DEVICE FOR NOISE SUPPRESSION OF A SMALL ELECTRIC MOTOR

The invention relates to a device in accordance with the preamble of claim 1.

Such a device is known from U.S. Patent 7,019,425 B2. According to the disclosure of the aforementioned document there is provided a bar core inductivity being installed between each of the first electrodes of a C _x capacitor and a connection clamp which is provided to be connected with a power source . By the known device there can be achieved a good electromagnetic noise or noise suppression, respectively, for small electric motors. Such electric motors may be in particular part of actuators used in vehicles, aircraft and the like. However, tightened up regulations regarding the maximum tolerable noise make it necessary to further improve the noise suppression of small electric motors. At the same time there exists a need to further miniaturize electric motors and to further reduce costs.

It is an object of the present invention to overcome the disadvantages in the art. It is a further object of the invention to provide a device allowing an improved noise suppression of electric motors. A further aim of the invention is to provide a device for noise suppression which allows a further miniaturization of an electric motor and a cost reduction.

This object is solved by the features of claim 1. Embodiments of the invention are described by the features of claims 2 to 29.

According to the invention it is provided that a first coil having a predetermined inductivity is connected with its first connector with the first electrode of a C _x capacitor

and its second connector with a first brush, wherein the first coil is provided on a first supporting element, and wherein the first supporting element is mounted on the surface of the disk-shaped element.

According to a feature of the invention the proposed inductivity is provided or supported, respectively, on a first supporting element which may be mounted on an upper or lower surface of the disk-element. By this measure there can be provided a device for noise suppression which is compact and allows the production of further miniaturized electric motors. If in accordance with the proposed invention there is provided one inductivity the improved noise suppression can only be achieved if the electric motor is operated always in the same sense of rotation.

According to an embodiment of the invention there is provided a second coil having the predetermined inductivity, which is connected with its third connector with the second electrode of the C _x capacitor and its fourth connector with a second brush, wherein the second coil is provided or supported, respectively, on a second supporting element, and wherein the second supporting element is mounted on the surface of the disk-shaped element. By the proposed embodiment there is achieved an improved noise suppression even if the electric motor is operated in both senses of rotation. The second supporting element carrying the second coil is preferably mounted on surface of the disk-shaped element where the first supporting element is mounted. I. e. both supporting elements are preferably mounted on the same surface of the disk-shaped element. This facilitates the production of the device and its mounting within a housing of an electric motor.

According to a further embodiment the first and/or the second supporting element/s is/are made from plastic. Said

supporting element/s can be produced very cheaply. Said plastic may be filled with a predetermined amount of a powder which improves the inductivity of the coil. Said powder may contain one or more materials selected from the following group: MeOFe ₂ O ₃ , MeFe ₂ O ₄ , wherein Me is one of the following ions: Mn ²⁺ , Fe ²⁺ , Co ²⁺ , Ni ²⁺ , Zn ²⁺ , Mg ²⁺ , Cu ²⁺ or mixtures thereof; Fe-; Co-; Ni- and rare earth powders. By using these materials or a composition thereof it is possible to set a predetermined inductivity and to adapt the inductivity with respect to the frequency ranges of the electric noise. It has to be understood that depending on constructional features each electric motor has a different noise characteristic. In order to effectively suppress the noise of an electric motor one may determine the noise characteristic and then adapt the inductivity by setting the material used for filling the plastic in an appropriate way so that a maximum suppression of the noise is achieved. The material used for producing the supporting element and the coil taken up on the supporting element is chosen such that the noise characteristic of the electric motor is suppressed most effectively.

According to a further advantageous feature the first and the second supporting elements are formed in one part. The part comprising both supporting elements can for example be made by injection molding or the like. By providing both supporting elements in one part production costs can be saved .

