GB2506932A

GB2506932A - Laminated rotor assembly

Info

Publication number: GB2506932A
Application number: GB201218450A
Authority: GB
Inventors: Iain Urquhart
Original assignee: Nissan Motor Manufacturing UK Ltd
Current assignee: Nissan Motor Manufacturing UK Ltd
Priority date: 2012-10-15
Filing date: 2012-10-15
Publication date: 2014-04-16
Also published as: GB201218450D0

Abstract

A rotor assembly comprises a rotor core 16 that is rotatable about a rotation axis, the rotor core comprising a stack of laminations 22, each lamination extending perpendicular to the rotation axis, the stack being arranged to receive one or more adhesive coated permanent magnets 18 and comprising a central bore 54 extending in a direction substantially parallel to the rotation axis, wherein the bore defines an inner surface 52 of the rotor core, and neighbouring laminations in the stack are joined at said inner surface by weld 58. As an alternative to a weld, a suitable element such as a caulking element may be used. End plates 24 having iron inserts 68 engage the outer laminations of the stack and moveable shunts 26 allow the radial field of the rotor to be diverted axially when applied against the end plates at high speed/low torque conditions of the electric machine so as to effect field weakening. A core assembly method is described.

Description

Rotor Assembly

Field of the Invention

The invention relates to a rotor assembly for a dynamo-electric machine, and a method for making the same. In particular, but not exclusively, the invention relates to a rotor assembly for use in a motor of an electric vehicle. The invention also extends to a dynamo-electric machine comprising the rotor assembly, and a method for making the same.

Background to the Invention

A dynamo-electric machine, such as an internal permanent magnet (1PM) machine, comprises an inner rotor core carrying a set of permanent magnets, and a surrounding stator carrying a set of stator coils. The rotor core is substantially cylindrical in shape, with a rotation shaft located at the rotation axis of the cylinder. The stack includes slots that receive permanent magnets, such that the magnets extend longitudinally along the length of the cylinder. The stator is shaped as an annular ring that is coaxial with the rotor core, and is arranged such that an outer wall of the rotor core faces an inner wall of the stator, the walls being separated by a small air gap.

In use, for example as a motor, the stator coils are energised sequentially to produce a rotating magnetic field, which effects rotation of the rotor.

The rotor core is made from a stack of steel sheets that are known in the art as laminations! The laminations extend in a plane that is generally orthogonal to the rotation axis, and confer anisotropic magnetic properties on the rotor core. Typically, the laminations are joined by an adhesive layer between neighbouring laminations, resulting in undesirable gaps between the stacked laminations. These gaps increase the reluctance in the axial direction (i.e. the direction parallel to the rotation axis), and therefore reduce the magnetic flux in the axial direction. Under normal operation of the machine, it is desirable to minimise flux in the axial direction in this way because axial flux leads to eddy currents, which cause magnetic losses and reduce the efficiency of the machine. Minimising the axial flux therefore increases the efficiency of the machine.

However, in some situations it is desirable to reduce the radial flux, by diverting a portion of the radial flux in the axial direction.

As the rotor is rotated, the permanent magnets induce an electro-motive force (known in the art as a back EMF"), which in turn induces an opposing voltage in the stator coils. As the rotor speeds up, this back EMF increases, thereby increasing the induced voltage in the statol coils.

This induced voltage must be minimised to avoid damage to the power supply, and to reduce current consumption of the machine. To minimise the induced voltage, the back EME induced by the permanent magnets must be reduced. This is particularly important under low torque, high speed conditions, where the resultant back EMF can exceed the supply voltage, potentially damaging the power supply. Thus, particularly undei low torque and high speed conditions, it is desirable to reduce the amount of radial flux.

One way to reduce the back EMF is to use a control technique known as flux weakening', in which a contiol cuilent is applied to suppiess the induced voltage allowing the machine to operate at higher speeds. However, this control current does not contribute to the generation of torque, resulting in high current consumption and reduced efficiency at high speeds. In addition, if too much flux weakening current is applied there is a high iisk of demagnetisation within the machine.

An alternative way to reduce the back EMF is to reduce the net flux across the air gap between the rotol and the stator. This can be achieved, for example, by applying short-ciicuit' plates to the ends of the rotor. When the plates contact the ends of the rotor, they provide a flux path of low reluctance at the ends of the rotor, and a proportion of the radial flux is rediiected through this low-reluctance path in the axial diiection. This rediiection increases the axial flux and reduces the radial tlux.

