The present invention relates to a multipole rotor for an electric motor according to the preamble of the independent claim 1.
A rotor of this type comprises a rotor core and a plurality of individual permanent magnets, which are distributed over the circumference of the rotor and which, when seen in a cross-sectional view of the rotor orthogonal to an axis of the rotor, have a convex curvature on the side facing the air gap between a stator of the electric motor and the rotor.
A rotor according to the preamble of the independent claim 1 is known e.g. from US 20050264122 A1. In the case of this rotor, the permanent magnets are, relative to the axis of the rotor, magnetized in a radial direction. Two respective neighboring permanent magnets form a magnetic pole pair. The convex curvature of the individual permanent magnets has the advantage that detent torques are largely avoided.
It is the task of the present invention to improve a multipole rotor of the generic kind in such a way that the detent torque is reduced still further and/or the content of certain harmonics, in particular the 3rd harmonic, is suppressed in delta connection.
This task is solved by the features of the independent claim 1. Accordingly, the task is solved by a solution according to the present invention in the case of a multipole rotor according to the preamble of the independent claim 1, when four respective permanent magnets, which are juxtaposed in the circumferential direction of the rotor, define together a magnetic pole pair, the magnetization direction of each individual permanent magnet being not a radial direction, but enclosing an angle α between 30° and 60° with a reference plane extending through the axis of the rotor and through the center of the respective permanent magnet.
The invention is also suitable for use with a rotor according to the preamble of the independent claim 2, which does not necessarily comprise a rotor core. Hence, the task is alternatively also solved by the features of the independent claim 2. The task is solved by a solution according to the present invention in the case of a multipole rotor according to the preamble of the independent claim 2, when four respective permanent magnets, which are juxtaposed in the circumferential direction of the rotor, define together a magnetic pole pair, the magnetization direction of each individual permanent magnet being not a radial direction, but enclosing an angle α≠0°, preferably an angle between 30° and 60°, with a reference plane extending through the axis of the rotor and through the center of the respective permanent magnet. In this case, the rotor does preferably not comprise a rotor core.
The invention offers the advantage that the detent torque is further reduced and/or the content of certain harmonics, in particular the 3rd harmonic, is suppressed in delta connection. The present invention allows both goals to be aimed at. Depending on the technology used, only one goal may be aimed at, e.g. reducing only the detent torque in a grooved motor in star connection or suppressing only the 3rd harmonic in a motor with ironless winding in delta connection, or both goals may be aimed at simultaneously, e.g. in a grooved motor that is to be operated in delta connection.
The individual permanent magnets are preferably evenly distributed over the circumference of the rotor. The axis of the rotor is the axis of rotation or the axis of rotational symmetry of the rotor. Further preferred, the rotor is an internal rotor, the permanent magnets being arranged on the outer circumference of the rotor. A pair of poles is defined by a group of four magnets. Every fourth permanent magnet has the same magnetization direction relative to its respective reference plane. The first two permanent magnets of a group of four define together a magnetic pole, e.g. a magnetic north pole. The third permanent magnet and the fourth permanent magnet of the group of four define together an opposite magnetic pole, e.g. a magnetic south pole. Therefore, the number of individual permanent magnets must be divisible by the number 4.
The embodiment of the present invention has a plurality of optimizable degrees of freedom: the angle of magnetization α, the air gap-side radius r of the permanent magnets, and the center of the radius of the permanent magnet can be selected freely. Furthermore, the rounding of the permanent magnet can also be chosen in a form other than a circular form and the inner contour of the permanent magnets or of the magnetic feedback element can be varied. Even if the suppression of a detent torque and of the harmonics is aimed at, not all optimizable degrees of freedom have to be used for achieving the optimization goal.
Advantageous embodiments of the present invention are the subject matter of the subclaims.
According to a preferred embodiment of the present invention, the magnetization direction of each individual permanent magnet encloses an angle between 40° and 50° with the respective reference plane. The detent torque can be avoided most effectively when the angle, which the magnetization direction of each individual permanent magnet encloses with the respective reference plane, is further preferred an angle of 45°.
Further preferred, two respective permanent magnets, which are juxtaposed in the circumferential direction of the rotor, define together a magnetic pole, the magnetization directions of these two permanent magnets being symmetric with respect to one another relative to an intermediate plane extending centrally between these two permanent magnets and through the axis of the rotor.
Further preferred, the third permanent magnet of a group of four permanent magnets following directly one after the other in a circumferential direction has, relative to the respective reference plane, a magnetization direction which is opposite to the magnetization direction of the first permanent magnet of this group of permanent magnets relative to the respective reference plane, the fourth permanent magnet of this group of permanent magnets having, relative to the respective reference plane, a magnetization direction which is opposite to the magnetization direction of the second permanent magnet of this group of permanent magnets relative to the respective reference plane.
