This invention generally relates to modulated pole machines. More particularly, the invention relates to a stator for such a modulated pole machine.
Over the years, electric machine designs such as modulated pole machines, e.g. claw pole machines, Lundell machines and transverse flux machines (TFM) have become more and more interesting. Electric machines using the principles of these machines were disclosed as early as about 1910 by Alexandersson and Fessenden. One of the most important reasons for the increasing interest is that the design enables a very high torque output in relation to, for instance, induction machines, switched reluctance machines and even permanent magnet brushless machines.
Further, such machines are advantageous in that the coil is often easy to manufacture. However, one of the drawbacks of the design is that they are typically relatively expensive to manufacture and that they experience a high leakage flux which causes a low power factor and a need for more magnetic material. The low power factor requires an up-sized power electronic circuit (or power supply when the machine is used synchronously) that also increases the volume, weight and cost of the total drive.
Stators of modulated pole electric machines generally use of a central single winding that will magnetically feed multiple teeth formed by a soft magnetic core structure. The soft magnetic core is then formed around the winding, while for other common electrical machine structures the winding is formed around a tooth of the core section. Examples of the modulated pole machine topology are sometimes recognised as e.g. Claw-pole-, Crow-feet-, Lundell- or TFM-machines. The modulated pole machine with buried magnets is further characterised by an active rotor structure including a plurality of permanent magnets being separated by rotor pole sections.
The transverse flux machine (TFM) topology is an example of a modulated pole machine. It is known to have a number of advantages over conventional machines. The basic design of a single-sided radial flux stator is characterized by a single, simple phase winding parallel to the air gap and with a more or less U-shaped yoke section surrounding the winding and exposing in principal two parallel rows of teeth's facing the air gap. Multi-phase arrangements include magnetically separated single phase units stacked perpendicular to the direction of motion of the rotor or mover. The phases are then electrically and magnetically shifted by 120 degrees for a three-phase arrangement to smooth the operation and produce a more or less even force or torque independent of the position of the rotor or mover. Note here that the angle referred to is given in electrical degrees which is equivalent to mechanical degrees divided by the number of pairs of magnetic poles.
A cylindrical motor uses a concentric stator and rotor, and the motion is then considered as rotational or as an end-less rotation. A linear machine uses translation motion that is normally not a closed motion pattern but may be a back-and-forward motion along a ‘line’. The linear machine or driver has a mover instead of a rotor. The magnetic circuit may be arranged by the same basic magnetic principles in both a rotor and mover, however the geometries will differ.
An example of an efficient rotor or mover arrangement is the use of so called buried magnets combined with soft magnetic pole sections or pieces to allow the permanent magnet field to flux-concentrate or be flexible in a direction transverse to the motion as e.g. described in the patent application WO2007/024184 by Jack et al.
WO2007/024184 discloses an electrical, rotary machine, which includes a first stator core section being substantially circular and including a plurality of teeth, a second stator core section being substantially circular and including a plurality of teeth, a coil arranged between the first and second circular stator core sections, and a rotor including a plurality of permanent magnets. The first stator core section, the second stator core section, the coil and the rotor are encircling a common geometric axis, and the plurality of teeth of the first stator core section and the second stator core section are arranged to protrude towards the rotor. Additionally the teeth of the second stator core section are circumferentially displaced in relation to the teeth of the first stator core section, and the permanent magnets in the rotor are separated in the circumferential direction from each other by axially extending pole sections made from soft magnetic material.
It is generally desirable to provide a stator for a modulated pole machine that is relatively inexpensive in production and assembly. It is further desirable to provide such a stator that has good performance parameters, such as one or more of the following: high structural stability, low magnetic reluctance, efficient flux path guidance, low weight, small size, high volume specific performance, etc.
An undesirable effect occurring in electrical machines is the so-called cogging torque, i.e. the torque due to the interaction between permanent magnets of the moving device and the stator. It is also known as detent or ‘no-current’ torque.
