Multi-stage generator

Abstract

A multi-stage generator 100 for an aircraft comprising: a generator shaft 101 which rotates about an axis 103; a main stage 150 which generates electrical power; a main exciter 130 providing excitatio

Description

Technical Field The invention relates to electrical power generation for aircraft. In particular, the invention relates to a multi-stage generator suitable for use in an aircraft. Background of the Invention Electric machines can be used for electrical power generation or to provide motive power in aircraft. An electric machine is typically formed of an assembly of magnetic circuit components comprising a stator, and a rotor configured to rotate within a bore of, or around the outer diameter of, the stator. As is well known, 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 electrical current. In a permanent magnet generator, the rotor's magnetic field is produced by permanent magnets, which induces an AC voltage in the stator windings as the stator windings pass through the moving magnetic field of the permanent magnet. In a motor, an alternating AC current supplied to the windings of a stator can induce torque in a rotor. Conventional three-stage generators used for electrical power generation in aircraft are made up of three different electric machines: a permanent magnet generator pilot exciter, a main exciter and a main stage. These electric machines are typically stacked on a common shaft. Such electric machines can rotate at high speeds up to 24000 revolutions per minute (rpm) which leads to some design and manufacturing constraints. There exists a need for an improved three-stage generator assembly. Summary of the Invention According to a first aspect of the invention, there is provided a multi-stage generator for an aircraft comprising any or all of the following features: a generator shaft configured to rotate about a rotational axis of the generator; a main stage configured to generate an electrical power output of the generator from rotation of the generator shaft; a main exciter configured to provide an excitation current to windings of the main stage; and a pilot exciter configured to provide an excitation current to windings of the main exciter; wherein the pilot exciter comprises an axial flux permanent magnet generator (PMG). The invention provides the use an axial flux permanent magnet generator in a pilot exciter of a three-stage generator suitable for use in an aircraft. The generator shaft may be configured to rotate at speeds of at least 6000 revolutions per minute (rpm), preferably at least 12000 rpm, preferably at least 18000 rpm, preferably at least 24000 rpm. The inventor has recognised constraints in the design and manufacture of three-stage generators, particularly for high-speed applications. For example, the permanent magnets in a radial flux PMG typically require sleeving around the circumference to counteract the centrifugal forces experienced during high-speed rotation, while attempting to minimise how much of the radial air gap is sacrificed by the sleeving. However, this can require tight tolerances and / or grinding processes prior to installing a metallic sleeve. By employing an axial flux PMG as the pilot exciter, there is less need to minimise the thickness of the support around the circumference because the air gap is arranged in the axial direction instead. Furthermore, this arrangement can simplify mounting of the stator of the pilot exciter because it can be part of the non-drive end housing or bolted thereto. Also, such an arrangement can eliminate the need for the pilot exciter stator to be secured with antirotation and axial lock components. Additionally, an axial flux PMG can simplify the arrangement of electrical connections from the pilot exciter stator. Therefore, the invention provides an improved multi-stage generator for an aircraft that is lighter, simpler to manufacture, and has a reduced footprint, especially in the axial direction. The axial flux PMG may comprise a pilot exciter rotor and a pilot exciter stator. The pilot exciter rotor may be separated from the pilot exciter stator by an air gap defined in a direction parallel to the rotational axis. The pilot exciter rotor may be disposed at a position along the rotational axis between the pilot exciter stator and the main stage. The pilot exciter rotor may be disposed at a position along the rotational axis between the pilot exciter stator and the main exciter. The pilot exciter rotor may comprise a support configured to restrict movement of the plurality of permanent magnets in a radially outward direction. The support may extend at least partially around a radially outwardly facing surface of the plurality of permanent magnets. This has the advantage of providing a more robust pilot exciter rotor that has greater resistance against centrifugal forces. The main exciter may comprise a main exciter rotor and a main exciter stator. The main exciter stator may comprise a plurality of main exciter stator windings. The main exciter stator windings may be arranged in a concentrated winding arrangement. The main exciter stator windings may be configured to receive power from the pilot exciter. The main exciter rotor may comprise a plurality of main exciter rotor windings. The main exciter rotor windings may be arranged in a distributed winding arrangement. The main exciter rotor windings may be configured to rotate with the generator shaft. The main exciter may be configured such that rotation of the generator shaft induces current in the plurality