GB2506922A

GB2506922A - Magnetic core location in a rotor assembly

Info

Publication number: GB2506922A
Application number: GB201218410A
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: GB201218410D0

Abstract

A rotor assembly comprises a rotor shaft 112 having a screw thread 44 on its outer surface, a locating shoulder 30 to locate a magnetic core 14 carried on the rotor shaft and a removable locking assembly 36. Removable locking assembly 36 comprises a retaining nut 36 received on the rotor shaft screw thread to hold the core against the shoulder by an axial retention force acting against the locating shoulder. Rotation of the nut with respect to the shaft can be prevented using annular washer 40 between the nut and core, and having a tooth (fig 7a, 52) in its inner diameter that is engaged by a grub screw in an aperture (fig 6a, 48) of the nut 36 to prevent the nut loosening. The retaining force can act through end caps 34,35 at opposite ends of the core. The locking arrangement can be used in a dynamo electric machine providing variable flux and is suitable for cores of different sizes such as may be used in test and development.

Description

Rotor Assembly

Field of the Invention

The invention relates to a rotor assembly for a dynamo-electric machine, and in particular, but not exclusively, to a rotor assembly for use in a mechanically actuated variable flux machine for an electric vehicle. The invention also relates to a locking assembly for the rotor assembly which serves to fix a magnetic core to a rotor shaft of the assembly.

Background to the Invention

Due to the increasing demands on fossil fuel resources, there is a drive towards the development of electric vehicles that offer a viable alternative for consumers to the vehicles powered by combustion engines that currently dominate the market.

In order to make electric vehicles more attractive to consumers, they need to meet a number of standards in terms of their performance and practicality. In particular, one of the key criteria for an electric vehicle to satisfy is that it must perform very efficiently. If they do not fulfil this requirement, there is little point in pursuing their development, as efficiency is the primary advantage that an electric vehicle should be able to offer over a traditional vehicle powered by fossil fuels.

As a part of a project to create ultra-efficient machines and drives for electric vehicles, the mechanically actuated variable flux machine was developed. This machine has a central rotor, around which a magnetic core is disposed. The core is arranged as a number of progressively larger annular sections which sit inside one another with small air gaps in between. In normal operation the magnetic flux is able to cross these air gaps and so propagates fully through the core. However, at either end of the core some metal plates are positioned such that they can be brought into contact with some of the inner core elements, and thus shod circuit some of the flux. When these plates are moved into position to create a magnetic short circuit, the strength of the flux that permeates through the core is reduced to about 85% of its normal operating strength. In this way the flux is said to be variable. By reducing the flux in this way, the current consumption is also reduced, especially under conditions of low torque and high speed. Therefore, as the current is reduced, the efficiency of the machine is improved.

During the testing of the core designs, the rotor core needs to be removed and replaced on a regular basis. Traditional methods for attaching the core to the rotor do not allow for regular removal and replacement, instead they provide a relatively permanent fixture. Therefore a need has been identified to develop a locking mechanism to assist in the testing of different rotor cores that is able to lock the core onto the rotor with sufficient force to retain its position at high running speeds, while also being practical to remove and replace.

It is one objective of the invention to address this need and to overcome the problems associated with the known methods of fixing the core to the rotor.

Summary of the Invention

According to a first aspect of the invention, there is provided a rotor assembly comprising: a rotor shaft having a screw thread on its outer surface; a magnetic core carried on the rotor shaft; locating means for locating the magnetic core on the rotor shaft; and a removable locking assembly for fixing the magnetic core to the rotor shaft; wherein the removable locking assembly comprises a retaining means received on the rotor shaft in screw threaded connection with the outer surface thereof, to hold the core against said locating means by means of an axial retention force which acts against the locating means.

A threaded connection such as this provides the technical benefit that an infinite range of adjustment is possible. This means that the rotor assembly can be used with a range of different types of core, of varying lengths. This also means that the axial retention force is directly adjustable, which removes any requirement to stress the core prior to locking it on to the rotor shaft. The removable locking assembly can be attached and removed repeatedly without causing damage to any components of the dynamo-electric machine. This is particularly useful for testing and development of the dynamo-electric machine, when different types of cores will need to be tested and therefore the ability to be able to swap cores easily and without damage to the other components is highly desirable.

The rotor assembly may further comprise means for preventing rotation of the retaining means with respect to the rotor shaft. This will provide the benefit that the retaining means will not loosen when the rotor shaft rotates.

