Rotating electrical machine

A rotating electrical machine includes a stator core, a rotor core, and permanent magnets. The stator core includes a yoke and tooth portions projecting from the yoke in a radial inward direction. Each tooth portion has a base joined to the yoke and an end opposite to the base. The rotor core includes a boss portion and projections. The projections project from the boss portion in a radial outward direction and spaced in a circumferential direction. Each permanent magnet is located between and spaced from adjacent projections to forma gap in the circumferential direction. A width of the gap is not greater than a width of the end of the tooth portion in the circumferential direction.

CROSS REFERENCE TO RELATED APPLICATION

This application is based on Japanese Patent Application No. 2012-146673 filed on Jun. 29, 2012, the disclosure of which is incorporated herein by reference.

FIELD

The present disclosure relates to a rotating electrical machine.

BACKGROUND

Permanent magnet materials such as rare-earth magnets have high energy density and therefore are essential materials to reduce the size of an electrical machine. However, it is hard to obtain an adequate amount of permanent magnet materials due to uneven distribution of resources in the world. For this reason, machines have been designed to reduce use of permanent magnet materials as much as possible. For example, in a rotating electrical machine disclosed in JP-A-2011-250508 corresponding to US 2011/0285243, a consequent-pole rotor is employed to reduce use of permanent magnet materials. The consequent-pole rotor has projections, projecting radially outward from a boss portion, and permanent magnets located between adjacent projections.

SUMMARY

After deep analysis of the rotating electrical machine disclosed in US 2011/0285243, the present inventor finds out that if a rotating electrical machine is designed by employing magnetic circuit data disclosed in US 2011/0285243, a variation in rotation of a rotor may occur. In particular, when the rotating electrical machine is used in an electrical power steering system of a vehicle, cogging torque may be increased. In a technique disclosed in US 2011/0285243, the magnetic circuit data is specialized for output torque. Specifically, the width of the permanent magnet is much greater than the width of the projection. This causes a disturbance in the space magnetic field distribution, and the disturbance results in the increase in the cogging torque. In summary, the present inventor finds out that the increase in the cogging torque is closely related to an interaction among the permanent magnet, the projection, and a tooth portion of a stator.

In view of the above, it is an object of the present disclosure to provide a rotating electrical machine for reducing cogging torque without a reduction in output torque.

According to an aspect of the present disclosure, a rotating electrical machine includes a supporting member, a stator core, a winding, a rotation shaft, a rotor core, and permanent magnets. The stator core includes a ring-shaped yoke fixed to the supporting member and tooth portions projecting from the yoke in a radial inward direction. Each tooth portion has a base joined to the yoke and an end opposite to the base. The winding is wound in a slot between the tooth portions. The rotation shaft extends through the stator core and rotatably supported by the supporting member. The rotor core includes a boss portion and projections. The boss portion is fixed to the rotation shaft. The projections project from the boss portion in a radial outward direction and spaced from each other in a circumferential direction. The permanent magnets are fixed to the boss portion. Each permanent magnet is located between and spaced from adjacent projections to form a gap in the circumferential direction. A width of the gap in the circumferential direction is equal to or smaller than a width of the end of the tooth portion in the circumferential direction.

DETAILED DESCRIPTION

Firstly, a cause of an increase in cogging torque found out by the present inventor is described below with reference toFIGS. 12,13, and14.

FIG. 12shows a change in magnetic flux over time t1to t3in a first comparison example in which a gap between a permanent magnet101and a projection102of a rotor in a circumferential direction is small.FIG. 13shows a change in magnetic flux over time t1to t3in a second comparison example in which a gap between a permanent magnet104and a projection105of a rotor in a circumferential direction is large.

As shown inFIG. 12, when the gap between the permanent magnet101and the projection102of the rotor in the circumferential direction is small, an end of a tooth portion103of a stator in a radial inward direction magnetically bypasses between magnetic poles easily. Thus, as indicated by broken lines inFIG. 12, a certain amount of main magnetic flux always flows in the tooth portion103so that cogging torque observed when no current is supplied can become small. However, since lateral magnetic flux (i.e., leakage magnetic flux) is increased, a reduction in output torque observed when a rated current is supplied is large.

