BI-PERMANENT MAGNETS IN SYNCHRONOUS MACHINES

Abstract

Rotor (12) for a permanent magnet synchronous machine, comprising: a rotor core structure with a cylindrical body, wherein the cylindrical body comprises a cylindrical outer wall (31) of the rotor core structure adjacent to an air gap (18); several arc-shaped cavities (22; 24, 26, 28, 30) formed in the rotor core structure, wherein the several arc-shaped cavities (22; 24, 26, 28, 30) are substantially concentrically layered with respect to the cylindrical outer wall (31) of the rotor core structure, each arc-shaped cavity (22; 24, 26, 28, 30) extending between first (32) and second (34) end sections and located adjacent to the surface of the cylindrical outer wall (31) of the rotor structure, and containing an intermediate middle section; and several permanent magnets (20) which are inserted into the several arc-shaped cavities (22; 24, 26, 28, 30); wherein each first end section (32) contains a respective first permanent magnet (40) having a first magnetic field strength, wherein each second end section (34) contains a respective second permanent magnet (42) having the first magnetic field strength, and wherein each middle section contains a respective third permanent magnet (44) having a second magnetic field strength which is less than the first magnetic field strength; and wherein the first permanent magnet (40) in a subsequent arc-shaped cavity layer (26, 28, 30) overlaps the first permanent magnet (40) in a previous arc-shaped cavity layer (24, 26, 28) by an angle, wherein a length of the angular overlap between the first permanent magnets (40) of the previous arc-shaped cavity layer (24, 26, 28) and a subsequent arc-shaped cavity layer (26, 28, 30) is determined on the basis of an optimization of a flux flow (50) towards the surface of the cylindrical outer wall (31) generated by the first permanent magnets (40).

Description

BACKGROUND OF THE INVENTION

[0001] One embodiment generally relates to permanent magnet motors.

[0002] A permanent magnet synchronous motor is an AC motor in which the shaft rotation rate, or speed, is synchronized with the frequency of the AC supply current. A rotating electric field is generated in the stator, and the rotor follows the rotating electric field of the stator. The rotor rotates in lockstep with the stator's field at the same rate, or speed. The rotor and stator are said to be synchronized.

[0003] The permanent magnets used in conventional rotors are designed to provide a uniform field strength. Neodymium magnets, also known as rare-earth magnets, are often desired due to their high magnetic field strength, which provides improved torque in motors. However, the use of rare-earth magnets in a motor is costly and increases the overall price of the motor.

[0004] The document JP 2006-20 376 A discloses an electric motor having a large through-hole for a motor shaft, in which a rotor with a plurality of arcuate grooves in which permanent magnets are arranged is used to reduce magnetic leakage flux.

[0005] DE 601 08 424 T2 discloses a permanent magnet rotor having several permanent magnets embedded in layers for each magnetic pole. The permanent magnets are arc-shaped and, in one embodiment, have different properties.

[0006] The document JP H10-290 542 A discloses an electric motor having a rotor having two arc-shaped magnets for each pole, a distance therebetween being greater than the width of a slot opening of a stator in order to reduce a reluctance torque against the direction of rotation of the rotor.

[0007] Document DD 2 22 745 A1 discloses a rotor with permanent magnets. The rotor core structure has radially extending cutouts in which permanent magnets are arranged. Auxiliary magnets with higher remanent induction and coercive field strength are arranged next to these, which serve to saturate stray ribs between the rotor poles.

[0008] The document JP S60-219 947 A discloses a permanent magnet synchronous motor with magnets with high coercive force and with magnets with high magnetic flux density, wherein the magnets with high coercive force face the stator, while the magnets with high magnetic flux density face the rotor shaft.

[0009] JP H10-271 722 A discloses a rotor with internal permanent magnets, which contains two arc-shaped permanent magnets in each pole. Their end portions have a higher coercive force than their central portions to prevent demagnetization.

[0010] The document DE 10 2007 014 973 A1 discloses rotors with internal permanent magnets, whose magnets made of different magnetic materials are injected in liquid / mobile form into rotor cavities in such a way that the materials partially mix.

[0011] The object of the invention is to reduce the overall price of a permanent magnet synchronous motor and to avoid demagnetization of permanent magnets in a rotor of the motor.

