GB2468694A

GB2468694A - Field pole layout in an inductor machine

Info

Publication number: GB2468694A
Application number: GB0904690A
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: 2010-09-22
Anticipated expiration: 2029-03-18
Also published as: GB2468694B; GB0904690D0; GB2468694B8

Abstract

An inductor machine comprises a passive rotor 104 and a stator 102, the stator comprising a plurality of circumferentially spaced-apart stator poles 114. Consecutive stator poles 114 alternately comprising armature poles 118 and field poles 120. Each armature pole 118 comprises an alternating current armature winding 122 which extends about a single stator tooth 116 and each field pole comprises a direct current field winding which extends about a single stator tooth. A current supply arrangement is disclosed in fig 2. The rotor comprised laminated magnetic material poles 110 supported by non-magnetic material hub 108, spacer members 112 magnetically isolating poles. The spacer members may be non-conducting.

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.

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 LUCIDARME, published in the EPE'97 conference proceedings, pages 3.903 to 3.908, 1997. Such machines are known as Flux Switching Permanent Magnet (FSPM) motors and comprise a passive salient pole rotor and a number of stator poles which include permanent magnets. Such arrangements have the disadvantage that permanent magnets generate losses in the motor and are an expensive component of a motor arrangement.

An alternative arrangement is described and illustrated in US 6,646,406. This document describes an electrical machine having a passive salient pole rotor and a stator having a number of stator poles. However, in this arrangement, the stator poles do not include magnets and instead a series of DC field windings and armature windings are provided. Each of the DC field windings and armature windings extend across pairs of stator teeth and, thus, overlap one another. Such motors are useful in, for example, automotive applications. However, the arrangement of overlapping windings is complex, and increases the size and cost of the motor.

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 improved performance, reduced size and lower losses.

According to the present invention there is provided an electrical machine comprising a passive rotor and a stator, the stator comprising a plurality of circumferentially spaced-apart stator poles, consecutive stator poles alternately comprising armature poles and field poles, each armature pole comprising an armature winding which extends about a single stator tooth and which is arranged, in use, to carry an alternating current, and each field pole comprising a field winding which extends about a single stator tooth and is arranged, in use, to carry a direct current.

By providing such an arrangement, the windings can be more compact. This reduces the overall size of the motor and the cost of manufacture. Losses are also reduced due to the reduced volume of copper.

In one arrangement, the stator poles are arranged such that no overlap occurs between the windings of adjacent poles. This arrangement is compact and reduces power losses.

The volume of the end windings is reduced, resulting in improved efficiency and reduced size.

Usefully, the windings of adjacent poles are circumferentially-spaced from one another.

Advantageously, each stator pole is pennanent magnet free. Through the use of field poles, permanent magnets (such as those used in a typical Flux-Switching Permanent Magnet (FSPM) motor) are not required. Consequently, losses due to the presence of permanent magnets are eliminated, and the cost of manufacture is reduced.

In one arrangement, the rotor comprises a plurality of rotor poles formed from a magnetic material, the rotor poles being preferably magnetically isolated from one another. By providing a plurality of magnetically isolated rotor poles, losses in the machine are reduced. Further, by providing isolated rotor poles, the windings on the stator poles can be made correspondingly smaller, reducing the size and cost of the motor.

It is useful for non-magnetic spacing portions to be located between the rotor poles.

This improves the magnetic isolation between the poles (i.e. reduces the leakage of magnetic flux between the poles), whilst also improving the structural and aerodynamic properties of the rotor. In a further variation, the spacing portions are non-conducting. This helps to reduce eddy currents which may form between the the rotor poles.

In one arrangement, the rotor comprises a rotor hub formed from a non-magnetic material, the rotor poles being located on the rotor hub. A non-magnetic rotor hub is a convenient approach to achieve magnetically isolated rotor poles.

In a specific embodiment, five rotor poles are provided. This number has been found to provide optimal performance in this motor arrangement.

It is useful that an equal number of armature poles and field poles are provided.

Desirably, six armature poles and six field poles are provided. This arrangement has been found to provide optimal perfonnance.

Preferably, the armature windings of the armature poles are connected to a source of AC power. In order to rotate the rotor, the current through the armature windings must be alternating. A variety of AC sources may be used as appropriate.

In one arrangement, the armature windings of the armature poles are connected in three phases. A three phase system is cost-effective to implement and has performance benefits.