Therefore, it is preferable that a gap between a permanent magnet and a projection of a rotor in a circumferential direction be as large as possible. However, as shown inFIG. 13, when the gap between the permanent magnet104and a projection105of the rotor in the circumferential direction is too large, an end of a tooth portion106of a stator in a radial inward direction cannot adequately bypass between magnetic poles. Thus, as indicated by broken lines inFIG. 13, main magnetic flux does not always flow in the tooth portions103. As a result, a magnetized condition varies largely depending on a rotor position so that cogging torque can become large.

As indicated by a solid line inFIG. 14, a waveform of magnetic flux in the tooth portion106of the second comparison example shown inFIG. 13is distorted largely and contains a lot of harmonics. Therefore, magnetic flux rotating in a stator varies so that cogging torque can become large.

On the other hand, as indicated by a broken line inFIG. 14, a waveform of magnetic flux in the tooth portion103of the first comparison example shown inFIG. 12is distorted a little. However, since the crest value of the waveform is reduced, effective magnetic flux is reduced accordingly. This phenomenon appears pronouncedly when a permanent magnet is wider than a projection as disclosed in JP-A-2011-250508.

Next, embodiments of the present disclosure are described based on the above findings.

A motor1(as a rotating electrical machine) according to a first embodiment of the present disclosure is described below with reference toFIGS. 1 and 2. As shown inFIG. 1, the motor1is a three-phase brushless motor. The motor1includes a housing10, a stator20, and a rotor30.

The housing10includes a tube11, a first side portion12, and a second side portion14. A first end of the tube11is closed with the first side portion12. A second end of the tube11is closed with the second side portion14. A bearing16is fitted in a through hole13in the center of the first side portion12. A bearing17is fitted in a through hole15in the center of the second side portion14.

The stator20includes a stator core21and a winding set22. The stator core21is located in the tube11of the housing10. The winding set22is wound on the stator core21.

The stator core21has a yoke24and tooth portions25. The yoke24is pressed into the tube11so that the yoke24can be pressed against and fixed to an inner surface of the tube11. The tooth portions25project from the yoke24in a radial inward direction of the yoke24. The yoke24and the tooth portions25are formed as a single piece. According to the first embodiment, the stator core21has twenty-four tooth portions25. That is, the number of the tooth portions25for every magnetic pole and every phase is one. The tooth portions25are arranged at a regular interval in a circumferential direction of the yoke24.

The winding set22includes a U-phase winding, a V-phase winding, and a W-phase winding. A slot28is formed between adjacent tooth portions25. Each winding of the winding set22is wound in every third slot25. In other words, each winding of the winding set22is wound at intervals of three slots25. It is noted thatFIG. 2shows a direction of an electric current flowing through the U-phase winding only.

The rotor30is a consequent-pole rotor. The rotor30includes a rotation shaft31, a rotor core32, and permanent magnets40.

The shaft31is rotatably supported by the bearings16and17.

The rotor core32is made from soft magnetic material. The rotor core32includes a boss portion33and projections34. The boss portion33is fixed to the shaft31, for example, by press-fitting the shaft31into the boss portion33. The projections34project from the boss portion33in a radial outward direction of the boss portion33and are spaced from each other in a circumferential direction of the boss portion33. The projections34serve as soft magnetic poles. According to the first embodiment, the rotor core32is made of steel plates that are laminated in a direction of an axis φ of the shaft31.

The permanent magnets40are fixed to the boss portion33. Each permanent magnet40is located between and spaced from adjacent projections34to form a gap50in the circumferential direction.

