A squirrel cage for an electric machine

Abstract

A squirrel cage 20 for an electric machine 10 suitable for a vehicle, the squirrel cage comprising a first end ring 30, a second end ring 40 and a plurality of bars 50, where the bars extend between t

Description

TECHNICAL FIELD The present disclosure relates to a squirrel cage. Aspects of the invention relate to a squirrel cage, to an electric machine, and to a vehicle. BACKGROUND Battery Electric Vehicles (‘BEVs’) and other vehicles having an electric machine have been used for a number of years in applications at various altitudes. For use at high altitudes, systems are employed to overcome any effects a thinner atmosphere can have on electrical components in the electric machine. Employing such systems may reduce the efficiency of the electrical components. It is an aim of the present invention to address one or more of the disadvantages associated with the prior art. SUMMARY OF THE INVENTION Aspects and embodiments of the invention provide a squirrel cage, an electric machine and a vehicle as claimed in the appended claims. According to an aspect of the invention, there is described a squirrel cage for an electric machine, the squirrel cage comprising a first end ring, a second end ring and a plurality of bars, wherein: the plurality of bars extend between the first end ring and the second end ring to define the squirrel cage and an axis of rotation of the squirrel cage; the first end ring has an axially inner face that is annular and has a first outer diameter, an axially outer face that is annular and has a second outer diameter, and a radially outer wall extending between the axially inner face and the axially outer face; the first outer diameter is greater than the second outer diameter; and the radially outer wall extends from the inner face to the outer face at an angle of 10 to 30 degrees to the axis of rotation. The angle provides a number of benefits, for example: increased electrical clearance for electrical components adjacent to the rotor; reduced mass compared to squirrel cages that do not have an angled end; the angle enables coolant to run more easily off the end of the squirrel change, therefore it reduces the likelihood of capillary action drawing in fluid to a stator-rotor connection; the taper enables electrical components to be located closer to the squirrel cage than if the taper were not there enabling a more compact electric machine without large air gaps. According to an additional aspect of the invention, there is described a squirrel cage for an electric machine, the squirrel cage comprising a first end ring, a second end ring and a plurality of bars, wherein: the plurality of bars extend between the first end ring and the second end ring to define the squirrel cage and an axis of rotation of the squirrel cage; the first end ring has an axially inner face having a first outer diameter, an axially outer face having a second outer diameter, and a radially outer wall extending between the axially inner face and the axially outer face; the first outer diameter is greater than the second outer diameter; and the radially outer wall extends from the inner face to the outer face at an angle of 10 to 30 degrees to the axis of rotation. According to either or both of the previous aspects there are further optional features which may be present as will be described below. Optionally, the squirrel cage may be for a high-altitude electric machine. This enables the electric machine to be used at higher altitudes. Optionally, the squirrel cage may be for a very high-altitude electric machine. This enables the electric machine to be used at higher altitudes. Optionally, the squirrel cage may be for an electric machine operatable at 1,500 m to 5,500 m above sea level. This enables the electric machine to be used at higher altitudes. Optionally, the squirrel cage may be for an electric machine operatable at 3,500 m to 5,500 m above sea level. This enables the electric machine to be used at higher altitudes. Optionally, the squirrel cage may be for an electric machine operatable at 1,500 m to 6,000 m above sea level. This enables the electric machine to be used at higher altitudes. Optionally, the squirrel cage may be for an electric machine operatable at 3,500 m to 6,000 m above sea level. This enables the electric machine to be used at higher altitudes. High altitude may be defined as 1,500-3,500 metres (4,900-11,500 ft). Very high altitude may be defined as 3,500-5,500 metres (11,500-18,000 ft). Extreme altitude may be defined as above 5,500 metres (18,000 ft). The squirrel cage works in normal altitude (sea level to 1500) to any one of: high altitude, very high altitude