DE102015011340A1 - Stator for an electric machine - Google Patents

Abstract

The invention relates to a stator for an electric machine with a stator core (10) comprising a return ring (12) and a number of stator teeth (14) distributed on the return ring (12), each having a tooth neck (14a) and one at the end of the Tooth neck arranged tooth tip (14b), According to the invention, the stator core (10) made of a powder composite material.

Description

The invention relates to a stator for an electric machine, in particular for a spindle motor with fluid dynamic bearing system.

Electric motors comprise a stator, the stator core of which usually consists of a stack of laminated steel sheets. Usually the thickness of the individual stator laminations is between 0.2 mm and 0.35 mm, for small spindle motors also between 0.1 mm and 0.15 mm.

From the US 7,62 6,302 B2 It is known to manufacture the stator core from a powder composite material. This method allows a relatively free shaping of the stator core.

Modern spindle motors for driving hard disk drives are very flat and have a height of typically 9.5 mm, 7 mm or even only 5 mm. Due to this low height also the electric drive system and in particular the stator must be constructed very flat, the drive system should still provide high torque and low electrical resistance with the highest possible performance.

An important feature of the drive system is also the value of Total Harmonic Distortion (THD) of the back EMF voltage. The THDs depend in particular on the material and the geometry of the stator and the windings arranged thereon.

Usually, the stator core of a spindle motor consists of a stack of laminated steel sheets with sheet thicknesses of 0.1 mm to 0.35 mm. As a result, the axial height of the stator core can be selected only in discrete values that correspond to a multiple of the sheet thickness. The stator laminations are punched out in a first production step and then connected to form a stack. The connection of the stator laminations is done by stamping, gluing or welding. Here, a large filling factor of the laminated core is sought, d. H. Between the individual stator laminations, there should be no air gaps which reduce the effective height (cross section) of the stator core. The smaller the cross section of the stator core, the greater the danger of magnetic saturation. A stamped package is only pointwise, so that no full-surface connection of the individual stator laminations is given. This can cause vibrations and acoustic problems in higher frequency ranges over about 10 kHz. If the spindle motor is inserted in a hard disk drive, these vibrations can interfere with the hard disk drive read / write devices.

Laminated stator cores also have relatively high electromagnetic losses that degrade the efficiency of the drive system.

Due to the structure of the stator core of discrete, flat stator laminations, the axial height of the stator core is the same everywhere, wherein the stator core has mutually parallel upper and lower sides. A free shaping of the stator core, in particular in the stacking direction, is not possible with a structure made of stator laminations. With limited space available in miniature motors, the possibility of free shaping of the stator core, especially in the axial direction, would be of great advantage.

In addition, laminated stator cores have only limited space for the stator windings in the radial direction, but this is necessary for a large electromotive constant Ke and a low magnetic saturation. In particular, the THD value depends on the electromotive constant Ke and the magnetic saturation, which in turn determine the acoustic properties of the spindle motor.

Practical experience has shown that laminated stator cores always have an asymmetric shape of the stator teeth due to manufacturing tolerances. These asymmetries create harmonic harmonics, such as 12th or 24th order, which can negatively affect the acoustics and engine characteristics. Due to the stamping sheets and the subsequent packaging of the sheets, it is almost impossible to achieve a symmetrical shape and homogeneous material properties of the stator cores produced in this way. Especially with small diameter stator cores, the tolerances are relatively strong.

Often it is desirable in spindle motors to optimize or maximize the electromotive constant Ke, whereas the THD and the ohmic resistance should be as low as possible. An optimization of the electromotive constant Ke can usually only by changing the design of the drive system, such. B. greater axial height, number of turns of the windings, higher magnetization or greater axial height of the rotor magnet done. Due to the limited space in spindle motors, in particular spindle motors of low overall height, these optimization options can not be realized or only very limited.