In many applications Gagging has to be low due to noise and vibration requirements. For example, if the machine is used as a generator in a windmill the cogging torque has to be low in order to have the generator to rotate at very low wind speeds. In case of smaller motors, up to some 50-100 Nm, cogging torque can easily be noticed by rotating the motor by hand.
In the context of a modulated pole machine (WPM), the amount of cogging torque depends on a large variety of factors. Even though some known measures for reducing torque exist, cogging reduction often increases the cost of the machine since the design will be more complex. Examples of methods that add costs and complexity are skewing the rotor and/or the stator. It is thus desirable to reduce the cogging of a modulated pole machine while avoiding an increase in machine complexity and/or cost. It is further desirable to provide a machine that can be manufactured efficiently and at low cost.
According to a first aspect, disclosed herein is a stator core section for a stator of a modulated pole machine, the modulated pole machine comprising the stator, a moving device, and an active gap between respective interface surfaces of the moving device and the stator for communicating magnetic flux between the stator and the moving device, the moving device being adapted to move relative to the stator in a direction of motion, wherein the stator core section comprises a stator core back from which a plurality of teeth extend, each tooth extending in a respective first direction defining a direction towards the rotor, the teeth being arranged along a second direction defining the direction of motion, each tooth having at least one side wall facing a neighbouring tooth and an interface surface facing the active gap, the interface surface and the side wall forming an edge connecting the interface surface and the side wall; wherein the edge is round defining a radius of curvature in a plane spanned by the first and second directions; and wherein the radius of curvature varies along a lateral direction normal to said plane.
Hence, disclosed herein are embodiments of a stator core section for a modulated pole machine that can be efficiently produced and that reduces the cogging torque of the machine. In particular, embodiments of the stator core sections described herein are well-suited for production by Powder Metallurgy (P/M) production methods.
The shaping of the stator sections can be made without significantly increasing the manufacturing cost or complexity of the resulting machine. Furthermore, a modification of the rotor is not required.
The active gap is often also referred to as an air gap, as it is normally filled with air.
In embodiments of the modulated pole machine, the moving device comprises a plurality of permanent magnets separated from each other in the direction of motion by pole sections. The pole sections may be formed as rectilinear rods elongated in the lateral direction. The plurality of permanent magnets may be arranged so that every second magnet along the direction of motion is reversed in magnetisation direction. Thereby individual pole sections may only interface with magnets showing equal polarity. Generally, the permanent magnets may also be rectilinear rods elongated in the lateral direction; the rods may extend across the lateral extent of the active air gap.
In some embodiments, the stator core section and/or the pole pieces of the moving device are made from a soft magnetic material such as soft magnetic powder, thereby simplifying the manufacturing of the components of the modulated pole machine and providing an efficient magnetic flux concentration, utilizing the advantage of effective three-dimensional flux paths in the soft magnetic material allowing e.g. radial, axial and circumferential flux path components in a rotary machine.
The soft magnetic powder may e.g. be a soft magnetic Iron powder or powder containing Co or Ni or alloys containing parts of the same. The soft magnetic powder could be a substantially pure water atomised iron powder or a sponge iron powder having irregular shaped particles which have been coated with an electrical insulation. In this context the term “substantially pure” means that the powder should be substantially free from inclusions and that the amount of the impurities O, C and N should be kept at a minimum. The average particle sizes are generally below 300 μm and above 10 μm.
However, any soft magnetic metal powder or metal alloy powder may be used as long as the soft magnetic properties are sufficient and that the powder is suitable for die compaction.
The electrical insulation of the powder particles may be made of an inorganic material. Especially suitable are the type of insulation disclosed in U.S. Pat. No. 6,348,265 (which is hereby incorporated by reference), which concerns particles of a base powder consisting of essentially pure iron having an insulating oxygen- and phosphorus-containing barrier. Powders having insulated particles are available as Somaloy® 500, Somaloy® 550 or Somaloy® 700 available from Höganas AB, Sweden.