The removable locking assembly may further comprise a fixing means that is arranged to cooperate with the means for preventing rotation of the retaining means, to prevent the retaining means from loosening when the rotor shaft is rotating.

The retaining means may comprise at least one aperture which is arranged to accept the fixing means. The fixing means may include a grub screw, which beneficially can be removed without causing any damage to the surrounding components.

The rotor assembly may further comprise an annular member disposed between the core and the retaining means. The annular member may take the form of a washer. The annular member may, for example, comprise the means for preventing rotation of the retaining means with respect to the rotor shaft. The means for preventing rotation of the retaining means may take the form of at least one tooth provided on an intemal diameter of the washer. Therefore, in a preferred embodiment, the or each grub screw is arranged to cooperate with the teeth formed on the washer in order to prevent the retaining means from loosening when the rotor shaft rotates. The locating means on the rotor shaft may take the form of a shoulder. A shoulder is mechanically simple and requires no assembly.

The rotor assembly may further comprise at least one end cap provided on the rotor and wherein the shoulder abuts against one of the end caps so that the axial retention force acts through said one of the end caps. This end cap may prevent the core from becoming damaged.

The rotor assembly may comprise a first end cap and a second end cap, each being provided at an opposite end of the core. In this embodiment, both ends of the core are protected from damage. The retaining means may take the form of a nut, which will allow for the retaining means to be fitted to the rotor shaft and tightened using conventional tools and methods. The rotor assembly may be arranged to accept magnetic cores of variable length in that the retaining means can tighten up against a core of any length that falls within the range set by the thread that is applied to the rotor. That is, if, when the core is pressed up against the shoulder of the rotor shaft, the other end of the core is located in the threaded section of the rotor, the locking assembly will be able to lock the core in place. It will be appreciated that the thread applied to the rotor shaft may be male, in which case the corresponding retaining means thread is female, but equally the threads may be of the opposite orientation. The threads are likely to be around M50 due to the typical dimensions of the rotor.

According to a second aspect of the invention there is provided a locking assemblyfor fixing a magnetic core to a rotor shaft, the locking assembly comprising a retaining means received on the rotor shaft in screw threaded connection with the outer surface thereof, to hold the core against said locating means by means of an axial retention force. The locking assembly may further comprise an annular member disposed between the core and the retaining means. The annular member may comprise the means for preventing rotation of the retaining means with respect to the rotor shaft. The means for preventing rotation of the retaining means may take the form of at least one tooth provided on the internal diameter if the washer.

The retaining means may comprise at least one small hole, arranged to accept a fixing means. The fixing means may be arranged to cooperate with the means for preventing rotation of the retaining means, to prevent the retaining means from loosening when the rotor shaft is spinning.

It will be appreciated that preferred and/or optional features of the first aspect of the invention may be incorporated alone or in appropriate combination within the second aspect 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 energy into electrical energy. However, it will be appreciated that this is intended to encompass electric machines in general and including those which are operable as motors for converting electrical energy into mechanical energy.

Brief Description of the Drawings

In order that the invention may be more readily understood, preferred non-limiting embodiments thereof will now be described with reference to the accompanying drawings, in which: Figure 1 shows an example of a mechanically actuated vaiiable flux machine to which a first embodiment of a rotor locking assembly is to be fitted, with the machine in an open state; Figure 2 shows the mechanically actuated variable flux machine in Figure 1 in a closed state; Figure 3 shows a first example of a known means of locking a rotor core to a rotor shaft; Figure 4 shows a second example of a known means of locking the rotor core to the rotor shaft; Figure 5 shows the rotor locking assembly of the first embodiment of the invention, when fitted to a rotor shaft, for use in the variable flux machine in Figures 1 and 2; Figure 6a shows a top view of a nut forming part of the rotor locking mechanism in Figure 5; Figure Sb shows a section view through the line Z-Z of the nut in Figure 6a; Figure 6c shows a bottom view of the nut in Figures 6a and 6b; Figure 7a shows a washer belonging to the rotor locking assembly in Figure 5; Figure 7b shows a section view through the line Z-Z of the washer in Figure 7a; and Figure 8 shows an exploded view of the rotor and rotor locking assembly in Figure 5.