During use of the machine, the short circuit plates can be selectively applied to the rotor core, foi example by a mechanical actuating means. In this way, when it is most crucial to minimise the back EME, such as under low torque, high speed conditions, the shod circuit plates can be applied to the rotor core to increase the axial flux and reduce the radial flux. By contrast, when minimising the back EME is less crucial, the short circuit plates can be removed from the rotor core, so as to minimise the axial flux and maximise the radial flux, thereby increasing the torque output of the machine. The effect is similai to that obtained by flux weakening, although this alternative method significantly reduces the risk of demagnetisation and reduces the current consumption at high speed, therefore increasing the efficiency of the machine.

A typical method of making the rotor core described above is illustrated in Figures 1A to 1 E. First, the laminations are coated with adhesive and stacked in a mandrel about the rotation shaft (Figures 1A and 1 B). Next, the stack of laminations is clamped (Figure 10), and the permanent magnets are coated with adhesive and inserted into slots in the stack (Figure 10).

Finally, the stack and magnets are heated in an oven to cure the adhesive (Figure 1 F).

As a result of this manufacturing method, in the assembled rotor core, adhesive layers are interposed between adjacent laminations. These adhesive layers provide effective bonding between the laminations, but create magnetically insulating layers between the laminations that interrupt the axial flux path and increase the axial reluctance. Although this increased axial reluctance is helpful in minimising undesirable axial flux during normal operation of the machine, in situations where it is desirable to reduce the back EMF by diverting the radial flux in the axial direction, the increased axial reluctance is problematic, as it reduces the degree to which the radial flux can be diverted. Therefore, the insulating adhesive layers limit the achievable reduction in the radial flux and hence the extent to which the back EMF can be reduced.

The relative size of the gaps between the laminations in the rotor core is measured using a parameter known in the art as the stack factor'. The method of determining the stack factor of a rotor core is illustrated in Figure 2. The stack factor is the total active length' (i.e. the sum of the thicknesses of each lamination in the rotor core), divided by the total length of the rotor core (i.e. the end-to-end length of the rotor core).

Thus, a rotor core having no gaps at all between the laminations has a stack factor of 1. As the relative thickness of the gaps between the laminations increases, the stack factor decreases.

For example, a rotor core having a gap between each pair of neighbouring laminations that is of equal magnitude to the thickness of the laminations would have a stack factor of approximately 0.5.

The interposed adhesive layers that result from the method described above limit the stack factor to values below 0.95. This limit sets a minimum value of the reluctance in the axial direction, which limits the extent to which the radial flux can be redirected, thereby limiting the amount by which the back EMF can be reduced. This is undesirable because it prevents the machine from achieving its optimum performance.

As shown in Figure 3, and as also described in further detail in the Applicant's patent application WO 2012/042844, to mitigate the effect of the limited stack factor, the reluctance in the axial direction can be further reduced by inserting rods of an isotropic material, such as a soft magnetic composite (SMC), into opposed ends of the rotol, such that the rods am exposed at their respective rotor ends, and extend a fixed distance into the rotor in the axial direction. The short-circuit plates are then selectively applied to the exposed ends of the rods to divert the radial flux in the axial direction as previously described. Since the anisotropic SMO rods are aligned in the axial direction, the SMC rods provide a path of even lower reluctance in the axial diiection, theieby rediiecting a greater proportion of the flux from the radial direction to the axial direction, and hence causing a greater reduction in the radial flux.

Whilst effective in reducing the amount of radial flux to acceptable levels, thereby reducing the current consumption of the machine, the incoipolation of the SMC rods into the rotor core results in a complex manufacturing process that is time-consuming and costly. Firstly, the materials and manufacture of the SMC rods add to the time and cost of the manufacturing process. Secondly, the extension of the SMC rods only paitially into the rotor core means that two different types of laminations must be machined: the laminations closest to the ends of the rotor core that include slots for the SMC rods, and those at the centre of the rotor core that do not include these slots.

Thus, although the use of SMC rods reduces current loss during operation of the dynamo-electric machine, it increases the complexity, and therefore the cost, of the manufacturing process.

It is one objective of the present invention to overcome or mitigate the above-mentioned problems.

Statements of the Invention

Against this background, according to a first aspect, the invention resides in a rotor assembly for a dynamo-electric machine, the rotor assembly comprising a rotor core that is rotatable about a rotation axis, and the rotor coie comprising a stack of laminations. Each lamination extends perpendicular to the rotation axis. The stack is arranged to receive one or more permanent magnets and comprises a central bore extending in a direction substantially paiallel to the lotation axis. The boie defines an innel surface of the iotoi core, and neighbouring laminations in the stack are joined at the inner surface.