and extreme altitude. This enables the electric machine to be used at higher altitudes. Optionally, the angle is 12 to 28 degrees. Providing an angle within the range 12 to 28 degrees provides the electrical clearance required for the end ring and other components located in the electrical machine in use but at the same time provides a suitable draft angle to enable the end ring to be removed a moulding into which the end ring is cast. Optionally, the angle may be 14 to 26 degrees, or 16 to 24 degrees, or 16 degrees. An angle of 16 degrees may enable the clearance and may provide sufficient draft angle to remove the cast from the mould. Optionally, the first end ring has a radially inner wall extending between the axially inner and outer faces, and the axially inner face has a first inner diameter, the axially outer face has a second inner diameter, the first inner diameter is smaller than the second inner diameter; and wherein the radially inner wall extends from the inner face to the outer face at an angle of 10 to 30 degrees to the axis of rotation. By providing a tapered radially inner wall there is a weight saving as compared to a non-tapered radially inner wall. Optionally, the angle may be 14 to 26 degrees, or 16 to 24 degrees, or 16 degrees. An angle of 16 degrees may enable the clearance and may provide sufficient draft angle to remove the cast from the mould. Optionally, the second end ring has an axially inner face that is annular and has a first outer diameter, an axially outer face that is annular and has a second outer diameter, and a radially outer wall extending between the axially inner face and the axially outer face; wherein the first outer diameter is greater than the second outer diameter; and the radially outer wall extends from the inner face to the outer face at an angle of 10 to 30 degrees to the axis of rotation. This provides the same or similar advantages as the first end ring. The angle may be 12 to 28 degrees. Providing an angle within the range 12 to 28 degrees provides the electrical clearance required for the end ring and other components located in the electrical machine in use but at the same time provides a suitable draft angle to enable the end ring to be removed a moulding into which the end ring is cast. Optionally, the angle may be 14 to 26 degrees, or 16 to 24 degrees, or 16 degrees. An angle of 16 degrees may enable the clearance and may provide sufficient draft angle to remove the cast from the mould. Optionally, the second end ring has a radially inner wall extending between the axially inner and outer faces, and the axially inner face has a first inner diameter, the axially outer face has a second inner diameter, the first inner diameter is smaller than the second inner diameter; and wherein the radially inner wall extends from the inner face to the outer face at an angle of 10 to 30 degrees to the axis of rotation. By providing a tapered radially inner wall there is a weight saving as compared to a non-tapered radially inner wall. Optionally, the angle may be 14 to 26 degrees, or 16 to 24 degrees, or 16 degrees. An angle of 16 degrees may enable the clearance and may provide sufficient draft angle to remove the cast from the mould. Optionally, each of the plurality of bars are spaced equidistantly around the first end ring and second end ring. Spacing the bars equidistantly around the first and second end rings may help to maintain the strength of the component in use. Optionally, the plurality of bars are parallel to the axis of rotation. Spacing the bars such that they are parallel to the axis of rotation may help to maintain the strength of the component in use. Optionally, the axially outer face of one or both of the first and second end ring is flat. A flat face of the axially outer face makes it easier to balance the squirrel cage and / or rotor assembly that the squirrel cage forms a part of during machining operations, machining operations. Optionally, the axially outer face of one or both of the first and second end ring may have a balancing feature configured to maintain the rotational balance of the squirrel cage in use. The balancing feature may aid in maintaining the rotational balance of the squirrel cage in use. Optionally, the balancing feature is a cutaway or machined away portion of the axially outer face. The cutaway or machined portion of the axially outer face aids in balancing the rotor assembly which the squirrel cage forms a part of in use. By having a flat axially outer face it is easier to machine the balancing feature into the axially outer face than a non-flat face for example, pie. According to a further aspect of the invention, there is described an electric