Detailed Description of the Embodiments of the Invention With reference to Figures 1 and 2, a dynamo-electric machine in the form of a variable flux machine 10, to which an embodiment of the invention is to be fitted, comprises a rotor shaft 12, and a core 14 arranged as a plurality of progressively larger circular elements of magnetic material, disposed annularly around the rotor shaft 12, with air gaps 16 defined therebetween, such that magnetic flux 18 must permeate across the air gaps 16 in order to traverse the core 14. The variable flux machine 10 further comprises two short-circuit plates 20, one at each end of the core 14, which are controlled by an actuator 22 to be held in either an open state or a closed state. Figure 1 shows the actuator 22 in the open state, in which the short circuit plates 20 are not magnetically connected with the core 14, and Figure 2 shows the actuator 22 in the closed state, in which each short circuit plate 20 is magnetically connected to the core 14 through a connection plate 24. When the variable flux machine 10 is in the closed state, some of the magnetic flux 18 permeates through a short circuit path 26, and in this way the total magnetic flux 18 leaving the core 14 is reduced.

In order to optimise this arrangement, it is necessary to test the variable flux machine 10 with a number of different types of core 14. This means that there is a requirement for the core 14 to be removed from the rotor shaft 12 and replaced with an alternative core 14 with a differing topology, so that the performance of the variable flux machine 10 with the new core 14 can be compared with the performance of the variable flux machine 10 with the previous core 14.

Figure 3 illustrates a standard means of fixing the core 14 in place on the rotor shaft 12, in which the core 14 is pushed onto the rotor shaft 12 with locking end caps 28 placed at either end, which are a push fit or a shrink fit onto the rotor shaft 12. The core 14 and the locking end caps 28 are located through the inclusion of a shoulder 30 which provides a mechanical stop. In order to provide an axial retention force, the locking end caps 28 are pressed up against the core 14 towards the shoulder 30 prior to a shrinking technique being applied to them. This type of fixture is considered permanent, as it is very difficult to remove the locking end caps 28 once they have been fitted. The only way to remove them is to cut them off, which is time consuming, and also risks damage to the surrounding components.

Figure 4 shows an alternative locking means to that described above, in which the locking end caps 28 have been replaced with a grub screw 32. First and second protective end caps 34,35 are also included to protect the core 14. The grub screw 32 is much easier to remove than the end caps 28. However, as with the arrangement in Figure 3, an axial force must be applied to the core 14 prior to installing the grub screw 32 in order for the retention to be effective. In addition to this, the grub screw 32 is susceptible to loosening when the rotor shaft 12 is spinning at high speed due to the centripetal force that it experiences as a result of the rotation.

A first embodiment of the locking assembly 36 of the invention is shown in Figures 5 to 8 when assembled onto a rotor shaft 112. The locking assembly includes a retention nut 38, a washer 40, and a pair of locking grub screws 42. The magnetic core 14 is provided with first and second end caps 34, 35, as in the previous figures. The rotor shaft 112 is provided with a screw thread 44 so that the retention nut 38 is arranged to screw onto the rotor shaft 112, and to tighten up against the core 14, such that the retention nut 38 and the shoulder 30 define an axial retention force therebetween to hold the core 14 in place. The axial retention force may be defined as the reaction force at the contact surface between the shoulder 30 and the second end cap 35 of the core 14, and between the first end cap 34 of the core 14 and the retention nut 38. This reaction force is caused by tightening the retention nut 38 50 as to compress and induce stress within the core 14 which is exerted across the contact surface areas as a reaction force. The washer 40 is an annular member which is received on the rotor shaft 112 so that a first side thereof abuts the second protective end cap 35 of the core 14. The retention nut is received onto the rotor shaft 112 to butt against a second side of the washer 40. As can be seen in Figures 7a and 7b, a central opening 50 of the washer is provided with opposed projections in the form of keyway teeth 52 which extend radially inwards from an internal surface 54 (i.e. the internal diameter) of the opening 50 of the washer 40. Because the first protective end cap of the core 14 abuts the shoulder, the axial retention force acts through the first protective end cap 34. In a similar way, the axial retention force acts through the second protective end cap 35.

The retention nut 38 is provided with small openings or apertures 48 which extend axially through the retention nut 38 and within each of which a respective one of the grub screws 42 are received. The grub screws 42 are arranged to protrude through the retention nut 38 to cooperate with the keyway teeth 52 of the washer 40 in order to ensure that the retention nut 38 is locked in place and cannot loosen as the rotor shaft 112 spins. The keyway teeth 52 engage with the grub screw 42 as the washer 40 rotates relative to the retention nut 38, thereby preventing further rotation of the washer 40 relative to the retention nut 38 and therefore locking the retention nut 38 to the shaft and preventing it from loosening.

