement includes a back iron with a plurality of teeth extending therefrom, the teeth having windings disposed therearound. Particularly in the case of air-cooled machines with a concentrated winding arrangement, the primary cooling path for the heat generated in the winding coils is through the teeth on which the windings are wound, and radially outwards to the back iron. Therefore, thermal contact between the back iron and the windings is important. The peak performance of an electric machine can be limited by the hot-spot temperature of the winding wires, i.e. the temperature found at the highest-temperature point on the wires, although overall temperature of the wires can also have an effect. Given that one path away from the windings that is taken by the heat is through the back iron to the housing, there can be variability in cooling due to variations in the effectiveness of clamping of the windings against the stator core. The effect is most prominent in air-cooled machines where conduction is a primary means of cooling, but beneficial effects of the clamping may be found in liquid cooled (e.g. oil-cooled) machines as well.

It is therefore preferable to clamp the windings against the tooth around which they are wound in order to improve thermal contact between the windings and the teeth. Existing solutions include a wedge, or opposing wedges, being forced into the gap between adjacent windings to spread them apart and push them towards their respective teeth. However, this has the potential to damage the winding wires or distort the stator core if the forces are too high. The latter problem is especially pronounced with stator cores having back irons with a relatively thin wall, which may in some examples be of the order of only 2 to 3 mm. In order to prevent short circuits between phases, existing stators may employ a phase separator of S-shaped cross-section, which is typically made from a polyimide film (for example Kapton (RTM)), disposed between adjacent windings in order to electrically isolate the windings from one another.

A stator for an electric machine is disclosed herein in the context of an aircraft. An electric machine typically includes a stator having a plurality of magnetisable stator teeth extending from a back iron, and a rotor configured to rotate about a longitudinal axis of the stator. The stator teeth are configured to receive conductors, for example in the form of conductive (e.g. copper) windings. The rotor of such an electric machine includes a plurality of magnetisable elements such as windings or permanent magnets. As such, the electric machine formed from the stator and the rotor can act as a generator when the rotor rotates within the stator thereby inducing an electric current in the windings of the stator, and can also act as an electric motor when an electric current provided in the windings induces rotation of the rotor.

A separator member for the stator is provided. The separator member is arranged in the slot adjacent to a first winding disposed around a first stator tooth. The separator member comprises a thermally-activated expandable material. As such, when the expandable material is activated, the material expands such that the first winding is retained against the first stator tooth. The slot may have a second winding wound around a second stator tooth adjacent to the first stator tooth. In this arrangement, the separator member can be provided between the first and second windings, such that when the expandable material is activated, the expandable material expands between the first and second windings pushing the first winding against the first stator tooth and pushing the second winding against the second stator tooth such that the windings are retained against their respective stator teeth. As such, the separator member can provide a dual function of electrically isolating the windings from one another, while also retaining the windings against the stator teeth in order to improve thermal contact.

Figure 1 illustrates schematically a stator 100 according to a prior art arrangement. The stator 100 comprises a stator core having a back iron 110 and a plurality of stator teeth including a first stator tooth 111 and a second stator tooth 112. The plurality of stator teeth are arranged in a circular array about a longitudinal axis of the stator core and define a plurality of stator slots therebetween. The first and second stator teeth 111, 112 define a stator slot 113 therebetween. The stator core can comprise a plurality of laminations stacked in a direction of the longitudinal axis of the stator 100. In the arrangement shown, every tooth in the plurality of teeth has a winding disposed therearound. The first stator tooth 111 has a first winding 121 wound therearound, and the second stator tooth 112 has a second winding 122 wound therearound. This is known as a double winding arrangement. In other arrangements, only every other tooth in the plurality of teeth has a winding disposed around it. This is known as a single layer winding arrangement. Each slot in the stator 100 comprises a separator 130. The separator 130 may comprise a flexible insulating material, such as Kapton (RTM), arranged in an S-shape within a slot 113 between the first and second windings 121, 122.

With reference to Figures 2 and 3, a method 300 of assembling a stator 200 according to an embodiment will be described. The features relating to the back iron, the plurality of teeth defining the plurality of slots, and the windings wound around the teeth of the stator may be similar to the corresponding features described in relation to Figure 1; a corresponding reference numeral in the form of '2xx' has been used in place of the reference numerals in the form of '1.xx'. The main difference between the stator 200 and the stator 100 relates to the separator.