The boss portion33of the rotor core32serves as a magnetic flux conductor for conducting a magnetic flux expelled from the permanent magnet40. The magnetic flux expelled from the permanent magnet40consists of a main flux and a leakage flux. The main flux flows from the permanent magnet40to the projection34through the tooth portions25and the yoke24. The leakage flux flows in a lateral direction from the permanent magnet40to the projection34through the tooth portions25and does not flow through the yoke24.

In the motor1, each winding of the winding set22is connected to a power converter (not shown) including an inverter, a controller, and a battery and energized in turn so that a magnetic field rotating in the circumferential direction can be generated. The rotor30rotates according to the rotating magnetic field.

Next, the stator20and the rotor30are described in detail with reference toFIGS. 2 and 3.

A width W1of the gap50between the permanent magnet40and the projection34in the circumferential direction is smaller than a width W2of an end of the tooth portion25in the circumferential direction. It is noted that the width W1is an outermost width of the gap50in the radial outward direction and that the width W2is an innermost width of the end of the tooth portion25in the radial inward direction. Here, an end surface of the permanent magnet40in the circumferential direction is defined as a first end surface41, an end surface of the projection34in the circumferential direction is defined as a second end surface35, an imaginary plane formed as an extension of the first end surface41is defined as a first imaginary plane IP1, and an imaginary plane formed as an extension of the second end surface35is defined as a second imaginary plane IP2. According to the first embodiment, when a center of the tooth portion25in the circumferential direction and a center of the gap50in the circumferential direction are aligned with each other in the radial direction (i.e., are on the same straight line in the radial direction), the tooth portion25is positioned within a region defined by the first imaginary plane IP1and the second imaginary plane IP2.

Specifically, each permanent magnet40has two first end surfaces41opposite to each other in the circumferential direction. The two first end surfaces41of the permanent magnet40are parallel to each other. Likewise, each projection34has two second end surfaces35opposite to each other in the circumferential direction. The two second end surfaces35of the projection34are parallel to each other. A distance between the first end surface41and the second end surface35, which face each other to form the gap50, increases in the radial outward direction. Accordingly, a width of the region defined by the first imaginary plane IP1and the second imaginary plane IP2increases in the radial outward direction.

A width of the permanent magnet40in the circumferential direction is equal to a width of the projection34in the circumferential direction. When any one of the gaps50is positioned to face any one of the tooth portions25in the radial direction, each of the other gaps50is positioned to face any one of the other tooth portions25in the radial direction. When the number of the tooth portions25for every magnet pole and every phase is defined as “k”, the number of the tooth portions25capable of facing each permanent magnet40in the radial direction is (3k−1), and also the number of the tooth portions25capable of facing each projection34in the radial direction is (3k−1). Since the number k is one, the number (3k−1) is two.

As described above, according to the first embodiment, the width W1of the gap50between the permanent magnet40and the projection34in the circumferential direction is smaller than the width W2of the end of the tooth portion25in the circumferential direction. Further, when the center of the tooth portion25in the circumferential direction and the center of the gap50in the circumferential direction are aligned with each other in the radial direction, the tooth portion25is positioned within the region defined by the first imaginary plane IP1and the second imaginary plane IP2.

In such an approach, the permanent magnets40and the projections34form a magnetic bypass having a suitable magnetic reluctance. Thus, since the main flux always flows so that a magnetic field variation can be reduced, cogging torque observed when no current is supplied to the winding set22is reduced. Further, since the leakage flux is reduced, output torque observed when a rated current is supplied to the winding set22is increased. As shown inFIG. 4, cogging torque in the first embodiment is about one-tenth of cogging torque in the first comparison example shown inFIG. 12.

Further, according to the first embodiment, the opposing first end surfaces41of the permanent magnet40are parallel to each other, and the opposing second end surfaces35of the projection34are parallel to each other.

Accordingly, the distance between the first end surface41and the second end surface35, which face each other to form the gap50, increases in the radial outward direction. Thus, the width W2of the end of the tooth portion25in the circumferential direction can be increased as much as possible. Therefore, the magnetic flux flowing from the permanent magnet40to the stator core21can be easily collected by the tooth portion25.

