GB2475811A

GB2475811A - A stator assembly incorporating permanent magnets for an inductor machine

Info

Publication number: GB2475811A
Application number: GB1102504A
Authority: GB
Inventors: Jintao Chen; Zi-Qiang Zhu; Shinichiro Iwasaki; Rajesh Deodhar
Original assignee: IMRA Europe SAS
Current assignee: IMRA Europe SAS
Priority date: 2009-03-18
Filing date: 2009-03-18
Publication date: 2011-06-01
Anticipated expiration: 2029-03-18
Also published as: GB2475811B; GB201102504D0

Abstract

The stator 102 comprises a plurality of circumferentially spaced-apart stator poles 106 magnetically coupled to one another, each stator pole 106 having an armature winding thereabout and comprises a slot having a radially-extending circumferentially-polarised permanent magnet located therein. The stator has 6n stator poles (where n is a positive integer not equal to zero) and the rotor has either 13n rotor poles or 14n rotor poles (where n is a positive integer not equal to zero). Each stator pole 106 may be spaced from an adjacent stator pole 106 by a distance which is at least equal to the width of a stator pole 106. In a further arrangement, consecutive stator poles alternately comprise magnet-containing stator poles and magnet-free stator poles. Each magnet-containing stator pole has an armature winding.

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 improved performance and reduced manufacturing costs. 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 AITMED and Jean LUCIDARME, 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.

Another electrical machine having a similar arrangement is shown in the paper "Fault-Tolerant Flux-Switching Permanent magnet Brushless AC Machines" by R.L. OWEN, A.S. THOMAS, G.W. JEWELL and D. HOWE, as presented at IEEE-lAS Annual Meeting 2008. These electrical machines show arrangements in which alternating magnets are removed, creating spaces around the periphery of the stator.

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 rotor 12 has ten salient rotor poles 16 and the stator has twelve stator poles 18. Each stator pole 18 comprises a tooth 20 having a slot 22 formed therein. A radially-extending permanent magnet 24 is located in each slot 22. The permanent magnets 24 are polarised circumferentially as indicated by the arrows. As indicated in Figure 1, adjacent magnets 24 are oppositely polarised. Armature windings 26 extend around the teeth 20 of the stator poles 18. The armature windings 26 are connected in three phases (A, B and C) as shown and can be energised by known machines.

It is known that magnets are a relatively expensive part of a motor system. Further, magnets can also lead to losses in a system by means of, for example, eddy currents.

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 losses and improved performance.

According to one aspect of the present invention there is provided an electrical machine comprising a salient pole passive rotor and a stator comprising a plurality of circumferentially spaced-apart stator poles magnetically coupled to one another, each stator pole having an armature winding thereabout and comprising a slot having a radially-extending circumferentially-polarised permanent magnet located therein, wherein each stator pole is spaced from an adjacent stator pole by a distance which is at least equal to the width of a stator pole.

By providing such an arrangement, the magnet volume in an electrical machine can be reduced significantly, concomitantly reducing cost and power losses due to eddy currents.

In one arrangement, each stator pole is spaced from an adjacent stator pole by a distance which is in the range of 1.1 to 1.4 times the width of a stator pole. This range has the advantage of maximising torque output from this configuration of motor.

In one arrangement, the stator comprises a plurality of unitary sections spaced about the circumference, each unitary section including a first tooth portion of a first stator pole and a second tooth portion of a second stator pole adjacent the first stator pole, the first and second tooth portions being connected by a connecting section. This arrangement ensures effective magnetic coupling between the stator poles and is straightforward to manufacture.

In one variation, each unitary section has an arcuate connecting section. In a further variation, first tooth portions of each unitary section are spaced from the second tooth portions of adjacent unitary sections by the slot of the respective stator pole.

In another variation, adjacent magnets are circumferentially polarised in opposite directions.

According to another aspect of the present invention there is provided an electrical machine comprising a salient pole passive rotor and a stator comprising a plurality of circumferentiaJly spaced-apart stator poles magnetically coupled to one another, each stator pole having an annature winding thereabout and comprising a slot having a radially-extending circumferentially-polarised permanent magnet located therein, wherein: a) the stator has 6n stator poles; and b) the rotor has 13n or 14n rotor poles; where n is a positive integer not equal to zero.

This relationship between the number of stator poles and the number of rotor poles has been found to provide the best performance in this motor configuration where the stator poles are magnetically coupled to one another. n can be any integer because the motor configuration is scalable. Therefore, the larger the motor, the larger the number of poles that are required. For the majority of applications, n is likely to be in the range of I to 4. However, any suitable value of n may be used.