The varying radius of the edge connecting the side wall and the interface surface may efficiently be implemented by compacting the stator core section from soft magnetic powder in a suitable compacting tool, such as a tool using a so-called shaped die.
Each stator core section may comprise a stator core back section and a set of teeth extending from the stator core section, wherein the stator core back section connects the teeth. A stator core section may further comprise a yoke section that provides a flux path in the lateral direction towards another stator core section comprising another one of the sets of teeth of the same phase. The core back section and the yoke section provide a flux path between neighboring teeth (which are displaced with respect to each other in the direction of motion) of the respective stator core sections.
In some embodiments the electric machine is a rotary machine, and the moving device is a rotor. In this case, the first stator core section, the second stator core section, the coil and the rotor may encircle a common geometric axis. In a rotary machine the lateral direction is an axial direction of the machine, and the direction of motion is a circumferential direction of the machine.
In some embodiments, the stator comprises: a first stator core section being substantially annular and including a plurality of teeth, a second stator core section being substantially annular and including a plurality of teeth, a coil arranged between the first and second circular stator core sections, wherein the first stator core section, the second stator core section, the coil and the rotor are encircling a common geometric axis defined by the longitudinal axis of the rotor, and wherein the plurality of teeth of the first stator core section and the second stator core section are arranged to protrude towards the rotor; wherein the teeth of the second stator core section are circumferentially displaced in relation to the teeth of the first stator core section.
In conventional machines, the coils explicitly form the multi-pole structure of the magnetic field, and the magnetic core function is just to carry this multi-pole field to link the magnet and/or other coils. In a modulated pole machine, it is the magnetic circuit which forms the multi-pole magnetic field from a much lower, usually two, pole field produced by the coil. In a modulated pole machine, the magnets usually form the matching multi-pole field explicitly but it is possible to have the magnetic circuit forming multi-pole fields from a single magnet. The modulated pole machine has a three-dimensional (3D) flux path utilizing magnetic flux paths in the transverse direction both in the stator and in the moving device, e.g. in the axial direction in a rotating machine, where the moving device is a rotor.
Thus in some embodiments the stator device and/or the moving device comprise a three-dimensional (3D) flux path including a flux path component in the transverse (i.e. lateral) direction relative to the direction of movement.
The present invention relates to different aspects including the stator device described above and in the following, and corresponding methods, devices, and/or product means, each yielding one or more of the benefits and advantages described in connection with the first mentioned aspect, and each having one or more embodiments corresponding to the embodiments described in connection with the first mentioned aspect and/or disclosed in the appended claims.
The above and/or additional objects, features and advantages of the present invention, will be further elucidated by the following illustrative and non-limiting detailed description of embodiments of the present invention, with reference to the appended drawings, wherein:
a and 1b show an example of a modulated pole machine.
a-2d show a schematic view of an example of a stator for a modulated pole machine.
a and 3b show a 3-phase modulated pole machine comprising a stator with 3 sets of stator component pairs, each holding one circumferential winding.
a-f schematically illustrate an example of the pressing process for manufacturing stator core sections as described herein.
In the following description, reference is made to the accompanying figures, which show by way of illustration how the invention may be practiced.
The machine comprises a stator 10 which comprises a central single winding 20 that magnetically feeds multiple teeth 102 formed by a soft magnetic core structure. The stator core is formed around the winding 20 while for other common electrical machine structures the winding is formed around the individual tooth core section.