According to a further preferred embodiment of the present invention, the permanent magnets are in contact with one another on their sides and define together an ideally closed ring. A high efficiency is accomplished in this way. Due to the repulsion between two magnets with like poles, the magnets will be distributed evenly at the circumference. The complicated positioning of the permanent magnets on the rotor core during production of the rotor is therefore no longer necessary, and this will reduce the effort as well as the cost of production.
According to a further preferred embodiment, the neighboring permanent magnets are in planar contact with one another at the respective pole transition, and the neighboring permanent magnets belonging to the same pole define a gap relative to one another or are in contact with one another, the gap having a width of less than 0.3 mm.
It will be particularly advantageous, when the side faces of the permanent magnets extend radially relative to the axis of the rotor, so that the sides of the permanent magnets are in planar contact with one another.
According to a further particularly preferred embodiment of the present invention, the convex curvature deviates from the curvature of a circle around the axis of the rotor, which circle envelops the permanent magnets directly. This embodiment allows to reduce the detent torque still further and to suppress the harmonics effectively.
For avoiding a detent torque as well as for suppressing the harmonics, it will be particularly advantageous when the radius of the convex curvature is smaller than the radius of a circle around the axis of the rotor, which circle envelops the permanent magnets directly, i.e. it envelops them at the air gap. Preferably, especially the average radius of the convex curvature is smaller than the radius of the circle around the axis of the rotor, which circle envelops the permanent magnets directly. The radius of the enveloping circle corresponds to the maximum outer diameter of the rotor, provided that the rotor is an internal rotor. Particularly preferred, the radius or the average radius of the convex curvature is between 15% and 70%, preferably between 20% and 50% of the radius of the circle around the axis of the rotor, which circle envelops the permanent magnets directly.
According to a further preferred embodiment of the present invention, the permanent magnets are fixed to one another and/or to the rotor core of the rotor by means of an adhesive. This allows the rotor according to the present invention to be produced in a particularly simple and cost-effective manner. The rotor core may either consist of a soft magnetic material, so that the rotor core represents a magnetic feedback element for the permanent magnets. Alternatively, a non-magnetic material may also be used for the rotor core.
According to a further preferred embodiment of the present invention, the rotor comprises an envelope, the permanent magnets being encompassed by the envelope on their outer side. Preferably, the magnets are connected to the envelope and/or an inner shaft by an adhesive or by a potting compound. According to another preferred embodiment of the present invention, the rotor is configured such that it comprises an envelope without a magnetic feedback element or a shaft extending therethrough.
Instead of adhesively fixing the permanent magnets to one another and/or to the rotor core or an envelope, the permanent magnets may also be fixed to the rotor core of the rotor by means of a bandage. Bandaging may also take place in addition to adhesive fixing.
According to a further embodiment of the present invention, the back of the permanent magnets positioned opposite the convexly curved side is flat. The back extends so to speak tangentially to the circumferential surface of the rotor core, provided that the rotor is an internal rotor. In the case of this embodiment, the production outlay for the permanent magnets is comparatively low.
The assembly of the rotor according to the present invention can be simplified, when the back of the permanent magnets located opposite the convexly curved side has, according to an alternative embodiment, a curvature which corresponds to the radius of the rotor core. This embodiment also provides a particularly high efficiency by optimizing the magnetic field built up by the rotor. The radius of the rotor core, to which the curvature of the back of the permanent magnets is adapted, is the outer radius of the rotor core in an internal rotor.
According to a further preferred embodiment of the present invention, the rotor may comprise a total of 8, 12 or 16 individual permanent magnets. What is decisive, however, is that the number of permanent magnets can be divided by the number 4.
According to a further embodiment of the present invention, the permanent magnets are preferably loaf-shaped in cross-section. This means that the cross-section of the permanent magnets comprises a base, two sides extending obliquely thereto and diverging from each other from the base, as well as a convexly curved outer side opposite the base. The two sides of the cross-section extend preferably radially with respect to the axis of the rotor.
According to an alternative but nevertheless preferred embodiment, the permanent magnets have the shape of a piece of cake in cross-section. This means that, in comparison with the loaf-shaped embodiment, the base of the cross-section is either shorter than the two sides or the cross-section has no base at all. The two sides of the cross-section extend obliquely to each other also in this case and define, together with the convexly curved outer side, the shape of a piece of cake.
Further preferred, all the permanent magnets have the same geometry and are further preferred each symmetric to their own reference plane.
The invention also provides an electric motor with a stator and a rotor according to the present invention. The rotor may here be configured according to one or a plurality of the above described embodiments.
Embodiments of the present invention are explained hereinafter with reference to the drawings, in which
As regards the statements made hereinafter, like parts will be designated with like reference numerals. If a figure comprises reference numerals that are not discussed in detail in the associated description of the figure, preceding or subsequent descriptions of the figures will be referred to.
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Number | Date | Country | Kind |
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17188593.2 | Aug 2017 | EP | regional |