Rotator of rotational electric machine

A rotor core has a regular decagon base portion in its cross section relative to a shaft center and ten convex portions each of which is located between each angle of the regular decagon base portion. Each convex portion has a first convex curve having a single curvature radius. Each permanent magnet is a curved plate and has a concave curve and a second convex curve. Each permanent magnet is provided so that the concave curve adheres on the first convex curve. When each permanent magnet moves in circumferential direction relative to the rotor core, the concave curve slides on the first convex curve of the rotor core. A position of the smallest gap clearance between the second convex curve and the stator does not change. A magnetic-flux-strength maximum position is not changed easily.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based on Japanese Patent Application No. 2013-241695 filed on Nov. 22, 2013, the disclosure of which is incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to a rotator of a rotational electric machine.

BACKGROUND

In a technical field of a rotational electric machine, it is required to reduce amount of the rare materials which forms a permanent magnet. Generally, the rotator has a ring-shaped permanent magnet. In order to reduce the amount of the permanent magnet, it can be configured that a plurality of permanent magnets are arranged circumferentially. Compared with the ring-shaped permanent, the amount of the permanent magnet can be reduced.

When multiple permanent magnets are arranged circumferentially, it is important to fix each permanent magnet on an iron core properly. For example, JP-2012-249354A describes that a flat surface of the permanent magnet is fixed on a flat surface of the iron core. A protrusion of the iron core is engaged with a groove of the permanent magnet.

However, in JP-2012-249354A, a gap clearance is necessary between the protrusion of the iron core and the permanent magnet to engage them. Thus, when assembling the permanent magnet to the iron core, it is likely that the permanent magnet moves in the groove relative to the iron core.

If the permanent magnet moves, a magnetic-flux-strength maximum position also deviates in a circumferential direction. The deviation of the magnetic-flux-strength maximum position causes an increase of a cogging torque and a torque ripple.

Moreover, when the magnetic-flux-strength maximum position deviates, the d-axis also deviates in performing a dq-transformation. As the result, controllability is deteriorated and a vibration is caused.

SUMMARY

It is an object of the present disclosure to provide a rotator of a rotational electric machine, which is capable of restricting an increase of a cogging torque and a torque ripple, and a deterioration of controllability.

According to a present disclosure, a rotator of a rotational electric machine has an iron core and a plurality of permanent magnets. In a cross section relative to a shaft center, the iron core includes a polygonal base portion having multiple sides of which number is twice of an integer “p”, and (2×p) pieces of convex portions. Each of the convex portions includes a first convex curve having a single curvature radius. The permanent magnet includes a concave curve adhering on the first convex curve and a second convex curve located on an opposite side relative to the first convex curve. Each permanent magnet is provided so that the concave curve adheres on the first convex curve.

According to the above configuration, when each permanent magnet is engaged with the iron core and moves in circumferential direction relative to the iron core, the concave curve slides on the first convex curve of the iron core. At this time, the position of the smallest gap clearance between the second convex curve and the stator does not change. Especially, in a case that the curvature radius of the first convex curve is equal to the curvature radius of the second convex curve, the position of the smallest gap clearance between the second convex curve and the stator does not change even when the permanent magnet moves relative to the iron core. Therefore, according to the present disclosure, the deviation of the magnetic-flux-strength maximum position in the clearance gap between the rotator and the stator can be restricted as much as possible. It is also avoided that the cogging torque and the torque ripple are increased and a controllability of the rotational electric machine is deteriorated.

DETAILED DESCRIPTION

Embodiments of the present disclosure will be described hereinafter.

First Embodiment

FIG. 1shows a rotator applied to a motor10as a rotational electric machine. The motor10is used as a driving force source of an electric power-steering device for a vehicle.