[0012] This object is achieved by the subject matter of the independent claims. Advantageous developments of the invention are described in the subclaims. SUMMARY OF THE INVENTION

[0013] One advantage of one embodiment is the reduction in the content and weight of rare-earth magnets, which reduces the overall cost of the motor. The internal permanent magnet synchronous machine uses bi-permanent magnets (i.e., the combination of two or more different types of magnetic materials) to reduce the number of rare-earth magnets used in the rotor. A combination of rare-earth magnets and ferrite magnets, or magnets with the same material composition but exhibiting different magnetic field strengths, is arranged in a multi-layered manner in the rotor. Between two adjacent cavities, the rare-earth magnets and ferrite magnets gradually overlap to minimize a reduction in the demagnetization of the ferrite magnets due to the magnetic field generated by the rare-earth magnets.

[0014] One embodiment contemplates a rotor for a permanent magnet synchronous machine. A rotor core structure includes a cylindrical body. The cylindrical body comprises a cylindrical outer wall of the rotor core structure adjacent an air gap. A plurality of arcuate cavities are formed in the rotor core structure. The plurality of arcuate cavities are layered substantially concentrically with respect to the cylindrical outer wall of the rotor core structure. Each arcuate cavity, extending between first and second end sections, is located adjacent the surface of the cylindrical outer wall of the rotor structure and includes a middle section therebetween. A plurality of permanent magnets are inserted into the plurality of arcuate cavities. Each first end section includes a respective first permanent magnet having a first magnetic field strength. Each second end section includes a respective second permanent magnet having the first magnetic field strength.Each central section contains a respective third permanent magnet with a second magnetic field strength that is smaller than the first magnetic field strength. The first permanent magnet, in a subsequent arcuate cavity layer, overlaps the first permanent magnet in a previous arcuate cavity layer by an angle, wherein a length of the angular overlap between the first permanent magnets of the previous arcuate cavity layer and a subsequent arcuate cavity layer is determined based on an optimization of a flux flow toward the surface of the cylindrical outer wall generated by the first permanent magnets.

[0015] One embodiment contemplates a permanent magnet synchronous machine comprising a stator and a rotor core structure. The rotor core structure is radially offset from the stator and separated from the stator by an air gap. The rotor core structure has a cylindrical body comprising a cylindrical outer wall of the rotor core structure. A plurality of arcuate cavities are formed in the rotor core structure. The plurality of arcuate cavities are layered substantially concentrically with respect to the cylindrical outer wall of the rotor core structure. Each arcuate cavity, extending between first and second end sections, is adjacent to the surface of the cylindrical outer wall of the rotor structure and has a middle section therebetween. A plurality of permanent magnets are inserted into the plurality of arcuate cavities. Each first end section includes a respective first permanent magnet having a first magnetic field strength.Each second end section contains a respective second permanent magnet with the first magnetic field strength. Each middle section contains a respective third permanent magnet with a second magnetic field strength that is less than the first magnetic field strength. The first permanent magnet, in a subsequent arcuate cavity layer, overlaps the first permanent magnet in a previous arcuate cavity layer by an angle, wherein a length of the angular overlap between the first permanent magnets of the previous arcuate cavity layer and a subsequent arcuate cavity layer is determined based on an optimization of a flux flow toward the surface of the cylindrical outer wall generated by the first permanent magnets. BRIEF DESCRIPTION OF THE DRAWINGS Fig. Figure 1 is a sectional view of a motor with internal permanent magnets. Fig. Figure 2 is a sectional view of the motor with internal permanent magnets showing demagnetization. Fig. Figure 3 is a sectional view of the motor with internal permanent magnets showing minimal demagnetization. DETAILED DESCRIPTION

[0016] In Fig. Figure 1 shows a sectional view of a permanent magnet motor 10. The permanent magnet motor 10 includes a rotor 12 coupled to a shaft 14. The rotor may include any number of poles (e.g., 2, 4, 6, etc.). The sectional view shown illustrates one pole of the rotor 12 (e.g., a north pole). A stator 16 is radially offset from the rotor 12 and separated from it by an air gap 18.

[0017] The function of rotor 12 is to drive a component coupled to shaft 14. When energized by an excitation voltage (e.g., a 3-phase supply), stator 16 generates a rotating magnetic field within motor 10. The rotor, which functions like a permanent magnet, locks onto the rotating magnetic field generated by stator 16. Rotor 12 rotates along with the rotating magnetic field. When the rotor locks onto the rotating magnetic field, motor 10 is synchronized.