In one variation, each stator pole has a stator tooth comprising first and second tooth portions spaced by the slot of the respective stator pole. In a further variation, the stator comprises a plurality of unitary sections spaced about the circumference, each unitary section including a first tooth portion of a first stator pole and a second tooth portion of an second stator pole adjacent the first stator pole connected by a connecting section.

This arrangement is simple to manufacture and ensures structural rigidity and magnetic coupling between the stator poles.

In one arrangement, the stator has six stator poles and the rotor has either thirteen or fourteen rotor poles. This arrangement (i.e. where n 1) is useful for smaller motor designs.

According to another aspect of the present invention there is provided an electrical machine comprising a salient pole passive rotor and a stator comprising a plurality of circumferentially spaced-apart stator poles magnetically coupled to one another, wherein consecutive stator poles alternately comprise magnet-containing stator poles and magnet-free stator poles, each magnet-containing stator pole having an armature winding thereabout and comprising a radially-extending circumferentially-polarised permanent magnet.

It is useful that the permanent magnets of consecutive magnet-containing stator poles have opposing polarity.

Desirably, the stator is formed from a magnetic material and is substantially circumferentially magnetically continuous. By this is meant that the stator provides a continuous magnetic loop between the stator poles and does not comprise isolated magnetic regions separated by non-magnetic regions.

In one variation, each magnet-containing stator pole comprises a stator tooth having first and second tooth portions spaced by a radially-extending slot in which the respective circumferentially-polarised permanent magnet is located.

In one arrangement, each magnet-free stator pole comprises a unitary stator tooth. This is straightforward to manufacture and ensures structural rigidity.

In a variation, the stator comprises a plurality of sections spaced about the circumference, each section including a first tooth portion of a magnet-containing stator pole, a second tooth portion of an adjacent magnet-containing stator pole and a unitary stator tooth of a magnet-free stator pole. In a further variation, wherein each section is unitary and comprises an arcuate connecting section which connects the first tooth portion, the second tooth portion and the unitary stator tooth of the magnet-free stator pole.

In one arrangement, the electrical machine may take the form of a motor. In a variation, the motor is a flux switching permanent magnet motor.

In an alternative arrangement, the electrical machine may take the form of a generator.

In one arrangement, the electrical machine comprising means for energising the stator, which may comprise 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 poles or slots.

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 2 is a front cross-section of the rotor and stator of a motor in accordance with a first embodiment of the present invention; Figure 3 is a front cross-section of the rotor and stator of a motor in accordance with a second embodiment of the present invention; Figure 4 is a graph showing torque (on the Y-axis) against current density (on the X-axis) for the first and second embodiments when compared to a conventional motor; Figure 5 is a front cross-section of the rotor and stator of a motor in accordance with a third embodiment of the present invention; Figure 6 is a front cross-section of the rotor and stator of a motor in accordance with a fourth embodiment of the present invention; Figure 7 is a graph showing torque (on the Y-axis) against current density (on the X-axis) for the third and fourth embodiments when compared to a conventional motor; Figure 8 is a graph showing magnet loss per location (on the Y-axis) against current (on the X-axis) for the third and fourth embodiments when compared to a conventional motor; Figure 9 is a graph showing torque ripple (on the Y-axis) against current (on the X-axis) for the third and fourth embodiments; and Figure 10 is a schematic of a suitable drive circuit for the motor of Figures 2, 3, 5 or 6.

Figure 2 shows an example of a three phase motor 100 according to a first embodiment of the present invention. The motor 100 comprises a stator 102 formed from a laminated ferromagnetic material (such as, for example, iron) and has a plurality of stator poles 106 (in this case, six) facing radially inwardly towards a stator bore 108.

The stator poles 106 are all magnetically coupled to one another through the stator * 102. A passive rotor 110 having a plurality of salient poles 112 (in this case, fourteen) 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 114 having a first tooth portion 114a and a second tooth portion 1 14b. The first and second tooth portions 114a, 1 14b are spaced by a radially-extending slot 116. A radially-extending permanent magnet 118 is located in each slot 116. The permanent magnets 1 18 are polarised circumferentially.

Alternate permanent magnets 118 are polarised in opposite directions as indicated by the arrows in Figure 2.

Armature windings 120 extend around the teeth 114 of the stator poles 106. The armature windings 120 are connected in three phases and can be energised by known machines. Consequently, a stator pole 106 comprises a stator tooth 114, in which a slot 116 is formed therein to receive a permanent magnet 118 and which an armature winding 120 extends thereabout.

Six stator poles 106 are provided in this embodiment. This is half the number provided in the conventional arrangement shown in Figure 1. This reduces the magnet volume in the stator 102, reducing magnet losses and enabling manufacture to be more cost effective. Further, the ratio of fourteen rotor poles 112 to six stator poles 106 has been found to be particularly effective. Multiples thereof (e.g. twelve stator poles to twenty eight rotor poles) are also effective.

