WO2002023701A1

WO2002023701A1 - Synchronous machine with permanent magnets

Info

Publication number: WO2002023701A1
Application number: PCT/IL2001/000589
Authority: WO
Inventors: Alexander Radovsky
Original assignee: S.H.R. Limited Bvi
Priority date: 2000-09-18
Filing date: 2001-06-28
Publication date: 2002-03-21
Also published as: AU2001267806A1

Abstract

A synchronous electrical machine the stator of which includes at least one core having an armature winding thereon. The core has spaced-apart ends which are aligned with an annular permanent magnet rotor whose cross-sectional geometry defines an even number of complementary pole sections, each having an inner and an outer surface, one having the curvature of a circle, the other having a curvature giving rise to an EMF that follows a sine curve. The magnet is axially polarized, the polarization of each pole section being the reverse of the adjacent pole sections. As a consequence, when the permanent magnet rotates to traverse the ends of the core it produces alternating sinusoidal variations in the lines of magnetic flux passing through the core.

Description

SYNCHRONOUS MACHINE WITH PERMANENT MAGNETS

FIELD OF THE INVENTION

This invention relates generally to synchronous electrical machines, and in particular to a machine of this type having a permanent magnet rotor.

BACKGROUND OF THE INVENTION A synchronous electrical machine is capable of functioning as a single or as a multi-phase AC generator, or as a motor depending on whether the torque applied thereto is negative or positive. A conventional machine of this type has a polyphase armature winding on a stator core and a rotor field winding which is excited by DC applied thereto by brushes engaging slip rings. But when the synchronous electrical machine has a permanent magnet, rotor brushes and slip rings are then unnecessary. A permanent magnet rotor for a synchronous machine comes in various configurations such as a star-like or a claw-like shape and a prismatic shape, the magnet being radially or tangentially magnetized.

The concern of the present invention is with a synchronous machine whose permanent magnet rotor has an annular or ring-like configuration and is axially magnetized. Of prior art interest therefore is U.S. Patent to Oudet 4,658,166 disclosing a synchronous machine whose annular, disc-shaped permanent magnet is formed of samarium-cobalt. This annular magnet traverses an air gap in the stator magnetic circuit with which it is associated. Also of prior art interest is U.S. Patent 5,523,637 to Miller which discloses a synchronous motor having a rotor formed of a series of permanent magnet rings mounted on a common shaft.

In a synchronous machine such as a machine functioning as an AC generator, it is important that the generated AC be in the form of a sinusoidal wave. An alternating current reverses direction periodically, and in its sinusoidal form, the current varies in intensity in accordance with a sine curve. Thus in the course of each

AC cycle, a positive sine curve is produced, followed by a negative sine curve.

Practical power systems require a sinusoidal form of current or voltage, for this makes possible a less expensive construction and greater efficiency in the operation of generators and motors, as well as transformers and other electrical devices.

A synchronous machine which yields only an approximation of a sinusoidal wave is not commercially acceptable. Thus in the above identified Miller patent which discloses a synchronous machine having a peπnanent magnet rotor, it is noted that while the magnetic flux density varies in a sinusoidal fashion, one area of the wave is not sinusoidal thereby making it necessary to minimize this area so that the wave approaches a truly synchronous form.

In a synchronous machine provided with a permanent magnet rotor and a stator core carrying armature windings, there are two possible ways by which one can ensure the creation of sinusoidal variations in magnetic lines of flux passing through the stator core. One way is to introduce a variable clearance between the rotor magnet and the stator core. Another way is to impart to the rotor magnet a spatially variable magnetization. However both of these approaches for creating a sinusoidal wave have disadvantages, for neither way makes optimum use of the mass of the permanent magnet. Another drawback of existing synchronous machines provided with a permanent magnet rotor, is that its stator is usually formed of a core body that is machined and notched to define a circular array of poles with slots therebetween. An armature winding distributed about the poles is received in the slots, which distributed winding includes winding ends that make no contribution to the magnetic field produced by the winding. Synchronous machines which include a stator structure of this type are difficult and costly to manufacture, and they are relatively large both in size and in weight.

SUMMARY OF THE INVENTION

In view of the foregoing, the main object of this invention is to provide an improved mono or multiphase synchronous machine having a permanent magnet rotor.

Among the significant advantages of a synchronous machine in accordance with the invention are the following:

(a) the machine is reliable and efficient in operation;

(b) the machine is formed of components that may readily be mass-produced and assembled, making it relatively inexpensive to manufacture;

(c) the size and weight of the machine are low as compared to existing synchronous machines having the same power rating;

(d) the machine yields an AC output which is truly sinusoidal and not just an approximation thereof.

