Rotor for a Brushless Direct-Current Motor, Particularly for an Electric Motor of the Inner Rotor Type, and Electric Motor Comprising Such a Rotor

Abstract

The disclosure relates to a rotor for a brushless direct-current motor comprising a shaft, a rotor core arranged on the shaft, the rotor core acting as a return body, and a ring magnet which surrounds the rotor core and is attached to same. The ring magnet is in the form of a circular disk, a radial direction and a peripheral direction being defined by the circular disk. Furthermore, a hole count q is defined by the equation q=N/(2 pm), N being the number of grooves in the rotor, p being the number of pole pairs of the rotor, and m being the number of phases. According to the disclosure, the winding of the rotor is connected in a delta connection.

Claims

1. A rotor for a brushless direct-current motor, the rotor comprising: a shaft; a rotor core arranged on the shaft, the rotor core configured as a magnetic return path body; and at least one ring magnet fastened to the rotor core and configured to surround the rotor core, the at least one ring magnet is having one of a circular disk shape and a cylindrical ring shape, a radial direction and a peripheral direction being defined by the one of the circular disk shape and the cylindrical ring shape, wherein a number q of holes is defined by the equation q=N/(2 pm), where N represents a number of slots in the rotor, p represents a number of pole pairs of the rotor, and m represents a number of phases, and wherein a winding of the rotor is connected as a delta connection.

2. The rotor as claimed in claim 1, wherein the winding of the rotor the number q of holes, where q=0.5.

3. The rotor as claimed in claim 1, wherein a waveform of an induced source voltage of the brushless direct-current motor is matched to a current waveform.

4. The rotor as claimed in claim 3, wherein the waveform of the induced source voltage has a trapezoidal profile.

5. The rotor as claimed in claim 3, wherein the waveform of the induced source voltage has a sinusoidal profile.

6. The rotor as claimed in claim 1, wherein the brushless direct-current motor uses a block commutation of 120°.

7. The rotor as claimed in claim 1, wherein the at least one ring magnet has a radially anisotropic grain structure.

8. The rotor as claimed in claim 1, wherein the at least one ring magnet is one of an SmCo ring magnet and NdFeB ring magnet and is magnetized at several poles over an outer periphery thereof.

9. The rotor as claimed in claim 1, wherein the at least one ring magnet has at least three pole pairs.

10. The rotor as claimed in claim 7, wherein the at least one ring magnet is a hot-pressed ring magnet comprised of one of SmCo powder and of NdFeB powder, the radially anisotropic grain structure being produced by a two-stage compaction process.

11. The rotor as claimed in claim 7, wherein the at least one ring magnet is a sintered ring magnet comprised of NdFeB powder, the radially anisotropic grain structure being produced by a two-stage compaction process.

12. The rotor as claimed in claim 1, wherein the at least one ring magnet is fastened to the rotor core using one of adhesive bonding, soldering, thermal shrink-fitting, and welding.

13. An electric motor comprising a stator having one of a circular disk stator yoke and a cylindrical ring stator yoke, a radial direction and a peripheral direction being defined by the one of the circular disk stator yoke and the cylindrical ring stator yoke, the stator having a defined number of pole teeth that project radially inward from the one of the circular disk stator yoke and the cylindrical ring stator yoke; a number of coils that corresponds to the defined number of pole teeth, coils of the number of coils being wound around corresponding pole teeth of the defined number of pole teeth; and a rotor that is enclosed by the stator in the radial direction, a gap having a defined width being defined between the stator and the rotor, the rotor having (i) a shaft, (ii) a rotor core arranged on the shaft, the rotor core configured as a magnetic return path body, and (iii) at least one ring magnet fastened to the rotor core and configured to surround the rotor core, the at least one ring magnet having one of a circular disk shape and a cylindrical ring shape, a number q of holes being defined by the equation q=N/(2 pm), where N represents a number of slots in the rotor, p represents a number of pole pairs of the rotor, and m represents a number of phases, a winding of the rotor being connected as a delta connection.

14. The electric motor as claimed in claim 13, wherein the electric motor has an idling rotation speed of at least 24,000 revolutions per minute and the rotor has a diameter of 30 mm.