In this embodiment, the stator 102 comprises a plurality of stator sections 122. Each stator section 122 comprises a first tooth portion 1 14a of a first stator pole 106 and a second tooth portion 114b of a second stator pole 106 which is adjacent to the first stator pole 106. The two tooth portions 1 14a, 1 14b are connected by an arcuate connecting portion 124.

Consequently, each stator section 122 has a C-shape is cross section and is separated from adjacent stator sections 122 by a permanent magnet 118. When the stator 102 is fully assembled, the stator sections 122 are all magnetically coupled together, enabling each of the stator poles 106 to be magnetically coupled together. In other words, the stator is circumferentially magnetically continuous because the magnets 118 separating the respective stator sections 122 enable magnetic flux to be channelled therethrough, magnetically coupling the stator sections 122 to one another.

Figure 3 illustrates a second embodiment of the present invention. In the second embodiment, identical reference numerals have been used to those in the first embodiment. In the second embodiment, the stator 102 is substantially similar to the stator 102 of the first embodiment. The difference between the first and second embodiments lies in that the rotor 110 of the second embodiment has thirteen poles, in contrast to the first embodiment which has fourteen poles. The ratio of thirteen rotor poles 112 to six stator poles 106 has also been found to be particularly effective.

Multiples thereof (e.g. twelve stator poles to twenty six rotor poles) has also been found to be effective.

Figure 4 show graphs of torque against current density for a conventional motor such as that shown in Figure 1 and for the first and second embodiments. It can be seen from these figures that at low current densities (below 15 A/mm2) that the torque output from the first and second embodiments is significantly improved when compared to a conventional arrangement, despite having lower magnet volume (which is cheaper to manufacture).

Figure 5 shows an example of a three phase motor 200 according to a third embodiment of the present invention. The motor 200 comprises a stator 202 formed from a laminated ferromagnetic material (such as, for example, iron) and has a plurality of stator poles 206a, 206b (in this case, twelve) facing radially inwardly towards a stator bore 208. The stator poles 206a, 206b are all magnetically coupled to one another through the stator 202. A passive rotor 210 having a plurality of salient poles 212 (in this case, ten) is rotatably located in the stator bore 208. The rotor 210 is rotatable about an axis X relative to the stator 202.

There are two types of stator pole in the third embodiment. The first type of stator pole is a magnet-containing stator pole 206a which comprises a stator tooth 214 having a first tooth portion 214a and a second tooth portion 214b. The first and second tooth portions 214a, 214b are spaced by a radially-extending slot 216. A radially-extending permanent magnet 218 is located in each slot 216. The permanent magnets 218 are polarised circuniferentially. Alternate permanent magnets 218 are polarised in opposite directions as indicated by the arrows in Figure 5.

Armature windings 220 extend around the teeth 214 of the magnet-containing stator poles 206a. The armature windings 220 are connected in three phases and can be energised by known machines.

Six magnet-containing stator poles 206a are provided in this embodiment. This is half the number of magnet-containing stator poles provided in the conventional arrangement shown in Figure 1. This reduces the magnet volume in the stator 202, reducing magnet losses and enabling manufacture to be more cost effective.

The second type of stator pole is the magnet-free stator pole 206b. Each magnet-free stator pole 206b comprises a unitary stator tooth 21 4c. The stator teeth 21 4c have neither slots nor magnets located therein, nor do they comprise armature windings as is the case for the magnet-containing stator poles 206a. Consequently, whilst the stator teeth 21 4c are still able to channel magnetic flux to the rotor 210, they are more cost effective to manufacture because they are structurally simpler and comprise fewer parts. Six magnet-free stator poles 206b are provided.

As shown in Figure 5, moving around the circumference of the stator 202, consecutive stator poles comprise alternate magnet-containing stator poles 206a and magnet-free stator poles 206b.

In this embodiment, the stator 202 comprises a plurality of stator sections 222. Each stator section 222 comprises a first tooth portion 214a of a first magnet-containing stator pole 206a and a second tooth portion 21 4b of a magnet-containing second stator pole 206a which is adjacent to the first magnet containing stator pole 206a. The two tooth portions 2l4a, 214b are connected by an arcuate connecting portion 224.

Intermediate the two tooth portions 21 4a, 21 4b, and formed on the arcuate connecting portion 224 is a magnet-free stator pole 206b. Consequently, each stator section 222 has an E-shape in cross section and contains a magnet-free stator pole 206b and two halves of a stator tooth 214 of a magnet-containing stator pole 206a. Each stator section 222 is separated from adjacent stator sections 222 by a permanent magnet 218.