Preferably, the field windings of the field poles are connected to a source of DC power, more preferably in series. The field poles are required to generate, in use, a substantially constant field and so require a DC power source. An advantage of a DC power source is that it does not require complex control mechanisms. Therefore, it is cost-effective to implement.

In one arrangement, the electrical machine may take the form of a motor.

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

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 rotor and stator of a motor in accordance with the present invention; Figure 2 is a schematic of a suitable drive circuit for the motor of Figure 1; and Figure 3 is a graph of torque (Y-axis) as a function of Phase RMS current density through armature windings (X-axis) for the motor of Figure 1 and for a known FSPM motor.

Figure 1 shows a three-phase motor 100 according to the present invention. The motor comprises a stator 102 and a rotor 104 rotatable with respect thereto. The stator 102 is formed from a laminated ferromagnetic material (such as, for example, iron) and has a stator bore 106 formed therein.

The rotor 104 is rotatably located in the stator bore 106 about an axis X relative to the stator 102. The rotor 104 is a passive rotor. By this is meant that the rotor does not comprise any windings (for example, armature windings) or permanent magnets which may generate a magnetic field. The rotor 104 has a substantially cylindrical rotor hub 108 formed from a non-magnetic material (such as, for example, aluminium, stainless steel or copper) and a plurality of magnetically isolated rotor poles 110 located on an outer surface thereof. This type of rotor is known as a segmental rotor because the rotor poles 110 form a plurality of individual segments. In this embodiment, five rotor poles 110 are provided.

Each rotor pole 110 is formed from a laminated ferromagnetic material (such as, for example, iron) and is magnetically isolated from the remaining rotor poles 110 by the non-magnetic rotor hub 108. Spacing members 112 are located between the rotor poles and are formed from a non-magnetic and non-conducting material such as a plastic. The spacing members 112 help to isolate magnetically the rotor poles 110 and to prevent eddy currents from being generated. The spacing members 112 also providing structural rigidity and improve the aerodynamic properties of the rotor 104.

In this manner, the rotor 104 has reduced drag when rotating (i.e. reduced windage losses).

The stator 102 comprises a plurality of stator poles 114 (in this case, twelve) facing radially inwardly towards the stator bore 106. The stator poles 114 are all magnetically coupled to one another through the stator 102. Each stator pole 114 comprises a stator tooth 116. Each stator tooth 116 is formed integrally with the remainder of the stator 102 and comprises a rectangular portion of laminated magnetic material (such as, for

example, iron)

Consecutive stator poles 114 alternately comprise armature poles 118 and field poles 120. By this is meant that, moving consecutively around the inner circumference of the stator 102, the stator poles 114 comprise an armature pole 118, a field pole 120, an armature pole 118 and so on. Consequently, a field pole 120 is located on either side of an armature pole 118.

Each annature pole 118 comprises a single stator tooth 116 and an armature winding 122 extending thereabout. Six armature poles 118 are provided in this embodiment.

Each armature winding 122 is wrapped around a respective stator tooth 116 in a racetrack shape when viewed in a radial direction. The armature windings 122 of the armature poles 118 are connected to a source of AC power and are connected in three phases A, B and C. Each phase group A, B, C comprises two armature windings 122 connected in series. Consequently, as shown in Figure 1, phase A has armature windings Al and A2, phase B has armature windings Bi and B2 and phase C has armature windings Cl and C2.

As shown in Figure 1, the two armature poles 118 in each phase group A, B or C are diametrically opposed to one another. The armature windings 122 are connected to a three phase AC source and can be energised by a circuit such as that shown in Figure 2 and which will be described later.

Each field pole 120 comprises a single stator tooth 116 and a field winding 124 extending thereabout. Six field poles 120 are provided in this embodiment. These are labelled Fl to F6 in Figure 1. Each field winding 124 is coiled (or wrapped) around a single stator tooth 116 in a racetrack shape when viewed in a radial direction. The field windings 124 of the field poles 120 are connected in series (i.e. Fl to F6 are connected in series). The field windings Fl to F6 are connected to a source of DC power such as that shown in Figure 2 and which will be described later.

