ROTORAN ASSEMBLY WITH COOLING DEVICE

Abstract

Rotor assembly (10; 110) comprising: a shaft (18; 118) with a hollow section (20; 120); a rotor core (24; 124) with at least one rotor stack (26; 126) positioned at least partially around the shaft (18; 118), the rotor core (24; 124) having a first end (28; 128) and a second end (30; 130); the rotor stack (26; 126) forming a first internal cavity (60; 160), the first internal cavity (60; 160) being only partially filled to define a gap (66; 166) in the first internal cavity (60; 160); a cooling mechanism comprising: a first channel (46; 146) configured to direct a fluid flow (44; 144) from the to direct the fluid from the hollow section (20; 120) of the shaft (18; 118) in a generally radial direction; to a second channel (48; 148) in fluid communication with the first channel (46; 146), which is defined at least partially by the gap (66; 166) in the first internal cavity (60; 160); and wherein the second channel (48;148) is designed to direct the fluid flow (44; 144) from the first channel (46; 146) to the first (28; 128) and / or second (30; 130) end of the rotor core (24; 124); wherein the rotor arrangement (10; 110) further comprises: first (54; 154) and second (56; 156) walls extending away from and around the shaft (18; 118); and wherein the hub opening (52; 145) is arranged between the first (54; 154) and second (56; 156) wall and the first (54; 154) and second (56; 156) wall are designed to direct the fluid flow (44; 144) into the hub opening (52; 145).

Description

TECHNICAL AREA The present invention relates generally to a rotor arrangement of an electric machine and in particular to a rotor arrangement with a cooling device. BACKGROUND An electric motor / generator generally contains a rotor assembly with multiple magnets of alternating polarity surrounding the rotor's outer circumference. The rotor rotates within a stator, which generally contains multiple windings and alternating magnetic poles. In generator mode, the rotation of the rotor causes the permanent magnets to pass by the stator poles and coils, thereby inducing an electric current in each coil. Conversely, when an electric current is passed through the stator coils, the energized coils cause the rotor to rotate, and consequently, the generator behaves like a motor. As is the case with any energy conversion device, motors / generators have an efficiency of less than 100 percent and release some of the energy as heat. Efficient removal of this excess heat is desirable. Document US 2009 / 0261667A1 discloses a rotor for an electric motor, to which a coolant is supplied through a hollow section of a rotor shaft for cooling purposes. The coolant flows radially through a first channel in the rotor and then axially through a second channel connected to the first channel. In publication WO 2010 / 128 632 A1, a cooling structure for engines is disclosed in which cooling oil is supplied to a rotor lamination stack through a hollow rotor shaft and a radially extending rotor hub bore. The cooling oil flows radially through the lamination stack via gaps formed by annular disks, thus cooling it. US Patent 5,757,094 A discloses a ventilation system for an AC motor having a rotor on a hollow rotor shaft. Cooling air flows through radial bores in the rotor shaft and along the axial ends of the rotor to the rotor windings and through channels in the stator lamination stack, as well as past the end windings, to cool the AC motor. The object of the invention is to efficiently dissipate excess heat generated during the operation of an electric motor / generator. This problem is solved by the features of the independent claims. Advantageous embodiments of the invention are the subject of the dependent claims. SUMMARY A rotor assembly with a cooling device comprising first and second channels is provided. The rotor assembly includes a shaft with a hollow section and a rotor core with at least one rotor pack positioned at least partially around the shaft. The rotor core has a first end and a second end. The rotor pack forms an internal cavity that is only partially filled with a permanent magnet to define a gap in the rotor pack. A first channel is configured to direct fluid flow from the hollow section of the shaft in a generally radial direction. A second channel is defined at least partially by the gap in the internal cavity of the rotor pack and is configured to direct fluid flow from the first channel to the first and / or second end of the rotor core. The first and second channels serve to cool the magnets to a lower operating temperature. In one embodiment, a first end ring is effectively connected to the first end of the rotor core. The first end ring is spaced from the rotor core such that it defines a radially and axially extending gap, which is referred to here as a gap. A hub opening is created and designed in the shaft to allow fluid flow from the fluid supply through the hub opening and into the gap. In this embodiment, the first channel through the fluid flow through the gap and across the hub opening is defined. The