FR2831345A1 - Electrical machine with automatic mechanical flux reduction has stator and two part permanent magnet rotor one part being connected to an output shaft and the other able to rotate relative to first - Google Patents

Abstract

The electrical machine (1) is synchronous, has a stator (2) and rotor (3). The rotor has four permanent magnets (13) is built in two longitudinal sections (9,10). The first section has an output shaft (8) and the second section is able to rotate through limited angles relative to the first. The rotation causes a portion of magnetic flux to be diverted through the rotor thus reducing the airgap flux for variable speed operation An Independent claim is included for: A longitudinally divided rotor with limited relative angular rotation able to reduce stator airgap flux by diversion through rotor

Description

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Electric machine with mechanical flux reduction
The present invention relates to the field of electrical machines, generally comprising a rotor and a stator, the stator being fixed for example to a casing and the rotor being coupled to a shaft.

 In the field of electric machines, synchronous permanent magnet machines are known, the use of which in electric drives is increasing. Indeed, the use of high performance magnets, in particular based on rare earths, makes it possible to obtain very compact and very efficient synchronous machines.

 The particularity of synchronous machines with permanent magnets is that they have an inductive flux created by permanent magnets, that is to say, not modifiable by external intervention. The stability of the inductive flux is used to obtain large mass torques.

 The stability of the inductor flow has a drawback in the case of variable speed operation. Indeed, the electromotive force at the terminals of the machine is proportional to the speed. If you want to operate at constant voltage and variable speed, you must modulate the resulting flow in the machine. This can be done by circulating currents in the stator winding so that the flux produced by these currents opposes the flux of the magnets. However, given the low reactance of these machines, the currents necessary to regulate the flow are large, of the order of the nominal current. The Joule losses thus created in the stator winding are very high and significantly reduce the efficiency of the machines, in particular under low load and high speed conditions.

 There is therefore a need to modify the flux generated by a rotor inductor with permanent magnets, mechanically, without an external command or control device, in particular without additional electric or hydraulic actuator, without stator magnet, without additional circuit not being inserted. not in the

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 classic geometry of a rotor, with automatic configuration change, for example based on the direction of rotation and making it possible to reduce the air gap induction and consequently the iron losses. The article "Efficiency of a permanent magnet polyphase machine with a mechanical field weakening" by MM. Rattei and Kraper published in ICEM 2000, pages 1688 to 1691, 28-30 August 2000 ESPOO Finland, describes an electric machine rotor provided with surface magnets arranged on four rings, two integral in rotation with the shaft and two capable of rotate so that the polarity of the magnets can be alternated in the axial direction, from which it follows that the flux resultant and the electromagnetic force induced in the stator winding are zero.

 However, the machine described in this article is with surface magnets. Consequently, only the vector component of the magnetic fields is variable.

 The present invention provides a rotor and an electrical machine with reduced loss in magnetic materials.

 The present invention provides a rotor with embedded or buried magnets constructed in such a way that the components of the magnetic fields can be reduced.

 The electric machine, according to one aspect of the invention, comprises a stator, a rotor, and an output shaft. The rotor comprises a first part connected to the output shaft and a second part capable of rotating relative to the first. The machine includes means for diverting part of the flow internally in the rotor so that the flow in the stator is reduced. The flux in the magnetic parts of the stator is thus reduced, which reduces the iron losses while benefiting from a rotor defluxing reducing the Joule losses. This gives a satisfactory yield.

 In one embodiment of the invention, the rotor comprises a carcass and a plurality of permanent magnets embedded in the carcass.

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 In one embodiment of the invention, the machine comprises a wound stator and a rotor with permanent magnets, the machine being of the synchronous type.

 In one embodiment of the invention, the second part is movable in rotation relative to the first part over a predetermined angular sector, the bypass of the flow internally in the rotor takes place between the poles of the second part and the poles of the first part, and varies depending on the angle.

 Said angular sector may be less than the angle between two poles of the machine.

 In one embodiment, the rotor comprises two mechanical stops limiting the rotation of the second part relative to the first.

 Advantageously, the rotor comprises a mechanical-elastic coupling between the first and second parts. Said elastic mechanical coupling can be equipped with a torsion bar. Said mechanical coupling can be active on a fraction of said angular sector.

 A simple construction machine is thus obtained, with a conventional stator and a simple rotor not requiring additional mechanical or electrical connections with the outside.

 The invention also provides a rotor for an electrical machine, comprising a first part connected to an output shaft and a second part capable of rotating relative to the first. The rotor includes means for diverting at least part of the flow internally in the rotor so that the flow in the air gap is reduced.

 The invention also provides an alternator-starter equipped with such a rotor. A vehicle can be equipped with a powertrain comprising an alternator-starter.

 The rotor can be divided into two parts, of similar or different dimensions. The movable part is able to move in rotation between a first position in which the flux sent to the stator is maximum and a second position in which the flux

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 sent to the stator is reduced, a large and determined part of the flow passing internally in the rotor. The minimum flux depends on the ratio of the air gap reluctances and the internal reluctances of the rotor.

