GB2256975A

GB2256975A - Vibration reduction by magnetic balancing in an electric drive motor.

Info

Publication number: GB2256975A
Application number: GB9214564A
Authority: GB
Inventors: Heinrich Cap
Original assignee: Papst Motoren GmbH and Co KG
Current assignee: Ebm Papst St Georgen GmbH and Co KG
Priority date: 1988-06-09
Filing date: 1992-07-08
Publication date: 1992-12-23
Anticipated expiration: 2012-07-08
Also published as: GB8913364D0; DE3918953C2; GB2256975B; GB2221583B; DE3918953A1; HK181996A; DE8807530U1; GB2221583A; GB9214564D0

Description

motor.

2 2 -) o?-'S AN ELECTRIC DRIVE MOTOR The present invention relates to an electric drive Brushless electric drive motors of the external rotor type, which are used for example in disk storage drives, usually have a mounting flange for the stator which also supports the bearing for the shaft. The noise from such motors when operational arises as rolling noise in the bearings, as electro -mechanical noise in the stator winding, and from electro- magnetic disturbing forces acting between the rotor and stator. The noise generating vibrations are transferred to the mounting flange, which usually has a large area and is often very thin. The mounting flange may thus act as a membrane which passes on or gives out noise generated.

It is known, from DE-OS 16 13 343, to provide an elastic material between the mounting flange and a shielding ring or printed circuit board. This "sandwich" construction dampens the vibrations.

The present invention seeks to provide drive motors having a much higher noise reduction than previously.

According to the present invention there is provided a drive motor of the external rotor type comprising a shaft extending through bearings, a stator carrying a winding, and a rotor arranged to substantially surround said stator and to define therewith a substantially cylindrical air gap, wherein said rotor is substantially bell-shaped, and said stator is fixed to a projection surrounding said shaft and acting to support said bearings, and wherein for noise reduction purposes the magnetic components of the rotor and stator are arranged with respect'to each other such that the magnetic field is axially symmetrical whereby axial and/or radial electromagnetic disturbing forces acting between the rotor and stator are minimised for noise reduction purposes.

In an embodiment of the invention, electromagnetically produced noise in the stator is prevented from being reflected backwards and forwards between the stator and the bearing. Such electro-magnetically produced noise can in particular occur due to axial and/or radial electromagnetic disturbing forces between rotor and stator and is minimised by the embodiment of the invention.

The minimisation of axial and/or radial electro- magnetic disturbing forces reduces noise between the rotor and stator. This is achieved by arranging magnetic components of the rotor and stator such that the magnetic field is axially symmetrical. The magnetic components of the rotor and stator may be arranged symmetrically to one another to minimise disturbing forces. However, this solution is not always practicable with constructional means.

Particularly when in the case of a brushless direct current motor for the rotor position dependent commutation of the currents in the motor windings, one or more galvanomagnetic sensors, eg Hall generators or Hall-IC's are used, which are located in the influence range of the rotor magnetic field, a certain axial projection of the rotor magnets is required on the side facing the sensor-or sensors, in order to ensure a magnetic flux density adequate for controlling the sensors. On the opposite side, the axial projection is appropriately much smaller, in order to economise on expensive magnetic material and/or the axial overall length. Thus, a permanent magnetic rotor is obtained, which is arranged asymmetrically with respect 1 1 to the axial plane of symmetry of the stator iron. The different sizes of the axial projections leads to an axial force being exerted on the rotor, whose magnet attempts to adjust itself symmetrically to the stator iron. This force is generally rotation position-dependent, eg because the air gap between the rotor and the stator does not have the same dimensions throughout. This can lead to the aforementioned electromagnetically produced noise. However, in a further development of the invention, this can be counteracted in such an asymmetrical arrangement in that the stator carries an end plate, which cooperates with the rotor magnet for the axial symmetrisation of the magnetic field.

Preferably, the rotor magnet projects axially to a varying degree over the stator iron at its two end faces and the end plate defines at least part of the air gap in the vicinity of the larger projection.

In an embodiment, the rotor magnet projects axially to a varying degree over the stator iron at its two end faces and in the vicinity of the larger projection the induction in the central area of the rotor magnetic poles is at least zonally weaker than in the marginal regions of the rotor magnetic poles adjacent to the pole clearance. This also ensures an axial force symmetrisation, together with reliable response or operation of the rotation position sensors.

What is decisive is that the magnetic components of the rotor and stator are arranged such that a magnetic symmetrisation takes place in such a way that the sum of the magnetic axial forces between the rotor and the stator are as small as possible, and preferably zero in the case of the finally fitted motor.

Preferably, said projection to which the stator is fixed is a projection of a mounting flange which extends radially and supports said stator.

In an embodiment, said mounting flange is arranged substantially coaxially relative to said shaft, and the space between facing surfaces of said stator and said mounting flange is filled with plastics such that said mounting flange and said stator are effectively a one piece 10 unit.