of main exciter rotor windings. A plurality of permanent magnets of the axial flux PMG may be directly connected to the main exciter rotor, such that the pilot exciter rotor and the main exciter rotor share a common yoke. The plurality of permanent magnets may be received by the main exciter rotor radially inwards of the main exciter rotor windings with respect to the rotational axis. This has the advantage of shortening the multi-stage generator in the rotational axis by positioning the pilot exciter and the main exciter at the same axial position. A plurality of permanent magnets of the axial flux PMG may be supported by a magnet carrier. The magnet carrier may be disposed around the generator shaft and configured to rotate therewith. This has the advantage of simplifying manufacture. A plurality of permanent magnets of the axial flux PMG may be directly connected to the generator shaft. The plurality of permanent magnets may be received in a recess on an axial end face of the generator shaft. This has the advantage of reducing the number of components required to mount the plurality of permanent magnets on the shaft. The main stage may comprise a main stage rotor and a main stage stator. The main stage rotor may comprise a plurality of main stage rotor windings. The main stage rotor windings may be arranged in a concentrated winding arrangement. The main stage rotor windings may be configured to receive power from the main exciter. The main stage stator may comprise a plurality of main stage stator windings. The main stage stator windings may be arranged in a distributed winding arrangement. The main stage may be configured such that rotation of the generator shaft induces current in the plurality of main stage stator windings. A plurality of permanent magnets of the axial flux PMG may be directly connected to the main stage rotor, such that the pilot exciter rotor and the main stage rotor share a common yoke. The plurality of permanent magnets may be received by the main stage rotor radially inwards of the main stage rotor windings with respect to the rotational axis. This has the advantage of shortening the multi-stage generator in the rotational axis by positioning the pilot exciter and the main stage at the same axial position. The main exciter may be disposed at a position along the rotational axis between the pilot exciter and the main stage. The main stage may be disposed at a position along the rotational axis between the pilot exciter and the main exciter. According to another aspect of the invention, there is provided an aircraft comprising a multi-stage generator as described hereinabove. 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 diagram of a multi-stage generator according to an embodiment; Figure 2A is a schematic sectional diagram of a multi-stage generator according to an embodiment; Figure 2B is a perspective view of the multi-stage generator of Figure 2A; Figure 3A is a schematic sectional diagram of a multi-stage generator according to another embodiment; Figure 3B is a perspective view of the multi-stage generator of Figure 3A; Figure 4A is a schematic sectional diagram of a multi-stage generator according to another embodiment; Figure 4B is a perspective view of the multi-stage generator of Figure 4A; Figure 5A is a schematic sectional diagram of a multi-stage generator according to another embodiment; Figure 5B is a perspective view of the multi-stage generator of Figure 5A; Figure 6 is a schematic diagram of an aircraft according to an embodiment. Detailed Description In examples described herein, there is disclosed a multi-stage generator for an aircraft. A multi-stage generator may have a generator shaft which rotates about a rotational axis of the generator. The generator shaft may be driven by an output shaft from a prime mover, such as an aircraft engine, or may be driven by air flow, for example from a ram air turbine. The multi-stage generator may have three stages: a main stage, a main exciter and a pilot exciter. The main stage is configured to generate an electrical power output of the generator from rotation of the generator shaft. In other words, the main stage is an electric machine which converts mechanical energy from rotation of the generator shaft into electrical energy, where the electrical energy can be used to supply electrical systems aboard the aircraft. The main exciter is configured to provide an excitation current to rotor windings of the main stage. The main exciter is also an electric machine which converts mechanical energy from rotation of the generator shaft into electrical energy, where the electrical energy can be used to supply an excitation current to field windings of the main stage. In this way, the main exciter can provide the electrical current required to produce a magnetic field in the main stage. The pilot exciter is configured to provide an excitation current to field windings of the main exciter. The pilot exciter is also an electric machine which converts mechanical energy from rotation of the generator shaft into electrical energy, where the electrical energy can be used to supply an excitation current to windings of the main exciter. In this way, the pilot exciter can provide the electrical current required to produce a magnetic field in the main exciter. The pilot exciter comprises an axial flux permanent magnet generator (PMG). That is to say, the pilot exciter is an electric machine of the axial flux type. As such, stator windings and permanent magnets of the pilot exciter are separated by an air gap along the