The invention provides a rotor assembly in which the laminations are joined at the inner surface of the rotor core, thereby avoiding the need for joining means such as adhesive layers between laminations in the stack. This reduces the stack factor of the rotor core, such that, when the rotor assembly is incorporated into a dynamo-electric machine with a stator assembly, radial magnetic flux interactions between the rotor assembly and a stator can be more readily reduced by means of magnetic shunts, and complex mechanisms such as SMC rods can be avoided, thereby reducing cost and increasing efficiency of the rotor assembly.

Preferably, the neighbouring laminations are joined by a joining element located on the inner surface. In this way, all laminations can be easily joined on the inner surface.

To ease manufacture of the rotor assembly further, a plurality of neighbouring laminations in the rotor core may each comprise a central opening, the central openings being aligned so as to define the central bore. Preferably, each central opening defines an inner surface of its respective lamination, and the inner surfaces of the laminations are aligned to provide the inner surface of the rotor core.

In this embodiment, to effect joining of neighbouring laminations, an inner surface of a first lamination may be joined to an inner surface of a neighbouring lamination. For example, the inner surface of the first lamination may be joined to the inner surface of the neighbouring lamination by a joining element, which may be a weld.

Preferably, the rotor core has a stack factor that is greater than 0.95. More preferably, the rotor core has a stack factor that is greater than 0.98, and most preferably the rotor core has a stack factor that is greater than 0.99.

In a preferred embodiment of the rotor assembly, no adhesive is located between the laminations in the stack.

The rotor may comprise a magnetic shunt moveable between an active position, in which the shunt is arranged to shunt the magnetic flux of the or each permanent magnet, and an inactive position, in which the shunt is arranged to avoid shunting the magnetic flux of the at least one permanent magnet. In such an embodiment, when arranged in the active position, the shunt preferably reduces the flux in a radial direction orthogonal to the rotation axis, and increases the flux in an axial direction parallel to the rotation axis.

The rotor may further comprise an end plate comprising a magnetic insert that is exposed at inner and outer surfaces of the end plate, the inner surface of the end plate being arranged such that the magnetic insert is in contact with a first end of the stack. In such an embodiment, when in the active position the shunt is arranged so as to contact the magnetic insert of the end plate, and/or when in the inactive position, the shunt is arranged to avoid contact with the magnetic insert of the end plate.

Preferably, the rotor core comprises at least one slot for receiving the at least one permanent magnet.

According to a second aspect, the invention also extends to dynamo-electric machine comprising a rotor assembly according to any embodiment described above, and a stator assembly comprising one or more stator coils, wherein the rotor core of the rotor assembly houses the or each permanent magnet, Preferably, the stator assembly surrounds the rotor assembly. More preferably, the stator assembly and the rotor are arranged such that an outer face of the rotor faces an inner face of the stator assembly, and an air-gap is defined between the faces.

The invention also resides in a method of making a rotor assembly for a dynamo-electric machine, the method comprising: (a) arranging a plurality of laminations in a stack to form a rotor core; (b) defining a central bore in the rotor core, the bore providing an inner surface of the rotor core; (c) joining neighbouring laminations in the stack at the inner surface of the rotor core; and (d) integrating one or more permanent magnets with the rotor core.

The method of the invention provides a method of making a rotor assembly that avoids the need for joining means such as adhesive layers between laminations in the stack. This reduces the stack factor of the rotor core, such that, when the rotor assembly is incorporated into a dynamo-electric machine with a stator assembly, radial magnetic flux interactions between the rotor assembly and the stator assembly can be more readily reduced by means of magnetic shunts. Complex structural features such as SMC rods can be avoided, thereby reducing the cost of the method, and the time required to manufacture the rotor assembly.

Preferably, step (c) comprises providing a joining element on the inner surface that joins neighbouring laminations.

To ease manufacture of the rotor assembly further, a plurality of laminations in the rotor core each may comprise a central opening, and step (b) may comprise aligning the central openings so as to define the central bore. In such an embodiment of the method, each central opening may provide an inner surface of its respective lamination, and the method may comprise aligning the inner surfaces of neighbouring laminations to provide the inner surface of the rotor core.

Preferably, the method comprises joining an inner surface of a first lamination to an inner surface of a neighbouring lamination. For example, step (c) may comprise welding the neighbouring laminations at the inner surface. Optionally, the welding may be selected from any one of the following: laser welding, arc welding. oxyfuel welding, ultrasonic welding, electron beam welding, or induction welding.

Preferably, the laminations are arranged such that the rotor core has a stack factor that is greater than 0.95. More preferably, the laminations are arranged such that the rotor core has a stack factor that is greater than 0.98, and most preferably the laminations are arranged such that the rotor core has a stack factor that is greater than 0.99.