According to a further embodiment the first and the second supporting elements are made of a further disk-shaped element. The further disk-shaped element may correspond in its shape to the shape of the disk-shaped element. In this case the disk-shaped elements may form a congruent stack. The further disk- shaped element may be laminated on the upper and/or lower surface of the disk-shaped element. The further

disk-shaped element may also be made from a resin which is coated on the upper or lower surface of the disk- shaped element and then cured. The resin may filled with the aforementioned powder.

The first and/or the second coil may be wound around a supporting unit comprising the further disk-shaped element. The further disk-shaped element may at least in sections project beyond the circumference of the disk-shaped element. Such a projection may serve to support the first and/or second coils. - However, the supporting unit may also comprise the disk-shaped element. In this case the first and/or second coil/s surround/s both, the disk-shaped element and the further disk-shaped element.

The disk-shaped element has another surface or lower surface, respectively, opposing the surface or upper surface, respectively. A disk-shaped ferrite-element may be mounted on the other surface. In this case the disk-shaped element is sandwiched between the further disk-shaped element and the disk-shaped ferrite-element . Also the disk-shaped ferrite- element may correspond in its shape to the shape of the disk- shaped element. However, the disk-shaped ferrite-element may also correspond in its shape to the shape of the further disk-shaped element.

The supporting unit may comprise the disk-shaped element and/or the disk-shaped ferrite-element . I. e. the first and/or the second coil may be wound around a supporting unit comprising the disk- shaped element and the disk- shaped ferrite-element or a supporting unit which additionally comprises the disk-shaped element.

Further, it is proposed that the first brush is provided on one end of a brush arm, the other end of which is fixed at

the first supporting element, and/or wherein the second brush is provided on one end of a second brush arm, the other end of which is fixed at the second supporting element. According to these features the supporting elements have not only the function to support or take up the coil but also to hold a, preferably flexible, brush arm at which the brush is mounted. Such a brush arm may be produced of a flexible metal strip in order to press the brush on a contact surface of a commutator.

According to a further advantageous embodiment the first brush is connected with the second connector via the first brush arm, and/or the second brush is connected with the fourth connector via second brush arm. This can easily be achieved when using as brush arms a metal with an electric conductivity.

The connection between the first connector and the one first electrode and the connection between the third connector and/or the second electrode are advantageously made by clamping, soldering, welding, or by means of an electrical conductive polymer. In particular a connection via clamping allows a very easy mounting of the brush arm at the supporting element. For this purpose there may be provided a snapping mechanism by which the first or second brush arm clamps the second or fourth connector and is thereby held at the same time at the supporting element.

According to a further embodiment the first connector may be connected with the first electrode by means of a first terminal being provided to be connected with a first power source, and/or the third connector may be connected with the second electrode by means of a second terminal being provided to be connected with the power source. By providing first and/or second terminal connectors a fixing of the first

and/or second connector with the first and/or second electrodes is facilitated. At the same time terminals for connecting the device with a power source are provided.

The disk-shaped element may be designed as a ceramic multilayer element. A suitable disk-shaped ceramic multi-layer element is for example described in U.S. Patent 7,019,425 B2. The disclosure regarding the structure, material, design etc. of the disk-shaped ceramic multi- layer element contained therein is herewith incorporated by reference.

According to an alternative embodiment the disk-shaped element may also be designed as a ceramic barrier- layer element. A suitable disk-shaped ceramic barrier- layer element is for example described in U.S. Patent 7,019,425 B2. The disclosure regarding the structure, material, design etc. of the disk-shaped ceramic barrier- layer element described therein is incorporated herewith by reference.

It has turned out to be advantageous that the first and the second electrode of the C _x capacitor are provided on an upper surface of the disk- shaped element. It has to be noted that the first and the second electrode may have any suitable, preferably symmetrical, shape. Preferably the first and the second electrode cover an essential part of the upper surface .

