GB2468842A

GB2468842A - Magnet assembly in an inductor machine

Info

Publication number: GB2468842A
Application number: GB0904689A
Authority: GB
Inventors: Shinichiro Iwasaki; Rajesh Pranay Deodhar
Original assignee: IMRA Europe SAS; IMRA Europe SAS UK Research Center
Current assignee: IMRA Europe SAS; IMRA Europe SAS UK Research Center
Priority date: 2009-03-18
Filing date: 2009-03-18
Publication date: 2010-09-29
Anticipated expiration: 2029-03-18
Also published as: GB2468842B; GB0904689D0

Abstract

There is provided an electrical machine 100 comprising a passive salient pole rotor 110 and a stator 102 comprising a plurality of stator poles 106 each having a winding 122 arranged thereabout, at least one stator pole 106 comprising a radially-extending permanent magnet arrangement 116. The magnet arrangement 116 has a first magnet portion 118 comprising a circumferentially-polarized permanent magnet having a first resistivity and a second magnet portion 120 comprising a circumferentially-polarized permanent magnet having a second resistivity which is greater than the first resistivity. The direction of polarization of each magnet may be the same (fig 4a) or opposite (fig 5a). Measurement regions A123 and B123 are indicated and the flux density measured at these points as a function of time as the rotor angle is varied is depicted graphically. A comparison table (Fig 6) is provided.

Description

An Electrical Machine The present invention relates to an electrical machine. Particularly, but not exclusively, the present invention relates to an electrical machine having reduced power losses. The present invention may be applied to motors and to generators.

A known electrical machine is described and illustrated in the paper "Switching flux permanent magnet polyphased synchronous machines" by Emmanual HOANG, Abdel Hamid BEN AHMED and Jean LUC1DARME, published in the EPE'97 conference proceedings, pages 3.903 to 3.908, 1997. Such machines comprise a passive salient pole rotor and a number of stator poles which include stator teeth.

Figure 1 shows an example of a known three phase salient pole motor. The motor 10 comprises a salient pole rotor 12 and a stator 14. The stator 14 is enclosed within an outer housing 16. The rotor has ten salient rotor poles 18 and the stator has twelve stator poles 20. Each stator pole 20 comprises a tooth 22 having a slot 24 formed therein. A radially-extending permanent magnet 26 is located in each slot 24. The permanent magnets 26 are polarised circumferentially as indicated by the arrows. As indicated in Figure 1, adjacent magnets 26 are oppositely polarised. Armature windings (not shown) extend around the teeth 22 of the stator poles 20. The armature windings are connected in three phases and can be energised by known machines.

It is known that eddy current losses can occur in electrical machines such as the motor described above. An example of this is shown in GB 2,428,903 wherein slots are provided in a non-magnetic housing of the motor to reduce eddy current losses.

The inventors of the present application have found that highly-localised induced eddy currents losses also occur in the permanent magnets of stator poles in a motor such as that shown in Figure 1. This can result in the magnet becoming excessively heated in localised regions. This heating may lead to at least partial demagnetisation of the permanent magnet This process is illustrated with respect to Figures 2a and 2b. Figure 2a shows an enlarged view of the region A of Figure 1. In this region, three measurement points are shown. These measurement points correspond to the three lines on the graph of Figure 2b. These lines show the magnetic field measured at these points as a function of time as the rotor angle is varied with respect thereto.

As can be seen in Figure 2b, points 1 and 2 experience a reduction of remnant flux density of approximately 0.1 T when compared to point 2 at all rotor angles. There is an even greater difference between the fields measured at these points towards the centre of the graph as the rotor passes close to the measurement points and temporarily partially demagnetises points 1 and 3 whilst increasing the flux density at point 2. The discrepancies in the field at these points also leads to eddy current heating at regions around points 1 and 3. This may result in further demagnetisation of these regions if rare earth magnets are used because the characteristic remnant flux density (Br) of such a magnet is inversely proportional to temperature and so decreases as the magnet heats up.

