WO2011110857A2

WO2011110857A2 - Fault tolerant flux switching machine

Info

Publication number: WO2011110857A2
Application number: PCT/GB2011/050476
Authority: WO
Inventors: Tsarafidy Raminosa; Chris Gerada
Original assignee: The University Of Nottingham
Priority date: 2010-03-10
Filing date: 2011-03-10
Publication date: 2011-09-15
Also published as: CN103081318A; GB2480229A; GB201003954D0; WO2011110857A3

Abstract

A stator for a flux switching permanent magnet machine has multiple pole pieces with windings around each pole piece. Each pole piece and the windings associated with each pole piece are physically separated from the other pole pieces and the associated windings by a separator. The purpose of the separator is to induce for each pole piece a higher self inductance profile than the mutual inductance profile for the other pole pieces. The separator substantially isolates the adjacent poles pieces electrically from each other, and may be of ferromagnetic material.

Description

Fault tolerant flux switching machine

The present invention relates to permanent magnet machines, more particularly, but not exclusively, to a topology for improving the fault tolerance of a flux switching permanent magnet machine.

Flux switching permanent magnet machines are known in the field of electrical motors. Typically, these machines consist of a simple, brushless rotor arranged in a stator, wherein the magnets and windings are provided in the stator. An example of a known flux switching permanent magnet machine (FSPM) is shown in Figure 1. Such machines are favoured for their high power/torque density and efficiency. Additionally, the simple rotor structure is robust and cheap to manufacture.

However, flux switching machines of the kind shown in Figure 1 are not fault tolerant. For example, a short circuit in the windings of the stator will often result in failure of the motor. In particular, failure of an element of the motor may result in overrating of the motor and burnout of the component pieces. A cause of this problem is the high mutual inductance between the windings of each pole piece between phases, typically approaching -50% of the self inductance. This lack of fault tolerance may prevent the use of such machines in critical applications, such as in the fields of aerospace and defence. Such concerns are also prevalent in the hybrid/electric vehicle industry where the high currents caused by driving converter failures can damage the expensive motors.

Some specially designed Permanent Magnet Surface Machines (PMSM) allow for fault tolerance. However, the magnets are held in position using retention sleeves, which are expensive to manufacture, and can reduce the overall reliability of the machine and give rise to additional losses. Furthermore, they do not have many of the benefits associated with an FSPM.

It is an object of the invention to mitigate or overcome at least some of the above problems. According to a first aspect of the invention, there is provided a stator for a flux switching permanent magnet machine, the stator having a plurality of pole pieces with windings around each pole piece, wherein each pole piece and the windings associated with each pole piece are physically separated from the other pole pieces and their associated windings by a separator, for inducing for each pole piece a higher self inductance profile than the mutual inductance profile for the other pole pieces.

According to another aspect of the invention, there is provided a flux switching permanent magnet machine including a direct drive rotor, a stator having a plurality of pole pieces, each pole piece surrounded by windings, and a ferromagnetic separator adjacent to each pole piece and the associated windings, wherein the ferromagnetic separator is arranged for isolating each pole piece physically from the other pole pieces, thereby inducing for each pole piece a high self inductance profile when compared to the mutual inductance profile for the other pole pieces.

For each of the above aspects of the invention, the stator configuration provides an improvement in the fault tolerance of the flux switching machine. A further aspect of the invention provides a linear motor incorporating a stator or flux switching permanent magnet machine in accordance with either of the above aspects of the invention.

Other advantages, aspects and features of the invention will be apparent from the appended claims and the following description of preferred embodiments, made by way of example only, with reference to the accompanying drawing, in which:

Figure 1 is an example of the topology of a flux switching permanent magnet machine (FSPM) in the prior art;

Figure 2 is an example of a FSPM according to a preferred embodiment of the invention; Figure 2a is a magnified view of the separators of Figure 2;

Figure 3 is a plot of split ratio versus average torque for an FSPM of the kind shown in Figure 2;

Figure 4 is a plot of relative rotor pole arc versus average torque for an FSPM of the kind shown in Figure 2;

Figure 5 is a plot of relative magnet thickness versus average torque and relative torque ripple for an FSPM of the kind shown in Figure 2;

Figure 6 is a plot of slot separator thickness versus inductance and average torque for an FSPM of the kind shown in Figure 2; Figure 7 is a plot of gap between adjacent tooth shoes versus inductance and average torque for an FSPM of the kind shown in Figure 2;