rotational axis of the generator. This is in contrast to a radial flux electric machine, where the stator windings and permanent magnets would be separated by an air gap in a radial direction. Figure 1 shows a schematic diagram of a multi-stage generator 100. The multi-stage generator 100 comprises a generator shaft 101 configured to rotate about a rotational axis of the generator 100. The multi-stage generator 100 further comprises a main stage 150. The main stage 150 is configured to generate an electrical power output of the generator 100 from rotation of the generator shaft 101. The multi-stage generator 100 further comprises a main exciter 130. The main exciter 130 is configured to provide an excitation current to the main stage 150. The multi-stage generator 100 further comprises a pilot exciter 110. The pilot exciter 110 is configured to provide an excitation current to the main exciter 130. The pilot exciter 110 comprises an axial flux PMG. The generator shaft 101 can be configured to rotate at speeds of at least 6000 rpm, at least 12000 rpm, at least 18000 rpm, or at least 24000 rpm. The multi-stage generator 100 may comprise a housing 102. The housing 102 may contain the generator shaft 101, the pilot exciter 110, the main exciter 130 and / or the main stage 150. In the arrangement shown, the housing 102 houses the pilot exciter 110, the main exciter 130 and the main stage 150. The housing 102 at least partly contains the generator shaft 101, which extends out of the housing 102 along the rotational axis thereof. The multi-stage generator 100 may define a drive end 100a and a non-drive end 100b on an opposite side of the generator shaft 101 to the drive end 100a along the rotational axis. The drive end 100a is the end of the multi-stage generator 100 at which the generator shaft 101 receives an external mechanical drive, for example from an aircraft engine. In the arrangement shown, the main stage 150 is closest to the drive end 100a and the pilot exciter 110 is furthest from the drive end 100a. As such, the pilot exciter 110 is closest to the non-drive end 100b and the main stage 150 is furthest from the non-drive end 100b. In the arrangement shown, the multi-stage generator 100 is arranged such that the main exciter 130 is disposed at a position along the rotational axis between the main stage 150 and the pilot exciter 110. In other arrangements, the multi-stage generator 100 can be arranged such that the pilot exciter 110 is disposed at a position along the rotational axis between the main stage 150 and the main exciter 130. Figures 2A and 2B show the multi-stage generator 100 in more detail. The main stage 150 may comprise a stator, a rotor, and two sets of windings configured such that rotation of a first set of windings relative to the second set of windings induces current in one of the sets of windings. In the illustrated arrangement, the main stage 150 comprises a main stage rotor 151 and a main stage stator 161. The main stage rotor 151 is configured to rotate with the generator shaft 101. In this respect, the main stage rotor 151 is fixed to the generator shaft 101. Throughout this disclosure, the generator shaft may be a unitary, cylindrical shaft, but is not limited thereto. Rather, the generator shaft may broadly describe a component that transfers rotational drive to the rotors (i.e. the main stage rotor 151, main exciter rotor 131, and pilot exciter rotor 111). The generator shaft may comprise a plurality of shaft elements configured to rotate together. In some arrangements, the generator shaft comprises one or more sleeve components provided around one or more of the rotors. The main stage rotor 151 may be configured to receive a plurality of rotor windings 152. The main stage rotor 151 may comprise a substantially cylindrical shaft portion having a plurality of teeth extending radially outward to define a plurality of longitudinal slots. In some arrangements, the main stage rotor 151 comprises a plurality of main stage rotor windings 152 configured to receive power from the main exciter 130. The main stage rotor windings 152 may be received on the plurality of teeth. The main stage rotor windings 152 may be configured to provide a plurality of electromagnets. In this respect, the main stage rotor windings 152 may be arranged in a concentrated winding arrangement. That is to say, the main stage rotor windings 152 may be provided on the plurality of teeth such that an electrically conductive element, such as a wire, is wound around a single tooth of the plurality of teeth, such that a given wire occupies a first slot and a second, adjacent slot. As such, each one of the plurality of rotor windings 152 may be wound around a different rotor tooth. In this manner, a winding of the plurality of rotor windings 152 may be coiled around a coil axis that is perpendicular to the rotational axis 103. The main stage stator 161 may be received in the housing 102 and may be fixedly connected thereto. The main stage stator 161 may comprise a main stage stator core 162. The main stage stator core 162 may be configured to concentrically surround the main stage rotor 151 and may be substantially cylindrical. The main stage stator core 162 may be configured to receive a plurality of stator windings 163. The main stage stator core 162 may comprise a plurality of teeth extending in a radially inward direction to define a plurality of longitudinal slots configured to receive a plurality of stator windings 163. In some arrangements, the main stage stator core 162 comprises a plurality of main