A first step 302 of the method 300 comprises providing a stator core. The stator core comprises a back iron 210 and a plurality of teeth 211, 212. In the arrangement shown, the first stator tooth 211 is adjacent to the second stator tooth 212. A slot 213 is defined between the first and second teeth 211, 212. The slot 213 extends longitudinally in a direction of the longitudinal axis of the stator core. At least one stator tooth 211 may extend from an annular portion of the back iron 210 in a radial direction towards the longitudinal axis. At an opposite end of the first stator tooth 211 to the back iron 210, the tooth 211 may comprise at least one tooth tip 214. The at least one tooth tip 214 may be in the form of a shelf or protrusion extending from an edge of the first stator tooth 211 in a circumferential direction. In the arrangement shown, the first and second stator teeth 211, 212 each comprise at least one tooth tip 214.

A second step 304 of the method 300 comprises winding a first winding 221 around the first stator tooth 211. The winding may be performed by an automated needle winding machine. The step may be repeated to provide at least one winding around some or all of the plurality of teeth in the stator 200. The step 304 may provide a stator 200 having windings around alternate teeth. In this type of arrangement, each slot defined between adjacent stator teeth 211, 212 comprises only a single winding. The step 304 may further comprise winding a second winding 222 around the second stator tooth 212. Therefore, the method 300 may provide the arrangement of Figure 2, wherein the slot 213 comprises a first winding 221 and a second winding 222.

A third step 306 of the method 300 comprises inserting a separator member 230 into the slot 213. In arrangements having only a first winding 221 in a slot 213, the separator member 230 is provided between the first winding 221 and the second stator tooth 212, or another structural member of the stator. The separator member 230 may be provided at a circumferential position (with respect to the longitudinal axis of the stator 200) or a lateral position that is between that of the first winding 221 and the second stator tooth 212. The separator member 230 may at least partially overlap the first winding 221 in the lateral or radial direction with respect to the longitudinal axis. In the illustrated arrangement, the separator member 230 is provided at the same radial position as the first winding 221 with respect to the longitudinal axis. Therefore, the step 306 can provide a separator member 230 arranged between the first winding 221 and the second stator tooth 212, or another structural member of the stator.

In arrangements having first and second windings 221, 222 in a slot 213, the separator member 230 is provided between the first winding 221 and the second winding 222, as shown in Figure 2. In this type of arrangement, the separator member 230 is provided at a circumferential position (with respect to the longitudinal axis of the stator 200) or a lateral position that is between that of the first winding 221 and the second winding 222. The separator member 230 may at least partially overlap the second winding 222 in the lateral or radial direction with respect to the longitudinal axis. In the illustrated arrangement, the separator member 230 is provided at the same radial position as the second winding 222 with respect to the longitudinal axis. Therefore, the step 306 can provide a separator member 230 arranged between the first winding 221 and the second winding 222.

The separator member 230 comprises a thermally activated expandable material 232. The expandable material 232 is arranged to expand upon activation with sufficient thermal energy. It will be understood that a thermally activated expandable material includes the material when in a pre-expanded state or when in a post-expanded state. In other words, the term "thermally activated expandable material" is used herein to describe a class of materials which can be expanded upon activation with heat. This term is not intended to cover any material which undergoes general thermal expansion as governed by its coefficient of thermal expansion, but rather materials which can be expanded upon reaching an activation temperature or temperature range. In contrast to the expansion governed by a material's coefficient of thermal expansion, the thermally activated expandable material may be configured to expand irreversibly. Furthermore, the expandable material may be configured such that upon the application of heat, the expandable material expands beyond the extent provided by its coefficient of thermal expansion, or that of one of its components. The thermally activated expandable material 232 may be a composite material such as Toray MicroPly ES72A or Hexcel Redux 212. In general, the expandable material 232 may comprise a foaming material which can be caused to expand by foaming upon thermal activation thereof. The expandable material 232 may be provided with distributed expandable or expanded portions, configured to provide even expansion of the material upon thermal activation.