In spindle motors often a magnetic offset between the stator and the rotor magnet is provided to the fluid dynamic bearing system, the pivotal mounting of the spindle motor is used to magnetically bias. However, this magnetic offset creates an asymmetry of the magnetic forces in the drive system with both constant and dynamic components, which can degrade the acoustic properties of the spindle motor. Corresponding acoustic problems may also arise if the axial height of the rotor magnet differs from the height of the stator arrangement. In a conventional spindle motor with a laminated stator core asymmetry of the drive system is unavoidable, because in order to obtain a sufficiently large electromotive constant Ke, the height of the rotor magnet must be greater than the height of the stator core. Furthermore, the bias for the fluid dynamic bearing is necessary.

Spindle motors often use rotor magnets made of NeFeB with a remanence of 0.55 to 0.75 Tesla, which are very expensive. As a rule, the higher the remanence of the magnet, the more expensive it is. In spindle motors with very low heights of 5-7 mm therefore very expensive rotor magnets with the highest possible remanence are necessary. On the other hand, a possible good price-performance ratio of the spindle motors is sought. The magnets used are a significant cost factor. The possibility of using inexpensive magnets would therefore be a great cost advantage.

Due to the limited space in spindle motors with a small overall height, in particular 7 mm or less, and the increased demands on the electromotive constant Ke and small distortions THD, it is necessary in the magnetization of the rotor magnets very high field strengths, d. H. very high currents in the magnetizing coils to use. In addition, shorter and shorter cycle times for magnetization are required in mass production. The use of high currents and short pulse duration in the magnetization can lead to damage or premature wear of the magnetizing coils, which increases the cost of production.

It is the object of the invention to provide a stator for an electrical machine, which is optimized in terms of its geometry and its magnetic behavior for very low-build motors, in particular spindle motors. A spindle motor with a drive system with such a stator should also be specified.

This object is achieved by a stator with the features of claim 1.

Preferred embodiments of the invention and further advantageous features are indicated in the dependent claims.

The stator comprises a stator core with a return ring and a number of stator teeth distributed on the return ring, each having a tooth neck and a tooth tip arranged at the end of the tooth neck. According to the invention, the stator core consists of a powder composite material, in particular a soft magnetic powder composite material (Soff Magnetic Composite).

This makes it possible to produce the stator core in almost any geometry. In contrast to a stator core consisting of stator laminations, punching out and connecting of the individual stator laminations is eliminated, and stator cores having an arbitrary shaping, in particular in the axial direction, can be produced. Furthermore, no waste from the punching remaining sheet remains. A powder composite stator core consists of a single integral block with no air gaps, so that a fill factor of 100% can be achieved.

For example, a soft magnetic powder composite consists of high purity iron powder with a special electrically insulating surface coating on each individual iron particle. This powder can be pressed under pressure and heat to a stator core of any desired shape. The electrically insulating surface coating of each particle ensures a high electrical resistance of the pressed stator core, so that eddy currents are avoided in the stator core. Suitable soft magnetic powder composites are for. B. Somaloy ^®.

Materials for the production of stator cores, whether iron sheet or powder composite material, have non-linear magnetic properties, which can be described for example by a BH diagram. In this case, a magnetic saturation of the stator material is to be avoided. The magnetic saturation occurs when the magnetic flux density in the material exceeds a certain value, which can be read from the BH diagram. Magnetic saturation causes distortion of the magnetic flux and, correspondingly, distortion of the voltage waveform of back EMF. Distortion of the voltage waveform of the back EMF can be described by the total harmonic distortion (THD). The THD indicates the deviation of the voltage curve of the back EMF from a pure sine voltage in percent (pure sine corresponds to 0% THD). If the voltage curve of the back EMF from the pure sine wave deviates, resulting from the interaction of the non-sinusoidal voltage components and the magnetic system magnetic forces that lead to vibration or noise emissions. For example, in a motor design with 12 magnetic poles and 9 grooves, the 36x, 48x and 60x harmonics are particularly excited. It is therefore desirable to keep the total harmonic distortion THD of the back EMF low, if possible below 2.5% and preferably below 1.5%. This can be achieved, inter alia, by reducing the magnetic flux density in the stator core. Local peak values of the magnetic saturation in the core material are to be avoided in particular.

