Abstract:
A polyphase motor, for example for applications for driving pumps or ventilators in the automobile industry, including a stator part excited by electric coils and a rotor exhibiting N pairs of poles that are magnetized radially in alternate senses, the stator part exhibiting wide teeth and narrow teeth extending radially from an annular ring. The wide teeth carry the coil windings and the distance between a wide tooth and a narrow tooth is greater than the width of a narrow tooth.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a polyphase electric motor, more particularly for applications for driving pumps or ventilators in the automobile industry. 
     In the industrial and automobile fields, the selection of a polyphase motor for driving pumps or ventilators is made, depending on criteria which can be cost, performances and lifetime criteria. As regards the performances, the output and noise are more particularly estimated. To reach a good output, it is necessary to have a sufficient volume of copper to emit the resistive losses and a short magnetic circuit to minimise the iron losses. To reduce the level of noise as much as possible, a correct balance of the radial forces as well as a low detent torque will be aimed at. The cost of the motor is related to the cost of materials but also to the cost of the manufacturing and it is particularly important to provide an economical solution for the manufacturing of the coil winding. 
     2. Description of Related Art 
     Motors are known in the state of the art, such as described in the German patent DE19905748, exhibiting a correct balance of the radial forces but the structure of such motors does not allow a simple manufacturing of the coil winding by separately winding the coils and by inserting them into the stator. As a matter of fact, mastering the detent torque requires poles having as wide as possible a shape, in such motors, and the coil winding must be made by inserting the copper wire through particularly narrow cut-outs. In order to make the coil winding easier, certain motors thus exhibit a stator made of several parts, but the simplification of the coil winding is thus at the expense of the design of the statoric circuit which becomes complex because of this assembling of such a large number of pieces and thus it becomes costly. 
     Motors are also known in the state of the art, which make it possible to reach a particularly simple and economical manufacturing of the coil winding on a stator made in one piece while providing a low detent torque. However, such motors are not totally balanced as regards the radial forces, which is a disadvantage as regards noise and also as regards the bearing lifetime. Such motors are for example described in the American patent U.S. Pat. No. 5,880,551 also assigned to the applicant. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention thus aims at providing an economic and strong solution, adapted to mass production and exhibiting a very good level of performances as regards the output, thanks to the volume of copper and the short magnetic circuit and as regards noise through the balanced radial forces an the low level of detent torque. 
     For this purpose, the invention relates to a polyphase motor formed by a stator part excited by electric coils and by a rotor magnetised and exhibiting N pairs of poles of the type magnetised radially in alternate senses, the stator part exhibiting large teeth and narrow teeth extending radially from an annular ring. The wide teeth which support the coil windings have a width which is greater than or equal to the double of the width of the narrow teeth. The size of the width of the teeth as measured on the inner diameter of the stator uses a couple of values, for the wide teeth and the narrow teeth respectively, which has the property of cancelling the detent torques in the motor. The shape of the stator which exhibits coiled teeth having a constant rectangular section allows the introduction of each coil around a wide tooth, with the coils being manufactured separately, outside the stator. For this purpose, the width of the cut-out making it possible to receive the coil winding is greater than the width of a narrow tooth, said width being measured on the inner diameter of the stator. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The invention will be best understood upon reading the following description and while referring to the appended drawings where: 
         FIG. 1  shows a cross-sectional view of a motor according to the invention. The motor includes a stator  1  exhibiting 6 wide teeth  11  to  16  and 6 narrow teeth  17  to  22  and a rotor  2  exhibiting N pairs of poles A 1  to A 10  which are magnetised radially in alternate senses, 
         FIG. 2  shows a cross-sectional view of a motor according to the prior art, with the stator including 6 poles  51  to  56 , 
         FIG. 3  shows a cross-sectional view of a motor according to the prior art, with the stator including 12 poles  71  to  82 , 
         FIG. 4  shows a cross-sectional view of a motor according to the prior art, with the stator including 6 wide poles P 1  to P 6  and 6 narrow poles P 7  to P 12 , 
         FIG. 5  shows a diagram showing the evolution of the constant of the couple per ampere/turn depending on the tooth width respectively, for 6- and 12-tooth stators, 
         FIG. 6  shows a curve showing the evolution of a detent torque with respect to the tooth width of a 6-tooth stator. 
         FIG. 7   a  shows a cross-sectional view of a motor according to the invention, with the ratio of the width of the wide tooth to the narrow tooth being particularly high, 
         FIG. 7   b  shows a cross-sectional view of a motor according to the invention, with the stator  1  exhibiting 6 recesses  101  to  106  on the outer surface thereof, positioned between the coils, 
         FIG. 8  shows a cross-sectional view of a motor according to the invention, with the stator  1  including 3 wide teeth  111  to  113  and 9 narrow teeth  117  to  125 , 
         FIG. 9  shows a cross-sectional of view of a motor according to the invention, with the stator  1  including 3 wide teeth  111  to  113  and 6 narrow teeth  126  to  131 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the motors of the prior art such as shown in  FIGS. 2 and 3 , a low detent torque is wanted by widening as much as possible the stator poles  51  to  56  and  71  to  82 , which leaves a small width of cut-out for introducing the winding wire. This makes the manufacturing of the coil windings  61  to  66  and  91  to  96  complex, difficult to automate and does not give very good fill factors. What is true for the 6-tooth stator is increased for the 12-tooth stator. As shown in  FIG. 5 , however, the performances are clearly more advantageous for the 12-tooth stator with a torque constant per ampere/turn which is increased by more than 30%. 
