Abstract:
The present disclosure relates to a mono- or polyphase electric motor including a stator carrying at least three coils and consisting of 12×N straight teeth extending radially, N being an integer greater than or equal to 1, and of an equal peak divergence alpha, measured at their end from the centre of the motor, and a rotor exhibiting P pairs of magnetized poles such that P=5+2×R, P being a non-multiple of three, R being an integer greater than or equal to 0, alpha lying between 360°(12×N)/3 and 360°(12×N)/2.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a National Phase Entry of International Application No. PCT/FR2013/051830, filed on Jul. 30, 2013, which claims priority to French Patent Application Serial No. 1257469, filed on Aug. 1, 2012, both of which are incorporated by reference herein. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to the field of electric motors, and more particularly to low to medium energy mono- or polyphase permanent magnet machines, i.e. capable of delivering output power up to a few kilowatts. 
       BACKGROUND 
       [0003]    Electric machines performances are characterized by a certain number of factors, the importance of which depends on the type of targeted applications. A major sizing element is the motor constant, also called Km, and expressed in torque units per Watt square root, i.e. reflecting the torque that the motor can generate for a given power consumption in the coils. Another factor which may be important is the torque without current, which must often be reduced as much as possible for various reasons (noise, loss, mechanical wear, geared motor reversibility . . . ). 
         [0004]    Within the scope of this patent, optimizing the motor constant for given axial overall dimensions and diameter will be our concern, while striving to have a motor with a minimum torque without current in motors having a “radial” topology, i.e. having a plane flow and radially extending teeth. The axial overall dimension will be defined here as the height of the stator plus the height of the coils heads above the stator. More generally, the capacity of industrializing such a motor is obviously a background factor to be taken into account. Eventually, we will also be preferentially interested by structures with balanced radial forces (making it possible to solve problems such as noise or wearing of rotor guide elements). The criteria mentioned above generally lead to compromises in the definition of structures that do not always enable to obtain an optimized structure. 
         [0005]    For low to medium power motors, topologies with 6 or 12 stator teeth are currently found, with the 12 teeth topology making it possible to obtain the best performance in terms of Km. The stator topologies having 12 teeth with equal peak divergences make it possible to eliminate, in association with magnetized rotors having a number of pairs of poles equal to 5+2R (with R being an integer) and not a multiple of 3, the harmonics of the torque without current up to the 12 th  row (excluded). The first appearing and majority harmonic thus has a period 12 times smaller than the electric period of the motor. As the amplitude of the harmonics decreases with their row, these motors are particularly valuable for applications requiring a minimum torque in the absence of current. 
         [0006]    The U.S. Pat. No. 7,595,577 is known in the prior art which relates to a motor having 12 equal teeth extended at their ends on the stator. This solution is a classic way to minimize the torque without current in demanding applications in the field. As a matter of fact, as the torque without current is linked to the preferred positions that the magnet rotor can take due to the gaps between the stator teeth, it seems wise to minimize these spaces, or even in some cases to eliminate these by contacting the teeth in order to reduce the twelfth harmonic. 
         [0007]    However, this type of solution has several drawbacks. First, the closer position of, or the contact between the teeth, induces leakage fluxes which affect the motor performances, even though the contact areas (commonly called necks) are so arranged as to be magnetically saturable. Moreover, these topologies require either a direct winding in the slots, or executing the stator in several subsets, which may be annoying in terms of industrial production. Eventually, the space available for the copper of the windings is not optimal. 
         [0008]    The German patent application DE102009000681 is also known, which describes an electric machine comprising: a rotor having 14 poles, with the rotor being so arranged as to be rotatable about a central axis, and a stator having 12 stator teeth which protrude in a radial direction relative to the central axis in the direction of the rotor characterized in that each stator tooth is provided with a stator coil, wherein two adjacent stator coils are each time connected in series to each other to form a pair of stator coils, with one pair of stator coils being associated with a phase being each time connected to a dedicated neutral point, so to form a star point circuit. For this type of embodiment of the prior art, the straight tooth synchronous machine provides for a tooth width as wide as that of the notches, generally a tooth width of about 15°. The figures of patent DE102009000681 do not make it possible to clearly understand the peak divergence of the teeth, which moreover varies from one tooth to another, with values of 15.1° or 15.8° in  FIG. 3  of this document of the prior art. 