machine comprising the squirrel cage and a rotor core. Optionally, the electric machine may be for a high-altitude electric machine. This enables the electric machine to be used at higher altitudes. Optionally, the electric machine may be for a very high-altitude electric machine. This enables the electric machine to be used at higher altitudes. Optionally, the electric machine may be for an electric machine operatable at 1,500 m to 5,500 m above sea level. This enables the electric machine to be used at higher altitudes. Optionally, the electric machine may be for an electric machine operatable at 3,500 m to 5,500 m above sea level. This enables the electric machine to be used at higher altitudes. Optionally, the electric machine may be for an electric machine operatable at 1,500 m to 6,000 m above sea level. This enables the electric machine to be used at higher altitudes. Optionally, the electric machine may be for an electric machine operatable at 3,500 m to 6,000 m above sea level. This enables the electric machine to be used at higher altitudes. The electric machine works in normal altitude (sea level to 1500) to any one of: high altitude, very high altitude and extreme altitude. Optionally, the electric machine may further comprise an electrical component, the electrical component being located at or greater than a minimum clearance distance from the radially outer wall; the minimum clearance distance (Imin) is calculated by: = wherein: kd is an altitude correction factor; and P is a pollution degree in mm. By utilising the equation to calculate Imin the electrical machine can be tailored to specific altitudes of use, for example, at lower altitudes a smaller clearance may be required than for higher altitudes the result is that more compact electric machines can be used where the use will at near sea-level as opposed to electric machines utilised in high altitude environments such as on mountain ranges. As barometric pressure decreases at higher elevations there is a decrease in the insulating effect of air in the environment in which the electric machine is located in. As a result, by locating the electrical component at the minimum distance or greater away from the radially outer wall (which in use has a current passing through it) reduces the likelihood of electrical arcing and shorting that could occur should a standard sealevel altitude electric machine be utilised at high altitudes. Optionally, altitude correction factor kd is determined by: = 2 * - S * W-V * 4 ; wherein h is maximum altitude in m the electrical machine is intended for use at. The kd can therefore be calculated to be for the maximum heigh the vehicle can be intended for use at. The maximum altitude may be measured as a height above sea level. For example, h may be 5000 m to 6000 m above sea level, h may be 5500 m above sea level. Optionally the pollution degree (P) is calculated as a function of the impulse voltage (Vi): F = s IS - wherein impulse voltage is measured in V. Impulse voltage may be a value during a voltage surge. The impulse voltage may be the maxima of an impulse waveform. Therefore, the pollution degree may be calculated for the maximum impulse voltage that is likely to be experienced in use. Optionally, wherein h is between 5000 m to 6000 m and Vi is between 2000 V to 3000 V. Such values of h and Vi may be particularly suitable for alpine or mountainous conditions. Optionally, h = 5500 m; Vi = 2500 V. Such values of h and Vi may be particularly suitable for alpine or mountainous conditions. Optionally, the electric machine is a 400 V electric drive unit or an 800 V electric drive unit. An 800 V electric drive unit is a relatively high voltage EDU as compared to more standard 400 V EDUs, 800 V EDUs can as a result provide greater power and at the same time be connected to batteries which charge more quickly. An 400 V EDU is an EDU configured to provide drive at a supply voltage of 400 V alternating current. An 800 V EDU is an EDU configured to provide drive at a supply voltage of 800 V alternating current. According to a yet further aspect of the invention, there is described a vehicle comprising a squirrel cage as previously described or an electric machine as previously described. Such a vehicle is usable in remote locations at high elevations. Optionally, the vehicle may be for a high-altitude electric machine. Optionally, the vehicle may be for a very high-altitude electric machine. Optionally, the vehicle may be for an electric machine operatable at 1,500 m to 5,500 m above sea level. Optionally, the vehicle may be for an electric machine operatable at 3,500 m to 5,500 m above sea level. Optionally, the vehicle may be for an electric machine operatable at 1,500 m to 6,000 m above sea level. Optionally, the vehicle may be for an electric machine operatable at 3,500 m to 6,000 m above sea level. The vehicle works in normal altitude (sea level to 1500) to any one of: high altitude, very high altitude and extreme altitude. Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and / or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and / or features of any embodiment can be combined in any way and / or combination, unless such features are incompatible. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and / or incorporate any feature of any other claim although not originally claimed in that manner. BRIEF DESCRIPTION OF THE DRAWINGS One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 shows a schematic of a vehicle with a sub-system in accordance with an embodiment of the invention; Figure 2 shows the vehicle of Figure 1 at different altitudes with respect to sea level; Figure 3 shows a schematic diagram of a sub-system of the vehicle in Figures 1 and 2; Figure 4 shows an electric machine with a squirrel cage in accordance with an embodiment of the invention; Figure 5 shows a close up view of the electric machine of Figure 4; Figure 6 shows an end-on view of the squirrel cage of Figure 4; Figure 7 shows a graph of altitude correction factor in respect to altitude above sea level; and Figure 8 shows a graph of pollution factor with respect to impulse voltage. DETAILED DESCRIPTION A vehicle 1, squirrel cage 20 and electric machine 10 will be described with the aid of Figures 1 to 8. As shown in Figure 1, the vehicle 1 is a wheeled passenger vehicle having a subsystem 4 and an optional charging port 6. The passenger vehicle 1 depicted is of the 4x4 or SUV type, however the vehicle 1 may be a car, a van, a light goods vehicle, or other such vehicle. The vehicle 1 may be suitable for cold weather use, for example having snow tyres or tracks enabling the vehicle to be driven over snow and ice. A sub-system 4 of the vehicle 1 including an electrical storage means 8 and a portion of the powertrain 2 is shown schematically in Figure 3. The vehicle 1 as shown schematically in Figure 2 is useable at different altitudes h above sea level to a maximum altitude hm. As will be apparent to the skilled reader Figure 2 is not to scale but is simply to aid explanation of the invention. For example, the vehicle 1 may be utilised in an alpine or mountain range 200 type environment. Such an environment may be classed as: • High altitude for elevations between 1500 m to 3500 m above sea level; • Very high altitude for elevations between 3500 m to 5500 m above sea level; and • Extreme altitude for elevations above 5500 m above sea level. Figure 2 shows the vehicle 1 at three different heights, 0 m (or sea level) at a maximum altitude hm and an intermediate height hi between sea level and hm. Whilst 0 m or sea level is shown as the minimum elevation in Figure 2 it will be apparent to the skilled reader that vehicle 1 will be useable on land below sea level. The maximum altitude hm of the vehicle 1 may be dependent upon a number of factors such as the minimum clearance distance Imin of one or more electrical components of an electric machine 10 of a vehicle as will be described below, minimum in the sense that a smaller clearance distance risks electrical shorting (arcing) between the components when atmospheric electrical resistance is too low over such a small clearance. The vehicle 1 has a powertrain 2. The powertrain 2 comprises a propulsion system comprising at least a prime mover 10. The prime mover(s) 10 are selectively operable for the purpose of providing drive torque for accelerating the vehicle 1. In configurations with two prime movers 10, the first prime mover 10 may provide drive to a front axle and the second prime mover 10 may provide drive to a rear axle, or vice versa. In such an embodiment both prime movers 10 may be supplied electrical energy from electrical storage means 8. The prime mover 10 is an electric traction motor 10 (also known as an electric machine 10). Where a plurality of prime movers are provided, they may be a plurality of electric traction motors 10. The vehicle 1 may be a fully electric vehicle having only an electric machine 10 as a power source for propulsion and no internal combustion engine or may be a hybrid vehicle having both an electric machine and an internal combustion engine arranged to propel the vehicle 1. The electric machine is powered by a battery which may be a traction battery or battery module. The vehicle 1 may be a plug-in hybrid electric vehicle (PHEV) or a mild hybrid electric vehicle (MHEV). It shall be appreciated that