More particularly the modulated pole electric machine of
Each stator section comprises an annular core back portion 261 and a flux bridge or yoke component 18 providing a flux path between circumferentially displaced teeth of the two stator core sections. In the machine in
The active rotor structure 30 is built up from an even number of segments 22, 24 wherein half of the number of segments—also called rotor pole sections 24—is made of soft magnetic material and the other half of the number of segments is made of permanent magnetic material 22. These segments may be produced as individual components. The permanent magnets 22 are arranged so that the magnetization directions of the permanent magnets are substantially circumferential, i.e. the north and the south poles, respectively, face in a substantially circumferential direction. Further, every second permanent magnet 22, counted circumferentially is arranged having its magnetization direction in the opposite direction in relation to its neighbouring permanent magnets. The magnetic functionality of the soft magnetic pole sections 24 in the desired machine structure is fully three dimensional and each soft magnetic pole section 24 is able to efficiently carry varying magnetic flux with high magnetic permeability in all three space directions.
This design of the rotor 30 and the stator 10 has the advantage of enabling flux concentration from the permanent magnets 22 so that the surface of the rotor 30 facing a tooth of the stator 10 may present the total magnetic flux from both of the neighboring permanent magnets 22 to the surface of the facing tooth. The flux concentration may be seen as a function of the area of the permanent magnets 22 facing each pole section 24 divided with the area facing a tooth. In particular, due to the circumferential displacement of the teeth, a tooth facing a pole section results in an active air gap that only extends partly across the axial extent of the pole section. Nevertheless, the magnetic flux from the entire axial extent of the permanent magnets is axially and radially directed in the pole section towards the active air gap. These flux concentration properties of each pole section 24 make it possible to use weak low cost permanent magnets as permanent magnets 22 in the rotor and makes it possible to achieve very high air gap flux densities. The flux concentration may be facilitated by the pole section being made from magnetic powder enabling effective three dimensional flux paths. Further, the design also makes it possible to make more efficient use of the magnets than in corresponding types of machines.
Still referring to
Each of the stator core sections 14, 16 may be compacted in one piece. Each stator core section 14, 16 may be formed as an annular disc having a central, substantially circular opening defined by a radially inner edge 551 of an annular core back portion 261. The teeth 102 protrude radially outward from a radially outer edge of the annular disc-shaped core back. The annular part between the inner edge 551 and the teeth 102 provides a flux path and a side wall of a circumferential cavity accommodating the coil 20. Each stator core section comprises a circumferential flange 18 at or near the inner edge 551. In the assembled stator the circumferential flange 18 is arranged on the inner side of the stator core section, i.e. the side facing the coil 20 and the other stator core section. In the embodiment shown in
Each of the teeth 102 has an interface surface 262 facing the air gap. During operation of the machine, the magnetic flux is communicated through the interface surface 262 via the air gap and through a corresponding interface surface of a pole piece of the rotor. The interface surface 262 is delimited in the circumferential direction, i.e. along the direction of motion of the rotor, by edges 263. Hence, the edges 263 connect the interface surface 262 with the respective side faces 266 of the tooth that face the neighbouring teeth. It will be appreciated that the P/M (Powder Metallurgy) technology imposes a certain minimum radius of the edges 263, i.e. they are rounded edges rather than complete sharp edges. Depending on the complexity of the compacting tool, the compaction pressure, the tool material etc., the smallest obtainable radius may e.g. be in the range of 0.25 to 0.5 mm. This radius is set by the shape of the die. However, as can be most clearly seen in
In the example of
Consequently, the teeth of the stator core sections 14, 16 are shaped such that the width—measured in the circumferential direction—of the interface surface gradually increases from the side proximal to the winding towards the side distal to the winding. During operation of the machine, the interface surface 262 thus gradually overlaps the corresponding surface of the rotor pole pieces and magnets instead of instantly. In embodiments of the stator described herein, the stator teeth thus have edges that are shaped in such a way so as to allow them to smoothly pass each other and the rotor pole sections. It will be appreciated from e.g.
