Abstract:
A rotary electrical machine comprises a rotor including a plurality of poles and a plurality of windings (E 0 -E 7 ) wound about the poles, a stator, and a compensation device configured to compensate the armature magnetic reaction of the stator. The compensation device comprises at least one permanent compensation magnet which is implanted in a pole(s) of the rotor. The magnet(s) is/are sized and placed in accord with the desired compensation effect desired.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS AND CLAIM TO PRIORITY 
     This application relates to International Application No. PCT/FR2010/050939 filed Mar. 8, 2010 and French Patent Application No. 09/52804 filed Apr. 29, 2009, of which the disclosures are incorporated herein by reference and to which priority is claimed. 
     FIELD OF THE INVENTION 
     The present invention relates to a rotary electrical machine such as an alternator, a starter, or an alternator-starter for a motor vehicle. More particularly, the invention relates to a rotary electrical machine which is equipped with means for compensation for the armature magnetic reaction. 
     BACKGROUND OF THE INVENTION 
     The phenomenon of the armature magnetic reaction is known to persons skilled in the art. The armature magnetic reaction occurs when a current is circulating in the armature of the rotary electrical machine. 
     Taking as an example an alternator for a motor vehicle in which the armature is the stator of the machine, and the inductor is its rotor, the armature magnetic reaction occurs when the alternator is live and is discharging a current. The stator coils in which a current is circulating then produce a so-called armature reaction magnetic field, which can give rise to saturation and downgrade the magnetic field of the inductor. Subsequently, the electromagnetic force emf) is distorted (with possible saturation), and the performance of the alternator, in particular in terms of output, is adversely affected. This distortion of the emf produces harmonics which give rise to an increase in the Joule losses and iron losses of the machine. In addition, the performance relating to the acoustic vibrations and electromagnetic compatibility can also be affected by the inductor reaction. 
     In the field of rotary electrical machines with a high power level, it is known in the prior art to provide so-called armature reaction compensation auxiliary coils. These coils, which are included in the stator of the machine, produce a compensation magnetic field which opposes the armature reaction magnetic field, such as to reduce the effects. 
     The above-described solution can provide satisfactory compensation for the armature magnetic reaction in a rotary electrical machine, but it is unsuitable for the motor vehicle field, in which the constraints of weight, compactness and cost are extremely strict. 
     It is therefore desirable to propose a solution for compensation for armature magnetic reaction which is designed for the rotary electrical machines which are used in motor vehicles. 
     SUMMARY OF THE INVENTION 
     According to a first aspect, the present invention provides a rotary electrical machine comprising an inductor in the form of a rotor, an armature in the form of a stator, and means for compensation for the armature magnetic reaction. According to the invention, the means for compensation for the armature magnetic reaction comprise at least one permanent compensation magnet which is implanted in a pole of the inductor. 
     According to a particular characteristic, the means for compensation for the armature magnetic reaction comprise a permanent compensation magnet which is implanted in each of the poles of the inductor. 
     According to another particular characteristic, the means for compensation for the armature magnetic reaction comprise two permanent compensation magnets which are implanted in each of the poles of the inductor. Preferably, each of the two permanent compensation magnets is implanted in one half of the corresponding pole. 
     According to particular embodiments, a length of the permanent compensation magnet occupies approximately 30 to 45% of a half-width of the corresponding pole. In addition, the permanent compensation magnet has a residual magnetic field of between 0.8 and 1.4 Tesla, it is positioned at a distance of between approximately 2 to 6 mm relative to an end of the corresponding pole, it has a length of between approximately 1 and 6 mm, and a thickness of between approximately 1 and 4 mm. 
     According to a preferred embodiment, the permanent compensation magnet is positioned at a distance equal to 5.3 mm relative to an end of the corresponding pole, it has a length equal to 4.4 mm, and a thickness equal to 2 mm. 
     According to another characteristic, the rotary electrical machine according to the invention which has been described briefly above also comprises a progressive air gap. According to a particular embodiment, this progressive air gap varies between approximately 0.4 and 0.7 mm. 