15. The electric motor as claimed in claim 13, wherein the number of coils of the electric motor are connected electrically in parallel.

16. A handheld power tool comprising: an electric motor comprising: a stator having one of a circular disk stator yoke and a cylindrical ring stator yoke, a radial direction and a peripheral direction being defined by the one of the circular disk stator yoke and the cylindrical ring stator yoke, the stator having a defined number of pole teeth that project radially inward from the one of the circular disk stator yoke and the cylindrical ring stator yoke; a number of coils that corresponds to the defined number of pole teeth, coils of the number of coil being wound around the corresponding pole teeth of the defined number of pole teeth; and a rotor that is enclosed by the stator in the radial direction, a gap having a defined width being defined between the stator and the rotor, the rotor having (i) a shaft, (ii) a rotor core arranged on the shaft, the rotor core configured as a magnetic return path body, and (iii) at least one ring magnet fastened to the rotor core and configured to surround the rotor core, the at least one ring magnet having one of a circular disk shape and a cylindrical ring shape, a number q of holes being defined by the equation q=N/(2 pm), where N represents a number of slots in the rotor, p represents a number of pole pairs of the rotor, and m represents a number of phases, a winding of the rotor being connected as a delta connection.

17. The rotor as claimed in claim 9, wherein the at least one ring magnet has at least 8 pole pairs.

18. The rotor as claimed in claim 17, wherein the at least one ring magnet has at least 18 pole pairs.

19. The electric motor as claimed in claim 13, wherein the electric motor is a brushless internal-rotor electric motor.

Description

DRAWINGS

[0025] The invention will be explained in more detail below with reference to preferred exemplary embodiments. In the drawings, in each case in schematic form:

[0026] FIG. 1 shows a detail of a rotor according to the invention and also of an electric motor according to the invention;

[0027] FIG. 2 shows an example of a delta connection with a parallel individual tooth winding;

[0028] FIG. 3 shows an example of an adapted waveform of an induced source voltage; and

[0029] FIG. 4 shows a schematic illustration of a ring magnet with a radially isotropic orientation of the preferred magnetic direction.

[0030] FIG. 1 shows a 120° segment of a partial cross section of an electric motor 100 according to the invention. The rotor of the electric motor 100 comprises, amongst other things, a shaft 12, a rotor core 14 which is arranged on the shaft 12, wherein the rotor core 14 serves as a magnetic return path body. The electric motor 100 further comprises at least one ring magnet 16 which is fastened to the rotor core 14 and surrounds the rotor core 14. The ring magnet 16 is of circular annular disk-like the cylindrical ring-like design, wherein a radial direction and a peripheral direction are defined by the circular disk shape and, respectively, the cylindrical ring shape.

[0031] The at least one ring magnet 16 is fastened to the rotor core 14 using one of the fastening processes from the group comprising adhesive bonding, soldering, thermal shrink-fitting or welding.

[0032] It can further be seen that the electric motor 100 comprises a stator 20, wherein the stator 20 has a circular disk-like stator yoke 22, by way of which a radial direction and a peripheral direction are defined, and also a defined number of pole teeth 24 which project radially inward from the stator yoke 22. A corresponding number of coils 30 are wound around the pole teeth 24. This basic construction is known per se in the case of internal-rotor electric motors and will not be described in further detail.

[0033] According to the invention, the ring magnet 16 has a radially anisotropic grain structure. In one embodiment, in which the ring magnet 16 is a ring magnet 16 which is hot-pressed from NdFeB powder, this radial anisotropy can be achieved in a compaction step which follows the first hot-pressing operation, therefore by a two-stage compaction process.

[0034] As an alternative to this and according to a further embodiment of the invention, the ring magnet 16 can be a sintered ring magnet 16 composed of SmCo powder or of NdFeB powder, wherein the radially anisotropic grain structure is likewise produced by a two-stage compaction process. The radially oriented anisotropic injection-molded ring magnets are usually produced by electromagnetic orientation technology. In contrast to the simple permanent magnet orientation, magnets which are produced by electromagnetic orientation are demagnetized before the lowering, and then polarized in line with the desired requirements. In this way, for example, a ring magnet 16, illustrated in FIG. 4, with a radially isotropic orientation of the preferred magnetic direction can be produced.