     In the motors of the prior art such as shown in  FIG. 4 , narrow poles P 7  to P 12  are introduced into the stator structure between the wide poles P  1  to P 6 . This efficiently makes it possible to take advantage of the increase in the couple per ampere/turn without reducing the volume of copper. But then again the stator poles fill almost the whole inner circumference of the stator and leave only narrow cut-outs to introduce the coil winding wire. This quasi continuity of the inner circumference of the stator makes it possible to obtain low torques detent torques for the motor. 
       FIG. 6  shows that, contrarily to what is usually made, narrow stator poles also make it possible to obtain very low detent torques. As a matter of fact, as the curve of the detent torque drawn according to the tooth width of a 6-tooth stator shows, there are 2 tooth widths for which the detent torque is null. It will thus be possible to determine 2 tooth widths for a motor, making it possible to obtain a low detent torque. These 2 tooth widths correspond on the one hand to a relatively wide tooth and on the other hand to a tooth having a width smaller than half the preceding one. 
     The motor according to the invention, as shown in  FIG. 1 , shows a 12-tooth structure which gives the best torque constant and uses for the stator  1  width teeth  11  to  16  and narrow teeth  17  to  22  making it possible to obtain a very low detent torque through an advisable selection of the poles widths. The width ratio of the wide teeth to the narrow teeth is then 2.2. 
     The coil windings  41  to  46  are positioned around the wide teeth  11  to  16 , which makes it possible to obtain the maximum torque per ampere/turn of the motor. As a matter of fact, this torque per ampere/turn depends on the width  31  of the coiled tooth but is not affected by the width  33  of the not coiled teeth, so long as the latter exhibit no saturation. 
     Using narrow poles for not coiled teeth makes it possible to have a very wide cut-out for the coil winding to go through. The width  32  of such cut-out is greater than the width  33  of a narrow tooth. This particular geometry of the stator makes it possible to manufacture the coil  41  to  46  separately and to insert them into the stator, around the wide teeth  11  to  16 . Thus, the length of the coil winding and thus the volume of copper are optimised and the coils can come flush with the pole shoe of the tooth, which minimizes the leakage flux. 
     Advantageously, the stator  1  includes, between the coils  41  to  46 , holes  47   a  to  47   f  for the passage of fixing members on a support, for a example screws or rivets. The geometry of the stator makes it possible to position the holes  47   a  to  47   e  within the outer diameter of the coils, which makes it possible to provide for an assembly of the motor without any outer tube. This makes it possible to reduce the number of parts and entails a gain in weight and in dimensions, with the diameter of the stator  1  thus becoming the outer diameter of the motor. 
     The motor in  FIG. 7   a  shows a preferred embodiment of the invention. The width ratio of the wide teeth  11  to  16  to the narrow teeth  17  to  22  is thus above 4, which makes it possible to increase the copper section of the coil windings and also to reduce the leakage permeance between the teeth. 
       FIG. 7   b  shows a preferred embodiment of the invention. As one of the limits of the electric motors is the temperature rise thereof, it is important to improve the surfaces of thermal exchange with the outside. For this purpose, the stator includes recesses on the outer surface thereof, which make it possible to increase the exchange surface thereof with the outside in an important way. Such recesses do not disturb the path of the magnetic field lines since the flux generated by each coiled pole is closed by the 2 adjacent poles. 
     The motors according to the invention and shown in  FIGS. 1 and 7  are perfectly balanced as regards the radial forces with or without current because of their perfect diametral symmetry. For cost reasons, reducing the number of coils may be preferred in certain applications. In this case, only one wide tooth out of 2 will be coiled and the presence of radial forces will be accepted during the power supply to the coils. 
       FIGS. 8 and 9  show embodiments with only 3 coils, but with stator structures which still guarantee the low detent torque of the motor. 
     The motor represented in  FIG. 8  shows 9 narrow teeth  117  to  125  and 3 wide teeth  111  to  113  supporting 3 coil windings  114  to  116 . Not coiled wide teeth have thus been converted into narrow teeth, which makes it possible to increase the angle  140  between the coiled large teeth and the adjacent teeth. 
     The motor represented in  FIG. 9  shows 6 narrow teeth  126  to  131  and 3 wide teeth  111  to  113  supporting 3 coiled windings  114  to  116 . In this case, non coiled wide teeth have been eliminated. This makes it possible, as in the previous case, to increase the angle  140  between the coiled tooth and the adjacent teeth and thus to free spaces  151  to  153  allowing for example the insertion of position magnetic sensors  154 ,  155 , and  156  cooperating with the rotor  2  of the motor.