Further, according to the first embodiment, the width of the permanent magnet40in the circumferential direction is equal to the width of the projection34in the circumferential direction. Accordingly, each gap50has the same width in the circumferential direction, and the gaps50are arranged at regular intervals in the circumferential direction. Thus, when any one of the gaps50is positioned to face any one of the tooth portions25in the radial direction, each of the other gaps50can be positioned to face any one of the other tooth portions25in the radial direction. Therefore, synchronizing timing in the circumferential direction becomes equal so that the cogging torque can be reduced without a reduction in the output torque.

Further, according to the first embodiment, the number k, which is the number of the tooth portions25for every magnet pole and every phase, is one, and the number of the tooth portions25capable of facing each permanent magnet40in the radial direction is (3k−1), which is two. Thus, the output torque can be maximized while reducing the cogging torque as much as possible.

A motor60according to a second embodiment of the present disclosure is described below with reference toFIGS. 5 and 6. A difference of the second embodiment from the first embodiment is as follows.

A stator core62of a stator61of the motor60includes the yoke24and twenty-four tooth portions63. Each tooth portion63has a leg64and a flange65. The leg64extends from the yoke24in the radial inward direction. The flange65extends from an end of the leg64in both directions along the circumferential direction.

The width W1of the gap50between the permanent magnet40and the projection34in the circumferential direction is smaller than a width W3of the flange65in the circumferential direction. Further, when a center of the flange65in the circumferential direction and the center of the gap50in the circumferential direction are aligned with each other in the radial direction (i.e., are on the same straight line in the radial direction), the flange65is positioned within the region defined by the first imaginary plane IP1and the second imaginary plane IP2.

Further, when any one of the gaps50is positioned to face any one of the tooth portions63in the radial direction, each of the other gaps50is positioned to face any one of the other tooth portions63in the radial direction. Further, when the number of the tooth portions63for every magnet pole and every phase is defined as “k”, the number of the tooth portions63capable of facing each permanent magnet40in the radial direction is (3k−1), and also the number of the tooth portions63capable of facing each projection34in the radial direction is (3k−1). Since the number k is one, the number (3k−1) is two.

The motor60of the second embodiment can have the same advantages as the motor1of the first embodiment. Further, since the slot66can be widened by narrowing the leg64of the tooth portion63up to the magnetic saturation limit, electrical loading can be increased. Thus, copper loss is reduced so that efficiency can be improved. For example, as shown inFIG. 7, the motor60is 8 percent more efficient than the first comparison example shown inFIG. 12. Accordingly, the size of the motor60can be reduced.

A motor70according to a third embodiment of the present disclosure is described below with reference toFIG. 8. A difference of the third embodiment from the preceding embodiments is as follows.

The motor70includes the stator61and a rotor74. An end71of the tooth portion63of the stator core62in the radial inward direction is separated by a first distance D1in the radial direction from an end73of an outer surface72of the permanent magnet40in the circumferential direction. The end71of the tooth portion63of the stator core62in the radial inward direction is separated by a second distance D2in the radial direction from an end78of an outer surface77of a projection76of the rotor74in the circumferential direction. The first distance D1is smaller than the second distance D2. Specifically, a distance between the end71of the tooth portion63and a center of the outer surface72of the permanent magnet40is equal to a distance between the end71of the tooth portion63and a center of the outer surface77of the projection76. Further, a curvature radius R1of the outer surface72of the permanent magnet40is larger than a curvature radius R2of the outer surface77of the projection76.

A width W4of a gap79between the permanent magnet40and the projection76in the circumferential direction is smaller than the width W3of the flange65in the circumferential direction. When any one of the gaps79is positioned to face any one of the tooth portions63in the radial direction, each of the other gaps79is positioned to face any one of the other tooth portions63in the radial direction. Further, when the number of the tooth portions63for every magnet pole and every phase is defined as “k”, the number of the tooth portions63capable of facing each permanent magnet40in the radial direction is (3k−1), and also the number of the tooth portions63capable of facing each projection76in the radial direction is (3k−1). Since the number k is one, the number (3k−1) is two.

