Electric machine

Abstract

An electric machine 100 for use in an aircraft, the electric machine comprising: a stator assembly 105 and a rotor assembly. The rotor assembly comprises a rotor shaft 102 configured to rotate about a

Description

Technical Field The invention relates to an electric machine for use in an aircraft. In particular, the invention relates to a dual-rotor electric machine. Background of the Invention Electric machines generally include a stator and a rotor which rotates relative to the stator to induce electrical current in windings of the stator when acting as a generator, or to rotate in response to electrical current provided to the windings when acting as a motor. Some arrangements include dual rotors. It is advantageous for electric machines to be fault-tolerant especially in the field of aircraft propulsion. There exists a need for an improved dual-rotor electric machine. Summary of the Invention The invention provides an electric machine in which dual rotors are mounted to a single shaft and are selectively engageable with the shaft, such that either rotor may be disconnected from the shaft while the shaft continues to drive or be driven by the other rotor. According to a first aspect, there is provided an electric machine for use in an aircraft comprising one or more of the following features: a stator assembly; a rotor assembly comprising one or more of the following features: a rotor shaft configured to rotate about a rotational axis relative to the static assembly; a first rotor which may be selectively engageable with the rotor shaft; a second rotor which may be selectively engageable with the rotor shaft; a first rotor disconnect mechanism which may be arranged to selectively connect and disconnect the first rotor from the rotor shaft; a second rotor disconnect mechanism which may be arranged to selectively connect and disconnect the second rotor from the rotor shaft. The stator assembly may be disposed around the rotor shaft, preferably at a position along the rotational axis between that of the first rotor and the second rotor. The first disconnect mechanism may comprise a first moveable shaft member configured to transfer rotational drive from the rotor shaft to the first rotor. The first moveable shaft member may be configured to rotate with the rotor shaft. The first moveable shaft member may be slidable axially along the rotor shaft. The first disconnect mechanism may comprise a first actuation member configured to exert a force on the first movable shaft member to disengage the first rotor from the rotor shaft. The moveable shaft member may comprise a flange configured to interface with the first actuation member. The stator assembly may comprise a first stator configured to cooperate with the first rotor. The stator assembly may comprise a second stator configured to cooperate with the second rotor. The first stator and second stator may be separated by non-ferromagnetic material. The electric machine may comprise a housing. The housing may be configured to contain the stator assembly. The housing may be configured to support the first rotor. The housing may be configured to support the second rotor. The first stator and the second stator may be separated from one another by a frame component of the housing. The stator assembly may comprise a stator core. The stator core may be defined by or may comprise a set of stacked laminations. The first stator and the second stator may be configured to share, or both comprise at least a part of, the set of stacked laminations. The electric machine may further comprise a cooling system, which may comprise a first cooling system and optionally a second cooling system. The first cooling system may be configured to cool the first stator. The second cooling system may be configured to cool the second stator. The first cooling system may be independent from the second cooling system. The electric machine may be an axial flux electric machine. According to a second aspect, there is provided an aircraft comprising the electric machine of the first aspect. 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 sectional schematic diagram of an electric machine according to an embodiment; Figure 2 is a sectional schematic diagram of the electric machine according to figure 1 with both rotors in a disconnected configuration; Figure 3 is a sectional schematic diagram of the electric machine according to figure 1 with one rotor in a connected configuration; Figure 4 is a schematic diagram of part of an electric machine according to an embodiment; Figure 5 is a schematic diagram of an aircraft according to an embodiment. Detailed Description Figure 1 illustrates an electric machine 100. The electric machine 100 comprises a stator assembly 105 and a rotor assembly. The rotor assembly comprises a rotor shaft 102. The rotor shaft 102 is configured to rotate about a rotational axis 101 relative to the stator assembly 105. The rotor assembly further comprises a first rotor 110 which is selectively engageable with the rotor shaft 102 and may be journaled to rotate about the rotational axis 101. The rotor assembly also comprises a second rotor 120 which is selectively engageable with the rotor shaft 102 and may be journaled to rotate about the rotational axis 101. The electric machine 100 further comprises a first rotor disconnect mechanism 117 arranged to selectively connect and disconnect the first rotor 110 from the rotor shaft 102. The electric machine 100 also comprises a second rotor disconnect mechanism 127 arranged to selectively connect and disconnect the second rotor 120 from the rotor shaft 102. In the illustrated arrangement, the electric machine 100 comprises an axial flux electric machine. That is to say, the direction of magnetic flux between a rotor (for example, the first rotor 110) and the stator is parallel to the rotational axis 101. In some arrangements the first rotor 110 and the second rotor 120 can each comprise surface mounted permanent magnets. These could be neodymium magnets, SmCo magnets, or any other appropriate permanent magnet. The permanent magnets may be mounted or otherwise supported on a rotor core of each of the first rotor 110 and the second rotor 120, which could be a laminated iron core, for example comprising FeSi or FeCo. The rotor shaft 102 may be connected to a drive transfer shaft of a driving or driven element of an aircraft (see figure 5). As such, the rotor shaft 102 can be configured to transfer drive between the drive transfer shaft and the first and second rotors 110, 120. For example, when operating in generator mode, the rotor shaft 102 is configured to transfer rotational drive from a prime mover of an aircraft (e.g. an