In a preferred embodiment, the method comprises integrating a rotation shaft with the rotor core, the rotation shaft being located in the central bore.

Preferably, the method further comprises integrating end plates with the rotor core, the end plates being arranged at opposed ends of the rotor core, and comprising magnetic inserts.

Optionally, the method may further comprise arranging magnetic shunts at opposed ends of the rotor core, the shunts being movable in use in a direction parallel to the rotation axis between an active position and an inactive position.

In a third aspect, the invention also extends to a method of making a dynamo-electric machine, the method comprising: making a rotor assembly according to any method described above and arranging a stator assembly comprising one or more stator coils such that the stator assembly surrounds the rotor core of the rotor assembly.

Optionally, the method further comprises arranging the stator assembly and the rotor such that an outer face of the rotor faces an inner face of the stator assembly, and an air-gap is defined between the respective faces.

Preferred and/or optional features of the first and second aspects of the invention may be incorporated alone, or in appropriate combination, in any other aspects of the invention also.

For the purpose of the following description, the invention has been described as a dynamo-electric machine which is a machine suitable for converting mechanical eneigy into electrical energy. However, it is intended that this encompasses electric machines in general and including those which are operable as motors for converting electrical energy into mechanical eneigy.

Brief description of the drawings

In oider that the invention may be more readily undeistood, refeience will now be made, by way of example, to the accompanying drawings, in which: Figures 1A to 1 E illustrate a known method of making a rotor assembly for a dynamo-

electiic machine of the prior art;

Figure 2 illustrates a known method of determining the stack factor' of a rotor core; Figure 3 is a cross-sectional view of a dynamo-electric machine of the prior art; Figure 4A is a cross-sectional view of an dynamo-electric machine comprising a rotor assembly of an embodiment of the present invention, with the magnetic shunts in an active position; Figure 4B is a cross-sectional view of the dynamo-electric machine of Figure 4A, with the magnetic shunts in an inactive position; Figure 5 is a perspective view of a lamination of the rotor assembly of Figure 4A; Figure 6 is a perspective cross-sectional view of a stack of laminations of the type illustrated in Figure 5, the laminations being joined togethei; and Figures JA to 7E illustrate a method of making the rotor assembly of Figures 4A and 4B.

Detailed description of the embodiments of the invention Figures 4A and 4B show a dynamo-electric machine 10 comprising an inner rotor assembly 12 and an outer stator assembly 14. The dynamo-electric machine 10 is exemplified here as a motor for use in an electric vehicle, although it will be appreciated that the dynamo-electric machine 10 is not limited to use in electric vehicles, and could alternatively be a generator.

The rotor assembly 12 comprises a rotor core 16 that houses a plurality of permanent magnets 18. The rotor core 16 is substantially cylindrical in shape, and has a central rotation axis. A rotation shaft 20 extending parallel to the rotation axis is located at the centre of the rotor core 16.

The rotor core 16 comprises a stack of substantially flat sheets 22 that are known in the art as laminations. These laminations 22 extend in a plane that is generally orthogonal to the rotation axis, and are stacked one on top of another in a stacking direction that is substantially parallel to the rotation axis. The rotor assembly 12 also comprises end plates 24 located at opposed ends of the rotor core 16, the end plates 24 also extending in a plane perpendicular to the rotation axis, and magnetic shunts 26, also located at opposed ends of the rotor core 16, the shunts 26 being moveable by an actuating means (not shown) towards and away from the rotor core 16 in an axial direction.

The stator assembly 14 is shaped as an annular ring that is coaxial with the rotor core 16, and carries a set of stator coils. The stator assembly 14 is airanged such that an outer wall 28 of the rotor core 16 faces an inner wall 30 of the stator assembly 14. The outer wall 28 of the rotor assembly 12 and the inner wall 30 of the stator assembly 14 are separated by an air gap.

In use as a motor, the stator coils are energised sequentially to produce a rotating magnetic field. This rotating magnetic field attracts and repels the permanent magnets 18 in the rotor core 16, causing them to move, and effecting rotation of the rotor assembly 12.

The rotor core 16 is formed from a plurality of substantially flat, circular laminations 22 that may be made from any suitable material, such as steel. The laminations 22 are fabricated by machining, for example by stamping or cutting a sheet.

Figure 5 shows one of the laminations 22 of the rotor core 16. The lamination 22 comprises opposed upper and lower surfaces 32, 34 that are circular and lie substantially orthogonal to the rotation axis of the rotor assembly 12. The spacing between the upper and lower surfaces 32, 34 defines the thickness of the lamination 22. An outer surface 36 of the lamination 22 extends between outermost edges 38, 40 of the upper and lower surfaces 32, 34. The outer surface 36 follows the circular shape of the upper and lower surfaces 32, 34.