first channel intersects the second channel in the gap. In an alternative embodiment, an additional rotor assembly is axially spaced from the at least one rotor assembly by a defined gap. A disk is positioned at least partially around the shaft and between the two rotor assemblies. The disk contains spaced-apart first and second fingers, which define a disk gap between them. A hub opening is formed in the hub and is configured such that it at least partially overlaps with the disk gap, allowing the fluid flow from the fluid supply to enter the disk gap via the hub opening. The first channel is defined by the fluid flow from the fluid supply, through the hub opening, and into the disk gap. The first channel intersects the second channel in the disk gap. The second channel contains a first fluid flow and a second fluid flow, which is oriented in the opposite direction to the first fluid flow. The foregoing features and advantages and other features and advantages of the present invention will be readily apparent from the following detailed description of the best ways of carrying out the invention when read in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a fragmentary schematic cross-sectional view of a section of an electric motor / generator with a rotor assembly; Fig. 2 is a schematic exploded view of the rotor assembly of Fig. 1; Fig. 3 is a fragmentary schematic cross-sectional view of the rotor assembly of Fig. 1 along an axis 3-3, which is also shown in Fig. 1; Fig. 4 is a schematic cross-sectional view of an alternative embodiment of a rotor assembly; Fig. 5 is a fragmentary schematic end view of the rotor assembly of Fig. 4; and Fig. 6 is a fragmentary schematic cross-sectional view of the rotor assembly of Figs. 4-5 along an axis 6-6, which is shown in Fig. 5. DETAILED DESCRIPTION With reference to the figures, in which the same reference numerals denote identical or similar components in the various views, Fig. 1 shows a section of an electric motor / generator 12 with a rotor assembly 10. The rotor assembly 10 is rotatable within a generally annular stator 14, which has several windings 16. Fig. 2 is a schematic exploded view of the rotor assembly 10. Fig. 3 is a fragmentary schematic cross-sectional view of the rotor assembly 10. With reference to Figs. 1-2, the rotor assembly 10 comprises a shaft 18 with a hollow section 20. The shaft 18 is arranged longitudinally and rotatable about a central axis 22 (shown in Figs. 2-3). A rotor core 24 is positioned at least partially around the shaft 18. As shown in Fig. 2-3, the rotor core 24 contains several rotor assemblies 26. The rotor core 24 has a first end 28 and a second end 30. As shown in Fig.As shown in Figure 2, a generally annular first end ring 32 is effectively connected to the first end 28 of the rotor core 24. A generally annular second end ring 34 is effectively connected to the second end 30 of the rotor core 24. Bearings 36 can be used to support the rotor assembly 10. A fluid source, here referred to as the fluid supply 40, is positioned within the hollow section 20 of the shaft 18 to supply a cooling fluid for cooling the rotor assembly 10 and the electric motor / generator 12. The cooling fluid can be an oil or any other suitable fluid. The fluid supply 40 can be an oil well, a device with a spray nozzle for spraying oil, or any other suitable device. If the rotor assembly 10 is used in a vehicle, the cooling fluid can be an oil coming from a gear set, clutch, or other internal component of the vehicle transmission (not shown). Depending on the specific application, more than one fluid supply 40 can be used, as shown in Fig. 3. The rotor assembly 10 includes a cooling device consisting of a first channel 46 and a second channel 48, which is in fluid communication with the first channel 46. With reference to Fig. 3, the first end ring 32 is spaced from the rotor core 24 such that it defines a gap extending radially and axially between the first end ring 32 and the rotor core 24, which is here referred to as a gap 50. With reference to Figs. 2-3, at least one hub opening 52 is formed and configured in the shaft 18 to allow the fluid flow 44 from the fluid supply 40 through the hub opening 52 and into the gap 50. The first channel 46 is defined by the fluid flow 44 through the hub opening 52 and the gap 50. Thus, the first channel 46 is designed to direct the fluid flow 44 from the hollow section 20 of the shaft 18 in a generally radial direction towards the first end 28 of the rotor core 24. Optionally, first and second walls 54, 56 can be formed, extending away from the shaft 18. The first and second walls 54, 56 allow the cooling fluid to converge or collect between them. The first and second walls 54, 56 are positioned on both sides of the hub opening 52 and are designed to direct the fluid flow 44 into the hub opening 52. The walls 54, 56 can have different axial wall thicknesses, as shown in Fig. 3. The walls 54, 56 can also have different radial thicknesses. A flow rate orThe flow rate through the hub opening 52 is determined from flow