in alternative embodiments, the vehicle 1 may be a hydrogen electric vehicle (HEV) and hence the electric machine may be powered by a hydrogen fuel cell arrangement comprising one or more hydrogen fuel cells. It shall be appreciated that the following description directed to a battery powered electric vehicle may also apply to a hydrogen electric vehicle. The at least one prime mover 10 comprises an electric traction motor 10. This is an electric motor arranged to convert electrical energy into kinetic energy in the form of mechanical torque and may also be arranged to convert kinetic energy into electrical energy. The electric traction motor 10 may be an alternating current induction motor or a permanent magnet motor, or another type of suitable electric machine. The electric traction motor 10 is a traction motor configured to enable at least an electric mode comprising electric-only driving. That is, the electric traction motor can drive the vehicle 1 by itself (without additional torque provided by a second torque source such as an engine). The electric traction motor 10 may form part of an electric drive unit 10 (EDU). The terms “electric traction motor” and “electric drive unit” may be used interchangeably throughout the present application. The EDU 10 may be a 400 V EDU 10. A 400 V EDU 10 is an EDU configured to provide drive at a supply voltage of 400 V alternating current. Alternatively, the EDU 10 may be an 800 V EDU. An 800 V EDU 10 is an EDU configured to provide drive at a supply voltage of 800 V alternating current. An 800 V EDU 10 is a relatively high voltage EDU as compared to more standard 400 V EDUs, 800 V EDUs can as a result provide greater power and at the same time be connected to batteries which charge more quickly. In order to store electrical energy for the electric traction motor 10, the vehicle 1 comprises an electrical energy storage means 8. The electrical energy storage 8 means may be a traction battery 8. The traction battery 8 provides a nominal voltage required by electrical power users such as the electric traction motor 10. The traction battery 8 may be a high voltage battery. The traction battery 8 may have a voltage and capacity to support electric only driving for sustained distances. The traction battery 8 may have a capacity of several kilowatt-hours, to maximise range. The capacity may be in the tens of kilowatt-hours, or even over a hundred kilowatt-hours. The traction battery 8 may be charged using a charging port 6 located on the vehicle as shown schematically in Figure 1. The traction battery 8 may also be charged by the conversion of kinetic energy into electrical energy by the electric traction motor 10. In alternative configurations there may be additional electrical storage means 8 located in different points of the vehicle 1 each of the electrical storage means 8 providing electrical energy to the prime mover 10. An inverter 12 may be provided to convert between the direct current (DC) output of the traction battery 8 and the alternating current (AC) input required for the electric machine 10. For example, the DC output may be converted to a three-phase electrical power AC input, or a two-phase electrical power AC input. The EDU 8 10 may therefore be a three-phase traction motor 10 or a two-phase traction motor 10 as appropriate. The electric machine 10 may have a squirrel cage 20. The prime mover(s) 10 are selectively operable for the purpose of providing drive torque for accelerating the vehicle 1. In alternative configurations with two prime movers 10, the first prime mover 10 may provide drive to a front axle and the second prime mover 10 provides drive to a rear axle, or vice versa. In such an embodiment both prime movers may be supplied electrical energy from the electrical storage means 8. The squirrel cage 20 and an electrical component 100 of the electric machine 10 will be now described in more detail with respect to the appended Figures 4-8. The squirrel cage 20 forms a part of the EDU 10, for reasons of clarity the rotor core of the rotor comprising the squirrel cage 20 and the stator of the EDU have been omitted from the Figures. Figure 4 shows a schematic of the squirrel cage 20 in cross-section. The squirrel cage 20 has a first end ring 30, a second end ring 40 and a plurality of bars 50 extending between the first and second end rings 30, 40. The bars 50 and end rings 30, 40 are connected to enable an electrical current to flow through them. The squirrel cage 20 in use rotates around an axis or rotation Z. The axis Z may is along the centreline of the squirrel cage 20. As Figure 4 shows only a cross-sectional view of the squirrel cage 20 only two bars 50 are shown. The