More particularly, an object of the invention is to provide a synchronous machine which in a preferred embodiment has at least one stator core formed by complementary U-shaped components, each having a long leg and a short leg, the two components being combined to cause the large legs thereof to abut each other and to cause the short legs to be spaced apart to define an air gap which is traversed by annular permanent magnet. Also an object of this invention is to provide a synchronous machine having an annular permanent magnet rotor whose cross-sectional geometry is such that when the rotor traverses the air gap in the stator core, this gives rise to alternating sinusoidal variations in the magnetic lines of flux passing through the core. Briefly stated these objects are accomplished in a synchronous electrical machine capable of functioning on an AC generator or as a motor. The stator of the machine includes at least one core having windings thereon, the core including spaced-apart ends defining an air gap. The rotor of the machine is provided with an annular permanent magnet which when rotating traverses the air gap of the stator core.

The annular magnet is axially magnetized so that its opposite surfaces which face the ends of the core in the air gap have alternate magnetic polarities. The cross-sectional geometry of the annular magnet defines complementary pole sections, each having a cross-section area that varies in accordance with a sine curve. For example, the outer surface may have the curvature of a circle while the inner surface may have a curvature giving rise to an EMF that follows a sine curve, or vice versa. Hence when the permanent magnet rotates, alternating sinusoidal variations are produced in the lines of magnetic flux passing through the stator core. When the machine functions as a generator, it then yields a sinusoidal AC output. In one preferred embodiment of a synchronous machine in accordance with the invention, the stator is provided with at least one U-shaped core whose legs define spaced-apart pole ends. The annular, axially magnetized permanent magnet rotor is axially aligned with the ends of the stator core, so that when the annular magnet rotates, alternate sinusoidal variations are produced in the lines of magnetic flux passing through the core.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention as well as other objects and features thereof, reference is made to the annexed drawing wherein: Fig. 1 is a longitudinal section taken through a three-phase synchronous machine in accordance with a first embodiment of the invention;

Fig. 2 is a transverse section taken through the machine;

Fig. 3 illustrates the manner in which a core for the stator of the machine is made;

Fig. 4 is a perspective view of the annular permanent magnet having two poles included in the rotor of the machine;

Fig. 5 is a cross-section of the peπnanent magnet;

Fig. 6 illustrates a sine wave; Fig. 7 schematically shows a second embodiment of a synchronous motor having a permanent magnet in which the motor is directly coupled to the wheel of a vehicle;

Fig. 8 illustrates a third embodiment of the machine;

Fig. 9 shows a laminated cylinder from which the stator of a fourth embodiment of the machine is foπned;

Fig. 10 shows the machine stator armature;

Fig. 11 shows the machine with the stator and the rotor; and

Fig. 12 shows the machine with a double stator.

DETAILED DESCRIPTION OF INVENTION First embodiment: Referring now to Figs. 1 and 2, shown therein is a synchronous electrical machine in accordance with the invention capable of functioning as a generator producing a three-phase AC output voltage, or as a three-phase motor.

The machine in this example is provided with a generally hexagonal casing 10, which encloses the stator and rotor of the machine. The opposite ends of the casing are closed by bearing plates 11 and 12 having ventilation windows 11W and 12W therein. Mounted within casing 10 is a circular array of equi-spaced cores 13 to 18, the planes of the cores extending radially with respect to the central axis of the machine. Casing 10 is hexagonal only in the context of a three-phase machine having six phase zones. The casing may have a different shape in machines of other phases. Cores 13 to 18 are fabricated of a material having high magnetic conductivity and low electrical conductivity such as a solid ferrite or other magnetoceramic material. Each core, such as core 13 shown in Fig. 1, is constituted by two complementary U-shaped components having a short leg 13S and a long leg 13L parallel thereto. When the two core components are combined, the long legs 13L thereof abut each other at a junction J, whereas the short legs 13S are then separated by an air gap G. Hence the free ends of the short legs face each other in the air gap G. The complementary U-shaped component of the core when combined then have a C-shaped configuration. In practice, the core of the stator may be manufactured from a wound steel band in a manner similar to which C-shaped transformer cores are produced. But instead of cutting the core in the middle, as when making a transformer core, the parallel legs of the core, as shown in Fig. 3, are cut at displaced positions. The cuts in the core are displaced by a distance α in accordance with the following equation:

a = l_m / 2 + S where l_m is equal to the thickness of the annular peπnanent magnet rotor which is accommodated in the air gap of the core δ is the clearance between the core and the magnet.