Figure 8 is an example of a fault tolerant PMSM against which preferred embodiments of the invention have been benchmarked;

Figure 9 is a plot of the comparison of back EMF for both an FSPM of the kind shown in Figure 2 and the PMSM of Figure 8;

Figure 10 is a plot of torque and clogging for both an FSPM of the kind shown in Figure 2 and the PMSM of Figure 8 ;

Figure 11 is a plot of mutual and self-inductances for both an FSPM of the kind shown in Figure 2 and the PMSM of Figure 8;

Figure 12 is a plot of the comparison of torque for one channel open for both an FSPM of the kind shown in Figure 2 and the PMSM of Figure 8; Figure 13 is a plot of the comparison of torque for one channel shorted for an FSPM of the kind shown in Figure 2;

Figure 14 is a plot of the comparison of torque for one channel shorted for the PMSM of Figure 8;

Figure 15 is a further embodiment of a FSPM according to a preferred embodiment of the invention; and Figure 16 is a still further embodiment of a FSPM according to a preferred embodiment of the invention.

Referring firstly to Figure 1, an example of a FSPM in the prior art is indicated generally at 10 and includes a stator 12 with an outer edge 11 and an inner edge 13, and a rotor 14 mounted within the stator 12 on a shaft 16.

The stator 12 has six radial pole pieces 22. In each pole piece 22 is a permanent magnet 18 (arranged radially). Wound around each pole piece 22 are windings (not illustrated, but the general location is indicated at 20). The configuration, number and density of windings 20 around each pole piece 22 may be varied according to design constraints. Similarly, the number of poles pieces 22 and associated permanent magnets 18 may be varied.

The rotor 14 is a simple brushless design having seven teeth 24. The number of teeth 24 compared to number of pole pieces 22 may be changed according to design choice. Often the number of pole pieces 22 and teeth 24 differ to increase efficiency, e.g. by reducing cogging torque.

As can be seen, the magnets 18 and windings 20 are part of the stator 12. The pole pieces 22 extend to the inner edge 13 of the stator 12, but the stator is configured to provide an air gap 23 between the pole pieces 22 at the inner edge 13. In the embodiment shown, the air gap 23 defines is a substantial arc along the inner edge 13 of the stator 12, e.g. having a length which is generally the same as the width of the pole pieces 22.

Stator flux lines and mutual inductance flux lines are indicated at 26 and 28, respectively. The stator flux lines 26 show that the flux between poles 22 is high. In Figure 1, the mutual inductance flux lines 28 are represented by an exaggerated dotted path, for ease of reference. As can be seen, the path of mutual inductance is between two pole pieces 22 along the outer edge of the stator 11. This results in a high level of mutual inductance and accordingly a low fault tolerance; a short circuit in a pole piece 22 will increase the flux, potentially to above the rated value.

Figure 2 shows an example of a FSPM according to a preferred embodiment of the invention, which has been designed to improve the fault tolerance of the FSPM. In this embodiment, the FSPM is a three phase machine. Figure 2a is a magnified view of the pole pieces and separators of the FSPM shown in Figure 2.

As in Figure 1, the machine 10 has a stator 12 with six pole pieces 22 and a rotor 14 with seven teeth 24. However, in accordance with preferred embodiments of the invention, the stator 12 includes a separator 30 between each pole piece 22. The separators 30 are provided to physically separate each pole piece 22 from the other pole pieces 22. The separators 30 do not physically abut the pole pieces 22 at the inner edge 13 of the stator 12. Rather, an air gap 23 is provided between the separators 30 and the adjacent pole pieces 22 at the inner edge 13 of the stator 12. The air gap 23 between the pole pieces 22 and separators 30 is shown clearly in Figure 2a. This air gap is significantly smaller than the air gap 23 between the pole pieces 22 in the embodiment of Figure 1.

In this preferred embodiment, the separators 30 take the form of radially extending arms, which extend from the outer edge 11 of the stator 12 to the inner edge 13 of the stator 12. As can be seen, the separators 30 fit in the gaps 23 between the pole pieces 22 of the embodiment of Figure 1, whilst leaving a small air gaps 23 between the adjacent parts of the stator 12 at the inner edge 13. The small air gap determines the self inductance of the coils and the size of the air gap can be adjusted accordingly to determine the resulting fault current.