Preferably, the plastics material is applied coaxially in annular form around the point of rotation of the motor and this allows a more economical production.

For a lost of cases it is sufficient to fill only the space between the winding head of the stator and the flange with plastics, and in some cases, due to reasons of economy, a partial filling is sufficient.

If there is a conductor plate, and/or a printed circuit board, and/or a shielding ring, these are not only sound dampened by the surrounding casting or moulding, but are also fixed to the stator at the same time. Thus the glueing of the conductor plate (PCB), of the shielding ring, and of the winding, which has been practised up to now, becomes superfluous.

An additional dampening of the stator is achieved by filling the grooves of the laminated sheet plate with plastics as well.

The dampening of the noises produced by the rotor is also achieved by connecting the parts with a hard and tough plastics layer. Even a point like distributed layer leads to good results.

m' 1 An embodiment of the present invention will hereinafter be described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is provided for clarity of explanation and shows a section of a drive motor which is not in accordance with the invention, and Figure 2 shows a section of half of an embodiment of a drive motor of the invention.

In Figure 1 there is shown in section a drive unit 1, which serves for example as a hard disk storage drive. This drive unit 1 incorporates a drive motor 2 having a substantially cup-shaped or bell-shaped rotor 3. A shaft 5 is connected at one end to the centre of the base 4 of the rotor 3. The shaft 5 is supported in ball bearings 6 and 7. The outer rings of these ball bearings 6, 7 are mounted on a mounting flange 8 and are arranged concentrically. To the upper ball bearing 7 there is connected a ferro- fluidic seal 9 which prevents liquid or solid particles from entering in an axial direction the "clean-room" environment from the motor environment. A hub 11 is connected rigidly to the end of the shaft 5 remote from its end connected to the rotor 3. Hard storage disks (not shown) can be mounted on this hub 11, for example.

A shielding ring 13 and a plate 14, for example, a printed circuit board, are received within a recess 12 in the inner wall of the flange 8 and are embedded therein by way of a hot setting, a thermo setting, a fusion or another suitable adhesive. A hard and tough plastics material is preferably provided as the fusion adhesive. In this way, a unitary or one piece part is formed comprised by the connection of the flange 8 with the shielding ring 13, the circuit board 14 and a stator 15. The adhesive is introduced into all or part of the area of a gap 17 defined between the flange 8 and a winding head 16 of the winding of the stator 15. The glueing of the circuit board, the shielding ring and the winding which has been required up to now, is replaced by a generous application of plastics material, preferably hot setting adhesive. The one piece part (8, 13, 14, 15) now formed is a poor sound transducer because of its increased weight and its increased rigidity, and this provides a distinct noise reduction. The stator laminations 19 of the stator 15 are generally grooved, as indicated at 18, and these grooves 18 can also be filled with plastics material.

Most or all of any cavities or gaps between the mounting flange 8 and the winding of the stator 15 are also filled, either partially or wholly, with plastics material.

Depending upon the size and construction of the motor 2, a partial casting or moulding with plastics material is sufficient. It has been found to be advantageous to apply the plastics annularly coaxially around the point of rotation of the motor.

The printed circuit board 14 and/or the shielding ring 13 arranged between theflange 8 and the winding head 16 are cast or moulded partially or entirely with plastics material, a partial casting being sufficient in many cases. Hot setting adhesives which can be introduced with a dosing device are particularly suitable for the casting or moulding. The use of a dosing device enables an economical use of material.

A permanent magnetic rotor magnet 21 is fastened to the inner surface of a substantially cylindrical wall of the rotor 3. One end surface of the rotor magnet 21 terminates approximately at the edge of the cylindrical wall at the open end of the rotor 3. The opposite end surface of the rotor magnet 21 is connected to the inner surface 23 of the base of the rotor 3 by a hard and tough plastics layer 22, for example, of a hot setting adhesive. Other parts of the rotor 3 can also be covered point-like, annularly, radially or sheetlike by means of plastics layers. The connection of the rotor 3 and the rotor magnet 21 by means of a plastics material to form a substantially one piece rotor part leads to further noise reductions.

Figure 2 shows an embodiment of a drive unit of the invention including a drive motor 30 having a substantially cup-shaped or bell-shaped rotor 33. The shaft 5 is substantially centrally mounted and fixed to the mounting flange 8. The inner rings of the axially spaced ball bearings 6, 7 are rigidly mounted on the shaft 5. The outer rings of the ball bearings 6, 7 are mounted at a predetermined spacing on the inner wall 34 of the central bore of hub 11. The hub 11 has a first projection 35 extending centrally into the base of the bell-shaped rotor 33 and is rigidly connected thereto. A second projection 36 of the hub 11 extends into an annular groove 37 of an annular part 38. A U-shaped gap defined between the projection 36 and the annular groove 37 acts as a labyrinth packing. The central bore of the annular part 38 is fixed rigidly to a hub-like projection 39 of the mounting flange 8. Stator laminations 19, preferably in the form of a grooved stack of sheets, are fixed with the winding to the radial outer side of the annular part 38. Hot setting adhesive or a different plastics is inserted into the gap 17 between the flange-sided winding head 16 and the inner wall of a recess 12 of the flange 8. A cover disk 41 sealing the mounting of the motor from the "clean-room" environment is arranged on the upper ball bearing 7. The dampening measures applied to the rotor described in Figure 1 are also applied to the embodiment of Figure 2 and lead to a reduction in the noise level.