     According to another characteristic, the permanent compensation magnet of the rotary electrical machine according to the invention is of the surface or buried type. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other characteristics and advantages of the invention will become apparent from reading the following description of one of its particular embodiments, with reference to the following figures, in which: 
         FIG. 1  shows a simplified general structure of a rotary electrical machine of the type with projecting poles; 
         FIGS. 2A to 2C  are skeleton diagrams used to explain the phenomenon of armature magnetic reaction; 
         FIG. 3  is a real measurement curve of the magnetic field present in an air gap of the machine; 
         FIG. 4  is a partial view in cross-section of a rotary electrical machine according to the invention, equipped with permanent magnets for compensation for the armature magnetic reaction; 
         FIG. 5  is a curve for measurement of the magnetic field present in an air gap of the machine, which shows the effect of compensation for the armature magnetic reaction provided by the permanent compensation magnets; 
         FIG. 6  is a partial view in cross-section showing the position of a permanent compensation magnet in a corresponding pole of the machine; 
         FIG. 7  is a partial view in cross-section of a rotary electrical machine according to the invention equipped with two permanent magnets for compensation for the armature magnetic reaction; 
         FIG. 8  shows the torque curves obtained with and without compensation magnets when the rotary electrical machine is functioning in engine mode; 
         FIG. 9  is a partial view in cross-section of a rotary electrical machine according to the invention comprising a progressive air gap; and 
         FIG. 10  is a curve for measurement of a magnetic field present in an air gap of the machine, showing the effect of compensation for the armature magnetic reaction provided by the progressive air gap of the machine in  FIG. 9 , and 
         FIG. 11  is a partial cross-sectional view of the rotary electrical machine according to the invention with the permanent compensation magnet of the buried type. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows in simplified form the structure of a rotary electrical machine  1  of the type with projecting poles. The machine  1  comprises a stator  10  and a rotor  11 , and is without current compensation means. 
     The stator  10  is provided with a plurality of notches  101  which are designed to receive stator coils (not shown). The rotor  11  comprises 4 pairs of poles with North (N) and South (S) polarities, consisting of poles S 110 , N 110 , S 111 , N 111 , S 112 , N 112 , S 113  and N 113 . 
     An excitation coil  114 , which defines a field coil generating a magnetic field, is provided in the rotor  11  of the machine  1 , and comprises eight windings in series, E 0  to E 7 , which are localized respectively at the level of the eight projecting poles of the machine  1 . Each of the windings E 0 -E 7  is wound about one of the projecting poles S 110 , N 110 , S 111 , N 111 , S 112 , N 112 , S 113  and N 113 . An excitation current Iex supplies the excitation coil  114 , such as to produce magnetic fields with the required polarities at the level of the poles S 110 , N 110 , S 111 , N 111 , S 112 , N 112 , S 113  and N 113 . 
     When the machine  1  is functioning in an alternator mode and is discharging current, an armature magnetic reaction occurs which is now described with reference to  FIGS. 2A, 2B and 2C . The armature magnetic reaction is a change in a magnetic field of the field coil of the rotor  11  caused by a magnetic field induced by current flowing through the stator coil of the stator  10  in the alternator mode. 
       FIGS. 2A, 2B and 2C  show schematically the magnetic fields in an air gap e of the machine  1 , at the level of a pole N of the latter,  FIGS. 2A, 2B and 2C  show the pole N with an excitation winding E, as well as conductors CS of the stator coil which are accommodated in notches  101  ( FIG. 1 ) in the stator. 
       FIG. 2A  shows a magnetic field FI, Which is a magnetic field produced by the pole N in the air gap e when the machine  1  is functioning without charge, i.e. when no current is circulating in the stator coil. The wave form of the field FI is then substantially regular in the air gap assembly in relation to the pole N. 