First, an entire configuration of the motor10will be explained with reference toFIGS. 1 and 2. The motor10is a three-phase circuit brush-less motor. The motor10has a housing20, a stator30, a shaft40, and a rotator50. The housing20has a cylinder case21, a first cover22closing one end of the cylinder case21, and a second cover23closing the other end of the cylinder case21. Each of the first cover22and the second cover23has a bearing24,25at its center portion.

The stator30is an armature of the motor10and has a stator core31and a winding32. The stator core31forms a cylindrical yoke33which is fixed on an inner wall of the cylinder case21, and multiple teeth34radially inwardly extending from the yoke33. The winding32is inserted in each slot and forms a U-phase winding, a V-phase winding, and a W-phase winding. It should be noted that the winding32is not illustrated inFIG. 2.

The shaft40is supported by the bearings24,25. The rotator50has a rotor core51and multiple permanent magnets52. The rotor core51is a cylindrical member fixed on the shaft40inside of the stator30. The rotor core51corresponds to an iron core. The permanent magnets52are arranged on an external wall surface of the rotor core51at regular intervals. Radially outer portions of each adjacent two permanent magnets52have different magnetic poles mutually.

The U-phase winding, the V-phase winding, and the W-phase winding are sequentially energized to generate a rotating field, whereby the rotator50rotates along with the shaft40.

Referring toFIGS. 2 and 3, the configuration of the rotator50will be explained in detail. As shown inFIG. 2, an outer diameter of the rotator50is 53 mm and the number of the permanent magnets52is twice the number of the integer “p”. In the present embodiment, “p” is five, and ten permanent magnets52are provided. The rotor core51has a cross-section of regular decagon relative to a shaft center61, as shown by two-dot chain line. The stator30has sixty slots62.

As shown inFIGS. 2 and 3, the rotor core51has a base portion63and ten convex portions64. The cross-section of the base portion63is regular decagon. Each convex portion64is positioned between the adjacent angle portions of the base portion63. That is, each convex portion64is located on each side of the base portion63of regular decagon. Each convex portion64has a first convex curve66of a single curvature radius.

As shown in anFIG. 3, in a cross section which perpendicularly intersects the shaft center61, both ends of the first convex curve66are located on the side65of the base portion63. In the cross section which perpendicularly intersects the shaft center61, a length of an imaginary straight line “L” connecting both ends of the first convex curve66is not less than 70% of a length of the side65of the base portion63.

Each permanent magnet52is a curved plate and has a concave curve67and a second convex curve68. The concave curve67is a curved surface along the first convex curve66of the rotor core51. The second convex curve68has a single curvature radius. The concave curve67and the first convex curve66have the same curvature radius. Each permanent magnet52is provided so that the concave curve67adheres on the first convex curve66.

In the cross section which perpendicularly intersects the shaft center61, a width of the permanent magnet52is slightly smaller than that of the convex portion64of the rotor core51. Thereby, when each permanent magnet52is engaged with the rotor core51, the concave curve67of the permanent magnet52is certainly adheres on the first convex curve66of the convex portion64of the rotor core51. Moreover, when each permanent magnet52moves in circumferential direction relative to the rotor core51with a contact between the concave curve67and the first convex curve66, the concave curve67slides on the first convex curve66of the rotor core51. This slide of the permanent magnet52is restricted when the permanent magnet52is brought into contact with the side65of the base portion63.

The curvature radius of the first convex curve66is defined as “R1”, the curvature radius of the second convex curve68is defined as “R2”, and a curvature radius of an imaginary circumscribed circle circumscribed to each permanent magnet52is defined as “R3”. In the present embodiment, the above-mentioned imaginary circumscribed circle corresponds to an inner wall of a cylindrical cover69which will be described later. The rotor core51and each permanent magnet52are formed in such a manner as satisfy following formulas (1) and (2).
0.6≦(R2/R1)≦1.0  (1)
R2≦R3  (2)

In the present embodiment, both the curvature radius R1 and R2 are 18 mm.
(R2/R1)=1.0
R2<R3