Supported by bearings 11B and 12B mounted on end plates 11 and 12 of the casing is a rotor shaft 19 which extends along the longitudinal axis of the machine. The circular array of cores 13 to 18 are symmetrically disposed with respect to the machine axis X and radiate therefrom. Mounted on rotor shaft 19 is an annular permanent magnet 20 formed of a magnetizable material having a high remanence and coercive force, such as Alnico, platinum-cobalt, samarium-cobalt or other suitable materials. Since an annular peπnanent magnet in accordance with the invention must have a specially contoured cross-sectional geometry, a preferred material for the permanent magnet is a material that can be readily molded or otherwise shaped to assume the desired configuration; such as a magnetic material foπned by compacting metal powders or ceramic magnetic material such as Ba Feι₂ Oι₉ (Feπocube).

Any known material capable of being peπnanently magnetized is usable for the rotor, provided that it can without difficulty be shaped to assume that desired geometry, the nature of which will later be explained. To mount annular magnet 20 on shaft 19, the shaft is provided with a hub 21 joined by ribs 22 to the magnet. Extending outwardly from the annular magnet 20 are the vanes 23 of a radial fan which act to cool the machine, the air circulating through ventilation windows 11W and 12 W. Ribs 22 can also function as vanes of a fan.

Each C-shaped core (13 to 18) on the stator caπies a set of four armature coils Al to A4. Coils Al and A2 are mounted on the long legs 13L, while coils A3 and A4 are mounted on the short legs 13S of the core. The pre-wound coils are in a tubular solenoid form dimensioned to fit snugly onto a leg of the coil, rather than being wound about the coil or in a conventional distributed winding. The air gap G in each core is defined by the spaced-apart short legs 13S of the C-shaped core which carry the coils A3 and A4, which air gap is traversed by the rotating permanent magnet.

As shown separately in Figs. 4 and 5, the annular permanent magnet is shaped to define the complementary pole sections 20A and 20B of a magnet having two poles. The annular magnet, however, may have a greater even number of pole sections. Each section has an arcuate outer surface whose curvature is that of a circle, and a contoured inner surface whose curvature is such as to give rise to a sine curve of flux variations. The annular magnet has a uniform width and planar ends. The magnet is received within the air gap G of the stator core whose planar ends face the planar ends of the magnet. The width of the gap is slightly greater than the width of the annular magnet to provide a fixed clearance therebetween so that the magnet is free to rotate.

The annular magnet is axially magnetized whereby its opposite ends are alternatively polarized. Hence section 20A of the magnet may be magnetized so that it is North at one end and South at the other end, and section 20B is then magnetized so that it is South at the one end and North at the other. Fig 1 illustrates this relationship with respect to the pair of cores 13 and 16 providing one phase of the machine. It will be seen that the upper section of the annular magnet which traverses gap G in core 13 has a North-South polarization while the section of the magnet which traverses gap G in core 16 has a South-North polarization.

In the course of a single cycle of a sine wave S during time period T as shown in Fig. 6, the voltage E rises from a zero to a positive peak level and then back to zero, from which point the voltage goes to a negative peak level and then back to zero. The cross-sectional geometry of the annular magnet 20 conforms to that of the illustrated sine wave, for the upper pole section 20A corresponds to the positive half cycle of the wave, and the lower pole section coπesponds to the negative half cycle. It will be seen that the thickness of each pole section of the annular magnet goes from a minimum value at one end to a maximum value at the center line and then returns to a minimum value at the other end in accordance with a sine wave.

When therefore the permanent magnet rotates so that in the course of rotation the upper and lower sections of the magnet successively traverse the gap in a stator core, this results in alternating sinusoidal variations in the magnetic lines of flux passing through the core.

Hence when the machine functions as an AC generator, it will yield a sinusoidal output. As pointed out previously there are many advantages to be gained by using sinusoidal voltages, for it makes possible design simplifications, as well as a reduction in the cost of generators and motors.

In practice, each section of the annular permanent magnet may have differently contoured inner and outer surfaces giving sinusoidal variation of the annular permanent magnet cross-section area, such as an inner surface of circular curvature and an outer surface of sine curvature. While the latter geometry is the reverse of that illustrated in Fig. 5, the magnet has the same properties, for when it rotates, it produces alternating sinusoidal variations in the lines of magnetic flux passing through the core. Figs. 1 and 2 illustrate a three phase synchronous electrical machine having three sets of diametrically opposed cores carrying armature coils, one for each phase.