In a preferred embodiment of the invention, it is provided that the ratio between the radial width A of the return ring and the outer diameter OD of the stator core is between 0.035 and 0.08.

In a further preferred embodiment of the invention, it is provided that the ratio between the radial width B of the tooth tips and the outer diameter OD of the stator core is between 0.045 and 0.08.

In a further preferred embodiment of the invention, it is provided that the ratio between the width E of the tooth necks and the outer diameter OD of the stator core is between 0.09 and 0.125.

In a further preferred embodiment of the invention, it is provided that the ratio between the outer radius R of the tooth tips and the outer diameter OD of the stator core is between 0.3 and 0.5.

By this above-described geometric conditions of the stator core in conjunction with an optimized dimensioning of powder composite material, a magnetic saturation in critical areas of the stator core can be prevented or reduced by increasing the cross section of the stator core in these areas and thereby reducing the magnetic flux density. Consequently, the harmonic distortions THD of the counter-emf voltage characteristic are also reduced. The THD depends in particular on the relative dimensions of the stator core.

The described design of the stator core is particularly suitable for spindle motors with n × 4 poles and n × 3 slots (teeth), where n is a natural number greater than or equal to 1.

In order to obtain sufficient space on the stator teeth for the windings in the axial and radial directions, the stator core made of powder composite material-in contrast to a laminated sheet metal core-can be given an optimized shape. In particular, the non-winded portions of the stator, d. H. the return ring and the stator heads can be increased in their axial height relative to the tooth necks, so that increases there the effective cross section of the stator core. So z. B. the axial height of the return ring and the ends of the stator teeth (tooth tips) preferably greater than the axial height of the stator teeth in the wound sections (stator necks).

According to the invention, a magnetic offset of the magnet system can be provided and optimized in accordance with the invention by seeking to improve the axial symmetry of the stator core and the rotor magnet. This improves the acoustic characteristics of the engine. In particular, the axial height of the stator heads and the rotor magnet can be selected approximately the same size (the same size in the magnetic sense and not necessarily in the mechanical sense). The magnetic offset can be adapted and adjusted by the almost arbitrary shape of the stator made of powder composite material relatively easy to the requirements.

By manufacturing the stator core from powder composite material, larger cross-sections can be achieved in critical areas of the stator core. As a result, a low-cost rotor magnet with low remanence can be used without the performance of the drive system deteriorating noticeably. By using a low-cost rotor magnet, the manufacturing cost of the spindle motor can be reduced overall.

The use of rotor magnets with a smaller remanence can also reduce the complexity of the magnetization, in particular the cycle times for the magnetization and the necessary current strengths in the magnetization coils.

The invention can be used both for stators of internal rotor motors and stators of external rotor motors.

1 shows a plan view of a stator core according to the invention according to a first embodiment of the invention.

2 shows a cross section through the stator core of 1 ,

3 shows an enlarged view of a stator tooth of 1 ,

4 shows a plan view of a stator according to a second embodiment of the invention.

5 shows a cross section through the stator core of 4 ,

5A shows a detailed view of the cross section of 5 ,

6 shows an enlarged view of a stator tooth according to 4 ,

7 shows a diagram of the tooth neck width E in relation to the outer diameter OD plotted on the harmonic distortion THD.

8th shows a diagram of the radial width of the return ring A in relation to the outer diameter OD and the radial width B of the tooth tips in relation to the outer diameter OD plotted against the harmonic distortion THD.

The 1 to 3 show a stator core 10 in a first embodiment of the invention. The stator core 10 consists of a soft magnetic powder composite material, such as Somaloy ^®. This composite material is in powder form and is pressed under pressure and heat to the intended shape of the stator core.

It is a stator core for an external rotor motor. However, the stator core according to the invention can also be designed equally for internal rotor motors.

The stator core 10 includes an inner return ring 12 from which radially outwardly a number of stator teeth 14 extend, distributed over the circumference of the return ring 12 are arranged.