stage stator windings 163 configured to be received by the slots. The main stage stator windings 163 may be arranged in a distributed winding arrangement. That is to say, the main stage stator windings 163 may be provided on the plurality of teeth such that an electrically conductive element is received by a first tooth and a second tooth, where the second tooth may be remote from the first tooth. In such an arrangement, the main stage stator windings 163 may comprise end windings 163a which extend beyond the axial end faces of the main stage stator core 162. The main stage stator windings 163 may be arranged as a double winding arrangement such that each of the plurality of slots comprises two windings provided at different radial positions with respect to the rotational axis. In other arrangements, there may be more than two conductors per slot. The main stage 150 may be configured such that the main stage stator 161 is radially separated from the main stage rotor 151 by an air gap 159. The main stage 150 may be configured such that providing an excitation current to the plurality of main stage rotor windings 152 produces a magnetic field which, due to the rotation of the main stage rotor 151 with respect to the main stage stator 160, induces current in the main stage stator windings 163. The main exciter 130 may comprise a stator, a rotor, and two sets of windings configured such that rotation of a first set of windings relative to the second set of windings induces current in one of the sets of windings. In the illustrated arrangement, the main exciter 130 comprises a main exciter rotor 131 and a main exciter stator 141. The main exciter rotor 131 is configured to rotate with the generator shaft 101. In this respect, the main exciter rotor 131 is fixed to the generator shaft 101. The main exciter 130 may have a similar arrangement to the main stage 150, for example in that the main exciter rotor 131 may comprise a plurality of teeth extending radially outward from the main exciter rotor 131 and configured to receive a plurality of main exciter rotor windings 132. Also, the main exciter stator 141 may comprise a main exciter stator core 142 comprising a plurality of teeth extending radially inward from the main exciter stator core 142 and configured to receive a plurality of main exciter stator windings 143. The main exciter 130 may be configured such that the main exciter stator 141 is radially separated from the main exciter rotor 131 by an air gap 139. Differently to the arrangement of the main stage 150, the main exciter 130 may be arranged such that the main exciter stator windings 143 are arranged in a concentrated winding arrangement and the main exciter rotor windings 132 are arranged in a distributed winding arrangement. The main exciter 130 may be configured such that providing an excitation current to the plurality of stator windings 143 produces a magnetic field which, due to the rotation of the main exciter rotor 131 with respect to the main exciter stator 141, induces current in the main exciter rotor windings 132. The pilot exciter 110 comprises an axial flux PMG. The axial flux PMG of the pilot exciter 110 may comprise a stator, a rotor, a plurality of permanent magnets 112 and a plurality of windings 123. The pilot exciter 110 is configured such that rotation of the plurality of permanent magnets 112 relative to the windings 123 induces an electrical current in the windings 123. The pilot exciter 110 comprises a pilot exciter rotor 111 and a pilot exciter stator 121. The pilot exciter rotor 111 is configured to rotate with the generator shaft 101. In this respect, the pilot exciter rotor 111 may be fixed to the generator shaft 101, or may be an integral part of the generator shaft 101, or may be fixed to or integral to a second shaft configured to rotate with generator shaft 101, for example by a geared connection. The pilot exciter rotor 111 comprises a plurality of permanent magnets 112. The plurality of permanent magnets 112 are distributed around the rotational axis 103. The plurality of permanent magnets 112 may have an aspect ratio such that the radial dimension is greater than an axial dimension. In other words, the plurality of permanent magnets 112 may be flat in a radial plane. The plurality of permanent magnets 112 can be fixed to an axially-facing mounting surface of the pilot exciter rotor 111, which may be by mating, mounting, bonding, or any other suitable fixing means. Such fixing can restrict the plurality of permanent magnets 112 from radial outward movement. Furthermore, the pilot exciter rotor 111 may comprise a support that extends at least partially around the radially-outward-facing surfaces of the plurality of permanent magnets 112, which may be configured to restrict, or further restrict, the plurality of permanent magnets 112 from radial outward movement with respect to the rotational axis. The support may extend at least partially around the plurality of permanent magnets 112 in a circumferential direction. The support may extend over all of the radially-outward facing surface of the plurality of permanent magnets 112 and may extend partially over the axial face of the plurality of permanent magnets 112 between the pilot exciter rotor 111 and the pilot exciter stator 121. The pilot exciter stator 121 may comprise a pilot exciter stator core 122. The pilot exciter stator core 122 may be axially offset from the pilot exciter rotor 111 along the rotational axis. The pilot exciter stator core 122 may be configured to receive a plurality of pilot exciter