In the cross section which perpendicularly intersects the shaft center61, a thickness of a center portion of the permanent magnet52is defined as “t1” and a thickness of both end portions of the permanent magnet52is defined as “t2”. The thickness “t1” is the maximum thickness of the permanent magnet52. Further, each permanent magnet52is formed in such a manner as to satisfy a following formula (3).
1.0≦(t1/t2)≦1.4  (3)

In the present embodiment, the thickness “t1” is equal to the thickness “t2”.
(t1/t2)=1.0

The rotator50further has the cylindrical cover69engaged with the outer surface of the permanent magnet52so that at least a center portion of the second convex curves68adheres on an inner surface of the cylindrical cover69. The cylindrical cover69is made from nonmagnetic materials, such as stainless steel. As shown inFIG. 3, the tension “T” is applied to the cylindrical cover69in two directions with respect to a contacting portion between the permanent magnet52and the cylindrical cover69. The tension “T” biases each permanent magnet52toward the first convex curve66of the rotor core51.

Comparison with Comparative Example

In the comparative example, an outer diameter of the rotator90is 53 mm. The rotator90has a rotor core91and ten permanent magnets92. The rotor core91has ten flat surfaces93on its outer surface. Each permanent magnet92has a flat surface94which adheres on the flat surface93of the rotor core91, and a convex curve95of which curvature radius is 18 mm. In the comparative example, a protrusion96is engaged with a groove97of the permanent magnet92. The protrusion96extends from the flat surface93of the rotor core91.

A gap clearance is defined in width direction between the protrusion96and the groove97in order to engage the protrusion96with the groove97. Thus, when assembling the permanent magnet92to the rotor core91, it is likely that the permanent magnet92moves in the groove97relative to the rotor core91. InFIG. 7, an imaginary line98shown by a two-dot chain line is a straight line which connects a shaft center and the center of the protrusion96, and an imaginary line99is a straight line which connects a shaft center and the center of the convex curve95. When the permanent magnet92moves 0.2 mm, a magnetic-flux-strength maximum position in the above gap clearance is positioned on the imaginary line99, which deviates about 26 minutes around the shaft center61. The angle 26 minutes corresponds to 2 degrees 10 minutes of the electrical degree in a five-pole motor.

The deviation of the magnetic-flux-strength maximum position causes an increase of a torque ripple.FIG. 4is a chart showing a general control block diagram of a three-phase circuit brush-less motor. According to the general control, angle errors are generated twice in a transformation from three-phase actual axis to dq-axis and an inverse transformation from dq-axis to three-phase actual axis.FIG. 5is a chart showing a transformation formula and an inverse transformation formula. In these formulas, θ represents a moved angle of the rotator from the d-axis. The deviation of the magnetic-flux-strength maximum position corresponds to a deviation of the d-axis. It should be noted that the angle error is not corrected by performing the transformation and the inverse transformation. In performing the inverse transformation, the angle error may be increased due to a hysteresis.

On the other hand, according to the present embodiment, a contacting surface between the rotor core51and the permanent magnet52is a curved surface. When each permanent magnet52is engaged with the rotor core51and moves in circumferential direction relative to the rotor core51, the concave curve67slides on the first convex curve66of the rotor core51. Since the curvature radius R1 of the first convex curve66is equal to the curvature radius R2 of the second convex curve68, the position of the smallest gap clearance between the second convex curve68and the stator30does not change even when the permanent magnet52moves. That is, according to the present embodiment, even if the permanent magnet52moves in a circumferential direction, the magnetic-flux-strength maximum position does not deviate, unlike the comparative example. InFIG. 3, an imaginary line71shown by a two-dot chain line is a straight line which connects a shaft center and the center of the first convex curve66, and an imaginary line72is a straight line which connects a shaft center and the center of the second convex curve68. The magnetic-flux-strength maximum position is located on the imaginary line71.

