Permanent magnet excited electric motor

Abstract

A permanent magnet excited motor is provided with a plurality of low circumferential hump-like protrusions on the face of each stator pole to partially decrease the air gap between the stator and rotor and act as so-called magnetic cams to exert a force on the rotor to smooth an otherwise uneven parasitic slot torque that occurs between the interaction of slot openings of the stator poles and the gaps between the permanent magnet poles of the rotor.

Description

This application FIG.wherein, the pulsating component is reduced so the height of the cog can be greatly reduced so its outer face toward the air gap is at approximately the same level as the tip of the stator pole at its end. In this way, a drastic air gap reduction is already achieved as described above. These two components are illustrated by curve 2. The curves 2 and 2' then do not yield a zero line as the sum of 1 and 1' but instead this sum curve for 2 and 2' has a certain waviness which is a relative disadvantage, but this effect can be further reduced or eliminated by providing a relative air gap enlargement T₃ between the stator pole middle and the stator pole end which is about 1/10 the stator pole width in the circumferential direction and on the whole is much smaller than the air gap enlargement next to the cog and at the pole arc end. Because of this enlargement T₃, the waviness in the sum group 2 and 2' can be further reduced, and again we have a sum moment of zero. This means that the average air gap reduction can be accomplished without the disadvantages of such a pulsating extra component.

FIG. 9a shows a complete stator section with six outer stator poles for an 8-pole inner rotor as shown in FIG. 9c. According to this invention, three cogs-303 cogs 303 are provided on each stator pole 311-316 in such a way that one sits centrally in the middle of the stator pole and another cog sits between the middle of the stator pole and the middle of one slot 317-322 so there is an equidistant distribution of three cogs 303 between two stator slots (adjacent). Between two adjacent cogs 303 or between one cog 303 and the adjacent stator pole end, the finer refinements as described above in combination with FIGS. 4 to 7 can also be used in addition.

FIG. 9b shows the dimensional relationships of a single stator pole in an enlarged detail. The cog height `h` here may be 0.2 mm. The cog width `a` in the peripheral direction may be 1.8 mm. The mechanical distance between two cogs and between one cog and the adjacent slot is 15 mechanical degrees. The air gap diameter may be about 50 mm.

FIGS. 7a and 8a also show the additional recess in the middle between the cog in the center of the stator pole and the adjacent slot as about 1.1 mm with an air gap diameter of about 50 mm.

FIG. 9, i.e., FIGS. 9a to 9d, show a practical example with an 8-pole rotor of the motor with 6 slots in the stator.

With a larger number of poles, each slot can be compensated by several cogs, e.g., three shown in FIG. 9. In this way the cog height can be reduced to 1/3 the height that would otherwise be required in the embodiments of FIGS. 1 to 8.

This invention can fundamentally also be used with other air gap forms, e.g., with a planar air gap, but this form of implementation requires additional measures because the stator pole width must be varied in the longitudinal direction of the slot, for example.

FIG. 9c shows an enlarged and more complete computer printout of the parts of the magnetic circuit for the practical example according to FIG. 9a where the laminated stator 300, the internal rotor 200 with its 8 permanent magnet poles 201 can be seen mounted on the magnetic flux return path 202 which is preferably designed as a soft iron hollow cylinder.

FIG. 9d shows a detail of an enlarged cutaway sector indicated as IXd in FIG. 9c. The field line pattern was also printed out by computer. Cutouts 304, 305 and 306 distributed uniformly on the outer perimeter of the stator yoke can be used for mounting in a motor casing, e.g., by having the parts of the motor casing fit in a form locking manner into these cutouts.

The six stator poles are each provided with a concentrated winding (not shown) where two diametrically opposed windings are excited simultaneously, e.g., by having the two windings connected in series.

Claims

1. A brushless dc motor having a cylindrical type air gap, a slotted stator core and permanent magnet excitation, the air gap being defined between circumferentially facing faces of the slotted stator core and an opposing ring of permanent magnets formed to rotate with respect to the core, the core having a plurality of radially extending winding slots, and the motor including at least one winding forming electromagnetic poles defined in the sectors between slots of the stator core with a plurality of raised portions on each of the electromagnetic pole faces extending radially into the air gap, the circumferential width of the raised portions being small compared to the circumferential width of the pole pitch, the raised portions on each of the pole faces being located such that the distance measured in the direction of rotor rotation from the center of a stator slot defined between any given pair of stator poles to the center of one of said raised portions on the stator face adjacent said given pair of stator poles is equal to n times the rotor pole pitch, n being a whole number.

2. A motor according to claim 1, wherein the circumferential extent of the raised portion corresponds to from 0.5 to 1 times the circumferential width of the stator slot orifice.

3. A motor according to claim 1, wherein the air gap directly next to the raised portion as seen in the circumferential direction has the maximum dimension, which dimension decreases in the direction of the stator pole ends, preferably to a value equal to that above the cog.

4. A motor according to claim 3, wherein the height of the raised portion is from 0.1 to 0.3 mm with an effective air gap of from 0.3 to 0.6 mm.

5. A motor according to claim 3, wherein the reduction in air gap dimension is gradual and continuous.

6. A motor according to claim 5, wherein the air gap next to the raised portion soon decreases drastically in dimension, then increases again near the stator pole ends and finally decreases directly at the stator pole ends.

7. A motor according to claim 1, wherein the rotor is an internal rotor. 8. A brushless dc motor having a cylindrical type air gap, a slotted stator core and permanent magnet excitation, the air gap being defined between circumferentially facing faces of the slotted stator core and an opposing ring of permanent magnets formed to rotate with respect to the core, a pole gap being defined between each adjacent pair of said permanent magnets, the core having a plurality of radially extending winding slots and a corresponding plurality of stator slot orifices, and the motor including at least one winding forming electromagnetic poles defined in the sectors between slots of the stator core wherein a parasitic torque is generated each time that a pole gap interacts with one of said stator slot orifices, the motor further including a plurality of raised portions on each of the electromagnetic pole faces extending radially into and thereby reducing the air gap, the circumferential width of the raised portions being small compared to the circumferential width of the pole pitch, the raised portions on each of the pole faces being (i) dimensioned and positioned thereon to generate a compensating torque which corresponds to and is generally opposite to one of said parasitic torques and (ii) located such that the distance measured in the direction of rotor rotation from the center of a stator slot defined between any given pair of stator poles to the center of one of said raised portions on the stator face adjacent said given pair of stator poles is equal to n times the rotor pole pitch, n being a non-negative and non-zero whole number. 9. A motor according to claim 8, wherein the circumferential extent of the raised portion corresponds to from 0.5 to 1 times the circumferential width of the stator slot orifice. 10. A motor according to claim 9, wherein the air gap directly next to the raised portion as seen in the circumferential direction has the maximum dimension, which dimension decreases in the direction of the stator pole ends, preferably to a value equal to that above the cog. 11. A motor according to claim 10, wherein the height of the raised portion is from 0.1 to 0.3 mm with an effective air gap of from 0.3 to 0.6 mm. 12. A motor according to claim 10, wherein the reduction in air gap dimensions is gradual and continuous. 13. A motor according to claim 12, wherein the air gap next to the raised portion soon decreases drastically in dimension, then increases again near the stator pole ends and finally decreases directly at the stator pole ends. 14. A motor according to claim 8, wherein the rotor is an internal rotor.