aircraft engine) via the drive transfer shaft to the rotors 110, 120. When operating in motor mode, the rotor shaft 102 is configured to transfer drive from the first rotor 110 and / or the second rotor 120 to a driven element (e.g. a propellor) via the drive transfer shaft. The electric machine 100 may comprise a housing 104. The housing 104 is configured to contain and / or support the stator assembly 105 and the rotor assembly. The first rotor 110 may be rotatably supported within the housing 104. In the arrangement shown in figure 1, the first rotor 110 is supported to rotate within the housing 104 by a first bearing 114. The first bearing 114 may be a roller bearing. In this arrangement, the outer race of the bearing 114 is fixedly connected to the housing 104 via the stator assembly 105. The inner race of the bearing 114 is fixedly connected to the first rotor 110. As such the first bearing 114 is configured to permit rotation between the first rotor 110 and the housing 104. It will be appreciated that the second rotor 120 can be supported to rotate within the housing 104 by a second bearing 124 in a similar manner, such that the second bearing 124 is configured to permit rotation between the second rotor 120 and the housing 104. The stator assembly 105 is configured to magnetically interact with the first rotor 110 and the second rotor 120. In this respect, the stator assembly 105 may be positioned along the rotational axis 101 between the first rotor 110 and second rotor 120, thereby providing a symmetrical arrangement. The stator assembly 105 can comprise a first stator 111 and a second stator 121. In the illustrated arrangement, the first stator 111 can be configured to magnetically interact with the first rotor 110 and the second rotor 120 can be configured to interact with the second stator 121. The first stator 111 may be separated from the second stator 121 by a non-ferromagnetic material. In alternative arrangements, the first stator 111 may be separated from the second stator 121 by an air gap. The nonferromagnetic material may be formed by a frame component 109, which may be fixedly connected to or an integral part of the housing 104. The frame component 109 may support the first stator 111 and the second stator 121 within the housing 104. The rotor shaft 102 may comprise a first coupling means 116 configured to transfer rotational drive between the rotor shaft 102 and the first rotor 110. Similarly, the rotor shaft 102 may comprise a second coupling means 126 configured to transfer rotational drive between the rotor shaft 102 and the second rotor 120. The first and / or second coupling means 116, 126 may be biased towards a connected configuration in which drive is transferred between the rotor shaft 102 and the respective rotor 110, 120. The first coupling means 116 and / or the second coupling means 126 may comprise any suitable means for transferring rotational drive such as a clutch. As mentioned above, the electric machine 100 comprises a first rotor disconnect mechanism 117 and a second rotor disconnect mechanism 127. The first rotor disconnect mechanism 117 may comprise a first actuation member 113 configured to actuate the first disconnect mechanism 117, and a first moveable shaft member 115 configured to be moveable with respect to the rotor shaft 102. The first moveable member 115 may be a first stub shaft. In the illustrated arrangement, the first moveable shaft member 115 comprises a main body 115a, an axially-movable torque transferring interface 115b and may include a flange 115c. The main body 115a is substantially annular and can comprise the torque transferring interface 115b, which may be a set of splines on an inner circumferential surface thereof. Alternative arrangements for securing the first moveable shaft member 115 rotationally, whilst allowing movement axially along the rotor shaft 102, can also be envisaged. The torque transferring interface 115b is configured to connect the first moveable shaft member 115 to the rotor shaft 102 in a slidable manner and to permit the transfer of torque therebetween. The flange 115c may extend radially outwardly from the main body 115a and may be arranged to interface with the first actuation member 113. In this manner, the first disconnect mechanism 117 can be configured such that actuation of the first actuation member 113 exerts a force on the first moveable shaft member 115 via the flange 115c so that the first moveable shaft member 115 slides along the rotor shaft 102 via the axially-movable torque transferring interface 115b so as to separate the first coupling means 116. The first actuation member 113 may employ a hydraulic mechanism to exert a force on the first moveable shaft member 115. In this way, the hydraulic mechanism may be configured to be actuated in the axial direction to interface with the first moveable shaft member 115, and in doing so displace the first movable shaft member 115 in the axial direction away from the first rotor 110. Alternatively the first actuation member 113 may employ a pneumatic mechanism or an electromagnetic mechanism. The housing 104 may have an opening on a first side at which the first rotor 110 is disposed. The first disconnect mechanism 117 may be disposed at the first side of the housing 104. The opening may be arranged to receive the first moveable shaft member 115. A first bearing 118, which may be a roller bearing, may be provided between the housing 104 and the first moveable shaft member 115 to permit the first moveable shaft member 115 to rotate with respect to the housing 104. It will be appreciated that the second disconnect mechanism 127 may be arranged in the same or similar manner to the first disconnect mechanism 117, including a second actuation member 123 and a second moveable shaft member 125 which may comprise a main body 125a, an axially-movable torque transferring interface 125b and a flange 125c. The second disconnect mechanism 127 may be disposed at the second side of the housing 104 at which the second rotor 120 is disposed, opposite to the first side. The housing 104 may have an opening at the second side of the housing to receive the second moveable shaft member 125 which may be permitted to rotate with respect to the housing 104 by a second bearing 128 provided therebetween, which may also be a roller bearing. Figure 2 shows the electric machine 100 in a disconnected configuration in which both the first rotor 110 and the second rotor 120 are disconnected from the rotor shaft 102, which may be by virtue of the first coupling means 116 and second coupling means 126 being in the disengaged state. The first coupling means 116 can comprise a first element 116a configured to interface with