equations based on the diameter of the hub opening 52, the fluid pressure, and the axial arrangement of the walls 54, 56, as understood by those skilled in the art. Precise control of this flow rate enables optimization of the rotor cooling in relation to the system efficiency (i.e., cooling requirements versus the energy expended to supply the cooling fluid and rotational losses). The rotor assembly 26 of the rotor core 24 contains at least one internal cavity that defines a gap or path for the flow of the cooling fluid. For example, the embodiment shown in Fig. 1 shows first and second internal cavities 60, 62, which are partially filled with magnets 64 and define gap 66. The embodiment shown in Fig. 1 includes a second set of internal cavities 64, 70, which are partially filled with magnets 72 and define gap 74, and a fifth internal cavity 78 that does not contain a magnet. The number of internal cavities, magnets, and their configuration can be selected by a person skilled in the art based on the desired performance of the electric machine. The internal cavities 60, 62, 68, 70 and the magnets 64, 72 within them can have different shapes and sizes. Optionally, the inner cavities 60, 62, 68, 70 can be symmetrical with respect to an axis 76. The second channel 48 is defined by the fluid flow 44 in the gaps 66, 74 of the internal cavities 60, 62, 68, 70 of the rotor core 24. Referring to Fig. 3, the second channel 48 contains a first flow 42 through the gap 74 and a second flow 43 through the gap 66. The first channel 46 intersects the second channel 48 in the gap 50. The second channel 48 is designed to direct the fluid flow 44 from the first end 28 of the rotor core 24 to the second end 30 of the rotor core 24. Referring to Fig. 3, a rib section 80 can optionally be formed on a section of the first end ring 32 radially outside the shaft 18. The rib section 80 reinforces the first end ring 32 and / or facilitates the flow. In summary, in the rotor assembly 10, the fluid flow 44 travels from the supply 40 first through the first channel 46, which is defined by the hub opening 52 and the gap 50. The fluid flow 44 then travels through the second channel 48, which includes a first flow 42 through the gap 74 and a second flow 43 through the gap 66 in the inner cavities 60, 62, 68, 70 of the rotor assembly 26. The magnets 64, 72 are each cooled by the fluid flow 44 in the gaps 66, 74. The fluid flow 44 then reaches the second end ring 34. In other words, the fluid flow 44 travels from the supply 40 to the first end 28 of the rotor core 24 and then to the second end 30 of the rotor core 24. Referring to Figures 2-3, the second end ring 34 of the rotor assembly can be configured with several cut-out sections 82 distributed circumferentially around the circumference of the second end ring 34. As shown in Figure 3, the cut-out sections 82 are at least partially aligned with the second channel 48 to allow the fluid flow 44 from the second channel 48 to exit the second channel 48 via the cut-out sections 82. Alternatively, a second end ring 34 with a reduced outer diameter 83 (shown in dashed lines in Figure 2) can be used to allow the fluid flow 44 from the second channel 48 to exit into a region radially outside the outer diameter 83. In other words, the outer diameter 83 is chosen to be small enough to expose at least a portion of the gap 66, 74 and allow the cooling fluid to escape. With reference to Fig. 1 and Fig. 3, the rotor assembly 10 optionally includes an inner shaft 84 concentric within the shaft 18. In this case, the shaft 18 transmits a torque to the gear set (not shown) to which it is attached. The inner shaft 84 can be mounted on the bearings 36. Spokes 86 extend radially between the shaft 18 and the inner shaft 84, defining respective grooves 88 between each pair of spokes 86. With reference to Fig. 3, a first secondary fluid path 90 extends from the fluid supply 40 to outside the first end ring 32 via respective holes 92, 94 in the shaft 18 and the first end ring 32, respectively. With reference to Fig. 3, a second secondary fluid path 96 extends from the fluid supply 40 to outside the second end ring 34 via an open end 98 of the arrangement 10. The cooling fluid in the first and second secondary fluid paths 90, 96 can be flung radially outwards onto an inner diameter of the stator windings 16 (shown in Fig. 1), thereby cooling the stator windings 16. An alternative embodiment of a rotor assembly 110 is shown in Figures 4-6. With reference to Figures 4 and 6, the rotor assembly 110 comprises a shaft 118 with a hollow section 120. The shaft 118 is arranged longitudinally and is rotatable about a central axis 122. With reference to Figure 6, a rotor core 124 is positioned at least partially around the shaft 118 and contains several rotor assemblies 126. The rotor core 124 has a first end 128 and a second end 130. As shown in Figure 6, a first end ring 132 is effectively connected to the first end 128 of the rotor core 124. A second end ring 134 is effectively connected to the second end 130 of the rotor core. Figure 