 The present invention will be better understood and other advantages will appear on reading the detailed description of a few embodiments taken by way of non-limiting example and illustrated by the appended drawings in which: - Figure 1 is a perspective view an electric machine according to the invention; Figures 2 and 3 are schematic perspective views of a rotor according to the invention; - Figure 4 is a diagram of the evolution of the air gap induction and the electromotive force as a function of the rotation of the mobile part of the rotor (electrical angle); - Figures 5 to 9 are evolution diagrams of the torque exerted between the two parts of the rotor (mechanical angle) according to different embodiments; and - Figure 10 is a schematic view in axial section of a rotor according to an embodiment of the invention.

 As can be seen in FIG. 1, the electric machine 1 comprises a stator 2 and a rotor 3. The stator 2 has the general shape of a hollow cylinder of revolution with an axial through hole in which the rotor 3 is placed.

 The stator 2 comprises a carcass 4 made of magnetic material, for example a stack of magnetic sheets. The carcass 4 comprises on its bore a plurality, here twelve, of axial notches 5 extending substantially over the entire length of the carcass 4. In the notches 5, is disposed a coil 6 connected in a manner not shown, to means supplying the electric machine 1. Between the notches 5, the carcass 4 has a corresponding number of radial teeth 7 extending between the bore of the stator 2 and the notional circle passing through the bottom of the notches 5.

 The rotor 3 is of the four-pole type and has a general shape of cylinder of revolution with an outside diameter slightly

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 lower than the bore of the stator 2. The rotor 3 comprises a shaft 8, on which are mounted a part 9 integral in rotation with the shaft 8 and a part 10 capable of moving in rotation relative to the shaft 8. Parts 9 and 10 are axially contiguous. The respective length of parts 9 and 10 can very easily be adapted to each application according to the desired flow variation. The construction of parts 9 and 10 is relatively similar in that they both comprise a carcass 11,12 respectively and four magnets 13,14 respectively forming four poles. The magnets 13 and 14 are said to be embedded in the sense that they extend substantially between the shaft 8 and the periphery of the rotor 3. The carcass 11, 12 is formed from a stack of magnetic sheets. Each carcass 11, 12 is formed of four parts each having a cross section in a quarter of a circle.

 As can be seen more particularly in FIGS. 2 and 3, the part 10 can rotate at an angle O relative to the part 9. To allow this rotation, provision may be made for mounting the part 10 on the shaft 8 by via bearings, possibly rolling bearings, not shown. More particularly, a bearing could be provided with a determined coefficient of friction to allow a friction torque to remain during the rotation of the part 10 relative to the part 9.

 In FIG. 2, the part 10 is in a first position relative to the part 9 in which the poles 13 and 14 are aligned. The electric machine 1 then behaves like a conventional machine, the rotor of which is constructed in one part, similar for example to part 9. The flux passing through the stator is then the maximum flux which is none other than the nominal flux in the case of a conventional construction machine.

 In FIG. 3, it can be seen that the part 10 is in a second position after having rotated by the angle 0 relative to the part 9. The magnets are then angularly offset by the angle 0. As a result, the flux can circulate at least partially inside the rotor 3 according to the flow path partially shown and referenced 15.

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 More generally, the air gap induction is maximum in the first position and then decreases as the angle between the poles 14 and 13 increases, that is to say between the parts 10 and 9. The electromotive force is created jointly by the two half-rotors (parts 9 and 10) and decreases faster than the induction in each air gap because a phase shift appears between the two induction components when the phase angle between parts 9 and 10 increases. It can be noted that the electromotive force is canceled for an electrical angle of 1800 of phase shift although the air gap induction does not cancel, and this due to the phase opposition between the two induction components. In addition, there are interaction forces between the two parts 9 and 10 of the rotor 3 due to the leakage fluxes exchanged between said parts 9 and 10. These interaction forces create a torque which tends to align the south poles d 'one part with the north poles of the other part, which corresponds to the minimum flux position.

 In other words, the rotor 3 tends by construction to take the second position, illustrated in FIG. 3, mechanically limited by stops. In addition, two other types of force can be applied to the mobile part 10, in particular the force created by a mechanical spring system placed between the shaft 8 and the mobile part 10 and / or the force created by the interaction. stator currents and magnets of said mobile part 10. The evolution of the air gap induction and of the electromotive force as a function of the rotation of the mobile part of the rotor is illustrated on the two curves of FIG. 4.

 The evolution of the torque exerted between the two parts 9 and 10 of the rotor 3 as illustrated in FIGS. 1 to 3 is illustrated in FIG. 5.

 We see that the torque changes between a minimum noted r for a high phase shift angle between the two parts 9 and 10 and a maximum noted Fc for a minimum phase shift angle between the parts 9 and 10.

The minimum and maximum angles are determined by mechanical stops which limit the rotational stroke of the mobile part 10.

 As long as the torque created by the currents on the mobile part 10 of the rotor 3 remains greater than-rg, the mobile part 10 does not change

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 position relative to the shaft 8. The position denoted B is stable. If, on the other hand, the torque drops below de-rob the mobile part 10 changes position. Its new equilibrium position is that for which the internal torque of the rotor is the opposite of the electromagnetic torque. For a torque less than-rc, the mobile part 10 finds itself in abutment at point C, that is to say in abutment for maximum flux.