a second element 116b. The first element 116a may be provided in fixed relation to the rotor shaft 102 and the second element 116b may be provided in fixed relation to the first rotor 110. Similarly, the second coupling means 126 may also comprise a first element 126a and a second element 126b, wherein the first element 126a is provided in fixed relation to the rotor shaft 102 and the second element 126b is provided in fixed relation to the second rotor 120. The first coupling means 116 and / or second coupling means 126 may be curvic couplings. It will be understood that many other coupling arrangements may also be employed. Figure 3 shows the electric machine 100 in a configuration in which only one rotor is connected to the rotor shaft 102 and the other rotor is disconnected from the rotor shaft 102. In the arrangement shown, the second rotor 120 is connected to the rotor shaft 102 via the second coupling means 126 and the moveable shaft member 125, such that drive can be transferred therebetween, while the first rotor 110 is disconnected from the rotor shaft 102 by virtue of the first coupling means 116 being disengaged, so that drive is not transferred. It will be appreciated that the opposite configuration can be achieved whereby the second rotor 120 is disconnected from, and the first rotor 110 is connected to, the rotor shaft 102. The disconnect mechanisms 117, 127 can be operated independently to provide four configurations. In a first configuration, both the first rotor 110 and the second rotor 120 are engaged with the rotor shaft 102 as per figure 1. In a second configuration, both the first rotor 110 and second rotor 120 are disengaged from the rotor shaft 102 as per figure 2. In a third configuration, the first rotor 110 is disengaged from and the second rotor 120 is engaged with the rotor shaft 102, as per figure 3. In a fourth configuration, the first rotor 110 is engaged and the second rotor 120 is disengaged (not shown). Figure 4 shows a partial cross section of part of a rotor assembly and stator assembly 205 according to an alternative arrangement. The rotor assembly comprises a first rotor 210 and second rotor 220. The first rotor 210 comprises a first set of permanent magnets 219 and the second rotor 220 comprises a second set of permanent magnets 229. The stator assembly 205 comprises a first stator 211 and a second stator 222. The stator assembly 205 can comprise a first set of windings 212 and second set of windings 222. The first windings 212 may be configured to magnetically interact with the first set of permanent magnets 219 and the second windings 222 may be configured to magnetically interact with the second set of permanent magnets 229. The first windings 212 may be received in a first set of slots 206a defined in the first stator 211 and the second windings 222 may be received in a second set of slots 206b in the second stator 221. The foregoing arrangement of figure 4 is applicable to the stator assembly 105 described in relation to figures 1-3. The main difference in the arrangement of figure 4 is that the stator assembly 205 is formed without any separation between the first and second stators 211, 222. Rather, the stator assembly 205 comprises a single stator core 206. The stator core 206 may be arranged so that the first stator 211 and the second stator 221 are formed from shared or common laminations. The stator core 206 may be configured so that the mutual inductances between first and second windings 212, 222, will be cancelled out and that any magnetic field originated by a rotor (e.g. the first rotor 210) will not be embraced by windings (e.g. the second windings 222) facing the other rotor (e.g. the second rotor 220). In this arrangement, multi-turn concentrated edge windings are used for the first stator 111 and second stator 121, with two sets of independent coils sharing the same laminations. It will be understood that other winding designs could be utilised. The laminations of the stator core 206 may comprise FeSi, but it will be appreciated that FeCo or other appropriate materials could be used instead of or in combination with FeSi. The individual laminations may have a thickness of approximately 0.2 mm. In normal operation of the electric machine in generator mode, the electric machine would be in a connected configuration as described in figure 1. Should a fault exist or occur in the first rotor 110, it can be disconnected by actuation of the first disconnect mechanism 117, as described in relation to figure 3. The second rotor 210 will continue to be driven by the rotor shaft 102, without experiencing drag from the faulty first rotor 110. It will be appreciated that the same process can be applied to the second rotor 120, in the event it fails and the first rotor 110 is still functional, by actuating the second disconnect mechanism 127. In the event that both the first rotor 110 and the second rotor 120 malfunction, they can both be disconnected from the rotor shaft 102 as shown in figure 2, in order to avoid causing further damage to the rotors and to avoid drag on the rotor shaft 102. It will be appreciated that a similar manner of disconnecting the rotor shaft from a faulty rotor is applicable when the electric machine is operating in motor mode. While the electric machine has been described and illustrated with reference to an axial flux electric machine, it will be appreciated that the inventive concepts in the disclosure could be applied to a radial flux electric machine, in which one or more stators is / are arranged radially inside or outside one or more rotors. The rotors may be selectively engageable with a common shaft, in accordance with any of the features of the rotor assembly otherwise described herein. Figure 5 is a schematic illustration of an aircraft 1. The aircraft 1 comprises a driving or driven element 2 and a drive transfer shaft 3 configured to transfer rotational drive between the driving / driven element 2 and the electric machine 100. The electric machine 100 may comprise separate cooling systems 131, 132, which may be direct or indirect, for each of the first stator 111 and the second stator 121. The first cooling system 131 may be independent from the second cooling system 132. In this way, the first cooling system 131 may be arranged so as to be fluidically isolated from the second cooling system 132. This is advantageous compared to a common cooling system because if a common cooling system were to leak, or otherwise fail, then both the first stator and the second stator of the electric machine may overheat. Employing two cooling systems that are independent from one another improves redundancy. 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