Since each of the armature windings 122 and field windings 124 only extend around a single stator tooth 116, the windings 122, 124 are circuniferentially spaced from one another. Consequently, there is no overlap between adjacent windings 122, 124. In other words, each stator tooth 116 only carries (or is located adjacent) a single type of winding (i.e. armature winding or field winding). By the above is meant that the windings 122, 124 do not overlap in the region in which the stator poles 114 are located. It will be appreciated that any positional relationship between the connections made to the windings outside of the stator 102 in order to connect the windings to an appropriate power source is not material to the present invention.

Figure 2 shows a drive circuit 200 suitable for driving the motor 100 shown in Figure 1. A DC voltage VDC 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 which drive the armature windings 122. 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 1, each of the armature windings 122 corresponding to phase A are connected in series. Each of the annature windings 122 corresponding to phase B and to phase C are connected in series respectively.

A DC current control is located between the positive and ground rails and is arranged in series with the six field windings Fl to F6 (here labelled F for brevity) which are connected in series. The DC current control can take any suitable form which enables current magnitude variation. The DC field can be varied as appropriate to provide control of torque as will be described later.

In use, the armature windings 122 are supplied with an alternating current which reverses in polarity with the passing of each rotor pole 110. At the point at which current reversal is required, the appropriate switch Si -S6 is switched. This process continues for each of the three phases A, B, C. The phases A, B, C are arranged in order to energise the relevant poles at the correct time. As shown in Figure 1, each armature pole 118 is offset from an adjacent armature pole 118 by an angle of 60 O The field windings 124 of the field poles 120 are energised by the DC current control shown in Figure 2. The field windings 124 are supplied with a constant current. A particular current value generates a particular torque output. Consequently, the torque output of the motor 100 can be varied by increasing the direct current supplied to the field windings 124. Consequently, the DC current control can provide the advantage of boost if required. If no current is supplied to the field windings 124, then no torque can be generated.

Figure 3 shows a graph of torque (on the Y-axis) against Phase RMS current density through the armature windings 122 (on the X-axis) for the motor 100. The winding packing factor is equal to 0.6. The top line (diamonds) indicates the performance of a known motor arrangement (labelled as an "FSPM" or Flux Switching Permanent Magnet machine) such as that described in the paper "Switching flux permanent magnet polyphased synchronous machines" by Emmanual HOANG, Abdel Hamid BEN AFIMED and Jean LUCIDARME, published in the EPE'97 conference proceedings, pages 3.903 to 3.908, 1997. The lower lines (squares, triangles, circles and crosses) show the performance of the motor 100 with varying DC current densities supplied to the field windings 124. As can be seen, the torque output is increased as the current density supplied to the field windings 124 is increased. Therefore, the present invention provides performance comparable to a conventional FSPM machine without requiring permanent magnets. By removing the need for permanent magnets, magnet losses in the motor arrangement are eliminated and manufacturing costs are reduced. Further, the present invention provides the advantages of boost because the torque output from the motor for a given phase RMS current density through the armature windings 122 can be varied depending upon the DC current density through

the field windings 124.

Variations will be apparent to the person skilled in the art. Whilst the invention has been described by way of example to a motor, the invention is also applicable to corresponding generators.

The arrangement is not limited to a motor having five rotor poles. Any suitable number of rotor poles may be used. Further, the rotor poles need not be magnetically isolated from one another -a conventional rotor may be used which comprises an integrally formed hub and rotor poles which are arranged such that magnetic flux can be channelled therebetween.

In addition, different numbers of stator poles may be provided as appropriate. The ratio of armature poles to field poles need not be 1:1 -other ratios may be used. In addition, more or less than three phases may be used. Such alternatives will be apparent to the skilled person.

Whilst it is desirable that the motor is permanent magnet free, this need not be so.

Permanent magnets may be included if desired to boost the field generated by the DC

field windings.