In order to keep the same winding area as the design shown of FSPM in Figure 1 (where the windings 20 are typically copper), the introduction of the separator 30 in the preferred embodiment of Figure 2 is accompanied by a reduction in the width of the teeth 24 of the rotor 14. This allows the machine to maintain the same torque output. The stator configuration shown in Figure 2 has a different magnetic and electric behaviour to that of the FSPM of Figure 1. In particular, a different flux path is created, mostly self-contained around each individual pole piece 22 (a flux line for a single pole piece is shown as an exaggerated dotted line at 32, for ease of reference). As shown, the flux line 32 for the pole piece 22 forms a loop down the pole piece 22, through the teeth 24 of the rotor 14, into the separator 30, along the outside edge of the stator 11 and back towards the originating pole piece 22; there is minimal flux 32 between different pole pieces 22 and accordingly the mutual inductance of the phase windings, each wound around one pole piece 22 remains very low. Therefore, the separator 30 may be considered to isolate the pole pieces 22 both physically and electromagnetically from the other pole pieces 22. Therefore, if a fault develops in a first pole piece 22, the flux path 32 ensures that the fault is mostly self-contained and accordingly there is little effect on the other pole pieces 22, ensuring that the machine 10 does not exceed its rated current value and does not adversely affect the other phase windings when a short circuit occurs.

In preferred embodiments, the separator 30 is made from a ferromagnetic material, such as Iron, to increase the electromagnetic isolation of the pole pieces 22. Figure 3 to 7 show experimental data relating to the design of the FSPM shown in Figure 2. The machine used to generate the experimental data has a fixed outer diameter of 75 mm and an axial length of 100mm and is optimised to achieve an inductance of lpu which is equal to 3.19mH. The machine 10 is as shown in Figure 2, that is to say as 6 pole stator and 7 teeth rotor. However, the skilled person will understand that the dimensions stated herein relate to the specific embodiment described with reference to Figure 2; these dimensions may be altered without departing from the inventive concept of a fault tolerant stator configuration or FSPM incorporating such a fault tolerant stator configuration.

Figure 3 shows a plot of the change in split ratio versus average torque produced. The split ratio is defined as the ratio of the stator bore diameter (i.e. as defined by the inner edge 13) to the stator outside diameter (i.e. as defined by the outer edge 11). As can be seen, the average torque is at a maximum between 0.5 and 0.6.

Figure 4 shows a plot of relative pole arc and average torque. The relative pole arc is measured in radians and it is apparent from the experimental data that the maximum torque is achieved between -0.9 to 1.2 radians.

From the experimental data, maximum torque is achieved with a split ratio in the region of 0.55 and a pole arc of approximately 1 radian.

Figure 5 shows the data used to determine the optimal relative magnetic thickness (the ratio of magnet width to rotor radius). Figure 5a shows average torque against relative magnet thickness and Figure 5b shows relative percentage of torque ripple against relative magnet thickness.

From Figure 5a it is clear that the maximum torque occurs at a relative magnet thickness of between 0.6 to 1.25, with a peak at approximately 1.0.

Figure 5b shows that torque ripple remains approximately constant to 0.75 and then begins to rapid increase at around 1.0. High torque ripple is undesirable as it indicates that the motor is not providing a constant output.

From Figures 5a and 5b it is clear that the high relative magnet thicknesses are undesirable as they result in a high torque ripple. It is found that the optimal torque output with minimal ripple is achieved for a thickness in the region of 0.8. Figure 6 shows the data used to optimise the separator thickness. For a fault tolerant machine, it is desirable to have a low mutual inductance as possible, whilst not affecting the torque.

Figure 6a shows a plot of mutual and self inductance against separator thickness and Figure 6b shows the torque produced against separator thickness.

From Figure 6a it is clear that the thicker the separator, the lower the mutual inductance, and that the self-inductance remains approximately constant. It is immediately apparent that even with a very thin separator (e.g. 0.5mm) the relative mutual inductance to self inductance is about 30% compared to 50% as is seen in the design of Figure 1. At a separator thickness of 2.5mm, the relative mutual inductance to self inductance is <10%.

Figure 6b shows that the average torque remains approximately constant to a thickness of approximately 2.5mm and at thicknesses greater than 2.5mm there is a slight decrease in torque. From the data in Figures 6a and 6b, the optimal separator thickness is 2.5mm.