       FIG. 2B  shows the effect of the armature magnetic reaction in the form of a field FRI, Which is an armature reaction magnetic field produced by the conductors CS of the stator coil, when a charge current is circulating in the conductors. As shown in  FIG. 2B , the field FRI comprises positive alternation and negative alternation. 
       FIG. 2C  shows a field FC which is a resultant magnetic field present in the air gap e when the machine is discharging a charge current. The field FC is the sum of the fields FI and FRI. As shown in  FIG. 2C , the field FC is significantly deformed and dephased in comparison with the field FI produced when the machine  1  is functioning without charge. In addition, saturation of the magnetic current (cf. reference SA in  FIG. 2C ) may occur as a result of this armature magnetic reaction. Saturation of this type may give rise to a loss of performance. 
     The wave forms of the magnetic fields FI, FRI and FC shown in  FIGS. 2A, 2B and 2C  are theoretical curves, which are simplified for the needs of the explanation. 
       FIG. 3  shows a real measurement curve of the magnetic curve FC which is present in the air gap of a rotary electrical machine with projecting poles. The x-axis shows the angular position of the rotor of the machine, and the y-axis shows the Tesla value corresponding to the magnetic field FC. The wave form of the field FC comprises peaks which are essentially caused by the presence of the notches in the stator. A wave form of this type comprises many harmonics which downgrade the performance of the rotary electrical machine. 
     According to the present invention, means are incorporated in the poles of the rotor of the rotary electrical machine, for compensation for magnetic reaction in the form of permanent magnets. The magnetic flow which is produced by the permanent magnet then opposes that of the armature magnetic reaction, thus decreasing the effect of the latter. 
       FIG. 4  shows permanent magnets AC N1  and AC S1  included in two successive poles of type N and S of a rotary electrical machine with projecting poles, such as the machine  1  in  FIG. 1 . 
     The permanent magnets AC N1  and AC S1  form part of the means for compensation for the armature magnetic reaction according to the present invention. In this embodiment, a permanent magnet AC is included in each of the poles of the rotary electrical machine, which is not necessarily the case in other applications of the invention. 
     The effect of the permanent magnets AC on the magnetic field FC in the air gap of the machine is shown in  FIG. 5 . The addition of the permanent magnets AC to the magnetic field FC is shown by the portions P AC  in bold on the curve in  FIG. 5 . The permanent magnets AC provide compensation for the armature magnetic reaction by correcting the intensity of the magnetic field FC, as shown in  FIG. 5 . This correction improves the frequential spectrum of the magnetic field FC in the sense of reduction of the harmonics which affect adversely the performance of the machine. 
     In order to obtain the required compensation for the armature magnetic reaction, it is desirable to optimise different parameters, in particular the position of the permanent magnet AC in the pole, as well as the dimensional and residual magnetic field characteristics of the magnet. 
     Tests and simulations have been carried out by the inventive organisation, in particular on a rotary electrical machine for a motor vehicle with a nominal power of 40 kW, an outer diameter of 200 mm, and a length of 240 mm. 
     Permanent magnets AC which have a residual magnetic field of between 0.8 and 1.4 T give good results for motor vehicle applications. However, other residual magnetic field intensities could also be appropriate, depending on the applications. 
     With reference to  FIG. 6 , the characteristics of dimensions and position of the magnets AC are defined by the following parameters: 
     La=Length of the magnet AC in the direction perpendicular to the radius of the rotor  11 ; 
     Ha=thickness of the magnet AC in the radial direction of the rotor  11 ; 
     Da=distance between the magnet AC and an end of the pole in the direction perpendicular to the radius of the rotor  11 ; 
     Lp=width of the pole in the direction perpendicular to the radius of the rotor  11 . 
     In this embodiment with a single magnet AC per pole, the magnet AC is situated in the first half of the width ½ LP of the pole. The first half of the pole is considered in this case to be the one which corresponds to the front part of the pole, determined by the direction of rotation of the rotor of the machine. In other words, the magnet AC is offset from a center line Lc of the pole, as best shown in  FIG. 6 . 