In practice however the machine may be aπanged to operate as a mono phase or dual phase machine.

Second Embodiment: As shown in Fig. 7, a synchronous motor in accordance with the invention may be used to directly drive the wheel W of a vehicle.

In this arrangement the structure of wheel W is integrated with that of the motor, for the cylindrical rim 21 of the wheel also functions as the rotor of the synchronous motor. Mounted on rim 21 is a tire 22, the wheel having a hub 23 within which is a bearing adapted to receive the axle on which the wheel turns.

Supported co-axially within cylindrical rim 21 by ribs 24 is an annular permanent magnet 25. The cross-sectional geometry and magnetization of this magnet is the same as that in the first embodiment. Associated with annular magnet

25 and disposed at a fixed position within rim 21 which now functions as the rotor of the machine is at least one set of stator cores 13 to 16. These correspond to cores 13 to 16 in Fig. 1 and have the same operative relationship to the annular rotor magnet.

Hence when the motor is electrically energized the rim of the wheel is caused to rotate to propel the vehicle. Third Embodiment: In the embodiment of the machine shown schematically in Fig. 8, the stator of this machine includes at least one U-shaped core 26 having parallel legs 27 and 28. Fitted onto leg 27 is an armature coil 29 and fitted onto leg 28 is an armature coil 30. In practice, the stator is formed by a circular array of such cores, the number of which depends on the number of phase zones. The U-shaped core has spaced-apart planar ends El and E2. The cross-sectional geometry of legs 27 and 28 may be rectangular, round or trapezoidal.

The annular multi-pole permanent magnet 31 is not mounted on a shaft as in the first embodiment, but is mounted against a steel disc 32 affixed to a shaft 33. Magnet 31 is supported on an insulating bushing 34 coaxial with the magnet. Suπounding annular magnet 31 is a ring 35 encircled by a steel band 36 of high-strength which protects the rotor from centrifugal forces. In practice, magnet 31 may be formed from a single piece or from several pieces which are joined together to create the desired annulus. The cross-sectional geometry of permanent magnet 31 is the geometry shown in Fig. 5 and defines complementary pole sections, each having a contoured inner surface whose curvature produces an EMF that follows a sine curve. The annular magnet is axially polarized, each two adjacent sections being polarized in reverse directions. It will be seen in Fig. 8 that when the polarization of the permanent magnet portion adjacent to end El of the core is North-South then that of the portion adjacent to end E2 of the core is South-North. Hence when annular magnet 31 rotates, then in the course of rotation, the ends of the core are traversed by reversely polarized sections of the magnet. As a result, induced in the lines of magnetic flux passing through the core are alternating sinusoidal variations, these being dictated by the sinusoidal variation of the cross-section of the annular magnet.

Fourth Embodiment: In this embodiment of a permanent magnet rotor synchronous machine, the stator cooperating with the rotor is derived from a hollow laminated cylindrical blank 37, as shown in Fig. 9, which is wound of a thin steel band.

As shown in Fig. 10, this cylindrical blank for a 3 -phase 6 phase-zone machine is machined to form a circular array of deep slots which define a crown 37C of six half-phase cores. The crown is composed of three pairs of half-phase cores

R-R, S-S and T-T. Each half-phase core has mounted thereon a semi-phase armature winding. In practice, the crown may have a different number of half-phase cores.

As shown in Fig. 11, this stator is associated with an annular permanent magnet rotor 38 whose contouring and polarization are the same as those of the annular magnet previously described. The peπnanent magnet rotor need not be machined of a single blank of metal, but its geometry must be such that as it rotates, it gives rise to alternating sinusoidal variations in the lines of magnetic flux passing through the stator core. Thus the permanent magnet rotor can be created by combining magnetic pieces inserted in a non-magnetic bushing. Magnet 38 is supported on the hub 39 of a shaft 40, one end of the annular magnet facing a pair of half-cores in the crown 37C of the stator.

In the course of each revolution of the annular permanent magnet, the magnet traverses all of the half-phase cores on the crown 37C of the stator. Since one end of the annular magnet is adjacent the stator cores whereas the opposite end of the magnet is magnetically vacant, in order to close the magnet flux lines, joined to the vacant end of the annular magnet is a steel ring 41. To decrease eddy current losses, the steel ring may be formed by a wound steel band. In operation, when the flux is at a maximal value in one half-phase cone, in the two neighboring half-phase cones, it is half of maximal and has the opposite direction. The synchronous machine may also be designed so that its permanent magnet rotor 38 as shown in Fig. 12 cooperates with a double stator 37A and 37B placed on either side of the rotor. In this assembly, there is no steel ring joined to the annular magnet. And instead of a stator made from a laminated-metal cylindrical blank, the stator may be cast from magnetic non-conductive material, such as magnetic ceramics.