In the present example, the stator core includes 10 a total of nine stator teeth 14 and nine between the stator teeth 14 arranged grooves, which serve as a winding space for the windings to be applied to the stator teeth 16 serve.

Of course, the stator core according to the invention 10 also a different number of stator teeth 14 However, preferably n · 3 stator teeth, where n is a natural number greater than or equal to 1.

Every stator tooth 14 includes a tooth neck 14a and one at the end of the cervix 14a arranged tooth head 14b , In the supervision according to 1 it can be seen that the stator teeth 14 are formed about hammer-shaped.

The width E of the tooth necks 14a in the circumferential direction is substantially less than the width of the tooth heads 14b in the circumferential direction, as can be seen in particular from the 1 and 3 results.

The stator core 10 has a defined inner diameter ID and a defined outer diameter OD and a constant axial total height H.

In the illustrated embodiment, the stator core 10 For example, an outer diameter OD of 16.7 mm and an inner diameter ID of 8.6 mm and a height H of 2.2 mm.

On the tooth necks 14a the stator teeth 14 are each stator windings 16 placed in the grooves between the stator teeth 14 extend and stand in the axial direction beyond the height H of the stator core, as is apparent from 2 results. The total height of the stator in the axial direction is therefore greater than the total height H of the stator core.

In order to improve the harmonic distortion THD, it is provided according to the invention to optimize certain dimensions of the stator core, wherein these dimensions are preferably given in relation to the outer diameter OD of the stator core.

First, according to the invention, the radial width A of the return ring in relation to the outer diameter OD of the stator core is selected between 0.035 and 0.08, ie. h., in the present example with OD = 16.7 mm, the value A is between 0.6 mm and 1.33 mm.

Similarly, the width E of the tooth necks in relation to the outer diameter OD of the stator core according to the invention selected between 0.09 and 0.125, resulting in the present example an effective width E of 1.5 mm to 2.09 mm.

In addition, according to the invention, it is provided to select the radial width B of the tooth heads in relation to the outer diameter OD of the stator core between 0.045 and 0.08, which effectively corresponds to a width B between 0.75 mm and 1.33 mm.

The outer radius R of the stator heads 14b does not necessarily have the radius of the stator core 10 , ie OD / 2, but is preferably smaller than OD / 2 and is in the present example between 5 mm and 8.35 mm, ie, in which R is in relation to the outer diameter OD between 0.3 to 0.5.

The 4 to 6 show a second embodiment of a stator core according to the invention 110 , which has an inner yoke ring 112 and a series of stator teeth 114 includes, each one a tooth neck 114a and a tooth head 114b exhibit.

On the tooth necks can turn windings 116 be applied.

Compared to the first embodiment, the total axial height H of the stator core 110 not the same size everywhere, but the height H_ts of the tooth necks 114a is less than the height H_tt of the tooth heads 114b or the height H_yk of the return ring 112 , in particular, from 5A results.

For example, the stator core 110 an outer diameter OD of 16.7 mm and an inner diameter ID of 8.6 mm.

The height H_ts of the cervicals 114a may be, for example, 2.2 mm, while the height H_yk of the return ring 112 or the height H_tt of the tooth heads 114b can be up to 3.65 mm.

The windings 116 be in the recess 118 that is due to the less high tooth necks 114a be formed, and preferably arranged so that the windings 116 not over the height H_tt of the tooth heads 114b and the height H_yk of the return ring 112 protrude. To the windings 16 respectively. 116 better on the tooth necks 14a respectively. 114a To be able to apply and to avoid damage to the winding wires, have the tooth necks 114a preferably rounded edges, ie, a more oval cross section.

Through these measures, the cross section of the stator core 110 overall, without the overall height of the stator (including the windings 116 ) compared to the embodiment according to the 1 to 3 gets bigger, since the windings 116 completely in the recess 118 are included and not over the surface of the stator core 110 protrude.

