Electrical Machine Having a Permanent-Magnet-Excited Rotor

The invention relates to an electrical machine having a permanent-magnet-excited rotor, and more particularly, a rotor of an electrical machine having several plate-like or lamellar permanent magnets arranged on its perimeter for each rotor pole at a point directly at the air gap adjacent to the stator of the machine.

An electrical machine having a permanent-magnet-excited rotor is described in German Patent No. U-80 18 686. Due to production engineering and cost considerations, the permanent magnets used for these types of rotors are designed as flat, rectilinear plates or strips. In the case of one rotor, if several permanent magnets are configured side-by-side on the rotor perimeter for each rotor pole, then irregularities in the magnetic field of the individual rotor poles result because of the finite gaps between the individual, adjoining magnets. These irregularities cause harmonic fields, which when combined with stator winding and stator slotting fields, can produce oscillating torques.

It would be advantageous to provide an electrical machine having a permanent-magnet-excited rotor designed to avoid oscillating torques caused by the interaction of stator and rotor harmonic fields.

This advantage is provided by the present invention by having the total number of permanent magnets, Z, configured on the rotor correspond to a number which is not integrally divisible by the stator slot number, N, nor by the pole number, p, of the machine. In a preferred embodiment, the total number of permanent magnets is a prime number greater than three. A rotor having such specifications prevents harmonic fields based on the division of the permanent magnets per rotor pole from interacting with harmonic fields caused by the stator slotting. Oscillating torque that would have been caused by such an interaction is eliminated.

In a further embodiment, the magnetic longitudinal axis of the rotor poles and/or the stator slots are skewed in relationship to the longitudinal axis of the rotor (e.g., see FIG. 3 where the stator slots 10 are skewed from the axis 11 of the rotor). This reduces the magnitude of the oscillating torques resulting from the interaction between the stator slotting fields and the permanent-magnet harmonic fields. Skewing the magnetic longitudinal axis of the rotor poles is easier from a production engineering standpoint than skewing the longitudinal axis of the stator slots, and it is expedient to undertake only one skewing of a magnetic longitudinal axis. Advantageous results are achieved with a skewing by the measure of one slot pitch. Skewing by the measure of one slot pitch means that the position of the magnetic longitudinal axis on the one front end of the rotor is shifted in the circumferential direction opposite the position on the other front end of the rotor by the center-to-center distance between two adjoining slots of the stator.

FIG. 1 is a cross-sectional view of an electrical machine.

FIG. 2 is a perspective view of a rotor equipped with permanent magnets constructed according to the present invention.

FIG. 3 is a partial view of a stator having skewed stator slots.

Referring to FIG. 1, an electrical machine is shown where reference numeral 1 denotes the stator. The stator winding 3 of the machine is arranged in the stator slots 2 of the stator 1 of circumferential shape. A rotatably supported rotor 5 is installed in the stator bore 4 of the stator 1.

The rotor 5 comprises a shaft 6, on which is mounted a laminated rotor core 7. Lamellar permanent magnets 8 are fitted on the periphery of the laminated rotor core 7, and a uniform distribution of the permanent magnets 8 on the rotor circumference is necessary. The number of permanent magnets 8 provided on the periphery of the laminated rotor core 7 corresponds to a prime number. In this embodiment, 31 permanent magnets 8 are arranged on the rotor circumference.

The permanent magnets 8 are magnetized such that several of the permanent magnets 8, arranged spatially side-by-side, form one rotor pole (north pole or south pole) in each case. Referring to FIG. 2, the magnetization of the permanent magnets 8 is such that the transition from one polarity to the other follows along a diagonally running line 9. Thus, the magnetic longitudinal axis of the rotor poles N, S is skewed in relationship to the longitudinal axis of the rotor 5, or rather in relationship to the stator slots 10 running parallel to the longitudinal axis of the rotor 5. This skewing angle, .alpha., is preferably 2.pi./N, where N is the number of slots 10 in the stator.

In the electrical machine of the present invention, all stator slots 10 can be wound, whereby the same coil pitch is obtained for all coils of the stator winding. Moreover, the rotor is uniformly covered with permanent-magnet material over its entire periphery, so that manifestations of imbalance are avoided. Thus, each rotor pole is covered with magnetic material over its full pole width.