As shown inFIG. 6, the electrical degree error which corresponds to the deviation of the magnetic-flux-strength maximum position becomes zero when the curvature-radius ratio (R2/R1) is 1.0. According the curvature-radius ratio (R2/R1) becomes more close to zero, the electrical degree error becomes larger. The electrical degree error is preferably 1 degree or less. The curvature-radius ratio (R2/R1) is 0.6, and a thickness ratio (t1/t2) is 1.4.

Advantages

As described above, according to the present embodiment, the rotator50has a rotor core51and multiple permanent magnets52. The rotor core51has the base portion63and ten convex portions64. The cross-section of the base portion63is regular decagon. Each convex portion64is positioned between the adjacent angle portions of the base portion63. Each convex portion64has the first convex curve66of a single curvature radius. Each permanent magnet52is a curved plate and has the concave curve67and the second convex curve68. Each permanent magnet52is provided so that the concave curve67adheres on the first convex curve66.

According to the above configuration, when each permanent magnet52is engaged with the rotor core51and moves in circumferential direction relative to the rotor core51, the concave curve67slides on the first convex curve66of the rotor core51. At this time, the position of the smallest gap clearance between the second convex curve68and the stator30does not change. Especially, in a case that the curvature radius R1 of the first convex curve66is equal to the curvature radius R2 of the second convex curve68, the position of the smallest gap clearance between the second convex curve68and the stator30does not change even when the permanent magnet52moves relative to the rotor core51. Therefore, according to the present embodiment, the deviation of the magnetic-flux-strength maximum position in the clearance gap between the rotator50and the stator30can be restricted as much as possible. It is also avoided that the cogging torque and the torque ripple are increased and a controllability of the motor10is deteriorated.

In present embodiment, the base of each convex portion64is not less than 70% of the length of the side65of the base portion63. Thus, each permanent magnet52is accurately fixed, whereby a stable manufacture is attained. Moreover, the width of the permanent magnet52is slightly smaller than that of the convex portion64of the rotor core5, whereby the magnetic loading is made proper.

In a cross section which perpendicularly intersects the shaft center61, both ends of the first convex curve66are located on the side65of the base portion63. Therefore, when each permanent magnet52moves in circumferential direction relative to the rotor core51, the movement of the permanent magnet52is restricted when the permanent magnet52is brought into contact with the side65of the base portion63.

Moreover, in present embodiment, the second convex curve68of the permanent magnet52is a curved surface having a single curvature radius. The rotator50further has the cylindrical cover69engaged with the outer surface of the permanent magnet52so that at least a center portion of the second convex curves68adheres on the inner surface of the cylindrical cover69. The tension “T” applied to the cylindrical cover69biases each permanent magnet52toward the first convex curve66of the rotor core51.

In present embodiment, the rotator50is used for the motor10of the power-steering device for a vehicle. Since the increase in cogging torque and torque ripple is controlled as mentioned above, the present rotator50is suitable for a motor10of the power-steering device.

Other Embodiments

In another embodiment of the present invention, the base of each convex portion64may be less than 70% of the length of the side65of the base portion63. In a cross section which perpendicularly intersects the shaft center61, both ends of the first convex curve66may not be located on the side65of the base portion63. For example, the both ends of the first convex curve66may be radially outside of the side65of the base portion63.

The curvature-radius ratio (R2/R1) may be less than 1.0 and more than 0.6. With this arrangement, the electrical degree error can be not greater than 1 degree. The curvature-radius ratio (R2/R1) may be less than 0.6 and more than zero. With this arrangement, although the electrical degree error exceeds 1 degree, the electrical degree error can be less than that of the comparative example. In a cross section which perpendicularly intersects the shaft center61, the outer shape of the base portion63is not limited to regular decagon.

The number of the slots62of the stator30is not limited to sixty. The permanent magnets52may be fixed by adhesive agent. The rotator may be used not only for the motor of the power-steering device for vehicles but also for other rotational electric machines in other technical field.

The present disclosure is not limited to the embodiment mentioned above, and can be applied to various embodiments.