In the assembled rotor core 16, shown in Figure 6, the laminations 22 are stacked one on top of another in the axial direction, such that the upper surface 32 of one lamination 22 contacts a lower surface 34 of a neighbouring lamination 22. The upper surface 32 of the uppermost lamination 22 in the stack defines an upper surface 42 of the rotor core 16, and, conversely, the lower surface 34 of the lowermost lamination 22 in the stack defines a lower surface 44 of the rotor core 16. The spacing between the upper and lower surfaces 42, 44 of the rotor core 16 defines the length of the rotor core 16. Together, the outer surfaces 36 of the laminations 22 define the outer surface 46 of the rotor core 16 that extends between the upper and lower surfaces 32, 34 of the rotor core 16.

Each lamination 22 comprises a plurality of openings 48, 60 that receive additional components of the rotor assembly 12. In the assembled rotor core 16, the laminations 22 are angularly arranged in a stack such that the openings 48, 60 in the laminations 22 are aligned.

A central opening 48 is arranged at the centre of each lamination 22, such that the rotation axis runs through the central opening 48. The central opening 48 is circular and extends throughout the thickness of the lamination 22. In this way, the central opening 48 provides an inner edge 50 on each of the upper and lower surfaces 32. 34 of the lamination 22, and an inner surface 52 of the lamination 22 that extends between these inner edges 50.

In the assembled rotor core 16, the circular central openings 48 of each of the laminations 22 in the stack are aligned, and together define a central bore 54 in the rotor core 16. In use, the central bore 54 receives the rotation shaft 20. The central bore 54 is of cylindrical shape and has a circular cross-section in a direction substantially orthogonal to the rotation axis. The central bore 54 extends axially (i.e. parallel to the rotation axis) through the entire length of the rotor core 16, such that the central bore 54 is open at the upper and lower surfaces 42, 44 of the rotor core 16. The central bore 54 confers an inner surface 56 on the rotor core 16. This inner surface 56 of the rotor core 16 is formed by the inner surfaces 52 of each of the central openings 48 of the respective laminations 22.

As best seen in Figure 6, neighbouring laminations 22 in the rotor core 16 are joined to one another at the inner surface 56 of the rotor core 16. Specifically, the inner surface 52 of each lamination 22 is joined to the inner surface 52 of a neighbouring lamination 22 by a joining element 58. The joining element 58 is provided as a weld on the inner surface 56 of the rotor core 16 that bridges the inner surfaces 52 of the neighbouring laminations 22.

In this way, there is no joining element, such as an adhesive layer, arranged between neighbouring laminations 22 in the rotor core 16. This is in contrast with known rotor cores, in which an adhesive layer is provided between neighbouring laminations. Because there is no joining element between the neighbouring laminations 22, the laminations 22 can be arranged more closely together, such that the gap between the laminations 22 is reduced.

In comparison with a rotor assembly of the prior art having laminations of the same thickness, in the rotor assembly 16 of the invention the gaps between the laminations 22 are decreased, which means that the end-to-end length (i.e. the distance between the upper and lower surfaces 42, 44) of the rotor core 16 decreases. Thus, the active length' of the rotor core 16 is the same, while the total length of the rotor core 16 is reduced, thereby leading to a higher stack factor. While rotor cores of the prior art typically have stack factors less than 0.95, the rotor core 16 of a rotor assembly 12 in accordance with the invention may have a stack factor greater than 0.95, and can even have a stack factor greater than 0.99.

Each lamination 22 also comprises further openings 60 that are arranged uniformly around the central opening 48, between the inner and outer edges 38, 40, 50 of the upper and lower surfaces 44, 46. These further openings 60 take the form of equi-angularly spaced slots and are substantially rectangular, extending throughout the thickness of the lamination 22.

In the same way that the central openings 48 of the stacked laminations 22 provide the central bore 54 of the rotor core 16, the further openings 60 of the stacked laminations 22 provide slots 62 in the rotor core 16. The slots 62 extend axially through the entire length of the rotor core 16 such that the slots 62 are open at the upper and lower surfaces 42. 44 of the rotor core 16. The slots 62 are of substantially cuboidal shape and have a rectangular cross-section in a direction substantially orthogonal to the rotation axis. In the assembled rotor assembly 12, each slot receives one of a plurality of permanent magnets 18.