[0018] The Fig. The rotor 12 shown in Figure 1 uses internal permanent magnets 20. It should be understood that the motor can be any type of permanent magnet motor and does not require internal permanent magnets. The internal permanent magnets 20 are inserted into a plurality of arcuate cavities 22 formed in the rotor 12. The plurality of arcuate cavities 22 have a multi-layered structure. The plurality of arcuate cavities 22 include a first arcuate cavity 24, a second arcuate cavity 26, a third arcuate cavity 28, and a fourth arcuate cavity 30. Each of the arcuate cavities is concentrically layered with respect to a cylinder outer wall 31 of the rotor 12.

[0019] Each arcuate cavity includes a first end section 32 and a second end section 34 located adjacent the surface of the cylindrical outer wall 31 of the rotor 12. Each arcuate cavity includes an intermediate middle section 36 extending between the first end section 32 and the second end section 34.

[0020] The internal permanent magnets 20 include a first magnet 40 disposed in a first end section 32 of each cavity layer and generating a magnetic field having a first magnetic field strength, and a second magnet 42 disposed in the second end section 34 of each cavity layer and having the first magnetic field strength. A third magnet 44 is disposed between the first magnet 40 and the second magnet 42 of each cavity layer. The third magnet 44 generates a magnetic field having a second magnetic field strength. The first magnetic field strength generated by the first magnet 40 and the second magnet 42 is greater than the second magnetic field strength generated by the third magnet 44. The third magnet 44 may comprise a single bent magnet conforming to the shape of a respective cavity, or the third magnet 44 may comprise more than one permanent magnet inserted into a respective cavity layer.The number of third magnets 44 used in a respective cavity layer may be based on the ease of assembly of the third magnets in the respective cavity. That is, due to the curvature of the cavity, it may be more efficient and practical to use smaller magnets to facilitate assembly.

[0021] The first and second magnets 40, 42 may have a different material composition than the third magnet 44, or the same material composition but different magnetic field strengths. For example, the first magnet 40 and the second magnet 42 may be rare-earth magnets (e.g., neodymium magnets), and the third magnet 44 may be a ferrite magnet. Alternatively, the first magnet 40, the second magnet 42, and the third magnet 44 may all be ferrite magnets or all be rare-earth magnets, with the first and second magnets 40, 42 having a stronger magnetic field than the third magnet 44. The overall cost of the magnets is reduced because fewer materials with the higher magnetic field strength are required. However, unless the relative placement of the different materials is correctly configured, the resulting magnetic field may not be effective in operating the motor.

[0022] One problem with the use of bi-permanent magnets is the demagnetization of the magnet that produces the weaker magnetic field by the stronger one. As in Fig. As shown in Figure 2, the flux generated by the ferrite magnets will be demagnetized by the adjacent rare-earth magnets if the magnets are not correctly positioned between the respective cavity layers. Fig. 2, both the rare-earth magnets 40 and the ferrite magnets 44 are north pole magnets. Due to the magnetic field exerted by the rare-earth magnets 40 on the ferrite magnets 44, the flux flow from the rare-earth magnets reverses direction away from the air gap 18. This is represented by the flux flow generally shown by arrows 48. As a result, the motor loses torque and efficiency.

[0023] To avoid demagnetization of the ferrite magnets, the rare-earth magnets are positioned so that the rare-earth magnets in a subsequent cavity gradually overlap the ferrite magnets of a previous cavity. As shown in Fig. 3, the first cavity layer 24, a second cavity layer 26, a third cavity layer 28, and a fourth cavity layer 30 each include rare earth magnets 40, 42 in the first and second end sections 32, 34, and ferrite magnets in the intermediate section 36. The rare earth magnets 40, 42 in the second cavity layer 26 overlap the ferrite magnet 44 in the first cavity layer 40 by an angle. Similarly, the rare earth magnets 40, 42 of the third cavity layer 28 overlap the ferrite magnet 44 of the second cavity layer 26 by an angle. The length of the angular overlap gradually increases between the second / third cavity layers compared to the first / second cavity layers. In addition, the rare earth magnets 40, 42 of the fourth cavity layer 30 overlap the ferrite magnet 44 of the third cavity layer 28 at an angle.The length of the angular overlap between the third / fourth cavity layers gradually increases compared to the second / third cavity layers. As a result, a progressive angular overlap between a ferrite magnet of a previous cavity layer and a rare-earth magnet of a subsequent cavity layer is designed for each cavity layer.