When the stator 202 is filly assembled, the stator sections 222 are all magnetically coupled together, enabling each of the stator poles 206a, 206b to be magnetically coupled together. In other words, the stator is circu.mferentially magnetically continuous because the magnets 218 separating the respective stator sections 222 enable magnetic flux to be channelled therethrough, magnetically coupling the stator sections 222 to one another.

Figure 6 illustrates a fourth embodiment of the present invention. In the fourth embodiment, identical reference numerals have been used to those in the third embodiment. In the second embodiment, the stator 202 is substantially similar to the stator 202 of the first embodiment. The difference between the third and fourth embodiments lies in that the rotor 210 of the second embodiment has eleven poles, in contrast to the third embodiment which has ten poles. The ratio of eleven rotor poles 112 to six magnet-containing stator poles 206a and six magnet-free stator poles 206b has also been found to be particularly effective.

Figure 7 shows a graph of torque against current density for a conventional motor such as that shown in Figure 1 and the third and fourth embodiments. It can be seen from these figures that at all measured current densities the torque output from the third and fourth embodiments is significantly improved when compared to a conventional arrangement, despite having lower magnet volume (which is cheaper to manufacture).

Figure 8 shows a graph of magnet loss per magnet location against RMS current for a conventional motor such as that shown in Figure 1 and the motors of the third and fourth embodiments. It can be seen from this figure that, up to currents of around 120 A, magnet loss per location is significantly reduced in the motors according to the third and fourth inventions when compared to a conventional arrangement.

Finally, with reference to Figure 9, it can be seen that the fourth embodiment has lower torque ripple than the third embodiment. Torque ripple is the variation in torque as each rotor pole passes each stator pole and a motive force is applied thereto.

Consequently, by providing eleven rotor poles in the fourth embodiment, the torque ripple can be reduced when compared to the third embodiment which has ten rotor poles.

Referring back to Figures 2, 3, 5 and 6, it can be seen that each phase (A, B and C) of the three phase system comprises two armature windings 120 corresponding to four separate spaced stator poles 106. Consequently, phase A has armature windings Al and A2, phase B has armature windings B 1 and B2 and phase C has armature windings Cl andC2.

Figure 10 shows a drive circuit 300 suitable for driving the motors 100, 200 shown in Figures 2, 3, 5 and 6. A DC supply voltage VL)c is applied between a positive rail and a ground rail. A capacitor C is connected between the positive and ground rails.

A switch arrangement including switches Si to S6 and an array of diodes are provided in order to control the current and voltage delivered to each of the three phases A, B, C. The switches Si to S6 can comprise any suitable switching devices; for example, power MOSFETs or Insulated Gate Bipolar Transistors (IGBTs) could be used. As shown in Figure 10, each of the armature windings 120 corresponding to phase A are connected in series. Each of the armature windings 120 corresponding to phase B and to phase C are connected in series respectively.

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.

The arrangement is not limited to a motor having a specified number of rotor andlor stator poles as described above. Variations will be apparent to the person skilled in the art and the numbers may be varied as appropriate. In particular, multiples of six stator poles and thirteen or fourteen rotor poles may 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 salient pole passive rotor and a stator comprising a plurality of circumferentially spaced-apart stator poles magnetically coupled to one another, each stator pole having an armature winding thereabout and comprising a slot having a radially-extending circumferentially-polarised permanent magnet located therein, wherein: a) the stator has 6n stator poles; and b) the rotor has 13n or 14n rotor poles; where n is a positive integer not equal to zero.
2. An electrical machine as claimed in claim 1, wherein each stator pole has a stator tooth comprising first and second tooth portions spaced by the slot of the respective stator pole.
3. An electrical machine as claimed in claim 2, wherein the stator comprises a plurality of unitary sections spaced about the circumference, each unitary section including a first tooth portion of a first stator pole and a second tooth portion of an second stator pole adjacent the first stator pole connected by a connecting section.
4. An electrical machine as claimed in claim 3, wherein each unitary section has an arcuate connecting section.
5. An electrical machine as claimed in any one of claims 1 to 4, wherein adjacent magnets are circumferentially polarised in opposite directions.
6. An electrical machine as claimed in any one of claims 1 to 5, wherein the stator has six stator poles and the rotor has either thirteen or fourteen rotor poles.
7. An electrical machine as claimed in any of the preceding claims in the form of a motor.
8. An electrical machine as claimed in claim 7, wherein the motor is a flux switcIiiiig permanent magnet motor.
9. An electrical machine as claimed in any of the preceding claims in the form of a generator.
10. An electrical machine as claimed in claim 7 to 8 further comprising means for energising the stator.
11. An electrical machine as claimed in claim 10, wherein the means for energising the stator comprises a three-phase drive.