[0035] An increased mechanical load-bearing capacity and, respectively, robustness of the ring magnet can be ensured by producing the ring magnet 16 by hot-pressing the NdFeB powder. In addition, the radial anisotropy of the grain structure of the ring magnet 16 which is introduced in a separate production step leads to a remanence flux density which is once again increased by approximately 10% in comparison to conventionally sintered ring magnets 16 and therefore to an increased power density. As an alternative, the production of the ring magnet 16 can also be produced in accordance with another process, for example in accordance with the impact extrusion process.

[0036] The anisotropy improves the magnetic remanence flux density by up to 10% in comparison to conventional sintered NdFeB magnets and by the factor 2.2 in comparison to the conventional plastic-bonded NdFeB magnets. Owing to this gain in magnetic flux across the ring magnet 16, the active axial length of the electric motor 100 and/or its electrical resistance can be reduced. According to the invention, the power density of the electric motor 100 and at the same time its mechanical robustness can be increased in this way. As a result, high rotation speeds are possible even in the case of large rotor diameters.

[0037] For example, it has been shown that an electric motor 100 which is constructed according to the invention can travel at a rotation speed of over 24,000 rpm during idling given a rotor diameter of 30 mm. Comparable values are currently provided in the prior art only by rotors with buried magnets, but with the abovementioned disadvantages which accompany this construction.

[0038] In a preferred embodiment, the ring magnet 16 has at least three pole pairs, preferably at least 8 pole pairs, particularly preferably at least 18 pole pairs. In general, the number of pole pairs of the ring magnet varies depending on the design in respect of size and power of the electric motor, wherein a radially anisotropic ring magnet is not subject to any restrictions in this respect.

[0039] It should once again be noted that, in contrast to this, the design with buried magnets has the disadvantage that the number of magnets and therefore of pole pairs is limited by the width of the webs of the rotor lamination between the magnets.

[0040] The higher magnetic flux in a rotor according to the invention also requires larger cross sections in the stator geometry. In this respect, it is advantageous, in principle, for the number of poles in the design according to the invention to not be limited since a higher number of pole pairs reduces the cross section of the iron magnetic return path. This is because the magnetic flux can be distributed between a higher number of pole pairs.

[0041] Furthermore, fewer turns are required in the stator for high rotation speeds given a higher magnetic flux. This in turn means that the copper wire cross sections have to increase in order to be able to equally fill the stator slot with fewer turns.

[0042] In general, needle winding machines are used here, wherein the needle, which guides the wire through the slots, can guide a wire with a maximum wire diameter of just over 1 mm.

[0043] As illustrated in FIG. 2, according to the invention, the winding of the rotor is connected in delta with parallel individual tooth windings, wherein a rotor winding preferably has a number q of holes of q=0.5.

[0044] In this case, the number q of holes is defined by the equation q=N/(2 pm), wherein N represents the number of slots in the rotor, p represents a number of pole pairs of the rotor, and m represents a number of phases.

[0045] As illustrated in FIG. 3a, a waveform of an induced source voltage, also called electromotive force or induced EMF voltage of the electric motor, is matched to a current waveform. Whereas the current waveform has a typical 120° block commutation in the figure, the induced source voltage is trapezoidal. This results in a high machine utilization and a largely uniform torque profile. In the illustrated configuration of the 120° block commutation, the trapezoidal waveform of the source voltage almost achieves the greatest possible machine utilization and, respectively, the greatest power factor of the electric motor. As illustrated in FIG. 3b, in an alternative embodiment, the induced source voltage is sinusoidal given the same current waveform.

[0046] FIG. 4b shows a plan view of the radially isotropic ring magnet 16 with an exemplary illustration of the magnetic preferred direction. The 4a shows a corresponding sectional view.

[0047] Further embodiments which can comprise further modifications and also combinations of features are conceivable in addition to the embodiments described and depicted.