[0024] The positioning to achieve the length of the angular overlap between the respective magnets of each cavity layer is based on optimizing a flux flow generated by both the rare-earth magnets and the ferrite magnets. The position of the rare-earth magnets of a respective subsequent cavity layer relative to the rare-earth magnets of a subsequent cavity layer is determined such that the flux flow generated jointly by the rare-earth magnets is directed perpendicularly to the rare-earth magnets and into the air gap 18. This is Fig. 3 generally illustrated at 50.

[0025] In addition, the positioning of the rare-earth magnets of each subsequent cavity layer relative to the ferrite magnets of a previous cavity layer is also used collectively when positioning the respective magnets. The optimal length of an intersection between the respective magnets of each cavity layer is determined so that the demagnetization of the magnetic field of the ferrite magnets is minimized. As shown in Fig. 3, a flux flow from a central portion of the ferrite magnets 44 is directed substantially perpendicular to the ferrite magnet and the air gap 18, as shown generally at 52. However, the flux flow at the end regions of the ferrite magnets 44 is directed substantially toward the rare earth magnets 40, 42, as shown generally at 54. Although the flux flow at the end regions is substantially not perpendicular to the ferrite magnets and is not directed toward the air gap 18, demagnetization of the ferrite magnets is prevented, in contrast to the flux flow shown in Fig. 2. The length of the magnet overlap between the respective cavity layers is determined by determining magnet positions that provide the minimum demagnetization of the ferrite magnets. The configuration of Fig. 3 produces improved torque for the motor by using a gradual and increasing overlap of the magnets between the respective cavity layers compared to the magnet configuration of Fig. 2. As a result, costs associated with using the same magnets in the rotor, such as rare earth magnets, can be reduced by using two sets of magnets with different magnetic field strengths in the configuration described here.

[0026] Although certain embodiments of the present invention have been described in detail, those skilled in the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention as defined by the following claims.