a illustrates an example of a 3-phase modulated pole machine, while
Hence, as in the example of
The embodiments of the stator described in connection with
However, the teeth of the stator of
As can be most clearly seen from
Embodiments of the stator core sections described herein may be efficiently manufactured and shaped using P/M technology. P/M is a well established technique for producing components out of metal powder using a tool and a press as illustrated by
For ease of presentation, the process will be illustrated with reference to a simple component 803. However, it will be appreciated that the process and tool described in the following is well-suited for manufacturing embodiments of the stator core sections described herein. The P/M process comprises filling a die 702 with metal powder 703 followed by a pressing using a number of punches 704 and 705. The pressing is followed by a sintering at high temperature (e.g. in the region of 1000° C.) or a heat treatment at lower temperature (e.g. up to approximately 650° C.). The tool, generally designated 700, may comprise a die 702 and at least one upper and/or one lower punch 704 and 705, respectively. The technology may be used for large scale production of high quality metal components. The MPM type of electrical machine is very well suited for utilizing the benefits of the P/M technology. For making electrical motors using P/M, the powder may be made out of electrically insulted iron powder called soft magnetic powder as described above.
In order to produce the stator with teeth shaped as described herein, the tool die 702 may be specifically adapted to have a so-called called ‘shaped die’ geometry, a technique known as such in the art. In embodiments of the tool only the die 792 that is shaping the tooth. The exact shape of the die depends on the actual component to be manufactured. In the case of MPM SMC stator components described herein, the shape may be made by gradually increasing a tooth fillet radius of the die from approximately 0.3 mm to a radius maximum of half the tooth width. In order to allow a reliable pressing process, the larger radius is close to the side where the flux-bridge flange 18 is situated, i.e. the ‘winding side’. This is true both with and without claw.
a illustrates the tool in a powder filling position where the lower punch 705 is withdrawn from the die but still blocking the bottom opening of the die, while the upper punch is withdrawn from the die leaving the top opening of the die unobstructed allowing powder to be inserted through the top opening.
The reduction of cogging torque by shaping the stator teeth as described herein made be analyzed using Finite element analysis (FEA) combined with Fast Fourier transform analysis (FFT). It can be shown that for an MPM the 3-phase cogging torque is the sum of the 6th and 18th harmonic amplitude of the 1-phase cogging torque times 3, according to the formula:
MPM 3-phase cogging torque=3*(6th harmonic ampl. of 1 phase+18th harmonic ampl. of 1 phase).
The 6th harmonic is dominating the contribution.
The cogging torques of the same example MPM are shown in
The cogging torque reduction obtained by the tooth shaping described herein can be close to 100%. In the example it is about 80%. How much reduction is obtained for a specific machine may depend on parameters such as the number of poles and the number of phases. In the above example, the number of phases is 3, but it will be appreciated that the tooth shaping described herein may be applied to any MPM stator having teeth in SMC. In case the number of phases differs from 3 it is not necessarily the 6th and 18th harmonic that contributes to the cogging torque.
Although some embodiments have been described and shown in detail, the invention is not restricted to them, but may also be embodied in other ways within the scope of the subject matter defined in the following claims. In particular, it is to be understood that other embodiments may be utilised, and that structural and functional modifications may be made without departing from the scope of the present invention.
Embodiments of the invention disclosed herein may be used for a direct wheel drive motor for an electric-bicycle or other electrically driven vehicle, in particular a light-weight vehicle. Such applications may impose demands on high torque, relatively low speed and low cost. These demands may be fulfilled by a motor with a relatively high pole number in a compact geometry using a small volume of permanent magnets and wire coils to fit and to meet cost demands by the enhanced rotor assembly routine.
In device claims enumerating several means, several of these means can be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims or described in different embodiments does not indicate that a combination of these measures cannot be used to advantage.
It should be emphasized that the term “comprises/comprising” when used in this specification is taken to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.
Number | Date | Country | Kind |
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PA 2010 70573 | Dec 2010 | DK | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/EP2011/073347 | 12/20/2011 | WO | 00 | 6/12/2013 |
Number | Date | Country | |
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61426017 | Dec 2010 | US |