Permanent Magnet Rotor With Sinusoidal Flux Pattern

1. Field of the Invention:

The present invention relates to a rotor which has a plurality of permanent magnets on the peripheral surface of a rotary shaft.

2. Description of the Prior Art:

A rotor having permanent magnets has been proposed in Japanese Patent Laid Open-Application No. 58-123355, wherein a rotary shaft is provided which has an orthogonal cross-sectional shape in a longitudinal direction thereof, and a plurality of circular-arc portions which center around an axis of rotation of the rotor shaft, as well as rectilinear portions which respectively connect adjacent circular arc portions with straight lines having no curved portion. A plurality of permanent magnets, equal in number to the circular-arc portions are respectively fixed in opposition only to the rectlinear portions.

In, for example, Japanese Utility Model Registration Application Laid-Open No. 55-120283, a rotor of the aforementioned construction is proposed which is easier to manufacture and which prevents the permanent magnets from turning relative to the axis of rotation against torques which act on the magnets.

The present invention has for its object to further improve the rotor disclosed in the aforementioned official gazette of Japanese Patent Application Laid-open No. 58-123355 and to provide a rotor which can increase output per unit volume.

In the present invention, a rotary shaft is provided which has an orthogonal cross-sectional shape in a longitudinal direction thereof, with a plurality of circular-arc portions which center around a rotary axis of the rotary shaft, and rectilinear portions which connect adjacent circular-arc portions with straight lines having no curved portion. Respective permanent magnets equal in number to the circular-arc portions are fixed in close contact or at uniform intervals while extending over the circular-arc portions and the rectilinear portions. In addition, the surface of each of the permanent magnets remote from the rotary shaft includes a sectional contour orthogonal to the longitudinal direction of the rotary shaft, forming a circular arc which juts out. These features are effective to enable a manufacturing of a rotor which exhibits improved magnetic characteristics and which is structurally immune to torque reactions acting on the permanent magnets.

FIG. 1 is a cross-sectional view of an embodiment of a permanent magnet rotor according to the present invention;

FIG. 2 is a diagram showing the symbols of dimensions in a rotary electric machine which comprises the rotor shown in FIG. 1;

FIG. 3 is a diagram of a B-H curve which is used for explaining the merit of the rotor shown in FIG. 1;

FIG. 4 is a diagram showing the magnetomotive force waveforms of the rotors shown in FIGS. 1 and 5 and a prior-art rotor;

FIG. 5 is a cross-sectional view of another embodiment of a permanent magnet rotor according to the present invention;

FIG. 6 is a diagram of a magnetomotive force waveform demonstrated when the machine shown in FIG. 2 is operated as a synchronous machine;

FIG. 7 is a vector diagram in the case where the rotor of the present invention is operated as a synchronous machine;

FIG. 8 is a cross-sectional view of yet another embodiment of a permanent magnet rotor constructed in accordance with the present invention;

FIG. 9 is a cross-sectional view of another permanent magnet rotor having two sets of curved surfaces;

FIG. 10 is a cross-sectional view of an embodiment similar to the rotor of FIG. 8;

FIG. 11 is a diagram of a magnetomotive force waveform demonstrated when the rotor shown in FIG. 10 is operated as a D.C. machine;

FIG. 12 is a vector diagram in the case where the rotor shown in FIG. 10 is operated as the D.C. machine; and

FIG. 13 is a still further embodiment of a permanent magnet rotor constructed in accordance with the present invention.

FIG. 14 is a sectional view showing a known rotor.

Referring now to the drawings wherein like reference numerals are used throughout the various views to designate like parts and, more particularly, to FIG. 1, according to this figure, a rotary shaft 1 includes an axis of rotation 2 and has an orthogonal cross-sectional contour in a longitudinal direction thereof. The rotary shaft 1 includes circular arc-portions 3-6 which are equal in number to the poles and which are centered around the axis of rotation 2. Rectilinear portions 7-10 are provided for connecting the adjacent circular-arc portions 3 and 4, 4 and 5, 5 and 6, and 6 and 3, respectively, with straight lines having no curved portion.

Permanent magnets 11-14 are disposed in the same number as that of the poles, and each permanent magnet 11-14 is fixed to two of the circular-arc portions and the rectilinear portion between these two circular-arc portions in close contact or at a uniform interval.

By way of example, the permanent magnet 11 is fixed to the two circular-arc portions 3, 4 and the rectilinear protion 7 between them by the use of an adhesive, not shown, while the permanent magnet 12 is fixed to the two circular-arc portions 4, 5 and the rectilinear portion 8 between them by the use of adhesive, not shown.

The permanent magnets 11 and 13 are so magnetized that their outer peripheral surfaces exhibit S-poles, and the permanent magnets 12 and 14 are so magnetized that their outer peripheral surfaces exhibit N-poles.

The adjacent permanent magnets 11 and 12, 12 and 13, 13 and 14, and 14 and 11 are bonded, desirably through members of a nonmagnetic material 15.

The center of a circle formed by the outer peripheries of the permanent magnets 11-14 lies on the axis 2.

A binder 16 is wound around a cylinder formed by the outer peripheries of the permanent magnets 11-14.