Claims

[1] Rotor (12) for a permanent magnet synchronous machine, comprising: a rotor core structure having a cylindrical body, the cylindrical body comprising a cylindrical outer wall (31) of the rotor core structure adjacent an air gap (18); a plurality of arcuate cavities (22; 24, 26, 28, 30) formed in the rotor core structure, the plurality of arcuate cavities (22; 24, 26, 28, 30) being substantially concentrically layered with respect to the cylindrical outer wall (31) of the rotor core structure, each arcuate cavity (22; 24, 26, 28, 30) extending between first (32) and second (34) end sections and being adjacent to the surface of the cylindrical outer wall (31) of the rotor structure, and including a central section therebetween; and a plurality of permanent magnets (20) inserted into the plurality of arcuate cavities (22; 24, 26, 28, 30); wherein each first end section (32) includes a respective first permanent magnet (40) having a first magnetic field strength, wherein each second end section (34) includes a respective second permanent magnet (42) having the first magnetic field strength, and wherein each middle section includes a respective third permanent magnet (44) having a second magnetic field strength that is less than the first magnetic field strength; and wherein the first permanent magnet (40) in a subsequent arcuate cavity layer (26, 28, 30) intersects the first permanent magnet (40) in a previous arcuate cavity layer (24, 26, 28) by an angle, wherein a length of the angular intersection between the first permanent magnets (40) of the previous arcuate cavity layer (24, 26, 28) and a subsequent arcuate cavity layer (26, 28, 30) is determined based on an optimization of a flux flow (50) towards the surface of the cylindrical outer wall (31) which is generated by the first permanent magnets (40). [2] The rotor (12) of claim 1, wherein the second permanent magnet (42) in the subsequent arcuate cavity layer (26, 28, 30) intersects the second permanent magnet (42) in the previous arcuate cavity layer (24, 26, 28) by an angle, wherein a length of the angular intersection between the second permanent magnets (42) of the previous arcuate cavity layer (24, 26, 28) and a subsequent arcuate cavity layer (26, 28, 30) is determined based on an optimization of a flux flow toward the surface of the cylindrical outer wall (31) generated by the second permanent magnets (42). [3] The rotor (12) of claim 2, wherein the angular intersection of the first permanent magnets (40) and the angular intersection of the third permanent magnets (44) between each subsequent (26, 28, 30) and previous (24, 26, 28) arcuate cavity layer prevents the flux flow (48) generated from the first permanent magnets (40) and the second permanent magnets (42) toward the third permanent magnets (44) in each respective arcuate cavity layer (22; 24, 26, 28, 30). [4] The rotor (12) of claim 3, wherein the first permanent magnet (40) in a respective preceding arcuate cavity layer (24, 26, 28) acts as a source of magnetic flux relative to the first permanent magnet (40) of a respective subsequent arcuate cavity layer (26, 28, 30), and wherein the first permanent magnet (40) of the respective subsequent arcuate cavity layer (26, 28, 30) acts as a sink of magnetic flux relative to the first permanent magnet (40) for the respective preceding arcuate cavity layer (24, 26, 28) to direct a flux flow generated by the first permanent magnet (40) of each arcuate cavity layer (22; 24, 26, 28, 30) toward the surface of the cylindrical outer wall (31). [5] The rotor (12) of claim 4, wherein the second permanent magnet (42) in the respective previous arcuate cavity layer (24, 26, 28) acts as a source of magnetic flux relative to the second permanent magnet (42) of the respective subsequent arcuate cavity layer (26, 28, 30), and wherein the second permanent magnet (42) of the respective subsequent arcuate cavity layer (26, 28, 30) acts as a sink of magnetic flux relative to the second permanent magnet (42) for the respective previous arcuate cavity layer (24, 26, 28) to direct a flux flow generated by the second permanent magnet (42) of each cavity layer (22; 24, 26, 28, 30) toward the surface of the cylindrical outer wall (31). [6] The rotor (12) of claim 5, wherein the first permanent magnets (40) arranged in the respective subsequent arcuate cavity layer (26, 28, 30) overlap the third permanent magnet (44) in a previous arcuate cavity layer (24, 26, 28) by an angle, wherein a length of the angular overlap between the first permanent magnets (40) of the subsequent arcuate cavity layer (26, 28, 30) and the third permanent magnet (44) of the previous arcuate cavity layer (24, 26, 28) is determined based on minimizing a reduction in demagnetization of the second permanent magnets (42). [7] The rotor (12) of claim 6, wherein the second permanent magnets (42) arranged in the respective subsequent arcuate cavity layer (26, 28, 30) overlap the third permanent magnet (44) in a previous arcuate cavity layer (24, 26, 28) by an angle, wherein a length of the angular overlap between the second permanent magnets (42) of the subsequent arcuate cavity layer (26, 28, 30) and the third permanent magnet (44) of the previous arcuate cavity layer (24, 26, 28) is determined based on minimizing a reduction in demagnetization of the second permanent magnets (42). [8] The rotor (12) of claim 1, wherein the first and second permanent magnets (40, 42) are rare earth magnets and the third permanent magnets (44) are ferrite magnets. [9] Permanent magnet synchronous machine, comprising: a stator (16); a rotor core structure radially offset from the stator (16) and separated from the stator (16) by an air gap (18), the rotor core structure comprising a cylindrical body including a cylindrical outer wall (31) of the rotor core structure; a plurality of arcuate cavities (22; 24, 26, 28, 30) formed in the rotor core structure, the plurality of arcuate cavities (22; 24, 26, 28, 30) being substantially concentrically layered with respect to the cylindrical outer wall (31) of the rotor core structure, each arcuate cavity (22; 24, 26, 28, 30) extending between first and second end sections (32, 34) being adjacent to the surface of the cylindrical outer wall (31) of the rotor structure and having a central section therebetween; and a plurality of permanent magnets (40, 42, 44) inserted into the plurality of arcuate cavities (22; 24, 26, 28, 30); wherein each first end section (32) includes a respective first permanent magnet (40) having a first magnetic field strength, wherein each second end section (34) includes a respective second permanent magnet (42) having the first magnetic field strength, and wherein each middle section includes a respective third permanent magnet (44) having a second magnetic field strength that is less than the first magnetic field strength; wherein the first permanent magnet (40) in a subsequent arcuate cavity layer (26, 28, 30) intersects the first permanent magnet (40) in a previous arcuate cavity layer (24, 26, 28) by an angle, wherein a length of the angular intersection between the first permanent magnets (40) of the previous arcuate cavity layer (24, 26, 28) and a subsequent arcuate cavity layer (26, 28, 30) is determined based on an optimization of a flux flow (50) towards the surface of the cylindrical outer wall (31) which is generated by the first permanent magnets (40).

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