It is an object of the present invention to provide an improved electrical machine. It is a further object of the present invention to provide an electrical machine with reduced power losses.

According to the present invention there is provided an electrical machine comprising a passive salient pole rotor and a stator, the stator comprising a plurality of stator poles each having a winding arranged thereabout, at least one stator pole comprising a radially-extending permanent magnet arrangement, wherein the magnet arrangement has a first magnet portion comprising a circutnferentially-polarised permanent magnet having a first resistivity and a second magnet portion comprising a circumferentially-polarised permanent magnet having a second resistivity which is greater than the first resistivity.

By providing such an arrangement, highly-localised eddy current losses in the permanent magnet can be reduced. This reduces the heating of the permanent magnet, in turn reducing the likelihood of further demagnetisation thereof.

It is desirable that the second magnet portion is located at a radial end of the magnet arrangement; preferably at a radial end of the magnet arrangement proximal the rotor.

It has been found that the greatest eddy current losses occur at the end of the magnet proximal the stator bore and adjacent the rotor. Therefore, by providing the second magnet portion in this region, such losses can be reduced.

It is useful that the first and second magnet portions are polarised in the same direction. Alternatively, the first and second magnet portions can be polarised in opposite directions. Both orientations of the poles of the second magnet portion relative to the first magnet portion can provide effective reductions in eddy current losses when compared to a standard permanent magnet arrangement.

In one variation, the second magnet portion has a radial length which is less than 30% of the total radial length of the magnet arrangement, preferably less than 15% of the total radial length of the magnet arrangement.

It has been found that the highest eddy current losses and resultant heating occurs in localised regions. By providing a second magnet portion which is a relatively small percentage of the radial length of the whole magnet arrangement, losses can be reduced without loss of performance or desirable magnet characteristics.

In one variation, the first magnet portion has a resisitivity in the range of 1 x 10 to 1 x iO Ohm/rn. In another variation, the second magnet portion has a resistivity in the range of lx 1Oto lx l0 Ohm/rn.

The electrical machine may, advantageously, take the form of a motor or a generator.

It is desirable that the electrical machine further includes means for energising the stator which comprises, preferably, a three-phase drive. However, the invention is not limited to three phase machines it may be applied to any number of phases. The invention is not limited to particular numbers of slots and poles.

An example of the invention is a three phase machine having 12 stator poles and 10 rotor poles. However, any suitable number of rotor poles and stator poles could be used, depending upon the application and/or requirements of the electrical machine.

An embodiment of the invention will now be described with reference to the accompanying drawings in which: Figure 1 is a front cross-section of a known three phase salient pole motor; Figure 2a is an enlarged view of the region A of Figure 1 showing three sample points

for magnetic field measurement;

Figure 2b is a graph showing the magnetic field (on the Y-axis) for each of the points shown in Figure 2a against time (on the X-axis) as a rotor pole passes thereby; Figure 3 is a front cross-section of the rotor and stator of a motor in accordance with the present invention; Figure 4a is an enlarged view of the region B of Figure 3 showing a first embodiment of the present invention and six sample points for magnetic field measurement; Figure 4b is a graph showing the magnetic field (on the Y-axis) for each of the points Al to A3 shown in Figure 4a against time (on the X-axis) as a rotor pole passes thereby; Figure 4c is a graph showing the magnetic field (on the Y-axis) for each of the points 81 to B3 shown in Figure 4a against time (on the X-axis) as a rotor pole passes thereby; Figure 5a is an enlarged view of the region B of Figure 3 showing a second embodiment of the present invention and six sample points for magnetic field measurement; Figure Sb is a graph showing the magnetic field (on the Y-axis) for each of the points Al to A3 shown in Figure 5a against time (on the X-axis) as a rotor pole passes thereby; Figure 5c is a graph showing the magnetic field (on the Y-axis) for each of the points Bi to B3 shown in Figure 5a against time (on the X-axis) as a rotor pole passes thereby; Figure 6 is a table detailing characteristics of a standard magnet arrangement and the first and second embodiments of the invention; Figure 7a is an image showing a computer calculation of the power losses in a standard stator pole having a conventional permanent magnet; Figure 7b is an image showing a computer calculation of the power losses in a stator pole having a magnet arrangement in accordance with the first embodiment of the invention; and Figure 7c is an image showing a computer calculation of the power losses in a stator pole having a magnet arrangement in accordance with the second embodiment of the invention.