Figure 7 shows the optimisation of "gap between adjacent tooth shoes". There is shown inductances versus gap in Figure 7a and torque versus gap in Figure 7b. The air gap 23 is as shown in Figure 2a.

It is clear from the data that a small gap between adjacent teeth results in a high self inductance but a low average torque. From the data the optimal slot opening is 0.8 deg.

The experimental data suggests that maximum efficiencies for a motor 75mm in diameter and 100mm in axial length are obtained when the measurements are: split ratio 0.55, relative rotor pole arc 1, relative magnet span 0.8, separator thickness 2.5 mm, and slot opening 0.8 deg. In order to measure the effectiveness of the machine and its fault tolerant behaviour the machine made to the above specification was tested against a 12 slots, 14 poles PMSM optimised to produce maximum torque density.

A representation of the PMSM machine is shown in Figure 8. As can be seen in Figure 8, the magnets are placed below the pole pieces. Such a configuration is known to be fault tolerant but requires expensive and difficult to produce sleeves to keep the magnets in place.

Figure 9 is a comparison of the back EMF produced by the FSPM invention in Figure 9a, and the prior art PMSM in Figure 9b.

The back EMFs produced by both machines are very similar. The FSPM has a slightly higher maximum output. The output of both machines are close to sinusoidal shape and are suitable for BLAC operation.

Figure 10 is a comparison of the torque and clogging torque for the FSPM (Figure 10a) and the PMSM (Figure 10b).

Both machines show extremely low cogging torque. The PMSM produces a greater amount of torque, when compared to the FSPM though as the PMSM has copper only pole pieces and the FSPM has a mixture between copper and magnets the higher torque is to be expected.

Figure 11 is a comparison of the self and mutual inductances for the FSPM (Figure 1 la) and the PMSM (Figure 1 lb).

The effectiveness of the magnetic isolation provided by the ferromagnetic separator in the FSPM is clearly shown. The resulting mutual inductance is very low compared to the PMSM design, where the mutual inductance is about half the self inductance.

The PMSM shows extremely negligible mutual inductance and thus provides very good magnetic isolation between phases. Both machines have high self inductances close to the lpu, and in the event of a short circuit current neither machine would go above the rated value, thus making them fault tolerant.

Both machines can operate as two separate three phases channels. Each channel could be fed by a separate three phase inverter. In the case of detection of an open circuit fault in one phase of a channel, the whole channel could be open through its converter and the remaining healthy channel would provide the required torque. The current overrating for the healthy channel would then be almost doubled. Similarly, in case of detection of inter- turn short circuit fault in one channel, the whole three phase terminals of this channel would have to be short circuited through its converter. This helps to reduce the fault current in the shorted turns as well as "symmetrising" the fault. In that case, the remaining healthy channel would have to be overrated in order to provide the pre-fault torque plus the braking torque due to the shorted channel .

When only one channel is operating normally, this channel actually behaves like a linear machine with some "magnetic end effects". This end effect can be minimized in reducing the mutual coupling between two nearby phases. Such coupling may occur at the junctions of the two channels.

Figure 12 shows the comparison of the torque for an open channel fault for the FSPM (Figure 12a) and the PMSM (Figure 12b). The torque waveforms in the case of an open channel show a 2nd harmonic low frequency ripple. This ripple is due to the above mentioned magnetic end effect at the junction of the two channels. This low frequency ripple is more noticeable in the FSPM but almost negligible in the PMSM machine thanks to the extremely low mutual coupling between phases in the single layer winding technology.

Figure 13 shows the torque for one channel shorted (graph subscript a) and the current produced (graph subscript b) for the FSPM (Figure 13A) and the PMSM (Figure 13B). As can be seen, the response for both machines is similar. For both machines, the short circuit current is very close but below the rated value (Figure 13Ab and Figure 13Bb). These currents are safe and would not overheat the machine. Therefore, in the event of a short both the FSPM and PMSM avoid motor burnout. In particular, both the FSPM and PMSM show currents that are almost balanced (Figure 13Ab and Figure 13Bb) and acceptable torque ripple (Figure 13Aa and Figure 13Ba).

The experimental data shows that the separators 30 provide means for magnetic isolation and allow the FSPM to be fault tolerant. The comparison data in Figures 9 to 13 of the FSPM show that the fault isolation and function of the machine is comparable to that of the known PMSM.