stator windings 123. The pilot exciter stator core 122 may comprise a substantially annular body with a plurality of teeth extending axially toward the pilot exciter rotor 111 to define a plurality of slots configured to receive the plurality of stator windings 123. In this respect, the plurality of teeth may be distributed around the rotational axis 103 of the generator 100 to define a plurality of radial slots, wherein each one of the plurality of windings 123 is wound around a stator tooth. In this manner, a winding of the plurality of windings 123 may be coiled around a tooth around a coil axis that is parallel to the rotational axis 103. In other arrangements, a distributed or overlapping winding arrangement may be employed. The pilot exciter 110 can be configured such that the pilot exciter stator 121 is axially separated from the pilot exciter rotor 111 by an air gap 119. The pilot exciter 110 may be configured such that rotation of the plurality of permanent magnets 112 relative to the plurality of windings 123 induces current in the pilot exciter stator windings 123. The foregoing description of the pilot exciter 110 of Figures 2A and 2B may apply to the arrangements of alternative multi-stage generators 200, 300, 400 as described with reference to Figures 3A to 5B. The main difference between the disclosed arrangements lies in the configuration of the pilot exciter rotor. With continued reference to Figures 2A and 2B, the plurality of permanent magnets 112 are comprised in a magnet carrier 113. The magnet carrier 113 may be configured to rotate with the generator shaft 101 and to support the plurality of permanent magnets 112, such that the magnet carrier 113 may facilitate rotation of the plurality of permanent magnets 112 around the rotational axis 103. The magnet carrier 113 may comprise an annular body disposed around the rotational axis 103 and may comprise one or more recesses configured to receive the plurality of permanent magnets 112. In some arrangements, the permanent magnets 112 may be fixed to the magnet carrier 113 by a suitable fixing means, such as an adhesive. The magnet carrier 113 may be configured to support the plurality of permanent magnets 112 such that the plurality of permanent magnets 112 extends axially toward the pilot exciter stator 121. The magnet carrier 113 may provide the above-mentioned fixing means including a support extending at least partially over the radially-outwardly-facing surface of the plurality of permanent magnets 112. The generator shaft 101 may be rotatably mounted in the housing by at least one bearing. In the arrangement shown, the generator shaft 101 is support by a first bearing 104a and a second bearing 104b. The first bearing 104a may be positioned along the rotational axis 103 adjacent the main exciter 130, and the second bearing 104b may be positioned along the rotational axis adjacent the main stage 150. In the illustrated arrangement, the first bearing 104a is disposed along the rotational axis between the main exciter 130 and the pilot exciter 110, and the second bearing 104b is disposed along the rotational axis such that the main stage 150 is between the second bearing 104b and the main exciter 130. As such, the second bearing 104b may be closer to the drive end 100a (see Figure 1) of the generator 100 than the main stage 150. As described above, rotation of the plurality of permanent magnets 112 induces current in the plurality of windings 123 on the pilot exciter stator 121. The generator 100 is configured to use this electrical output to supply current to the main exciter stator 141 to generate an excitation field. The excitation field may be received via a generator control unit. In this respect, the multi-stage generator 100 may comprise a generator control unit connected between the pilot exciter 110 and the main exciter 130, in particular between the plurality of pilot exciter windings 123 and the plurality of main exciter stator windings 143. The generator control unit may be configured to rectify an alternating current (AC) output of the pilot exciter 110 into a direct current (DC) input for the main exciter stator windings 143. In addition to its generator excitation function, the pilot exciter 110 may also be configured to provide power for other components of the electrical system of the multi-stage generator 100. As described above, rotation of the main exciter rotor 131 relative to the main exciter stator 141 induces current in the main exciter rotor windings 132. The generator 100 is configured to use this electrical output to supply current to the main stage rotor 151 to generate an excitation field. In this respect, the multi-stage generator 100 may comprise a rectifier between the main exciter 130 and the main stage 150 configured to rectify an AC output of the main exciter 130 into a DC input for the main stage rotor 151. Figures 3A and 3B show another arrangement of a multi-stage generator 200. The multistage generator 200 comprises a generator shaft 201, a main stage 250, a main exciter 230, and a pilot exciter stator 221 which all comprise similar features and are arranged in a similar manner to the corresponding components of the multi-stage generator 100 described in relation to Figures 1 to 2B. Differently to the arrangement of Figures 2A and 2B in which the pilot exciter rotor 111 includes a magnet carrier 113, the multi-stage generator 200 comprises a pilot exciter rotor 211 that is integrated into the generator shaft 201. This eliminates the need for a separate magnet carrier. In the illustrated