5 is an end view of the rotor assembly 110, looking at a section of the first end ring 132 and the shaft 118. The first end ring 132 has a generally circular shape. The central axis 122 extends through the center point 131 of the first end ring 132 from the leaf or...the side. Furthermore, in Fig. 5, cavities 160, 162, partially filled with magnets 164 and 172 respectively, are shown in the rotor assembly 126, defining the gaps 166 and 174 respectively. The rotor assembly 110 can contain several cavities that are partially unfilled, similar to the one shown in Fig. 1. Any suitable configuration of cavities and magnets can be used. With reference to Fig. 6, the rotor core 124 comprises a first core section 135, which is separated from a second core section 137 by a defined axial gap to allow a disk 139 to be placed between the first and second core sections 135, 137. The disk 139 is made of a non-magnetic material. For example, the disk 139 can be made of stainless steel, a polymer, a glass fiber composite, or other suitable materials. Optionally, the disk 139 can be made of a material that is a poor or low electrical conductor to minimize eddy current losses. In one example, the disk 139 has an axial wall thickness of about 1 mm. With reference to Fig. 4, the disk 139 comprises several spaced-apart fingers 141 that define a disk gap 143 between each pair of fingers 141. Each of the core sections 135, 137 comprises at least one rotor stack 126.Figure 6 shows several rotor packages 126. Optionally, as shown in Figure 6, the rotor packages 126 can be inclined relative to each other. With reference to Figs. 4 and 6, a fluid supply 140 is located within the hollow section 120. Depending on the application, more than one fluid supply 140 can be used. A fluid flow 144 travels from the fluid supply 140 through a first channel 146, which is designed to direct the fluid flow 144 in a generally radial direction away from the hollow section 120 of the shaft 118. At least one hub opening 145 is formed in the shaft 118 and is designed such that it at least partially overlaps with the disk gap 143, allowing the fluid flow 144 to enter the disk gap 143 from the fluid supply 140 via the hub opening 145. With reference to Figures 5-6, a first channel 146 is defined by the fluid flow 144 from the fluid supply 140, through the hub opening 145, and into the disk gap 143. With reference to Figure 6, a second channel 148 is designed to direct the fluid flow 144 from the first channel 146 to both the first and second ends 128, 130 of the rotor core 124. The first channel 146 intersects the second channel 148 in the disk gap 143. In the disk gap 143, the fluid flow 144 splits into a first fluid flow 147 to the first end 128 of the rotor core and a second fluid flow 149 to the second end 130 of the rotor core 124. As shown in Fig. 6, the second fluid flow 149 is oriented in a direction opposite to that of the first fluid flow 147. The first and second fluid flows 147, 149 flow in the gaps 166 and 174 in the inner cavities 160, 162 of the rotor assembly 126.The magnets 164, 172 are cooled by the fluid flow 144. Optionally, the magnets 164, 172 can be aligned to the left or right of the cavities 160 and 162, respectively. Optionally, the magnets 164, 172 can be positioned in the center of the cavities 160 and 162, respectively, to allow the fluid flow 144 to flow along all sides of the magnets 164, 172. With reference to Fig. 6, a section 151 of the fluid flow 144 in the second channel 148 can flow in a generally radial direction between adjacent rotor assemblies 126. As mentioned above, the rotor assemblies 126 can optionally beveled relative to each other, resulting in the stepped pattern shown in Fig. 6. Even if the rotor packages 126 are not chamfered relative to each other, there may be a radial component of the fluid flow 144 due to the cooling fluid being flung against the axially outermost section of the gaps 166 and 174 when the rotor arrangement 110 rotates. Optionally, the first and second walls 154, 156 can be configured to extend away from the shaft 118. The first and second walls 154, 156 allow the cooling fluid to pool or accumulate between them. The first and second walls 154, 156 are positioned on both sides of the hub opening 145 and configured to direct the fluid flow 144 into the hub opening 145. The walls 154, 156 can have different axial and / or radial thicknesses. A flow rate through the hub opening 145 is determined from flow equations based on the diameter of the hub opening 145, the fluid pressure, and the axial arrangement of the walls 154, 156, as is well understood by those skilled in the art. Precise control of this flow rate enables the optimization of rotor cooling in relation to the system efficiency (i.e.,Cooling requirements compared to the energy and rotation losses used to deliver the cooling fluid). Referring to Figures 5-6, the first and second end rings 132, 134 can contain several notches 182 distributed circumferentially around the circumference of the respective first and second end rings 132, 134. The multiple