         [0009]    The patent EP0872943 discloses another example of an electrodynamic rotating machine with permanent magnets having a stator with a concentrated winding. 
         [0010]    The patent application WO93/07672 is also known, which relates to a structure of motor with straight teeth. However, the proposed topology has 4 pairs of magnetic poles on the rotor, which does not allow a natural compensation of the sixth harmonic of the residual torque and requires modifications to be brought to the shape of the rotor magnets in order to reduce the residual torque. Besides, if this structure uses straight teeth, the latter have a large width (approximately 50% of the pole pitch), which is not optimized. 
         [0011]    In a certain number of its patents, the applicant provided solutions making it possible to remedy all or part of the disadvantages mentioned above by using straight teeth topologies associated with rotors having 5 or 7 pairs of poles while showing that configurations could be found, in terms of width of stator teeth, for which the torque without current could be eliminated or at least significantly minimized. In particular, the U.S. Pat. No. 8,102,093 relates to a motor having 12 straight stator teeth, with 6 being wound, and 6 not being wound, with the particularity that the wound teeth are at least twice as wide as the non wound teeth. The wide teeth carrying the coils thus make it possible to maximize the Km value under certain conditions thanks to an increased permeance. The teeth width is so selected as to obtain an optimal compensation of the sixth harmonic of the residual torque appearing due to the not equal width between the teeth. 
         [0012]    This type of structure perfectly meets the needs for a reduction in the torque without current, for balanced radial forces, enables an independent winding on coil bodies added on the stator subsequently and leads to satisfactory levels of performance. However, if constant axial overall dimensions and diameter are used, it surprisingly appears to the persons skilled in the art that, in some cases, this topology does not lead to the optimal result and that it would be profitable to have equal but narrower teeth than the teachings of the prior art would suggest in order to optimize the motor Km value. For example, even though the height of the stator has to be reduced, when using larger winding coil heads, to keep a constant overall dimension, better configurations, in terms of Km value, are obtained with 12 narrow teeth having an equal width, than with 12 teeth having different sizes, and this with teeth widths smaller than half the pole pitch. 
       SUMMARY 
       [0013]    The present invention thus aims at providing a solution to the problems mentioned above. More particularly, the invention aims at providing a mono- or polyphase electric motor comprising a stator carrying at least three coils and consisting of 12×N straight teeth extending radially (i.e. perpendicularly to the axis of rotation of the motor), with N being an integer greater than or equal to 1, and of an equal peak divergence alpha, measured at their ends from the centre of the motor, and a rotor having P pairs of magnetized poles such that P=5+2×R, with P being a non-multiple of three, R being an integer greater than or equal to 0, with alpha being between 360°/(12×N)/3 and 360°/(12×N)/2. 
         [0014]    Preferably, the electric motor is characterized in that one tooth out of two carries a coil. In an alternative solution, the electric motor is characterized in that all the teeth carry coils. In a particular embodiment, the electric motor has three coils installed on teeth spaced by 60° and generally distributed over a tooth pitch (theta) of less than or equal to 120°. 
         [0015]    If an even greater reduction in the torque without current is desired, methods can also be used for reducing, at least locally, the air gap between the teeth. The stator structure can thus be modified to obtain saturable necks over a part of the height, or to add one or more ferromagnetic ring(s). Thus, in a particular embodiment, the electric motor has a stator made of a stack of two different types of metal plates, with the adjacent stator teeth being in contact on the first type of metal plate by means of saturable magnetic necks. 
         [0016]    In an alternative solution, the electric motor has at least one ferromagnetic ring inserted into the radial air gap between the stator teeth and the magnetic rotor. An electric motor having a stator which has two types of metal plates forming a recess adapted to receive a ferromagnetic ring can also be considered. According to another possible alternative solution, the electric motor has a stator with an outer ring positioned after the winding of the teeth so as to provide the closing of the magnetic circuit. 