arrangement, the pilot exciter rotor 211 comprises a plurality of permanent magnets 212. The plurality of permanent magnets 212 can be comprised in the generator shaft 201 such that the plurality of permanent magnets 212 can be directly connected to the generator shaft 201. That is to say, the generator shaft 201 can perform the function of the pilot exciter rotor 211. In this respect, the generator shaft 201 may comprise one or more recesses on an axial end face thereof and configured to receive the plurality of permanent magnets 212. In some arrangements, the plurality of permanent magnets 212 are adhered to the generator shaft 201, for example by an adhesive. The generator shaft 201 may provide the above-mentioned fixing means. In this respect, the generator shaft 201 may comprise an axially-extending support configured to restrict radially outward movement of the plurality of permanent magnets 212. The generator shaft 201 may be configured to support the plurality of permanent magnets 212 such that the plurality of permanent magnets 212 extend axially toward the pilot exciter stator 221. In this way, the pilot exciter 210 can be configured such that rotation of the generator shaft 201 rotates the plurality of permanent magnets 212 relative to the stator core 222 of the pilot exciter stator 221, such that current is induced in the plurality of pilot exciter stator windings 223. Figures 4A and 4B show another arrangement of a multi-stage generator 300. The multistage generator 300 comprises a generator shaft 301, a main stage 350, a main exciter 330, and a pilot exciter stator 321 which all comprise similar features and are arranged in a similar manner to the corresponding components of the multi-stage generator 100 described in relation to Figures 1 to 2B. Differently to the arrangement of Figures 2A and 2B in which the pilot exciter rotor 111 includes a magnet carrier 113, the multi-stage generator 300 comprises a pilot exciter 310 that is integrated with the main exciter 330. As described earlier in relation to Figures 2A and 2B, the main exciter 330 comprises a main exciter rotor 331 having a plurality of main exciter rotor windings 332, and a main exciter stator 341 having a plurality of main exciter stator windings 342. The main exciter rotor windings 332 are distributed around the rotational axis of the generator 300 in a circular array having a first radius. In the illustrated arrangement, the main exciter rotor 331 comprises the pilot exciter rotor 311. That is to say, the main exciter rotor 331 performs the function of the pilot exciter rotor 311 by transferring rotational drive from the generator shaft 301 to the plurality of permanent magnets 312. The main exciter rotor 331 may comprise a yoke connected to the generator shaft 301 and comprising a plurality of radially extending teeth configured to receive the plurality of main exciter rotor windings 332. The main exciter rotor 331 may be configured to receive the plurality of permanent magnets 312 of the pilot exciter 310. In some arrangements, the main exciter rotor 331 may comprise a plurality of recesses configured to support the plurality of permanent magnets 312. The main exciter rotor 331 may provide the above-mentioned fixing means. In this respect, the main exciter rotor 331 may comprise an axially-extending support configured to restrict radially outward movement of the plurality of permanent magnets 312. In the arrangement shown, the plurality of permanent magnets 312 are directly connected to the main exciter rotor 331 and distributed around the rotational axis of the generator 300 in a circular array having a second radius. The second radius is smaller than the first radius. Therefore, the radius of the array of main exciter rotor windings 332 is greater than the radius of the array of permanent magnets 312. As such, the plurality of permanent magnets 312 may be disposed on the main exciter rotor 311 at a radially inner position with respect to the main exciter rotor windings 332. The plurality of permanent magnets 312 may be disposed concentrically within the plurality of rotor windings 332 and therefore may be disposed concentrically within the main exciter stator 341. The main exciter rotor 331 may be configured to support the plurality of permanent magnets 312 such that the plurality of permanent magnets extend axially toward the pilot exciter stator 321. In this way, the pilot exciter 310 can be configured such that rotation of the generator shaft 301 rotates the main exciter rotor 331 which rotates the plurality of permanent magnets 312 relative to the pilot exciter stator 321, such that current is induced in the plurality of pilot exciter stator windings 323. Similarly to the arrangements described in relation to Figures 2A and 2B, the generator shaft 301 may be rotatably mounted in the housing by a first bearing 304a and a second bearing 304b. The second bearing 304b can be disposed along the rotational axis such that the main stage 350 is between the second bearing 304b and the main exciter 330. In the illustrated arrangement, the first bearing 304a is disposed along the rotational axis adjacent the main exciter 330, but is instead between the main stage 350 and the main exciter 330. While the pilot exciter rotor 311 has been described as being integrated into the main exciter rotor 331, it will be appreciated that the pilot exciter 310 could instead be integrated into the main stage 350 in a similar manner, by providing the plurality of permanent magnets 312 on the main stage rotor 351, such that the main stage rotor 