Figure 3 shows an example of a three phase motor 100 according to the present invention. The motor 100 comprises a stator 102 located within a housing 104. The housing 104 can be formed from a variety of materials such as aluminium, nylon or steel.

The stator 102 is formed from a laminated ferromagnetic material (such as, for example, iron) and has a plurality of stator poles 106 (in this case, 12) facing radially inwardly towards a stator bore 108. A passive rotor 110 having a plurality of salient poles 111 (in this case, 10) is rotatably located in the stator bore 108. The rotor 110 is rotatable about an axis X relative to the stator 102.

Each stator pole 106 comprises a stator tooth 112 having a radially-extending slot 114 formed therein. A permanent magnet arrangement 116 is located in each slot 114. Each permanent magnet arrangement 116 comprises a first magnet portion 118 and a second magnet portion 120. Each first magnet portion 118 extends radially through the stator tooth 112 and is polarised circumferentially as indicated by the respective arrows. As shown in Figure 3, adjacent first magnet portions 118 are oppositely polarised. Each first magnet portion 118 is formed from Sintered NdFeB. This material has a resistivity of 1 x 106 Ohm-rn.

The second magnet portion 120 of each magnet arrangement is located at a proximal end of the magnet arrangement 116 adjacent the bore 108 and the rotor 110. Each second magnet portion 120 has radial length (i.e. a length in the radial direction) which is less than 15% of the total radial length of the particular magnet arrangement 116. In the described embodiments, the total length of the magnet arrangement 116 is 27 mm and the length of the second magnet portion 120 is, preferably, 2-3 mm.

The second magnet portion 120 is formed from a material having a higher resistivity than the first magnet portion 118. The second magnet portion 120 may be formed from, for example, Bonded NdFeB (having a resistivity of 1 x 10 Ohm-rn) or Ferrite (having a resistivity of 1 x i04 Ohm-rn). Further details of the second magnet portion 120 will be described with reference to the specific first and second embodiments set out below.

Armature windings 122 extend around the teeth 112 of the stator poles 106. The armature windings 122 are connected in three phases and can be energised by known arrangements.

Figure 4a illustrates a first embodiment of the invention in more detail. Figure 4a shows an enlarged view of the region B of Figure 3. In this embodiment, the first magnet portion 118 and the second magnet portion 120 are circumferentially polarised in the same direction as shown by the arrows. Also, in this embodiment, the second magnet portion 120 is formed from bonded NdFeB.

Figure 4a also shows six measurement regions -three (Al, A2, A3) are taken at points at an end of the first magnet portion 118 adjacent the second magnet portion 120. The other three (Bi, B2, B3) are taken at points on the second magnet portion 120.

Measurement points Al, A2, A3 correspond to the three lines represented by points 1, 2 and 3 respectively on the graph of Figure 4b. Measurement points Bi, B2, B3 correspond to the three lines represented by points 1, 2 and 3 respectively on the graph of Figure 4c. These lines show the magnetic flux density measured at these points as a function of time as the rotor angle is varied with respect thereto.

As can be seen in Figure 4b, points 1, 2 and 3 have less of a discrepancy in magnetic flux density that the equivalent points shown in Figure 2b. Whilst there is a temporary difference between the fields measured at these points towards the centre of the graph as the rotor passes close to the measurement point, this discrepancy is much smaller than that shown in Figure 2b.