         [0017]    As regards the rotor, according to a preferred alternative solution, the magnet is of the ring type with alternating magnetic poles corresponding to a radial magnetization, or at least close to it. In this case, the rotor is inscribed in the stator. However, a disk-shaped magnet with a magnetization of the axial type may also be used, with the magnet then turning above the radially extending teeth, with the coils being radially recessed relative to the magnet. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    The invention will be better understood when examining the following figures: 
           [0019]      FIG. 1  shows a motor having 12 extended teeth according to the prior art. 
           [0020]      FIG. 2  shows a motor having 12 straight, 6 wide and 6 narrow teeth, according to the prior art. 
           [0021]      FIG. 3  shows a motor according to the invention in a first embodiment. 
           [0022]      FIG. 4  shows the theoretical variation of the motor torque constant as a function of the width of the teeth in a motor having 12 equal narrow teeth, with a constant axial overall dimension. 
           [0023]      FIG. 5  shows a sectional view of a stator according to the invention in a secondary embodiment using alternating metal plates having, for some of them, saturable necks. 
           [0024]      FIG. 6  shows a sectional view of a stator according to the invention in another secondary embodiment using ferromagnetic rings in the air gap in a first alternative solution. 
           [0025]      FIG. 7  shows a sectional view of a stator according to the invention in another secondary embodiment using ferromagnetic rings in the air gap in a second alternative solution. 
           [0026]      FIG. 8  shows a front view of a motor according to the invention in an embodiment using 5 pairs of poles and 12 wound narrow teeth. 
           [0027]      FIG. 9  shows a front view of a motor according to the invention in an embodiment using 7 pairs of poles and ferromagnetic rings in the air gap. 
           [0028]      FIG. 10  shows a front view of a motor according to the invention in an embodiment using 11 pairs of poles and ferromagnetic rings in the air gap. 
           [0029]      FIG. 11  shows a front view of a motor according to the invention in another embodiment using three coils positioned in a limited tooth pitch. 
           [0030]      FIGS. 12 and 13  show an alternative solution of the invention with an axially magnetized disk rotor in two different views. 
           [0031]      FIG. 14  shows an alternative embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0032]      FIG. 1  is a figure of U.S. Pat. No. 7,595,577 which relates to a motor having 5 pairs of magnetic poles on the rotor and 12 teeth wound on the stator, with each of said teeth having an extension  1  at their ends opposite the magnetized rotor. This embodiment is a typical representation of the solutions with teeth extended to minimize the torque without current with the defects identified in the preamble to this patent: leakage flux between the teeth, complexity of the winding, reduced space for the coils . . . . 
         [0033]    The motor in  FIG. 2  provides an appropriate solution to remedy these defects according to the applicant&#39;s patent FR2899396, which implements alternating large 2 and narrow 3 teeth, the peak divergence of which, as measured at the teeth ends facing the rotor varies from a factor above 2, typically 22° for the wide teeth and 10° for the narrow teeth, this still for a rotor having 5 pairs of magnetic poles like in the previous example. These different widths lead to an original compensation of the harmonics of the residual torque, while making it possible to clear a large space for the winding. However, under some circumstances, this solution may be restrictive if the maximum torque density for a given axial overall dimension is our concern, while aiming at a very low torque without current. In the first place, in spite of the method of harmonic compensation, the imperfections of the magnet will always prevent falling below a certain value of the torque without current. 
         [0034]    Now, a motor having 12 equal teeth has a natural compensation of the sixth harmonic (theoretically the first harmonic present is the twelfth harmonic). If this is combined with the reduction in the width of the teeth, it appears that, in some cases, better performances can be obtained, thanks to a greater volume of copper while avoiding an excessive saturation in these teeth.  FIG. 3  illustrates, in a front view and in section, a motor having 12 teeth and 5 pairs of magnetic poles according to the invention having the same outer dimensions as that in  FIG. 2  with identical radially extending narrow teeth  4   a  and  4   b  having a peak divergence alpha=13°—as measured on the ends of the teeth—. The section in a plane perpendicular to the radial median axis is rectangular, as well as the section along a transverse plane. The teeth  4   a ,  4   b  are oriented radially. 