notches 182 are at least partially aligned with the second channel 148 such that the fluid flow 144 exits the second channel 148 via the multiple notches 182 in the first and second end rings 132, 134. Alternatively, the first and / or second end rings 132, 134 can be selected with a reduced outer diameter 183 (shown in dashed lines in Figure 5) to allow the fluid flow 144 to exit from the second channel 148 into a region radially outside the outer diameter 183. In other words, the outer diameter 183 is chosen to be small enough to expose at least a section of the column 166, 174 and allow the cooling fluid to escape. Referring to Fig. 6, a first secondary fluid path 190 extends from the fluid supply 140 through a hole 192 (also shown in Fig. 6) in the shaft 118 to the outside of the first end ring 132. A second secondary fluid path 196 extends from the fluid supply 140 through an open end 198 of the arrangement 110 to the outside of the second end ring 134. The cooling fluid in the first and second secondary fluid paths 190, 196 can be flung radially outwards onto an inner diameter of the stator windings 16 (shown in Fig. 1), thereby cooling the stator windings 16. In summary, in one embodiment (the rotor arrangement 10 shown in Figs. 1-3), the fluid flow 44 migrates from the fluid supply 40 to the first end 28 of the rotor core 24 and then to the second end 30 of the rotor core 24. In an alternative embodiment (the rotor arrangement 110 shown in Figs. 4-6), the fluid flow 144 migrates from a fluid supply 140 into a disk gap 143, which is created by placing a disk 139 between the first and second core sections 135, 137 of the rotor core 124. The fluid flow 144 splits into two paths: a generally axial first flow 147 to the first end 128 of the rotor core 124 and an opposite generally axial second flow 149 to the second end 130 of the rotor core 124. Although the best ways of carrying out the invention have been described in detail, experts in the field to which this invention relates will recognize various alternative designs and embodiments to put the invention into practice within the scope of the attached claims.

Claims

Rotor assembly (10; 110) comprising: a shaft (18; 118) with a hollow section (20; 120); a rotor core (24; 124) with at least one rotor stack (26; 126) positioned at least partially around the shaft (18; 118), the rotor core (24; 124) having a first end (28; 128) and a second end (30; 130); the rotor stack (26; 126) forming a first internal cavity (60; 160), the first internal cavity (60; 160) being only partially filled to define a gap (66; 166) in the first internal cavity (60; 160); a cooling mechanism comprising: a first channel (46; 146) configured to direct a fluid flow (44; 144) from the to direct the fluid from the hollow section (20; 120) of the shaft (18; 118) in a generally radial direction; to a second channel (48; 148) in fluid communication with the first channel (46; 146), which is defined at least partially by the gap (66; 166) in the first internal cavity (60; 160); and wherein the second channel (48;148) is designed to direct the fluid flow (44; 144) from the first channel (46; 146) to the first (28; 128) and / or second (30; 130) end of the rotor core (24; 124); wherein the rotor arrangement (10; 110) further comprises: first (54; 154) and second (56; 156) walls extending away from and around the shaft (18; 118); and wherein the hub opening (52; 145) is arranged between the first (54; 154) and second (56; 156) wall and the first (54; 154) and second (56; 156) wall are designed to direct the fluid flow (44; 144) into the hub opening (52; 145). Arrangement (10) according to claim 1, further comprising: a first end ring (32) effectively connected to the first end (28) of the rotor core (24), wherein the first end ring (32) is spaced apart from the rotor core (24) to define a gap (50) between them; a hub opening (52) formed in the shaft (18) and configured to allow the fluid flow (44) from a fluid supply (40) through the hub opening (52) into the gap (50); and wherein the first channel (46) is defined by the fluid flow (44) through the hub opening (52) and through the gap (50). Arrangement (10) according to claim 2, further comprising: a second end ring (34) effectively connected to the second end (30) of the rotor core (24); several cut-out sections (82) distributed circumferentially around a circumference of the second end ring (34); wherein the several cut-out sections (82) overlap at least partially with the second channel (48) to allow the fluid flow (44) to exit the second channel (48) via the several cut-out sections (82). Arrangement (110) according to claim 1, further comprising: a second rotor assembly (126) spaced axially apart from the at least one rotor assembly (126) by a defined gap; a disk (139) positioned at least partially around the shaft (118) and between the two rotor assemblies (126), the disk (139) containing spaced-apart first and second fingers (141) defining a disk gap (143) between them; a hub opening (145) formed in the shaft (118) and configured to overlap