     For the above-described rotary electrical machine for a motor vehicle, the following optimum values have been determined: La=4.4 mm and Da=5.3 mm for a thickness of magnet of Ha=2 mm. 
     It will be appreciated that the aforementioned optimum values are non-limiting, since, depending on the applications, different values can be determined by persons skilled in the art. 
     Thus, depending on the applications, the length La of the magnet AC can represent approximately 30 to 45% of the half-width ½ LP of the pole. In addition, the magnet AC can advantageously be positioned at a distance Da Which will be between approximately 2 and 6 mm. The length La of the magnet AC can be between approximately 1 and 6 mm, and the thickness Ha can be between approximately 1 and 4 mm. 
       FIG. 7  shows an embodiment in which two armature magnetic reaction compensation magnets AC 1  and AC 2  are provided for each pole. 
     In this machine in  FIG. 7 , the magnets AC 1  and AC 2  have opposite polarities, and are situated in the first and second halves of the pole. With alternation of the magnetic field FC, as shown in  FIG. 3 , one of the magnets, for example AC 1 , will have the effect of increasing the intensity of the magnetic field FC in the first part of the alternation, and the other magnet, for example AC 2 , will have the effect of reducing the intensity in the second part of the alternation. Correction of the distortion, caused by the armature reaction, of the wave form of the magnetic field, is thus carried out such as to obtain a more regular wave form, comprising fewer harmonics. As illustrated in  FIG. 7 , a direction of magnetization of the magnets AC 1  and AC 2  extends in the radial direction of the rotor  11 . 
     The present invention also provides the advantage of a clear improvement in the mechanical torque which is provided when the rotary electrical machine is functioning in engine mode. 
       FIG. 8  shows, according to the density of current in the stator, the torque provided by the aforementioned 40 kW machine functioning in engine mode at a speed of 3,000 rpm, depending on whether or not the machine is equipped with permanent magnets AC for compensation for the armature magnetic reaction. 
     A first curve with square points is that of the machine without the magnets AC. A second curve with round points is that of the machine with the magnets AC, and has torque values which are higher than those of the first curve. 
     The above-described embodiments comprise permanent magnets AC of the surface type, i.e. which abut the surface of the pole situated opposite the air gap, as shown in  FIGS. 4 and 9 . However, it will be noted that permanent magnets AC of the buried type (shown in  FIG. 11 ) can also be used in certain applications of the invention. 
     According to the present invention, a progressive air gap ep, as shown in  FIG. 9 , can be associated with the permanent magnets AC for compensation for the armature reaction. Each of the permanent compensation magnets AC produces a magnetic flow in the direction D MF , which is opposite to the direction D MR  of the armature magnetic reaction of said stator. 
     In the embodiment in  FIG. 9 , the air gap ep varies in the angular direction of the rotor  11  progressive from approximately 0.4 mm to approximately 0.7 mm, from one end to the other of the pole. The surface of the pole which is opposite the air gap ep and the notches in the stator is machined so as to obtain this progressive air gap ep. 
       FIG. 10  shows an example of the compensation which is provided by the progressive air gap ep. The effect of the progressive air gap ep on the wave form of the magnetic field is indicated, by the portions in bold P EP . The curve in  FIG. 10  shows only the compensation effect provided by the progressive air gap ep, i.e. without the compensation magnets AC. 
     It will also be noted that a compensation magnet AC can be provided in a divided manner, i.e. in the form of at least two magnets which are disposed side by side, and are separated by a thin wall of magnetic material (the rotor iron). An embodiment of this type makes it possible to reduce the losses in the magnet AC. 
     The invention has been described here within the context of a particular rotary electrical machine. It should be clear that the invention will have applications in a broader field, in other words that of synchronous machines. More particularly, the invention has a significant application in rotary electrical machines with a toothed rotor, i.e. machines of the Lundell type which are very widely used in motor vehicles. It will also be noted that the invention can be used in rotors which comprise interpolar permanent magnets.