The permanent magnets 18 may be made from any suitable permanently magnetic material such as ferrite, alnico or rare earth materials such as Neodymium or Samarium cobalt. The permanent magnets 18 are substantially cuboidal, having a rectangular cross-section of substantially the same dimensions as the rectangular cross-section of the slots 62. In this way, the permanent magnets 18 can be inserted into and housed within the slots 62. When arranged in the rotol core 16, the permanent magnets 18 extend axially along the entile length of the rotor core 16. Because the slots 62 are open at the upper and lower surfaces 42, 44 of the rotor core 16, and because the permanent magnets 18 extend along the entire length of the rotor coie 16, upper and lower surfaces 42, 44 of the permanent magnets 18 are exposed at, and lie substantially flush with, the upper and lower surfaces 42, 44 of the lotor core 16.

As best seen in Figures 4A and 4B, the rotor assembly 12 also comprises upper and lower end plates 24 arranged respectively at opposed upper and lower ends of the rotor corn 16.

Each end plate 24 is of circular shape and has a radius that is equal to or less than the radii of the laminations 22.

Consideiing only the upper end plate 24, the end plate 24 comprises inner and outel surfaces 64, 66. The inner surface 64 of the end plate 24 is arranged parallel to and in contact with the upper surface 42 of the rotor core 16. The spacing between the inner and outer surfaces 64, 66 of the end plate 24 defines its thickness. One oi moie magnetic inserts 68 aie provided in the end plate 24, the or each insert 68 being made from any suitable magnetic material, such as iron. The inserts 68 extend through the thickness of the end plate 24, and inner and outer surfaces 70, 72 of the inserts 68 are exposed at respective inner and outer surfaces 64, 66 of the end plate 24. In this way, the inner surface 70 of the insert 68 contacts the uppei surface 42 of the rotor core 16.

The rotor assembly 12 further comprises upper and lower magnetic shunts 26, provided at respective upper and lower ends of the rotor core 16. The magnetic shunts 26 are exemplified here as plates that are made from a magnetic mateiial such as iron. The plates 26 are circular and of substantially the same radius as the end plates 24.

The shunts 26 are mounted to the shaft 20 of the lotor assembly 16 by any suitable means and aie moveable in an axial direction between an active position and an inactive position by an actuating means (not shown). Suitable mounting means and actuating means are described, for example, in the Applicant's published patent application WO 20121042800.

Considering the upper shunt 26 only, when the shunt 26 is in the active position as shown in Figure 4A, an inner surface 74 of the shunt 26 is arranged in contact with the outer surface 66 of the end plate 24. Since the magnetic inserts 68 of the end plate 24 are exposed at the outer surface 66 of the end plate 24, when the shunt 26 is in the active position, the inner surface 74 of the shunt 26 is also in contact with the outer surfaces 72 of the magnetic inserts 68 of the end plate 24. The upper shunt 26 therefore bridges the magnetic inserts 24 of the upper end plate 24.

In the inactive position, the shunt 26 is arranged such that a clearance is provided between the inner surface 74 of the shunt 26 and the outer surface 66 of the end plate 24. In the inactive position, the shunt 26 does not contact the magnetic inserts 68 of the end plate 24 and therefore does not bridge the magnetic inserts 68 of the upper end plate 26.

In use as a motor, the stator assembly 14 is arranged to surround the rotor assembly 12 such that an air gap is defined between the outer surface 46 of the rotor core 16 and the inner surface 30 of the stator assembly 14. The coils of the stator assembly 14 are energised sequentially to effect rotation of the rotor core 16 about the rotation axis. This rotation causes a back EMF as previously described.

Under normal operating conditions, (i.e. when the back EMF is less problematic) the shunts 26 remain in the inactive position, as shown in Figure 4B. The majority of the flux in the rotor core 16 flows in the radial direction, across the air gap, with some flux flowing tangentially between the magnets 18 in the core. There is very little flux in the axial direction due, firstly, to the large stray field that would be associated with axial flux, and, secondly, to the anisotropic properties of the laminations 22 in the rotor core 16.

Under conditions where the back EME is more problematic, such as high speed, low torque conditions, the actuating means moves the magnetic shunt 26 to the active position, shown in Figure 4A. When the shunt 26 is in the active position, the shunt 26 and the magnetic inserts 68 of the end plate 24 form a low-reluctance path. This low reluctance path helps to shod circuit' the magnetic flux between the permanent magnets 18, and also reduces the stray field associated with the axial flux. Thus, a proportion of the radial flux is redirected in the axial direction. The axial flux therefore increases, while the radial flux decreases.