While prefeπed embodiments of a peπnanent magnet synchronous machine have been disclosed herein, it is to be understood that many changes may be made therein without departing from the spirit of the invention. Thus the annular permanent magnet instead of being fabricated from a single piece of material may be formed of sections which are combined to form an annulus.

Claims

CLAIMS:

1. A synchronous electrical machine comprising:

(a) a stator provided with at least one core having an armature winding thereon, said core having spaced-apart ends; (b) a rotor provided with a pennanent magnet which when the rotor revolves then traverses the spaced-apart ends in the stator core, said magnet being axially magnetized whereby ends of the magnet which face the spaced ends of the core have alternate magnetic polarities, said magnet having a shape configured to cause the magnet in the course of rotation to produce alternating sinusoidal variations in magnetic flux passing through the core.

2. A machine as set forth in Claim 1, in which the magnet has an annular shape and a cross-sectional geometry which gives rise to said alternating sinusoidal variations.

3. A machine according to Claim 1, in which the cross-sectional geometry of the annular magnet creates an even number of complementary pole sections each having an inner and an outer surface one of which has the curvature of a circle, the other of which has the curvature that gives rise to a sine curve of magnetic flux.

4. A machine as set forth in Claim 3, in which the outer surface has the curvature of a circle and the inner surface has the curvature that gives rise to a sine curve of magnetic flux.

5. A machine according to Claim 2, in which the ends of the annular magnet are planar.

6. A machine according to Claim 3, in which the pole sections of the magnet are axially magnetized, each two adjacent pole sections being magnetized in reverse directions.

7. A machine according to Claim 1, in which the core is foπned of material having a high magnetic conductivity and a low electrical conductivity.

8. A machine according to Claim 7, in which the material is a feπite.

9. A machine according to Claim 1, in which the core is composed of complementary U-shaped sections, each having a short leg and a long leg, the sections being combined to cause the long legs to abut each other and the short legs to be spaced apart to create said ends.

10. A machine according to Claim 9, in which the long and short legs of the sections each have a coil mounted thereon.

11. A machine according to Claim 10, in which the coil is a tubular solenoid that fits onto the core leg.

12. A machine according to Claim 2, in which the stator includes at least one pair of diametrically opposed cores each having an air gap, and the annular magnet concurrently traverses the air gaps in the pair.

13. A machine according to Claim 1, having 3 or 3n pairs of diametrically opposed cores in a circular array to provide a three-phase machine.

14. A machine according to Claim 2, in which the annular magnet is mounted on a rotor shaft that extends along a central axis of the machine.

15. A machine according to Claim 14, in which the shaft is provided with a hub, 5 and the magnet is supported on the hub by ribs radiating therefrom.

16. A machine according to Claim 15, further including vanes of a ventilating fan radiating from the annular magnet.

10 17. A machine according to Claim 2, in which said annular magnet is supported co-axially within an annular rotor whereby the annular rotor surrounds the stator.

18. A machine according to Claim 17, in which the annular rotor is combined with a rim of a wheel whereby the machine functions as a motor to turn the wheel.

15

19. A machine according to Claim 1, in which the stator is formed from a hollow, laminated, cylindrical, metal blank that is notched to form a crown of half-phase cores, each having an armature winding thereon.

₀ 20. In a synchronous electrical machine, a peπnanent magnet rotor which is axially magnetized whereby surfaces of the magnet which in the machine face ends of the core have alternate magnetic polarities, said magnet having an annular shape and varying cross-sectional area configured to cause the magnet in the course of rotation to induce alternating sinusoidal variations in lines of magnetic flux passing 5 through the core.

21. A rotor as set forth in Claim 20, wherein said rotor has a cross-sectional geometry defining an even number of complementary pole sections, each section having an inner and an outer surface, one surface having curvature of a circle, the other surface having a curvature which is that of a sine curve.

22. A rotor as set forth in Claim 21, in which the magnet is fabricated of a magnetizable material having a high-remanence and coercive force.

23. A synchronous electric machine as set forth in Claim 1, in which said stator is a double stator on either side of the rotor.