The separator member 230 may comprise a carrier 231. The carrier 231 may be configured to carry the expandable material 232. In the arrangement shown, the carrier 231 comprises a strip of material configured to extend along the longitudinal extent of the slot 213. At least one surface of the carrier member 231 may be provided with the expandable material 232. The expandable material may be provided on a surface which faces windings in the slot. In the arrangement shown, the expandable material 232 is provided on both sides of the carrier 231. The expandable material may be provided on surfaces of the carrier which face opposing windings in the slot. As such, the expandable material 232 can be provided between the first winding 221 and the carrier 231, and between the second winding 222 and the carrier 231. It will be understood that in arrangements having only a single winding 221 in the slot 213, the expandable material 232 may be provided only on one side of the carrier 231, between the first winding 221 and the carrier 231. The carrier 231 may comprise any material capable of supporting the expandable material, for example a material which is stiffer than the expandable material and electrically insulating. The carrier may comprise a thermoplastic, such as polyether ether ketone (PEEK), but any non-conductive material could be used.

The separator member 230 may comprise insulation 233. The insulation may be provided over the expandable material 232 and can be configured to secure the expandable material 232. The insulation 233 may be configured to secure the expandable material 232 against the carrier 231. The insulation 233 may be a flexible insulating material such as Kapton (RTM). As shown in Figure 2, the expandable material 232 may be sandwiched between the carrier 231 and the insulation 233. It will be understood that the sectional schematic view of Figure 2 showing the separator 230 may represent the separator 230 along the entire longitudinal extent of the slot 213. In other words, the expandable material 232 and the insulation 233, where present, may be provided along the entire longitudinal extent of the slot 213, or along a portion of it where clamping against the stator tooth is most beneficial, which may be along a mid-length of the slot or stator.

The stator 200 may comprise a slot closer 234. The slot closer 234 may be configured to close or seal the slot 234 in a radial direction. In the arrangement shown, the slot closer 234 may be slidably received at an inner radial end of the first and second stator teeth 211, 212 in a direction parallel to the longitudinal axis. The slot closer 234 may be configured to rest against an inner surface of the tooth tips 214. The slot closer 234 may be positioned inside the slot 213 so as to abut a radially facing surface of the tooth tips 214. In this way, the slot closer may be positioned radially outwards from the tooth tips 214 with respect to the longitudinal axis of the stator 200. In an alternative arrangement, the tooth tips 214 of the first and/or second stator teeth 211, 212 may comprise a recess configured to receive the slot closer 234 in a slidable manner. In another arrangement, which may be employed in a stator without tooth tips 214, the recess may be provided in the first and/or second stator teeth 211, 212.

In the illustrated arrangement, the slot closer 234 is integral with the separator member 230. The slot closer 234 is configured to extend across the circumferential extent of the opening of the slot 213 while the separator member 230 may be configured to extend across the radial extent of the slot 213. Therefore, the separator member 230 and the slot closer 234 may be provided by a component having a substantially T-shaped cross-section, with the expandable material 232 and optionally the flexible insulation 233 provided on the stem of the T-shaped cross section. In this arrangement, the separator member 230 comprises: the carrier 231, the expandable material 232, the insulation 233 and the slot closer 234. While the slot closer 234 helps to locate the separator member 230 within the slot 213, it will be understood that the slot closer 234 is not essential, because the separator member 230 can be held within the slot 213 using other methods.

A fourth step 308 of the method 300 comprises activating the separator member 230.

Activating the separator member 230 may comprise heating the expandable material 232.

The expandable material 232 may be activated by curing, such as thermal curing. The step 308 may be performed by heating the stator 200 in an oven. Heating the expandable material 232 causes it to expand such that the expandable material 232 exerts a force on the first winding 221 such that the first winding 221 is pressed against the first stator tooth 211. This improves the thermal contact between the winding and the stator tooth.

This effect is particularly pronounced halfway along the longitudinal extent of the stator tooth 211, because after winding the first winding 221 around the first stator tooth 211, the gap between the first stator tooth 211 and the first winding 221 is generally the greatest at this position. Therefore, the expansion of the expandable material 232 can cause the gap to reduce significantly to thereby improve the thermal contact.

The step 308 may therefore provide a separator member 230, wherein the thermally activated expandable material 232 is in its expanded state. In this state, the expandable material 232 may comprise a foamed composite structure, which may be characterised by a gaseous phase, having an open or closed cell structure, formed in a continuous solid phase.