bars 50 may be spaced equidistantly around the first and second end rings 30, 40 as shown in Figure 6. Whilst ten bars 50 are shown in Figure 6 there may in practice be more than ten bars 50. For example, there may be 10 to 100 bars, or 10 to 50 bars. The bars 50 shown in Figures 4-6 are parallel to the axis of rotation Z. In alternative configurations the rods 50 may not be parallel to axis of rotation Z but may be disposed at a skew or angle such that a first and second ends of each bar 50 are angularly offset relative to each other at the connection to the first and second end rings 30, 40 respectively. The first end ring 30 is annular and has an axially inner face 32, an axially outer face 34, a radially outer wall 36 and a radially inner wall 38. The radially inner and outer walls 38, 36 extend between the axially inner and outer faces 32, 34. Figure 6 shows an end on view of the squirrel cage 20 from direction A shown in Figure 4. In order to aid the balancing of the end ring 30 the end ring may have one or more balancing features 60 which may be weights added onto the axially outer face 32 or cutaway portions of the axially outer face 32. The axially outer face 34 has an inner diameter D1 and an outer diameter D2. The axially inner face 32 has an inner diameter D3 and an outer diameter D4. D1 is greater than D3 such that the axially inner wall 38 tapers away from the axis of rotation Z as shown in Figure 5. D4 is greater than D2 such that the axially outer wall 36 tapers towards the axis of rotation Z as shown in Figure 5. Figure 5 shows a close-up view of a portion of the squirrel cage 20 and the first end ring 30. As shown in Figure 5 the radially outer wall 36 extends from the axially inner face 32 to the axially outer face 34 at an angle 0 of 10 to 30 degrees in relation to the axis of rotation Z. The angle 0 may be 12 to 28 degrees, 14 to 26 degrees, or 16 to 24 degrees, or 16 degrees. The angled axially outer face 34 helps to maintain the minimum distance Imin between the squirrel cage 20 and an electrical component 100. As shown in Figure 5 the radially inner wall 38 extends from the axially inner face 32 to the axially outer face 34 at an angle (p of 10 to 30 degrees in relation to the axis of rotation Z as is shown in Figure 5. The angle (p may be 14 to 26 degrees, or 16 to 24 degrees, or 16 degrees. The second end ring 40 may be identical to the first end ring 30. That is, the second end ring 40 may be annular and have an axially inner face 42, an axially outer face 44, a radially outer wall 46 and a radially inner wall 48. The radially inner and outer walls 48, 46 extend between the axially inner and outer faces 42, 44 and the end ring 40 may have one or more corresponding balancing features 60. The axially inner faces 32, 42 of the end rings 30, 40 may face each other with the bars 50 extending between the two faces 32, 42 as can be seen in Figure 4. Like the end ring 30, the end ring 40 may be similarly dimensioned and tapered, although not necessarily identically. The electrical component 100 may be any uninsulated wiring, electrical harness, cabling or any electrical component which has a current passing through during use. The minimum clearance distance Imin (in mm) between the axially outer wall 36, 46 will now be described in more detail. The minimum clearance distance Imin can be determined in dependence upon the equation: W = (1) where kd is an altitude correction factor (dimensionless) and P is a pollution degree in mm. The altitude correction factor kd may be determined by the following equation (2): = 2 s W 4 4 MW (2) Where h is the altitude in m. Where a vehicle is to be used at a maximum altitude, for example at a maximum altitude, hm = 5500 m, the maximum altitude should be input into equation (2) to obtain the altitude correction factor kd. Equation 2 is shown graphically in graph 300 in Figure 7. Graph 300 shows the relationship between altitude correction factor kd (y-axis) and altitude h (x-axis). The pollution degree P may be determined by the following equation (3): 10 F = <80 * (3) Where Vi is the impulse voltage in V. Equation 3 is shown graphically in graph 400 in Figure 8. Graph 400 5 shows the relationship between pollution degree P and voltage Vi. Equation (3) may alternatively be rewritten as equation (4): P = - 0.9658 (4) 10 Where kP is a calibration factor equal to 0.001 mV-1. Imin may be 2 to 3 mm, Imm may be 2.2 to 2.5 mm. Imin may be 2.4 mm. 15 Imin may be calculated using an impulse voltage of 2500 Vi and a hmax of 5500 m. It will be appreciated that various changes and modifications can be made to the present invention without departing from the scope of the present application.