351 performs the function of the pilot exciter rotor 311. In such an arrangement, the pilot exciter stator could be provided adjacent the main stage 350, such that the pilot exciter stator windings 323 axially face the plurality of permanent magnets 312. Figures 5A and 5B show another arrangement of a multi-stage generator 400. The multistage generator 400 comprises a generator shaft 401, a main exciter 430, a main stage 450, and a pilot exciter 410 which all comprise similar features to the corresponding components of the multi-stage generator 100 described in relation to Figures 1 to 2B. Differently to the arrangement of Figures 2A and 2B in which the main exciter 130 is disposed along the rotational axis 103 between the main stage 150 and the pilot exciter 110, the multi-stage generator 400 in Figures 5A and 5B is arranged such that the main stage 450 is disposed along the rotational axis between the main exciter 430 and the pilot exciter 410. In this way, the main exciter 430 is closest to the drive end (where the generator shaft 401 is connected to a rotational drive input) compared to the main stage 450 and the pilot exciter 410. In the arrangement shown, the multi-stage generator 400 comprises a first bearing 404a and a second bearing 404b to support the generator shaft 401. The first bearing 404a is disposed along the rotational axis between the pilot exciter 410 and the main stage 450. The second bearing 404b is disposed along the rotational axis such that the main exciter 430 is between the second bearing 404b and the main stage 450. While the multi-stage generators 100, 200, 300 are generally arranged in the manner shown schematically in Figure 1, Figures 5A and 5B illustrate one arrangement of a multistage generator 400 with alternative positioning of the stages of the multi-stage generator 400 along the rotational axis of the generator. However, it will be appreciated that the disclosure extends to other alternative arrangements that are not illustrated. For example, the pilot exciter 410 and the main exciter 430 of Figures 5A and 5B could be swapped, such that the pilot exciter 410 is positioned closest to the drive end of the generator 400. Furthermore, the positions of any bearings along the rotational axis may differ from the arrangements described above. For example, in Figure 5A, the second bearing 404b may instead be positioned along the rotational axis between the main exciter 430 and the main stage 450. Figure 6 is a schematic illustration showing an aircraft 1. The aircraft 1 may comprise the multi-stage generator 100 as described above. It will be understood that the aircraft may comprise any of the multi-stage generators described above, such as those described in relation to Figures 2A to 5B. The aircraft 1 comprises a prime mover, which may be an aircraft engine 2. The aircraft engine 2 comprises an engine output shaft 3 which may be configured to provide a rotational drive to the multi-stage generator 100. The engine output shaft 3 may be connected to the generator shaft 101 by any suitable drive transfer device. The operation of the multi-stage generator 100 will be described in relation to Figures 2A and 2B, but it will be appreciated that the steps are equally applicable to any arrangement of a multi-stage generator disclosed herein. The multi-stage generator 100 receives a rotational input, for example from an aircraft engine, which rotates the generator shaft 101 about its rotational axis 103. Rotation of the generator shaft 101 also rotates the pilot exciter rotor 111, the main exciter rotor 131 and the main stage rotor 151. Rotation of the pilot exciter rotor 111, and therefore rotation of the plurality of permanent magnets 112 connected to the pilot exciter rotor 111, provides a rotating magnetic field which induces current in the plurality of pilot exciter stator windings 123. The electrical output of the pilot exciter stator windings 123 is used to drive a current in the main exciter stator windings 143, which generates a static magnetic field. This may be performed using a generator control unit (GCU) configured to rectify the pilot exciter AC voltage to drive the main exciter with DC voltage. Since the main exciter rotor 131 is rotating with the generator shaft 101, the plurality of main exciter rotor windings 132 experience a changing magnetic field which induces current therein. The electrical output of the main exciter rotor windings 132 is used to drive a current in the main stage rotor windings 152, which generates a rotating magnetic field by virtue of the rotation of the main stage rotor 151 with the generator shaft 101. This may be performed using a plurality of rotating diodes which connect the main exciter rotor windings to the main stage rotor windings. The plurality of main stage stator windings 163 experience the rotating magnetic field such that current is induced therein. The electrical output of the main stage 150 can be used to provide power to electrical loads on the aircraft. 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. A multi-stage generator for an aircraft comprising:a generator shaft configured to rotate about a rotational axis of the generator;a main stage configured to generate an electrical power output of the generator from rotation of the generator shaft;a main exciter configured to provide an excitation current to windings of the main stage; anda pilot exciter configured to provide an excitation current to windings of the main exciter;wherein the pilot exciter comprises an axial flux permanent magnet generator (PMG).