Figure 14 shows a further embodiment of a fault tolerant FSPM according to preferred embodiment of the invention, having the features of a fault tolerant FSPM as described above with reference to Figure 2, but wherein the machine is a 3 phase machine with 14 rotor teeth and 12 pole pieces. Such a machine is an extension of the principles discussed above.

Figure 15 is a further embodiment of a fault tolerant FSPM 40, having a stator 42, a "rotor" 44, magnets 46, separators 48 and pole pieces 50. This embodiment is a linear motor, in which the "rotor" 44 moves in the direction of the arrow shown in the Figure. The stator 42, rotor 44, magnets 46 and separator 48 function as described above, wherein a series of separators 48 are used to separate the pole pieces 50 physically and magnetically.

The skilled man will appreciate that the above principles may be applied to any flux switching machine, in order to physically and electrically isolate the pole pieces. Such a configuration allows for a high self inductance and low mutual inductance of the pieces thereby allowing for electrical isolation of the pole pieces.

It will therefore be understood that the separator 30 may be considered to isolate the pole pieces 22 both physically and electromagnetically from the other pole pieces 22. Therefore, if a fault develops in a first pole piece 22, the flux path 32 ensures that the fault is mostly self-contained and accordingly there is little effect on the other pole pieces 22, ensuring that the machine 10 does not exceed its rated value when a short circuit occurs.

Claims

1. A stator for a flux switching permanent magnet machine, the stator having a plurality of pole pieces with windings around each pole piece, wherein each pole piece and the windings associated with each pole piece are physically separated from the other pole pieces and the associated windings by a separator, for inducing for each pole piece a higher self inductance profile than the mutual inductance profile for the other pole pieces.

2. A stator according to claim 1 wherein the mutual inductance profile for the pole pieces is less than 30% of the self inductance profile.

3. A stator according to claim 1 or claim 2 wherein the separator substantially isolates the adjacent poles pieces physically and electrically from each other.

4. A stator according to any of claims 1 to 3 wherein the separator substantially isolates the adjacent poles pieces physically and electromagnetically from the other pole pieces.

5. A stator according to any of claims 1 to 4 wherein the pole pieces and separators do not physically abut.

6. A stator according to claim 5 configured with an air gap between the separators and the adjacent pole pieces.

7. A stator according to any of claims 1 to 6 wherein the separator is ferromagnetic.

8. A stator according to any of claims 1 to 7 wherein the stator is annular.

9. A stator according to claim 8 wherein the split ratio of the stator is between 0.5 and 0.65, more preferably 0.55.

10. A stator according to claim 8 or claim 9 wherein the relative rotor pole arc is between 0.9 and 1.2 radians, more preferably 1 radian.

11. A stator according to any of claims 8 to 10 wherein the relative magnet span is between 0.6 to 1.25, more preferably 0.8.

12. A stator according to any of claims 8 to 11 wherein the slot opening is between 0.7 and 0.9 deg, more preferably 0.8 deg.

13. A stator according to any preceding claim wherein the stator is annular and has an outer diameter of approximately 75mm, an axial length of approximately 100mm, and the separator thickness is approximately 2.5mm.

14. A stator according to any preceding claims wherein the stator is configured for use in a linear motor.

15. A fault tolerant flux switching permanent magnet machine including a stator according to any of claims 1 to 14.

16. A flux switching permanent magnet machine including a direct drive rotor, a stator having a plurality of pole pieces, each pole piece surrounded by windings, and a ferromagnetic separator adjacent to each pole piece and the associated windings, wherein the ferromagnetic separator is arranged for isolating each pole piece physically from the other pole pieces, thereby inducing for each pole piece a high self inductance profile when compared to the mutual inductance profile for the other pole pieces.

17. A machine according to claim 16 wherein the stator is annular

18. A machine according to claims 16 or 17 wherein the rotor has 7 teeth and the stator has 6 pole pieces.

19. A machine according to claims 13 or 14 wherein the rotor has 14 teeth and the stator has 12 pole pieces

20. A machine according to any of claims 16 to 19 wherein the stator diameter is approximately 75mm and the separator thickness is approximately 2.5mm.

21. A machine according to any of claims 16 to 20 wherein the stator is configured for use in a linear motor.

22. A machine according to any of claims 16 to 21 wherein the stator is a stator according to any of claims 1 to 13.

23. A linear motor incorporating a stator according to any of claims 1 to 13.

24. A linear motor incorporating a flux switching permanent magnet machine according to any of claims 16 to 22.