Figure 8 illustrates the assembly process of the rotor locking assembly 36. The first protective end cap 34 is pushed onto the rotor shaft 112 until it reaches the shoulder 30. The core 14 is then received on the rotor shaft 112, to abut the first protective end cap 34, and then the second protective end cap 35 is pushed onto the rotor shaft 112 to abut the core 14. After this the washer 40 is pushed onto the rotor shaft 112 to meet the second protective end cap 35, and the retention nut 38 is screwed onto the thread 44 of the rotor shaft 112, and tightened against the washer 40. In this way the retention nut 38 and the shoulder 30 create the axial retention force which holds the washer 40, the core 14 and the first and second protective end caps 34, 35 in compression, and prevents the core 14 from coming off the rotor shaft 112 when it rotates. Finally, the locking grub screws 42 are screwed into the small holes 48 in the retention nut 38 to cooperate with the keyway teeth 52 in the washer 40, thus locking the retention nut 38 in place. To remove the core 14 from the rotor shaft 112, this process is simply reversed, and hence the removal process does not risk damage to any of the components and can therefore be repeated many times.

It will be appreciated that a threaded system such as this allows for infinite adjustment, which means that the invention can be used to secure different cores 14 having a range of lengths to the rotor shaft 112, which is a particularly useful feature of the invention. So long as the length of the core 14 does not cause it to go beyond the end of the threaded section of the rotor shaft 112, this locking assembly can be used to secure the core 14 to the rotor shaft 112. Therefore this locking assembly is suitable for use with a range of different cores 14 of varying shapes and sizes, which is an advantage for testing and development.

Furthermore the threaded system allows for as much tightening as is required, meaning that a high axial force can be provided without the need to apply a force to the core 14 prior to installing the locking assembly.

It will be appreciated by a person skilled in the art that the invention could be modified to take many alternative forms to that described herein, without departing from the scope of the appended claims.

Claims

Claims 1. A rotor assembly comprising: a rotor shaft having a screw thread on its outer surface; a magnetic core carried on the rotor shaft; locating means for locating the magnetic core on the rotor shaft; and a removable locking assembly for fixing the magnetic core to the rotor shaft; wherein the removable locking assembly comprises a retaining means received on the rotor shaft in screw threaded connection with the outer surface thereof, to hold the core against said locating means by means of an axial retention force which acts against the locating means.
2. A rotor assembly according to Claim 1, further comprising means for preventing rotation of the retaining means with respect to the rotor shaft.
3. A rotor assembly according to Claim 2, wherein the removable locking assembly further comprises a fixing means that is arranged to cooperate with the means for preventing rotation of the retaining means, to prevent the retaining means from loosening when the rotor shaft is rotating.
4. A rotor assembly according to Claim 3, wherein the retaining means comprises at least one aperture which is arranged to accept the fixing means.
5. A rotor assembly according to Claim 3 or Claim 4, wherein the fixing means includes a grub screw.
6. A rotor assembly according to any one of Claims 2 to 5, wherein the removable locking assembly further comprises an annular member disposed between the magnetic core and the retaining means.
7. A rotor assembly according to Claim 6, wherein the annular member takes the form of a washer.
8. A rotor assembly according to Claim 6 or Claim 7, wherein the annular member comprises the means for preventing rotation of the retaining means.
9. A rotor assembly according to Claim 8, wherein the means for preventing rotation of the retaining means takes the form of at least one tooth provided on an internal diameter of the annular member.
10. A rotor assembly according to any one of Claims 1 to 9, wherein the locating means on the rotor shaft takes the form of a shoulder.
11. A rotor assembly according to Claim 10, further comprising at least one end cap provided on the rotor and wherein the shoulder abuts against one of the end caps so that the axial retention force acts through said one of the end caps.
12. A rotor assembly according to Claim 11, wherein the rotor assembly comprises a first end cap and a second end cap, each being provided at an opposite end of the core.
13. A rotor assembly according to any one of Claims ito 12, wherein the retaining means takes the form of a nut.
14. A dynamo-electric machine comprising a rotor assembly according to any one of Claims ito 13.
15. A dynamo-electric machine according to Claim 14, wherein the dynamo-electric machine is arranged to provide variable magnetic flux.
16. A rotor assembly substantially as herein described with reference to the accompanying Figures 1 and 2 and 5 to 7.
17. A dynamo-electric machine substantially as herein described with reference to the accompanying Figures 1 and 2 and 5 to 7.