In the arrangement shown in Figure 2, the expandable material 232 facing the second stator tooth 212 likewise expands when heated such that the expandable material 232 exerts a force on the second winding 222 such that the second winding 222 is pressed against the second stator tooth 212. In this way, the gap between the separator member 230 and the windings on either side of the separator member 230 is at least partially filled by the expanding material 232. The step 308 may therefore provide a separator member 230 arranged between the first winding 221 and the second stator tooth 212, wherein the separator member 230 comprises a thermally activated expandable material 232 in its expanded state. Furthermore, the step 308 may provide a separator member 230 arranged between the first winding 221 and the second winding 222, wherein the separator member 230 comprises a thermally activated expandable material 232 in its expanded state.

A fifth step 310 of the method 300 may comprise encapsulation by providing a potting compound such as a resin into the slot 213. The potting compound may be provided in the slot 213 by dipping at least one end of the stator 200 into the potting compound. The potting compound is configured to fill remaining air gaps in the slot 113. The resin may be a thermal resin, for example epoxy resin including aluminium oxide. The fifth step 310 may comprise impregnation, typically performed before encapsulation. Impregnation can involve the addition of a low-viscosity fluid into the slot 213 which may be subsequently cured. Therefore, after step 310, the method 300 provides a slot 213 wherein a separator member 230 is arranged between a first winding 221 and the second stator tooth 212, wherein the separator member 230 comprises a thermally activated expandable material 232 in its expanded state, wherein the arrangement of the separator member 230 with at least the first winding 221 is set by impregnation and/or the encapsulation by the potting compound. Furthermore, the step 306 may provide a slot 213 wherein a separator member 230 is arranged between the first winding 221 and the second winding 222, wherein the separator member comprises a thermally activated expandable material in its expanded state, and wherein the arrangement of the separator member 230 with the first winding 221 and the second winding 222 is further fixed by the impregnation of the potting compound.

Figure 4 is a schematic diagram illustrating an aircraft 1. The aircraft 1 comprises a driving or driven element 2 and an electric machine 10 connected to the driving or driven element 2 by a drive shaft 3. The electric machine 10 comprises the stator 200. A rotor 11 is connected to the drive shaft 3 and configured to rotate within the stator 200.

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

Claims

Claims 1. A stator for an electric machine comprising: a stator core comprising: a back iron; and a plurality of stator teeth arranged in a circular array about a longitudinal axis of the stator core and defining a plurality of stator slots therebetween; a first winding wound around a first stator tooth of the plurality of stator teeth; and a separator member arranged between the first winding and a second stator tooth of the plurality of stator teeth located adjacent the first stator tooth, the separator member comprising a thermally-activated expandable material.The stator according to claim 1, wherein the separator member is arranged adjacent to the first winding such that, when activated, the thermally-activated expandable material exerts a force on the first winding towards the first stator tooth.The stator according to claim 1 or claim 2, further comprising a second winding wound around the second stator tooth, the separator member being arranged between the first winding and the second winding.The stator according to claim 3, wherein the separator member is arranged such that, when activated, the thermally-activated expandable material exerts a force on the first winding towards the first stator tooth and exerts a force on the second winding towards the second stator tooth.The stator according to any preceding claim, the separator member further comprising a carrier and an insulating layer, wherein the thermally-activated expandable material is arranged between the carrier and the insulating layer.The stator according to any preceding claim, the separator member further comprising a slot closer, the slot closer configured to connect the separator member to the first stator tooth and/or the second stator tooth.The stator according to any preceding claim, wherein the thermally-activated expandable material is a foamable composite. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.The stator according to any preceding claim, wherein the thermally-activated expandable material is in its pre-expanded state.The stator according to any of claims 1 to 7, wherein the thermally-activated expandable material is in its expanded state.The stator according to claim 9, wherein the separator member is configured such that the thermally-activated expandable material exerts a force on the first winding to retain the first winding against or towards the first stator tooth.An electric machine comprising the stator according to any of claims 1 to 10.A method for assembling the stator comprising: providing a stator core comprising: a back iron; and a plurality of stator teeth arranged in a circular array about a longitudinal axis of the stator core and defining a plurality of stator slots therebetween; winding a first winding around a first stator tooth of the plurality of stator teeth; providing a separator member between the first winding and a second stator tooth of the plurality of stator teeth located adjacent the first stator tooth, the separator member comprising a thermally-activated expandable material.A method according to claim 12, further comprising activating the thermally-activated expandable material by applying heat thereto.A method according to claim 12 or claim 13, further comprising providing a potting compound to the first winding.