 The operation which has just been described above can be largely modified by adding a mechanical spring device placed for example between the movable part 10 and the shaft 8.

 In Figure 6, we see that the torque of the springs is added and tends to shift the torque curve upwards. The reduced flow stop position is reinforced in the sense that the mobile part 10 leaves it only for a higher torque value with greater than Fg.

 In FIG. 7, a couple of negative springs is provided which tends to decrease the value of total torque with a position B2 reached for a negative torque r B2. To reach the position B2, this time it is necessary to apply a positive torque to the mobile part 10. The position C2 at the maximum flow stop is reached for a lower torque than the position C illustrated in FIG. 5.

 It is also possible to use a spring system which creates a torque only for a fraction of the angular extent, see FIG. 8. In this configuration, good rigidity is maintained in order to reach the point C3 of maximum flow stop and it facilitates the position changes for low flows. The rest position in the absence of current supply to the electric machine is not the position B3 of reduced flow stop but the point for which the torque is canceled.

 In FIG. 9, a more elaborate spring torque curve is shown which can be obtained with a passive mechanical device made up of springs, rollers and cams giving the possibility of adjusting the shape and the amplitude of the mechanical torque. The torque resulting from the torque of the springs and electromagnetic torque is very low, which facilitates changes in the angular position of the

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 movable part 10. For any positive torque, the stable position remains the position r with minimum flux. With a small negative torque, the mobile part 10 changes position and the flow varies.

 In FIG. 10 is illustrated an example of the mechanical device with an elastic coupling between the mobile part 10 and the shaft 8.

 The shaft 8 is hollow, pierced with an axial hole 16 and having axially at the level of the fixed part 9 an additional thickness 17 on its periphery. The fixed part 9 is mounted directly on the periphery of the extra thickness 17. The movable part 10 is mounted on a sleeve 18 of which it is integral, said sleeve 18 being able to rotate relative to the shaft 8 on which it is mounted. The sleeve 18 and the extra thickness 17 have the same outside diameter which makes it possible to standardize the manufacture of the movable part 10 and of the fixed part 9 which may be identical or at least of the same thickness.

 In addition, the rotor 3 comprises a transverse finger 19 integral with the sleeve 18 and projecting into two diametrically opposite circular holes in the sleeve 18 and in two diametrically opposite holes 21 of elongated shape, formed in the hollow shaft 8. The finger 19 passes through the axial hole 16. A torsion bar 22 is disposed in said axial hole 16 while being integral with one end of the finger 19 and therefore with the movable part 10 and at its opposite end integral with the shaft 8. The torsion bar 22 is coaxial with the rotor 3 and extends over a major part of its length. An elastic connection is thus produced between the two parts 9 and 10 of the rotor 3.

 Thanks to the invention, there is an electric machine equipped with mechanical defluxing means, the operation of which is entirely autonomous and does not require either an actuator, a stator magnet or an additional electrical circuit. In addition, the variation of the flow can be automatic, for example based on the direction of rotation. The defluxing reduces the air gap induction and consequently the iron losses and avoids the use of defluxing current generating Joule losses. The electric machine according to the invention is particularly suitable for variable speed drives, especially those requiring a large positive torque at low speed

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 and a low negative torque at medium or high speed, in particular the starter alternators associated with thermal engines of motor vehicles.

Claims (10)

1. Electric machine (1), comprising a stator (2), a rotor (3), and an output shaft (8), the rotor comprising a first part (9) connected to the output shaft, and a second part (10) capable of rotating relative to the first, characterized in that it comprises means for deriving part of the flow internally in the rotor so that the flow in the stator is reduced.  2. Machine according to claim 1, characterized in that the rotor comprises a carcass (11) and a plurality of permanent magnets (13) embedded in the carcass.  3. Machine according to claim 1 or 2, characterized in that it comprises a wound stator and a permanent magnet rotor, the machine being of the synchronous type.  4. Machine according to any one of the preceding claims, characterized in that the second part is movable in rotation relative to the first part over a predetermined angular sector, the bypass of the flow internally in the rotor being a function of the angle between the poles of the second part and the poles of the first part.  5, Machine according to claim 4, characterized in that said angular sector is less than the angle between two poles of the machine.  6. Machine according to claim 4 or 5, characterized in that the rotor comprises two mechanical stops limiting the rotation of the second part relative to the first.  7. Machine according to any one of the preceding claims, characterized in that the rotor comprises an elastic mechanical coupling between the first and second parts.  8. Machine according to claim 7, characterized in that said elastic mechanical coupling comprises a torsion bar (22). <Desc / Clms Page number 11>  9. Machine according to claim 7 or 8, characterized in that said mechanical coupling is active on a fraction of said angular sector.  10. Rotor (3) for an electrical machine, comprising a first part (9) connected to an output shaft (8), and a second part (10) capable of rotating relative to the first, characterized in that it comprises means for diverting part of the flow internally in the rotor so that the flow in the stator is reduced.  He. An alternator-starter comprising a stator and a rotor according to claim 10, intended to be mounted on a vehicle powertrain.