The anisotropic nature of the laminations 22 impedes the diversion of the flux in the rotor core 16 to some extent. However, due to the location of the joining elements 58 at the inner surface 56 of the rotor core 16, the rotor core 16 has a stack factor greater than 0.95. This high stack factor means that the rotor core 16 does not impede the axial flux to as great a degree as a rotor core of the prior art. Thus, when the shunt 26 is in the active position, the flux can be redirected in the axial direction to a greater extent, providing the required reduction in the radial flux, and thereby the optimum efficiency of the motor.

Furthermore, because the joining elements 58 are provided on the inner surface 56 of the rotor core 16, rather than the outer surface 46, the joining elements 58 themselves do not interfere with the radial flux path, since they are not arranged between the permanent magnets 18 and the stator coils. This means that the joining elements 58 do not affect the efficiency of the machine 10.

Figures 7A to 7E illustrate a method of making the rotor assembly 12 described above. First, the laminations 22 are formed by machining (not shown). Next, as shown in Figures 7A and 7B, the laminations 22 are arranged in a stack on a mandrel, such that the openings 48, 60 in the laminations 22 are aligned. The central shaft 20 is absent from the central bore 54 at this stage, so as to allow for access to the inner surface 52 of rotor core. Once the stack has been formed, it is clamped as shown in Figure JC so as to hold the laminations 22 in place, and force them into close contact.

Next, the laminations 22 in the stack are joined by welding the inner surface 56 of the rotor core 16, as shown in Figure i'D. The welding may be any suitable type of welding, for example laser welding, arc welding, oxyfuel welding, ultrasonic welding, electron beam welding, induction welding or any other suitable welding technique. Laser welding is particularly suitable, as this method allows accurate welding in the confined space of the central bore 54.

The welds 58 are arranged such that the inner surface 52 of each lamination 22 is joined to the inner surface 52 of a neighbouring lamination 22 or laminations 22 in the stack. In this way, a joining element 58 is provided at the inner surface 56 ot the rotor core 16.

As shown in Figure YE, the permanent magnets 18 are then coated with an adhesive, and inserted into the slots 62 in the rotor core 16. The entire rotor core 16 is heated to cure the adhesive and secure the permanent magnets 18. The shaft 20 is inserted into the central bore 54 of the rotor core 16. the end plates 24 are arranged at opposed ends of the rotor core 16! and the actuating means and magnetic shunts 26 are mounted to the shaft 20.

To make a dynamo-electric machine 10, the rotor assembly 12 is arranged with a stator assembly 14, such that the stator assembly 14 surrounds the rotor assembly 12. The outer face 28 of the rotor assembly 12 faces the inner face 30 of the stator assembly 14, and an air gap is defined between them, the air gap being approximately 0.2 to 1.0 mm.

It should be appreciated that various modifications and improvements can be made without departing from the scope of the invention as defined in the claims. For example, the joining element need not be a weld, but may be any suitable element, such as a caulked element.

The laminations may each be provided with a central hole prior to stacking, or alternatively the laminations may be stacked, and the central holes of the laminations (and thereby the central bore of the rotor core) may then be provided by machining through the stacked laminations.