The noise level is also reduced by arranging the magnetic components of the rotor 33 and stator 19 such that the magnetic field is axially symmetrical. By this means axial and/or radial electro-magnetic disturbing forces acting between the rotor and- stator are minimised.

Instead of hot setting adhesive, other plastics can be used for the sounddampening connection of the motor parts as, for example, easy to work casting means.

The invention is especially suitable for motors whose diameter D is large compared to the over-all axial length L, that is, D/L amounts to at least 2.

it will be appreciated that modifications and variations to the invention as described and illustrated can be made.

The drive motors described and illustrated herein are also described and claimed in our copending application No. 2221583 f rom which the present application is divided.

J.

Claims

1. A drive motor of the external rotor type comprising a shaft extending through bearings, a stator carrying a winding, and a rotor arranged to substantially surround said stator and to define therewith a substantially cylindrical air gap, wherein said rotor is substantially bell-shaped, and said stator is fixed to a projection surrounding said shaft and acting to support said bearings, and wherein for noise reduction purposes the magnetic components of the rotor and stator are arranged with respect to each other such that the magnetic field is axially symmetrical whereby axial and/or radial electromagnetic disturbing forces acting between the rotor and stator are minimised for noise reduction purposes.

2. A drive motor as claimed in Claim 1, wherein the magnetic components of the rotor and stator are arranged symmetrically to one another.

3. A drive motor as claimed in Claim 1, with a permanent magnetic rotor, which is arranged asymmetrically with respect to the axial plane of symmetry of the stator iron, wherein the stator carries an end plate, which co-operates with the rotor magnet for the axial symmetrisation of the magnetic field.

4. A drive motor as claimed in Claim 3, wherein the rotor magnet projects axially to a varying degree over the stator iron at its two end faces and the end plate defines at least part of the air gap in the vicinity of the larger projection.

5. A drive motor according to any preceding claim, wherein the rotor magnet projects axially to a varying degree over the stator iron at its two end faces and in the vicinity of the larger projection the induction in the central area of the rotor magnetic poles is at least zonally weaker than in the marginal regions of the rotor magnetic poles adjacent to the pole clearance.

6. A drive motor as claimed in any preceding claim, wherein said projection to which the stator is fixed is a projection of a mounting flange which extends radially and supports said stator.

7. A drive motor as claimed in Claim 6, wherein said shaft is fixed to said mounting flange, and wherein said bearings are ball bearings and inner rings thereof are rigidly mounted on said shaft.

8. A drive motor as claimed in Claim 7, wherein outer rings of said ball bearings are mounted in a central bore of a hub which is arranged coaxially of said shaft, and wherein said hub has a first projection extending centrally into the base of said bell-shaped rotor and rigidly connected thereto.

9. A drive motor as claimed in Claim 8, wherein said hub has a second projection extending into an annular groove of an annular part which is arranged coaxially of said shaft and has a central bore fixed rigidly to said projection to which the stator is fixed, and wherein said second projection and said annular groove together define a labyrinth packing.

10. A drive motor as claimed in any preceding claim, wherein said projection to which the stator is fixed is a projection of a mounting flange supporting said stator and arranged substantially coaxially relative to said shaft, and wherein the space between facing surfaces of said stator and said mounting flange is filled with plastics -1 such that said mounting flange and said stator are effectively a one piece unit.

11. A motor as claimed in Claim 10, wherein the plastics material is applied coaxially in annular form around the point of rotation of the motor.

12. A motor as claimed in Claim 10 or 11, wherein the space between the stator and the flange is partially or 10 entirely cast or moulded with plastics.

13. A motor as claimed in any preceding claim, wherein the stator is laminated, the laminations thereof are grooved and said grooves, at least partially, filled with plastics.

14. A motor as claimed in any preceding claim, wherein parts of the rotor are connected to each other by a plastics layer, wherein the layer is applied point-like, annularly, radially, or sheetlike.

15. A motor as claimed in any preceding claim, wherein the bottom surface of said bell-shaped rotor is connected to a face of a rotor magnet by way of a plastics layer.

16. A motor as claimed in any of Claims 10 to 15, wherein the plastics used for the connection or the surrounding of motor parts is an easy to work casting resin.

17. A motor as claimed in any of Claims 10 to 15, wherein the plastics used for the connection or the surrounding -of motor parts is a hot-settable adhesive.

18. A motor as claimed in Claim 17, wherein the hot-settable adhesive is injected by means of a. dosing device.