Permanent Magnet Excited Rotor for a Synchronous Machine

1. Field of the Invention

This invention relates to a permanent magnet-excited rotor for a synchronous machine which has a short-circuit cage with shorting rings arranged at both end faces and the lamination stack of which, secured against centrifugal stresses, has preferably radial slots for receiving the permanent magnets.

German Pat. No. 24 12 307 shows a rotor having a lamination stack in which individual segments are held by positively engaging undercuts at the shaft. Permanent magnets inserted into the radial slots are magnetized from the outside, after the short-circuit cage has been cast.

German Utility Model No. 77 26 439 suggests closing each of the radial slots at the outer circumference by a narrow strip, which also holds the lamination stack segments together and thereby secures them against centrifugal stresses. Such a securing device is substantially simpler than fastening of the laminated stack segments to the rotor shaft by positive engagement.

If the radial slots are closed at the outer circumference of the rotor, the permanent magnets must be inserted axially into the slots. Inserting the permanent magnets from the shaft opening is impossible as a rule since the radial dimension of the permanent magnets is usually larger than the diameter of the shaft opening. Axial insertion of the permanent magnets, however, must take place before the short circuit cage and the short rings are made, since the slots are covered up by the shorting rings. Permanent magnets which are inserted before the short circuit cage is cast are demagnetized by the high temperature developed during the casting of the cage and must, therefore, be remagnetized from the outside. Such subsequent remagnetizing is not possible with rotors with six or more poles and generally not if rare earth magnets are used.

It is an object of the invention to provide a permanent magnet-excited rotor for a synchronous machine in which high strength of the lamination stack against centrifugal stresses is achieved in a simple manner and in which the full magnetic force of the permanent magnets is preserved if the short circuit cage is cast.

According to the invention, a rotor for a synchronous motor has slots that are closed at the outer circumference of the rotor and one or both shorting rings have recesses that clear the slots in the axial direction to admit elongated magnets into the slots. The individual lamination stack segments are securely held together by closing the slots at the outer circumference and are capable of withstanding large centrifugal forces. Since the radial slots remain axially accessible because of the recesses in the shorting rings, the premagnetized permanent magnets can be axially inserted into the slots after the short circuit cage has been cast. Thus, the full magnetic force of the permanent magnets is preserved.

If the radial dimension of the permanent magnets exceeds the radial width of the shorting rings, the shorting rings are subdivided into segments and each segment extends from one slot to the second-following slot. The segments at one end face are offset circumferentially by a length of arc corresponding to the space between two slots relative to the segments at the other end face so that adjacent slots are accessible from opposite ends of the rotor. This permits virtually the full radial thickness of the lamination stack to be utilized for the arrangement of the permanent magnets.

According to a further embodiment of the invention, a saving of material in the fabrication of a short-circuit cage can be achieved by making the radial dimension of the slots less than the radial thickness of the lamination stack and the radial depth of the recesses equal to the radial dimension of the slots. Each shorting ring has only half as many recesses as the number of the slots and the recesses are arranged in meander-fashion and are offset circumferentially by a length of arc corresponding to the spacing between two slots. The meander shape reduces the volume of the shorting rings and thereby saves material.

Material is also saved and the casting tool is simplified by another embodiment in which the radial dimension of the slots is less than the radial thickness of the lamination stack and the radial depth of the recesses is equal to the radial dimension of the slots. In this embodiment, a number of recesses corresponding to the number of slots is provided in only one shorting ring, and the segment parts of the shorting ring protruding outward between the recesses are connected to a radially inner portion of the shorting ring.

FIG. 1 shows a perspective view of a rotor with shorting rings subdivided into segments;

FIG. 2 a front view of a rotor with a meander-shaped shorting ring; and

FIG. 3 a front view of a rotor in which a number of recesses corresponding to the number of slots is provided in only one shorting ring.

FIG. 1 shows a laminated rotor that has a lamination stack 2 and a short-circuit cage 3. The shorting rings 4 of the cage 3 are not continuous but are each subdivided into three segments 5 with recesses 6 between the individual segments. A radial slot 7 extends axially the full length of the lamination stack 2 in alignment with each of the recesses 6. At the outer circumference of the rotor 1, the slots 7 are closed by narrow strips 8. These narrow strips 8 may be stamped out in the formation of each lamination, or they can consist of nonmagnetic material which is welded on subsequently. Permanent magnets are inserted into the radial slots 7 and pushed axially through the respective recess 6 into each slot 7.

The arcuate length of the segments 5 of the shorting rings 4 is designed so that these segments extend from one slot past the next following slot to the slot thereafter. In addition, the segments 5 of one shorting ring are offset circumferentially relative to the segments 5 of the other shorting ring 4 by a length of arc corresponding to the arcuate spacing between two slots 7. In this manner a short circuit is obtained over the entire circumference of the rotor 1 even though the segments 5 of the two shorting rings 4 are completely separate from each other. In this embodiment of the rotor, half of the permanent magnets are pushed into the slots 7 from one side and the other half from the other side. This arrangement of the rotor makes it possible to form the slots 7 so that they are closed at the outer circumference of the rotor 1 for reasons of strength fabrication of the short circuit cage by a casting process without having to tolerate losses with respect to the magnetic properties of the permanent magnets.

In the rotor shown in FIG. 2, the radial dimension of each of the slots 7 is substantially less than the radial thickness of the lamination stack 2. The radial depth of the recesses 6 corresponds in this embodiment to the radial dimension of the slots 7. The shorting ring 4 at each end of the rotor can therefore bridge across the region between the slots 7 and the shaft 9. Since the number of recesses 6 in each shorting ring 4 is only half as great as the number of slots 7, each shorting ring can be formed in meander-fashion to save material and the two shorting rings can be offset circumferentially by a length of arc corresponding to the spacing between two adjacent slots 7. Half of the permanent magnets can then be pushed through the recesses 6 from one end of the lamination stack 2 and the other half from the other end of the stack. A recess 10 is provided over each of the slots 7 at the other circumference of the rotor 1, and a bar of non-magnetic material is welded into each of these recesses. The bars must be welded in before the magnets are put in place. If the permanent magnets were inserted into the slots 7 first, they would be demagnetized by the heat generated by the arc welding, and the performance of the machine would be impaired thereby. Electron beam welding after the permanent magnets are inserted also would not be possible because the electron beam would be deflected by the magnetic field of the magnets.

In a rotor according to FIG. 3, in which the radial dimension of the slots 7 is, as in FIG. 2, substantially less than the radial thickness of the lamination stack 2, the recesses 6, which have a depth matched to the radial dimension of the slots 7, can all be provided in one shorting ring 4. The segment parts 11 of the shorting ring 4 protruding radially outwardly between the recesses 6 are connected here to an inner ring portion 12 surrounding the shaft 9. With such an embodiment of the shorting ring 4, only one of the two shorting rings need be manufactured in this form; the other shorting ring can be manufactured without recesses, as is customary.