Claims

1. A squirrel cage for an electric machine, the squirrel cage comprising a first end ring, a second end ring and a plurality of bars, wherein:the plurality of bars extend between the first end ring and the second end ring to define the squirrel cage and an axis of rotation of the squirrel cage;the first end ring hasan axially inner face that is annular and has a first outer diameter, an axially outer face that is annular and has a second outer diameter, and a radially outer wall extending between the axially inner face and the axially outer face; and whereinthe first outer diameter is greater than the second outer diameter; andthe radially outer wall extends from the inner face to the outer face at an angle of 10 to 30 degrees to the axis of rotation.

2. The squirrel cage of claim 1, wherein the angle is 12 to 28 degrees.

3. The squirrel cage of claim 1 or 2, wherein the first end ring has a radially inner wall extendingbetween the axially inner and outer faces, andthe axially inner face has a first inner diameter,the axially outer face has a second inner diameter, andthe first inner diameter is smaller than the second inner diameter; and whereinthe radially inner wall extends from the inner face to the outer face at an angle of 10 to 30 degrees to the axis of rotation.

4. The squirrel cage of any preceding claim, wherein the second end ring has an axially inner face that is annular and has a first outer diameter, an axially outer face that is annular and has a second outer diameter, and a radially outer wall extending between the axially inner face and the axially outer face; andwhereinthe first outer diameter is greater than the second outer diameter; andthe radially outer wall extends from the inner face to the outer face at an angle of 10 to 30 degrees to the axis of rotation.

5. The squirrel cage of claim 4, wherein the angle is 12 to 28 degrees.

6. The squirrel cage of claim 4 or 5, wherein the second end ring has a radially inner wall extendingbetween the axially inner and outer faces, andthe axially inner face has a first inner diameter,the axially outer face has a second inner diameter,the first inner diameter is smaller than the second inner diameter; and whereinthe radially inner wall extends from the inner face to the outer face at an angle of 10 to 30 degrees to the axis of rotation.

7. The squirrel cage of any preceding claim, wherein each of the plurality of bars are spaced equidistantly around the first end ring and second end ring.

8. The squirrel cage of claim 7, wherein the plurality of bars are parallel to the axis of rotation.

9. The squirrel cage of any preceding claim, wherein the axially outer face of one or both of the firstand second end ring is flat.

10. The squirrel cage of any preceding claim, wherein the axially outer face of one or both of the first and second end ring has a balancing feature configured to maintain the rotational balance of the squirrel cage in use.

11. The squirrel cage of claim 9 and 10, wherein the balancing feature is a cutaway or machined away portion of the axially outer face.

12. An electric machine comprising the squirrel cage of any of claims 1 to 11 and a rotor core.

13. The electric machine of claim 12, wherein the electric machine is intended to be operated at animpulse voltage Vi and above an altitude of h metres above sea level, wherein the electric machine further comprises an uninsulated electrical component, the electrical component being located at or greater than a minimum clearance distance Imin from the radially outer wall;the minimum clearance distance Imin being:= feFwherein:kd is a dimensionless altitude correction factor when h is in metres, calculated by:= 2 - 3 * 4 MW® ♦ ; andP is a pollution degree in mm calculated by: F = &W1 — 8^^638.

14. The electric machine of claim 12 or 13, wherein the electric machine is a 400 V electric drive unit or an 800 V electric drive unit.

15. A vehicle comprising a squirrel cage of any of claims 1 to 9 or an electric machine according to any of claims 12 to 14.

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