at least partially with the disk gap (143) such that the fluid flow (144) enters the disk gap (143) from a fluid supply (140) via the hub opening (145); and wherein the first channel (146) is defined by the fluid flow (144) from the fluid supply (140) via the hub opening (145) into the disk gap (143). Arrangement (110) according to claim 4, wherein the second channel (148) contains: a first fluid flow (147); and a second fluid flow (149) which is oriented in an opposite direction relative to the first fluid flow (147). Arrangement (110) according to claim 4, further comprising: a first end ring (132) effectively connected to the first end (128) of the rotor core (124); several notches (182) distributed circumferentially around a circumference of the first end ring (132); wherein the several notches (182) are at least partially aligned with the second channel (148) such that the fluid flow (144) exits the second channel (148) via the several notches (182) in the first end ring (132). Arrangement (110) according to claim 4, further comprising: a second end ring (134) effectively connected to the second end (130) of the rotor core (124), wherein the second end ring (134) has an outer diameter small enough to allow the fluid flow (144) to escape from the second channel (148) into a region radially outside the outer diameter of the second end ring (134). Electric motor (12) comprising: a stator (14); a rotor (10) mounted within the stator (14), the rotor (10) comprising a shaft (18) with a hollow section (20); a rotor core (24) comprising at least one rotor assembly (26) positioned at least partially around the shaft (18), the rotor core (24) having a first end (28) and a second end (30); a fluid supply (40) positioned in the hollow section (20) of the shaft (18); a cooling device configured to allow a fluid flow (44) from the fluid supply (40) to the first (28) and / or second (30) end of the rotor core (24); the cooling device comprising a first channel (46) in fluid communication with a second channel (48), the first channel (46) directing the fluid flow (44) in a generally radial direction enables and the second channel (48) enables the fluid flow (44) in a generally axial direction;at least one inner cavity (60) formed in the rotor assembly (26); a permanent magnet (64) that only partially fills the inner cavity (60) such that a gap (66) is defined in the inner cavity (60) of the rotor assembly (26); wherein the second channel (48) is defined by the fluid flow (44) through the gap (66) in the inner cavity (60) of the rotor assembly (26); a first end ring (32) that is effectively connected to the first end (28) of the rotor core (24), wherein the first end ring (32) is spaced apart from the rotor core (24) to define a gap (50) between them; a hub opening (52) formed in the shaft (18) and designed to allow the fluid flow (44) from the fluid supply (40) through the hub opening (52) and into the gap (50); and wherein the first channel (46) is defined by the fluid flow (44) through the hub opening (52) and through the gap (50);wherein the electric motor (12) further comprises: first (54) and second (56) walls extending away from and around the shaft (18); and wherein the hub opening (52) is arranged between the first (54) and second (56) wall and the first (54) and second (56) wall are designed to direct the fluid flow (44) into the hub opening (52). Method for cooling a rotor assembly (10; 110), the method comprising: providing a shaft (18; 118) with a hollow section (20; 120); positioning a rotor core (24; 124) at least partially around the shaft (20; 120), the rotor core (24; 124) having a first end (28; 128) and a second end (30; 130); forming at least one internal cavity (60; 160) in the rotor core (24; 124); partially filling the internal cavity (60; 160) with a permanent magnet (64; 164) to define a gap (66; 166) in the internal cavity (60; 160); and a first channel (46; 146) is formed to define a fluid flow (44; 144) in a generally radial direction from the hollow section (20; 120) of the shaft (18; 118); a second channel (48; 148) is formed which is in fluid communication with the first channel (46; 146) and is defined at least partially by the gap (66; 166) in which at least one internal cavity (60; 160) is located;wherein the second channel (48; 148) is configured to direct the fluid flow (44; 144) from the first channel (46; 146) to the first (28; 128) and / or second (30; 130) end of the rotor core (24; 124); a fluid supply (40; 140) is positioned within the hollow section (20; 120) of the shaft (18; 118); and a cooling fluid is directed through the first (46; 146) and second (48; 148) channels to cool the rotor (10; 110); wherein first (54; 154) and second (56; 156) walls are provided extending away from and around the shaft (18; 118); and wherein a hub opening (52; 145) is arranged between the first (54; 154) and second (56; 156) wall and the first (54; 154) and second (56; 156) wall are designed to direct the fluid flow (44; 144) into the hub opening (52; 145).

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