2. The multi-stage generator according to claim 1, wherein the generator shaft is configured to rotate at speeds of at least 6000 revolutions per minute (rpm), preferably at least 12000 rpm, preferably at least 18000 rpm, preferably at least 24000 rpm.

3. The multi-stage generator according to claim 1 or claim 2, wherein the axial flux PMG comprises:a pilot exciter rotor; anda pilot exciter stator;wherein the pilot exciter rotor is separated from the pilot exciter stator by an air gap defined in a direction parallel to the rotational axis.

4. The multi-stage generator according to claim 3, wherein the pilot exciter rotor is disposed at a position along the rotational axis between the pilot exciter stator and the main stage or the main exciter.

5. The multi-stage generator according to claim 3 or claim 4, wherein the pilot exciter rotor comprises a support configured to restrict movement of the plurality of permanent magnets in a radially outward direction, wherein the support extends at least partially around a radially outwardly facing surface of the plurality of permanent magnets.

6. The multi-stage generator according to any preceding claim, wherein the main exciter comprises:a main exciter rotor; anda main exciter stator;wherein the main exciter stator comprises a plurality of main exciter stator windings arranged in a concentrated winding arrangement and configured to receive power from the pilot exciter; andwherein the main exciter rotor comprises a plurality of main exciter rotor windings arranged in a distributed winding arrangement and configured to rotate with the generator shaft, the main exciter being configured such that rotation of the generator shaft induces current in the plurality of main exciter rotor windings.

7. The multi-stage generator according to claim 6, wherein a plurality of permanent magnets of the axial flux PMG is directly connected to the main exciter rotor.

8. The multi-stage generator according to claim 7, wherein the plurality of permanent magnets is received by the main exciter rotor radially inwards of the main exciter rotor windings with respect to the rotational axis.

9. The multi-stage generator according to any of claims 1 to 6, wherein a plurality of permanent magnets of the axial flux PMG is supported by a magnet carrier, the magnet carrier being disposed around the generator shaft and configured to rotate therewith.

10. The multi-stage generator according to any of claims 1 to 6, wherein a plurality of permanent magnets of the axial flux PMG is directly connected to the generator shaft, for example in a recess on an axial end face of the generator shaft.

11. The multi-stage generator according to any preceding claim, wherein the main stage comprises:a main stage rotor; anda main stage stator;wherein the main stage rotor comprises a plurality of main stage rotor windings arranged in a concentrated winding arrangement and configured to receive power from the main exciter; andwherein the main stage stator comprises a plurality of main stage stator windings arranged in a distributed winding arrangement;the main stage being configured such that rotation of the generator shaft induces current in the plurality of main stage stator windings.

12. The multi-stage generator according to claim 11, wherein a plurality of permanent magnets of the axial flux PMG is directly connected to the main stage rotor.

13. The multi-stage generator according to claim 12, wherein the plurality of permanent magnets is received by the main stage rotor radially inwards of the main stage rotor windings with respect to the rotational axis.5 14. The multi-stage generator according to any preceding claim, wherein the mainexciter is disposed at a position along the rotational axis between the pilot exciter and the main stage.

15. The multi-stage generator according to any of claims 1 to 13, wherein the main 10 stage is disposed at a position along the rotational axis between the pilot exciter and the main exciter.

16. An aircraft comprising the multi-stage generator according to any of claims 1 to 15.15 17. Use of an axial flux permanent magnet generator in a pilot exciter of a three-stagegenerator suitable for use in an aircraft.18

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