The motor70of the third embodiment can have the same advantages as the motor1of the first embodiment. Further, since the magnetic resistance can be increased by increasing the gap in the leakage flux path, the leakage flux can be effectively reduced. For example, as shown inFIG. 9, output torque of the motor70is 3/2 (i.e., 1.5) times greater than that of the first comparison example shown inFIG. 12. Accordingly, the size of the motor60can be reduced.

For example, like the first embodiment, the rotor core75can be made by stamping steel into a predetermined shaped plate and by laminating the steel plates. In such an approach, the curvature radius R2of the outer surface77of the projection76can be easily made smaller than the curvature radius R1of the outer surface72of the permanent magnet40.

A motor80according to a fourth embodiment of the present disclosure is described below with reference toFIGS. 10 and 11. A difference of the fourth embodiment from the preceding embodiments is as follows.

The motor80includes a stator81and a rotor86. A stator core82of the stator81includes the yoke24and tooth portions83. Each tooth portion83has a leg84and a flange85. The leg84extends from the yoke24in the radial inward direction. The flange85extends from an end of the leg84in both directions along the circumferential direction.

A width W6of a gap90between a permanent magnet87and a projection89of the rotor86in the circumferential direction is smaller than a width W5of the flange85in the circumferential direction. Further, when a center of the flange85in the circumferential direction and a center of the gap90in the circumferential direction are aligned with each other in the radial direction (i.e., are on the same straight line in the radial direction), the flange85is positioned within a region defined by a first imaginary plane IP3and a second imaginary plane IP4. The first imaginary plane IP3is a plane formed as an extension of an end surface of the permanent87in the circumferential direction. The second imaginary plane IP4is a plane formed as an extension of an end surface of the projection89in the circumferential direction.

The stator core82has forty-eight tooth portions83. Therefore, the number of the tooth portions83for every magnetic pole and every phase is two. The tooth portions83are arranged at regular intervals in the circumferential direction. When the number of the tooth portions83for every magnet pole and every phase is defined as “k”, the number of the tooth portions83capable of facing each permanent magnet87in the radial direction is (3k−1), and also the number of the tooth portions83capable of facing each projection89of the rotor core88in the radial direction is (3k−1). Since the number k is two, the number (3k−1) is five. When any one of the gaps90is positioned to face any one of the tooth portions83in the radial direction, each of the other gaps90is positioned to face any one of the other tooth portions83in the radial direction.

The motor80of the fourth embodiment can have the same advantages as the motor1of the first embodiment. Further, since the width of the flange85of the tooth portion83in the circumferential direction can be reduced, the width of the gap90in the circumferential direction can be reduced accordingly. As the width of the gap90in the circumferential direction becomes smaller, the widths of the permanent magnet87and the projection89in the circumferential direction become larger. Thus, magnetic loading can be increased so that output torque of the motor80can be increased.

The number of poles of the rotor is not limited to eight. The number of phases is not limited to eight. The number of tooth portions for every magnet pole and every phase can vary depending on the intended use.

The rotor is not limited to a surface permanent magnet type rotor. The rotor can be an embedded permanent magnet type rotor.

The rotor can have a consequent-pole type structure partially in the axis direction.

The width of the gap between the permanent magnet and the projection in the circumferential direction can be equal to the width of the end of the tooth portion in the circumferential direction.

In the embodiments, a full pitch, distributed winding is employed. Alternatively, a different winding design such as a short pitch, distributed winding can be employed.

In the embodiments, the rotor core is made by laminating steel plates. Alternatively, the rotor core can be made by a different method. For example, the rotor core can be made by compression molding of magnetic powders.

The rotating electrical machine to which the present disclosure is applied is not limited to a motor. For example, the rotating electrical machine can be an alternator. When the present disclosure is applied to an alternator, cogging torque can be reduced without a reduction in output electrical power.