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 rotor and a stator, the stator comprising a plurality of circumferentially spaced-apart stator poles, consecutive stator poles alternately comprising armature poles and field poles, each armature pole comprising an armature winding which extends about a single stator tooth and which is arranged, in use, to carry an alternating current, and each field pole comprising a field winding which extends about a single stator tooth and is arranged, in use, to carry a direct current.
2. An electrical machine as claimed in claim 1, wherein the stator poles are arranged such that no overlap occurs between the windings of adjacent poles.
3. An electrical machine as claimed in claim 2, wherein the windings of adjacent poles are circumferentially-spaced from one another.
4. An electrical machine as claimed in claim 1, 2 or 3, wherein each stator pole is permanent magnet free.
5. An electrical machine as claimed in any one of the preceding claims, wherein the rotor comprises a plurality of rotor poles fonned from a magnetic material.
6. An electrical machine as claimed in claim 5, wherein the rotor poles are magnetically isolated from one another.
7. An electrical machine as claimed in claim 6, wherein non-magnetic spacing portions are located between the rotor poles.
8. An electrical machine as claimed in claim 7, wherein the spacing portions are non-conducting.
9. An electrical machine as claimed in any one of claims 5 to 8, wherein the rotor comprises a rotor hub formed from a non-magnetic material, the rotor poles being located on the rotor hub.
10. An electrical machine as claimed in any one of claims 5 to 9, wherein five rotor poles are provided.
11. An electrical machine as claimed in any one of the preceding claims, wherein an equal number of armature poles and field poles are provided.
12. An electrical machine as claimed in claim 11, wherein six armature poles andsix field poles are provided.
13. An electrical machine as claimed in any one of the preceding claims, wherein each armature winding is connected to a source of AC power.
14. An electrical machine as claimed in claim 13, wherein the armature windings of the armature poles are connected in three phases.
15. An electrical machine as claimed in any one of the preceding claims, wherein the field windings of the field poles are connected to a source of DC power.
16. An electrical machine as claimed in claim 15, wherein the field windings of the field poles are connected in series to the source of DC power.
17. An electrical machine as claimed in any one of the preceding claims in the form of a motor.
18. An electrical machine as claimed in claim 17, wherein the motor is a flux-switching motor.
19. An electrical machine as claimed in any one of claims 1 to 16 in the form of a generator.
20. An electrical machine substantially as hereinbefore described with reference to the accompanying drawings.Amendment to the claims have been filed as followsCLAIMS1. An electrical machine comprising a passive rotor and a stator, the stator comprising a plurality of circumferentially spaced-apart stator poles, consecutive stator poles alternately comprising armature poles and field poles, each armature pole S comprising an armature winding which extends about a single stator tooth and which is arranged, in use, to carry an alternating current, and each field pole comprising a field winding which extends about a single stator tooth and is arranged, in use, to carry a direct current, wherein the number of stator teeth and rotor poles is in the ratio of 12n:5n where n is a whole integer.2. An electrical machine as claimed in claim 1, wherein the stator poles are arranged such that no overlap occurs between the windings of adjacent poles. * .3. An electrical machine as claimed in claim 2, wherein the windings of adjacent poles are circumferentially-spaced from one another. ****4. An electrical machine as claimed in claim 1, 2 or 3, wherein each stator pole is permanent magnet free. * * * ..*5. An electrical machine as claimed in any one of the preceding claims, wherein the rotor comprises a plurality of rotor poles formed from a magnetic material.6. An electrical machine as claimed in claim 5, wherein the rotor poles are magnetically isolated from one another.7. An electrical machine as claimed in claim 6, wherein non-magnetic spacing portions are located between the rotor poles.8. An electrical machine as claimed in claim 7, wherein the spacing portions are non-conducting.9. An electrical machine as claimed in any one of claims 5 to 8, wherein the rotor comprises a rotor hub formed from a non-magnetic material, the rotor poles being located on the rotor hub.10. An electrical machine as claimed in any one of claims 5 to 9, wherein five rotor poles are provided.11. An electrical machine as claimed in any one of the preceding claims, wherein an equal number of armature poles and field poles are provided.12. An electrical machine as claimed in claim 11, wherein six armature poles andsix field poles are provided. *S..13. An electrical machine as claimed in any one of the preceding claims, wherein :: 15 each armature winding is connected to a source of AC power.14, An electrical machine as claimed in claim 13, wherein the armature windings of the armature poles are connected in three phases. * * * ****:. 20 15. An electrical machine as claimed in any one of the preceding claims, wherein the field windings of the field poles are connected to a source of DC power.16. An electrical machine as claimed in claim 15, wherein the field windings of the field poles are connected in series to the source of DC power.17. An electrical machine as claimed in any one of the preceding claims in the form of a motor.18. An electrical machine as claimed in claim 17, wherein the motor is a flux-switching motor.19. An electrical machine as claimed in any one of claims 1 to 16 in the form of a generator.20. An electrical machine substantially as hereinbefore described with reference to the accompanying drawings. * * *.** *S..... * . S... S.. * S * *.. *S.S