Claims

Claims: 1. A rotor assembly for a dynamo-electric machine, the rotor assembly comprising a rotor core that is rotatable about a rotation axis, the rotor core comprising a stack of laminations, each lamination extending perpendicular to the rotation axis, the stack being arranged to receive one oi more permanent magnets and comprising a cential bore extending in a direction substantially parallel to the rotation axis, wherein the bore defines an inner surface of the rotor core, and neighbouring laminations in the stack are joined at said inner surface.
2. The rotor assembly of claim 1, wheiein the neighbouring laminations are joined by a joining element located on the inner surface.
3. The rotor assembly of claim 1 or claim 2, wherein a pluiality of neighbouring laminations in the rotor core each comprise a central opening, the central openings being aligned so as to define the central bore.
4. The rotor assembly of claim 3, wherein each central opening defines an inner surface of its lespective lamination, and the innel surfaces of the laminations are aligned to provide the inner surface of the rotor core.
5. The rotor assembly of claim 4, wherein an inner surface of a first lamination is joined to an innel surface of a neighbouiing lamination.
6. The rotor assembly of claim 5, wherein the inner surface of the first lamination is joined to the inner surface of the neighbouring lamination by a joining element.
7. The rotor assembly of claim 2 or claim 6, wherein the joining element is a weld.
8. The rotor assembly of any of claims 1 to 7, wherein the lotol core has a stack factor that is greater than 0.95.
9. The rotor assembly of claim 8, wherein the rotor core has a stack factor that is greater than 0.98.
10. The rotor assembly of claim 9, wherein the rotor core has a stack factor that is greater than 0.99.
11. The rotor assembly of any of claims 1 to 10, wherein there is no adhesive located between the laminations in the stack.
12. The rotor assembly of any of claims 1 to 11, wherein the rotor comprises a magnetic shunt moveable between an active position, in which the shunt is arranged to shunt the magnetic flux of the or each permanent magnet, and an inactive position, in which the shunt is arranged to avoid shunting the magnetic tlux of the at least one permanent magnet.
13. The rotor assembly of claim 12, wherein, when arranged in the active position, the shunt reduces the flux in a radial direction orthogonal to the rotation axis, and increases the flux in an axial direction parallel to the rotation axis.
14. The rotor assembly of claim 12 or claim 13, wherein the rotor comprises an end plate comprising a magnetic insert that is exposed at inner and outer surfaces of the end plate, the inner surface of the end plate being arranged such that the magnetic insert is in contact with a first end of the stack.
15. The rotor assembly of claim 14, wherein, when in the active position, the shunt is arranged so as to contact the magnetic insert of the end plate.
16. The rotor assembly of claim 14 or claim 15, wherein, when in the inactive position, the shunt is arranged to avoid contact with the magnetic insert of the end plate.
17. The rotor assembly of any of claims 1 to 16, wherein the rotor core comprises at least one slot for receiving the at least one permanent magnet.
18. A dynamo-electric machine comprising a rotor assembly of any of claims ito 17, the rotor core of the rotor assembly housing the or each permanent magnet, and a stator assembly comprising one or more stator coils.
19. The dynamo-electric machine of claim 18, wherein the stator assembly surrounds the rotor core of the rotor assembly.
20. The dynamo-electric machine of claim 19, wherein the stator assembly and the rotor core of the rotor assembly are arranged such that an outer face of the rotor core faces an inner face of the stator assembly! and an air-gap is defined between the faces.
21. A method of making a rotor assembly for a dynamo-electric machine, the method comprising: (c) arranging a plurality of laminations in a stack to form a rotor core; (d) defining a central bore in the rotor core, the bore providing an inner surface of the rotor core; (c) joining neighbouring laminations in the stack at the inner surface of the rotor core; and (e) integrating one or more permanent magnets with the rotor core.
22. The method of claim 21, wherein step (c) comprises providing a joining element on the inner surface that joins neighbouring laminations.
23. The method of claim 21 or claim 22, wherein a plurality of laminations in the rotor core each comprise a central opening, and step (b) comprises aligning the central openings so as to define the central bore.
24. The method of claim 23, wherein each central opening provides an inner surface of its respective lamination, and the method comprises aligning the inner surfaces of neighbouring laminations to provide the inner surface of the rotor core.
25. The method of claim 24, wherein the method comprises joining an inner surface of a first lamination to an inner surface of a neighbouring lamination.
26. The method of any of claims 21 to 25, wherein step (c) comprises welding the neighbouring laminations at the inner surface.
27. The method of claim 26, wherein the welding is selected from any one of the following: laser welding, arc welding, oxyfuel welding, ultrasonic welding, electron beam welding, or induction welding.
28. The method of any of claims 21 to 27, wherein the laminations are arranged such that the stack has a stack factor that is greater than 0.95.
29. The method of claim 28, wherein the laminations are arranged such that the stack has a stack factor that is greater than 0.98.
30. The method of claim 29, wherein the laminations are arranged such that the stack has a stack factor that is greater than 0.99.
31. The method of any of claims 21 to 30, further comprising integrating a rotation shaft with the rotor core, the rotation shaft being located in the central bore.
32. The method of any of claims 21 to 31, further comprising integrating end plates with the rotor core, the end plates being arranged at opposed ends of the rotor core, and comprising magnetic inserts.
33. The method of any of claims 21 to 32, further comprising arranging magnetic shunts at opposed ends of the rotor core, the shunts being movable in use in a direction parallel to the rotation axis between an active position and an inactive position.
34. A method of making a dynamo-electric machine, the method comprising: making a rotor assembly according to the method of any of claims 21 to 33; arranging a stator assembly comprising one or more stator coils such that the stator assembly surrounds the rotor core of the rotor assembly.
35. The method of claim 34, turther comprising arranging the stator assembly and the rotor core such that an outer face of the rotor core faces an inner face of the stator assembly, and an air-gap is defined between the respective faces.
36. A method of making a rotor assembly for a dynamo-electric machine substantially as herein before described with reference to Figures 4 to 7.
37. A method of making a dynamo-electric machine substantially as herein betore described with reference to Figures 4 to 7.
38. A rotor assembly for a dynamo-electric machine substantially as herein before described with reference to Figures 4 to 7.
39. A dynamo-electric machine substantially as herein before described with reference to Figures4to7.