Turning to Figure 4c, which is a measurement of the flux density at points B!, B2 and B3 in the second magnet portion 120, it can be seen that the remnant flux density is correspondingly lower. Power losses in the second magnet portion 120 are also reduced due to the increased resistivity thereof.

The power losses are greatly reduced when compared to a standard permanent magnet arrangement (as shown in Figure 2a). Figure 6 is a table comparing physical characteristics of the first embodiment with those of a standard (Figure 2a) permanent magnet arrangement. For only a small second magnet portion 120 (in this embodiment, 3 nun compared to a total magnet arrangement radial length of 27 mm), the total power losses (measured at 15,000 rpm, 100 Arms, 58 Degrees current angle) are 44W.

This compares well with a standard permanent magnet arrangement which has a power loss of 163W. In other words, the power loss in embodiment 1 is less than 27% of that in a standard arrangement.

Other important factors include the averaged loss density, which is reduced from 1 17W/kg in a standard permanent magnet arrangement to jusi 32WIkg in embodiment 1. This is a reduction of over 72%. Further evidence is provided by the reduced localised maximum loss density in the first embodiment, which is reduced from 26667W/kg in a standard arrangement to 1333W/kg in the first embodiment. This is shown in a comparison of Figures 7a and 7b which show computer generated images indicating the loss regions. The lighter areas correspond to regions of higher loss. Note that the peak of the scale in Figure 7b is over an order of magnitude lower than that for Figure 7a.

Figure 5a illustrates a second embodiment of the invention in more detail. Figure 5a shows an enlarged view of the region B of Figure 3. In this embodiment, the first magnet portion 118 and the second magnet portion 120 are circumferentially polarised in opposite directions as shown by the arrows. Also, in this embodiment, the second magnet portion 120 is formed from Ferrite.

Figure 5a also shows six measurement regions -three (Al, A2, A3) are taken from at points at end of the first magnet portion 118 adjacent the second magnet portion 120.

The other three (B!, B2, B3) are taken at points on the second magnet portion 120 in the manner of Figure 4a.

Measurement points Al, A2, A3 correspond to the three lines represented by points 1, 2 and 3 respectively on the graph of Figure 5b. Measurement points B!, B2, B3 correspond to the three lines represented by points 1, 2 and 3 respectively on the graph of Figure 5c. These lines show the magnetic flux density measured at these points as a function of time as the rotor angle is varied with respect thereto.

As can be seen in Figure 5b, points 1, 2 and 3 have less of a discrepancy in magnetic flux density that the equivalent points shown in Figure 2b. This concurs with Figure 4b of the first embodiment. Whilst there is again a temporary difference between the fields measured at these points towards the centre of the graph as the rotor passes close to the measurement point, this discrepancy is much smaller than that shown in Figure 2b.

Turning to Figure Sc, which is a measurement of the flux density at points Bi, B2 and B3 in the second magnet portion 120, it can be seen that the remnant flux density is correspondingly lower. Power losses in the second magnet portion 120 are also reduced due to the increased resistivity thereof.

The power losses in the second embodiment are greatly reduced when compared to a standard permanent magnet arrangement (as shown in Figure 2a). The table of Figure 6 also includes physical characteristics of the second embodiment which can be compared against those of the first embodiment (Figure 4a) and a standard (Figure 2a) permanent magnet arrangement. In the second embodiment, the second magnet portion is even smaller in radial length than that of the first embodiment (in this embodiment, the radial length is 2 mm compared to a total magnet arrangement radial length of 27 mm).

Further, the total power losses (measured at 15,000 rpm, 100 Arms, 58 Degrees current angle) are 59W. This compares well with a standard permanent magnet arrangement which has a power loss of 163W. In other words, the power loss in the second embodiment is less tha.n 37% of that in a standard arrangement.

Other important factors include the averaged loss density, which is reduced from 117W/kg in a standard permanent magnet arrangement to 43W/kg in embodiment 2.