1. An electric machine for use in an aircraft, the electric machine comprising:a stator assembly; anda rotor assembly comprising:a rotor shaft configured to rotate about a rotational axis relative to the stator assembly;a first rotor selectively engageable with the rotor shaft;a second rotor selectively engageable with the rotor shaft;a first rotor disconnect mechanism arranged to selectively connect and disconnect the first rotor from the rotor shaft; anda second rotor disconnect mechanism arranged to selectively connect and disconnect the second rotor from the rotor shaft.

2. The electric machine according to claim 1, wherein the stator assembly is disposed around the rotor shaft at a position along the rotational axis between that of the first rotor and the second rotor.

3. The electric machine according to claim 1 or clam 2, wherein the first disconnect mechanism comprises a first moveable shaft member configured to transfer rotational drive from the rotor shaft to the first rotor, the first moveable shaft member being configured to rotate with the rotor shaft and to be slidable axially along the rotor shaft.

4. The electric machine according to claim 3, wherein the first disconnect mechanism comprises a first actuation member configured to exert a force on the first movable shaft member to disengage the first rotor from the rotor shaft.

5. The electric machine according to claim 4, wherein the movable shaft member comprises a flange configured to interface with the first actuation member.

6. The electric machine according to any preceding claim, wherein the stator assembly comprises a first stator configured to cooperate with the first rotor, and a second stator configured to cooperate with the second rotor.

7. The electric machine according to claim 6, wherein the first stator and the second stator are separated by a non-ferromagnetic material.

8. The electric machine according to claim 6 or claim 7, further comprising a housing configured to contain the stator assembly and support the first rotor and the second rotor, wherein the first stator and the second stator are separated by a frame component of the housing.

59. The electric machine according to claim 6, wherein the stator assembly comprises a stator core defined by a set of stacked laminations, wherein the first stator and the second stator are configured to share the set of stacked laminations.10 10. The electric machine according to any of claims 6 to 9, further comprising a firstcooling system configured to cool the first stator and a second cooling system configured to cool the second stator, wherein the first cooling system is independent of the second cooling system.15 11. The electric machine according to any preceding claim, wherein the electricmachine is an axial flux electric machine.

12. An aircraft comprising the electric machine according to any preceding claim.11

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