Centrifugal forces acting on the permanent magnets 11-14 are countered with the binder 16 and the adhesive interposed between the rotary shaft 1 and the respective permanent magnets 11-14, and torque reactions are parried with the rectilinear portions 7-10 of the rotary shaft 1.

According to this embodiment, a magnetic flux distribution results, based on the permanent magnets 11-14, which approximates a sinusoidal wave. It is well known as to rotary electric machines that magnetic flux distribution close to the sinuosoidal wave is desirable when the enhancement of a precise control or the reduction of noise or vibrations is intended.

A magnetic flux density to be established by the permanent magnets 11-14 is determined by the properties of materials and the operating point of a magnetic circuit including the permanent magnets. The operating point is given as the intersection point between a demagnetization curve and a straight line whose gradient is a permeance coefficient. The permeance coefficient P.sub.g is determined in accordance with the following relationship:

by P.sub.g =L.sub.m /A.sub.m .times.A.sub.8 /L.sub.8, wherein:

L.sub.m =a thickness of the magnet,

A.sub.m =an area of the magnet,

A.sub.g =a gap area, and

L.sub.g =a gap length between an armature core 17 and an outer peripheral surface of each of the permanent magnets 11-14.

With magnets having the shape illustrated in FIG. 1, the permeance coefficient becomes large in a middle portion having a great thickness L.sub.M and small in the end portion of a relatively small thickness L.sub.M '.

Concerning a magnetic flux density at the operating point, the flux density B.sub.1 of the middle portion becomes higher than that of B.sub.2 at the end part as shown most clearly in FIG. 3. The magnet flux wave form which is established by a magnet of uniform thickness becomes rectangular with somewhat rounded shoulders as shown at I in FIG. 4 because some flux leaks at the end parts of the pole. In contrast, when the middle portion is made thicker as in the present embodiment, a magnetic flux wave form having a high middle portion, indicated at II in FIG. 4, is obtained due to the difference of the permeance coefficients described hereinabove. A broken line II is nearer to a sinusoidal wave than the solid line I. That is, according to the present embodiment, the ratio of fundamental wave/total effective value is great, so that a rotary electric machine of high magnetic flux utilization factor and good controllability can be fabricated. A horizontal axis H in FIG. 3 indicates the electromotive force, and numeral 19 in FIG. 2 indicates an armature slot.

An embodiment in FIG. 5 is an example wherein the difference of the thicknesses is more enlarged, to bring the magnetic flux waveform still closer to the sinusoidal wave as shown at III in FIG. 4.

In the embodiment of FIG. 5, the center of the circular arc of the outer peripheral surface of the each of permanent magnets 11-14 deviates toward tbhe side of the permanent magnet with respect to the axis rotation 2 of a rotary shaft 1. Numeral 23 designates a compound, and numeral 16 a binder.

Since a demagnetization yield strength is proportional to the thickness of a magnet, the former is enhanced by increasing the latter.

In a case where a permanent-magnet rotary electric machine is operated as a synchronous motor, the magnetic field of a rotar revolves with a slight lag over the revolving magnetic field of a stator as illustrated in FIG. 6. A vector diagram in this case is shown in FIG. 7, wherein .PHI..sub.u denotes rotor magnetic flux, I armature current, r armature resistance, X synchronous reactance, E.sub.0 induced voltage, and V supply voltage. Accordingly, a demagnetizing field in this operation arises in a deceleration mode in which a torque opposite to the direction of rotation is produced, and it becomes great near the middle of a pole and small at the end part thereof. When, in consideration of this phenomenon, the permanent magnet is so shaped as to be thick at the middle part of the ple and thin at the end parts, a high demagnetization yield strength can be attained without increasing the volume considerably.

FIG. 8 shows an example wherein a rotary shaft 1 is prevented from turning by a key 21, and wherein the outer surface of a snugly-fitted boss 20 is provided with circular-arc portions 3-6 and rectilinear portions 7-10. FIG. 9 shows an example wherein small-diameter circular-arc portions indicated at numerals 3-6 are connected by gentle curve portions 7-10 which are gentler in curvature than the small-diameter circular-arc portions.

An example in FIG. 10 is advantageous when a permanent-magnet rotary electric machine is rotated in the mode of a D.C. machine, namely, as a no-commutator motor. A vector diagram in the case of the D.C. machine is shown in FIG. 12. In the mode of the D.C. machine, the magnetic field of a stator is controlled so as to shift by 90.degree. in terms of an electrical angle from the magnetic field of a rotor as illustrated in FIG. 11. Therefore, a demagnetizing field becomes great at the ends parts of a pole and small at the middle part thereof contrariwise to the mode of the synchronous motor described before. Accordingly, when the magnet is so shaped that the end parts of the pole are thicker as shown in FIG. 10, a structure which is efficient as to the demagnetization yield strength is obtained. In this embodiment, each of the permanent magnets 11-14 is fixed to two rectilinear portions and a circular-arc portion located between them. FIG. 13 shows an example wherein small-diameter circular-arc portions indicated at numerals 3-6 are connected by gentle curve portions 7-10 which are gentler in curvature than the small-diameter circular-arc portions.