         [0035]    The stator  7  consists of teeth  4   a ,  4   b  and coils  5 . One tooth  4   a  out of two is surrounded by an electric coil  5 —typically made of copper or aluminum—with two coils  5  being spaced by a mechanical 180° around the centre of the motor, thus forming an electric phase and all the 6 coils  5  forming a three-phase motor. The teeth  4   b  which do not carry the coils  5  advantageously, but not restrictively, have bores  6  to enable the attachment of the metal plates composing the stator without affecting the magnetic performances and the magnetic flux return occurring at the back of the teeth  4   a  and  4   b . These bores  6  are provided in the development area where the tooth enlarges to extend through a peripheral belt zone. 
         [0036]    The rotor, separated by an air gap  15  from the stator  7 , is typically composed of a ferromagnetic yoke  9  carrying magnetised poles  8  in the form of a radially magnetized—alternating North/South—multipolar uniform ring or quantized sectors uniformly magnetized so as to produce a magnetization close to a radial direction. A rotor may also be made by embedding magnets in a yoke following the numerous teachings in the art that can currently be found in this field. 
         [0037]    For an equal motor height, formed by the metal plates thickness and the coils thickness, but thanks to a different distribution of such heights, a gain of about 7% in the Km value can be obtained with this motor having equal narrow teeth as compared to the motor shown in  FIG. 2 . In addition, such a motor has a better torque without current, less sensitive to variations induced by the magnet (inhomogeneity of the material, magnetization . . . ) thanks to the natural compensation of the sixth harmonic. 
         [0038]      FIG. 4  justifies selecting (for the case of the previous figure) the tooth width at 13° via a graph showing the variation in Km for three different motor configurations (5 and 7 pairs of poles with 6 wire wound teeth—called respectively 5PP and 7PP—, and 5 pairs of poles with 12 wire wound teeth—called 5PP-12-), and this with an iso-axial dimension (ie index), including the thickness of the coil buns, and with an iso-active length (ia index). The active length is the height of the stator metal plate measured in the direction of the axis of rotation  18  of the rotor. This  FIG. 4  thus shows the evolution, as a function of the (alpha) width of the teeth, of the Km ratio for each given tooth width, to the maximum Km value observed on the angular range studied. 
         [0039]    As can be seen, the optimum Km value for 5 pairs of poles/6 wound teeth is about 13° (or 360°/(12×N)/2.3), and the optimum values generally are in a range between 360°/(12×N)/3 and 360°/(12×N)/2, and more particularly 10° and 14° for 5 and 7 pairs of poles, if N=1 (12 teeth per stator) and whether an iso-overall dimension or an iso-active length comparison is considered. The claimed range is symbolized in this  FIG. 4 , by the width of the double arrow. 
         [0040]    As discussed above, a classic solution to reduce the torque without current consists in using extended or even contacting teeth via saturable zones (called necks). In addition to the winding problems, this incurs leakages reducing the motor performance. Now, it is not necessary to use such a tooth profile along the whole height of the stator. 
         [0041]    Using a stator  7 —viewed in a section here—consisting of alternating metal plates  13   a  having necks  10  and metal plates  13   b  with no neck can thus be considered, like in  FIG. 5 . The winding of the stator teeth is then executed from the back of the teeth prior to the positioning of the outer ring  11  which provides the closing of the magnetic circuit. The ratio between the metal plates  13   a  having necks  10  and the metal plates  13   b  having none may be less than 1. Typically, using necks  10  on the equivalent of one third of the height only already leads to a good compromise for the residual torque without affecting the effective torque with a leaking flow at the necks  10 . 
         [0042]    This solution is efficient, but may thus complicate the winding process. Another solution consists in keeping the original stator structure, and then in adding one or more ring(s) made of a ferromagnetic material. 