This is a reduction of over 63%. Further evidence is provided by the reduced localised maximum loss density in embodiment 1, which is reduced from 26667W/kg in a standard arrangement to 4180W/kg in embodiment 2. This is shown in a comparison of Figures 7a and 7c which show computer generated images indicating the loss regions. The lighter areas correspond to regions of higher loss. Note that the peak of the scale in Figure 7c is four times lower than that for Figure 7a.

Although the invention has been described with reference to the above specific examples, the invention is not limited to the detailed description given above.

Variations will be apparent to the person skilled in the art.

For example, each stator pole of a motor or generator need not have a magnet arrangement including first and second portion as set out in the above embodiments.

Some of the magnets may be standard magnets (as shown in Figure 2a), with only one or more stator poles including a magnet arrangement as set out in the above embodiments.

Further, the second magnet portion need not be located at a radial end of the first magnet portion. It may be located elsewhere; for example, at the sides, in the middle or wherever a particular magnet configuration has a localised loss region. The radial length of the second magnet portion when compared to the first magnet portion may be less or more than the proportions set out in the embodiments. These dimensions can be varied as appropriate for the configuration of motor in which the invention is used.

The arrangement is not limited to a motor having ten rotor poles and twelve stator poles. The number of such poles is not material to the present invention; any appropriate number of stator and rotor poles could be used.

Whilst the invention has been described by way of example to three phase machines, it may be applied to machines of other numbers of phases. Whilst the invention has been described by way of example to a motor, the invention is also applicable to corresponding generators.

While the invention is susceptible to various modifications and alternative forms, specific embodiments are shown by way of example in the drawings and are herein described in detail. It should be understood, however, that drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the invention is to cover all modifications, equivalents and alternatives falling within the scope of the appended claims.

Claims

CLAIMS1. An electrical machine comprising a passive salient pole rotor and a stator, the stator comprising a plurality of stator poles each having a winding arranged thereabout, at least one stator pole comprising a radially-extending permanent magnet arrangement, wherein the magnet arrangement has a first magnet portion comprising a circumferentially-polarised permanent magnet having a first resistivity and a second magnet portion comprising a circumferentially-polarised permanent magnet having a second resistivity which is greater than the first resistivity.
2. An electrical machine as claimed in claim 1, wherein the second magnet portion is located at a radial end of the magnet arrangement.
3. An electrical machine as claimed in claim 2, wherein the second magnet portion is located at a radial end of the magnet arrangement proximal the rotor.
4. An electrical machine as claimed in claim 1, 2 or 3, wherein the first and second magnet portions are circumferentially polarised in the same direction.
5. An electrical machine as claimed in claim 1, 2 or 3, wherein the first and second magnet portions are circumferentially polarised in opposite directions.
6. An electrical machine as claimed in any one of the preceding claims, wherein the second magnet portion has a radial length which is less than 30% of the total radial length of the magnet arrangement.
7. An electrical machine as claimed in claim 6, wherein the second magnet portion has a radial length which is less than 15% of the total radial length of the magnet arrangement.
8. An electrical machine as claimed fri any one of the preceding claims, wherein the first magnet portion has a resisitivity in the range of 1 x 106 to 1 x i0 Ohm/rn.
9. An electrical machine as claimed in any one of the preceding claims, wherein the second magnet portion has a resisitivity in the range of 1 x 10 to 1 x i04 Ohm/rn.
10. An electrical machine as claimed in any one of the preceding claims, wherein each stator tooth comprises a radially-extending permanent magnet arrangement having first and second portions.
11. An electrical machine as claimed in any of the preceding claims in the form of a motor.
12. An electrical machine as claimed in any of the preceding claims in the foth of a generator.
13. An electrical machine as claimed in claim 11 or 12 further comprising means for energising the stator.
14. An electrical machine as claimed in claim 13, wherein the means for energising the stator comprises a three-phase drive.
15. An electrical machine substantially as hereinbefore described with reference to Figures 3 to 7 of the accompanying drawings.