The size ratios A / OD, B / OD, E / OD and R / OD as in 6 are preferably correspond to the size ratios of the embodiment according to the 1 to 3 ,

The positive effects of the invention, in particular with regard to a reduction of the harmonic distortions THD, are shown in particular in the diagrams of 7 and 8th ,

7 FIG. 12 is a graph showing the relationship between the cog width E and the outer diameter OD of the stator core plotted against the harmonic distortion THD in percentage. FIG.

It can be clearly seen that the larger the ratio E / OD becomes, the smaller the harmonic distortion THD becomes.

According to the invention, it is desired to keep the harmonic distortions THD less than about 3%, which corresponds to a ratio E / D of 0.09 and greater.

Preferably, a THD of less than 1.5% or less than 1% should be achieved, which corresponds to a ratio E / OD of 1.11 and greater about.

The 8th shows a graph of the return width A in relation to the outer diameter OD plotted on the harmonic distortion THD and on the other the tooth head thickness B in relation to the outer diameter OD, also plotted on the THD.

It can be seen that the harmonic distortions THD improve overall at larger values of A / OD, giving the best values of THD at values of A / OD between 0.035 and greater. Total THD remains at 1% or less in this area.

The THD also decreases to larger values at a ratio of B / OD and reaches values of 1.5% and less at a ratio of B / OD between 0.045 and 0.08.

LIST OF REFERENCE NUMBERS

10, 110

stator core

12, 112

Return ring

14, 114

stator tooth

14a, 114a

cervical

14b, 114b

addendum

16, 116

winding

118

recess

20, 120

rotor magnet

OD

Stator core outer diameter

ID

Stator core inner diameter

H

Total height of the stator core

H_yk

Height of the return ring

H_ts

Height of the cervix

H_tt

Height of the tooth head

A

Return ring thickness (radial)

B

Head thickness (radial)

e

Cervical width

R

Tooth tip outer radius

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• US 7626302 B2 [0003]

Claims (12)

Stator ( 10 ; 110 ) for an electrical machine, comprising a stator core with a return ring ( 12 ; 112 ) and a number of at the return ring ( 12 . 112 ) arranged and radially projecting stator teeth ( 14 . 114 ), each one tooth neck ( 14a . 114a ) and a tooth tip arranged at the end of the tooth neck ( 14b . 114b ), characterized in that the stator core ( 10 . 110 ) consists of a powder composite material. Stator according to claim 1, characterized in that the ratio between the radial width A of the return ring ( 12 . 112 ) and the outer diameter OD of the stator core ( 10 . 110 ) is between 0.035 and 0.08. Stator according to one of claims 1 or 2, characterized in that the ratio between the radial width B of the tooth heads ( 14b . 114b ) and the outer diameter OD of the stator core ( 10 . 110 ) is between 0.045 and 0.08. Stator according to one of claims 1 to 3, characterized in that the ratio between the width E of the tooth necks ( 14a . 114a ) and the outer diameter OD of the stator core ( 10 . 110 ) is between 0.09 and 0.125. Stator according to one of claims 1 to 4, characterized in that the ratio between the outer radius R of the tooth heads ( 14b . 114b ) and the outer diameter OD of the stator core ( 10 . 110 ) is greater than 0.3 and less than 0.5. Stator according to one of claims 1 to 5, characterized in that the axial height H_ts of the tooth necks ( 114a ) is smaller than the axial height H_tt of the tooth heads ( 114b ). Stator according to one of claims 1 to 6, characterized in that the axial height H_ts of the tooth necks ( 114a ) is smaller than the axial height H_yk of the return ring ( 112 ). Stator according to one of claims 1 to 7, characterized in that it has 3 · n stator teeth and slots, where n = {1; 2; ... N} Electric machine with a stator according to one of claims 1 to 8. Electrical machine according to claim 9, characterized in that the axial height H_m of the rotor magnet ( 20 . 120 ) is the same size as the axial height H_tt of the tooth heads ( 114b ). Electrical machine according to one of claims 9 or 10, characterized in that it is designed as a spindle motor with fluid dynamic bearing system. Hard disk drive with an electrical machine according to one of Claims 9 to 11.

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