         [0043]      FIG. 6  shows a first solution using ferromagnetic rings  12 , directly in the air gap between the stator  7  and the rotor (not visible). As a matter of fact, these rings  12  are positioned in front of the narrow teeth  4  and make it possible to reduce the magnitude of the torque without current and to use only one type of metal plates  13   b  for the stator assembly  7 . On the contrary, the air gap  15  between the stator  7  and the rotor (not visible) is slightly increased as a whole. 
         [0044]    Thus, another alternative consists in positioning the ferromagnetic rings  12  on either side of the metal plates stack, as illustrated in  FIG. 7 , using metal plates  13   c  and  13   d  having two different inner diameters so as to create a recess  14  adapted to receive the ring(s)  12 . Although requiring a slight reduction in the depth of the coils  5 , this solution has the advantage of making it possible to keep a small air gap  15 . 
         [0045]      FIG. 8  shows a motor, according to the invention, having 12 identical narrow teeth  4   a  with each one carrying a coil  5 . The rotor has 5 pairs of magnetic poles  8  carried by a yoke  9 .  FIG. 9  shows a motor having 12 narrow teeth with an equal peak divergence and 7 pairs of magnetic poles  8 . One or more ferromagnetic ring(s)  12 , the function of which has been described above, is/are positioned in the air gap  15  between the rotor and the stator  7 . 
         [0046]      FIG. 10  shows a motor having 12 narrow teeth with an equal peak divergence and 11 pairs of magnetic poles  8 . One or more ferromagnetic ring(s)  12 , the function of which has been described above, is/are positioned in the air gap  15  between the rotor and the stator  7 . 
         [0047]      FIG. 11  shows another alternative embodiment intended to be used in an application requiring a small axial overall dimension (in the thickness of the stator, along the axis of rotation of the rotor). As a matter of fact, the motor has only three electric coils  5  carried by three teeth  4   a  on the stator, with the teeth being separated by a mechanical 60°, so that only one tooth out of two is wound and in an tooth pitch (theta) of only 120° as a whole. This configuration makes it possible to clear space on the opposite side of such formed tooth pitch (theta), so that, for example, a set of gears (not shown) providing a reduction in the speed of the rotor to move an external member (not shown) at a lower speed and a higher torque can be positioned. This geared motor can then be positioned as close as possible to the stator in a small axial and radial overall dimension opposite the tooth pitch (theta), thanks to a first outer radius  16  of the stator  7  smaller than the second outer radius  17  of the stator  7 , wherein the part of the stator  7  where the coils are positioned  5  is inscribed. 
         [0048]      FIGS. 12 and 13  illustrate, in the case of a stator  7  having 6 coils  5 , the possibility of using a disk-shaped magnet  8  having alternating axially magnetized poles (either parallel to the axis of rotation  18  or perpendicular to the radial extension of the stator  7 ). This type of magnet may be advantageous from an economic point of view, since a disk-shaped magnet is less expensive to develop as compared to a ring-shaped magnet, but also in terms of homogeneity of the material. This type of structure leads however to shortening the coils  5 , or at least to recessing these radially, so that the magnet  8  can be positioned. In this exemplary non-restrictive embodiment, the stator  7  consists of a single metal plate  13   a , but using several metal plates may be considered. 
         [0049]      FIG. 14  shows an exemplary non-restrictive embodiment of a geared motor using a motor according to the invention. The motor according to invention will be particularly advantageous when used in combination with a motion mechanical transformation system (reduction or multiplication) thus making it possible to obtain an efficient geared motor. As a matter of fact, the characteristics thereof of high torque density with current will make the geared motor compact and the low torque without current thereof will make a geared motor able to return in an often called “failsafe” position, when an elastic return system is positioned upstream or downstream of the mechanical transformation system. 
         [0050]      FIG. 14  shows a motor according to the invention associated with a motion reduction system  19 . The rotor of the motor meshes with a first meshing gear  20  and the last meshing gear  21  is integral with a mechanical output shaft  22  which an external member (not shown) to be moved may be associated with.