According to a further advantageous feature a first isolation path electrically separates the first and the second electrodes, and second isolation paths electrically separate the third electrode from the first and the second electrodes. The first and the second isolation paths may have the form of a "Y" or the like. The proposed design of the electrodes and the isolation paths allows for an integration of a C _x

capacitor with a high capacity with C _y capacitors within one disk.

The first and the second supporting elements may at least partially cover the first and the second electrodes. It has been observed that in this arrangement first and second inductivities are not negatively affected by the electrodes.

According to a further embodiment a first discrete capacitor being mounted on the disk-shaped element is switched between the first and the second electrodes. I. e. the first discrete capacitor is connected in parallel with the C _x capacitor. In a similar way a second discrete capacitor being mounted on the disk-shaped element may be switched between the third electrode and at least one of the first and the second electrodes. Also, the second discrete capacitor is connected in parallel with the C _y capacitor. By the proposed features the capacity of the C _x and/or C _y capacitors can be increased. Therewith, in dependence of the noise characteristic of the electric motor, the noise can be suppressed effectively.

Advantageously, first and/or second discrete capacitor/s is/are SMD element/s. Such elements are available commercially at a low price. A mounting of such elements on the surface of the disk- shaped element is easy. The coefficients of expansion of the discrete capacitor and the disk-shaped element are very similar so that a damage of the discrete capacitors due to thermal effects is highly unlikely.

According to a further embodiment the first electrodes are connected with a resistor having a resistance in the range of 10 ω to 100 kω. The provision of the proposed resistor

enhances the protection against high voltage breakthrough through the dielectric layers.

Furthermore, the first and second electrodes may be connected with NTC-resistor or a VDR-device. In particular the resistor may be produced by printing technique, e. g. screen-printing. By providing the proposed resistor and/or the other elements current and/or voltage peaks can be compensated and the durability of the proposed device can be enhanced.

The disk-shaped element may have a central breakthrough for feeding through a shaft. This makes it possible to combine the proposed device with conventional electric motors . In particular the proposed device may be combined with an end cap for a small electric motor and may be mounted therein. In this connection it has turned out to be advantageous that the disk-shaped element covers an inner end face of the end cap. Advantageously, the disk-shaped element fully covers the inner end face. Thereby, the end cap is shielded against a radiation of electromagnetic noise or noise, respectively, from the interior of a housing surrounding the electric motor.

According to a further aspect of the invention there is proposed a small electric motor, comprising a housing which is closed with the aforementioned end cap.

Preferred embodiments are now described with reference to the enclosed figures.

Fig. 1 a plan view of a disk-shaped element,

Fig. 2A a sectional view of a first device,

Fig. 2B a sectional view of a first device with a feedthrough capacitor,

Fig. 3 a plan view of a mounting element,

Fig. 4 a sectional view of a second device,

Fig. 5 a perspective view of an end cap comprising a third device,

Fig. 6 a substitute circuit diagram,

Fig. 7 a plot of the conducted emissions in dependency of the frequency of a conventional electric motor and an electric motor provided with a device according to the invention,

Fig. 8 a sectional view of a third device and

Fig. 9 a sectional view of a fourth device.

Fig. 1 shows a plan view of a disk- shaped element 1 in which a C _x capacitor and two C ₇ capacitors are combined. The disk- shaped element 1 of Fig. 1 is designed as a ceramic barrier layer element. The disk-shaped element 1 comprises a disk which is produced of a dielectric ceramic, e.g. strontium titanate, barium titanate, calcium titanate or mixtures thereof having a VDR characteristic, NDK mass (COG) and similar, microwave ceramic and similar. On an upper surface of the disk there are provided a first electrode 2, a second electrode 3 and a third electrode 4. The third electrode 4 is connected with ground, e. g. with a housing of the electric motor. The electrodes 2, 3, 4 may be made from a silver or copper layer or any other suitable electrically conductive material. Such electrodes 2, 3, 4 may be manufactured by

burning in pasteous systems. Reference sign 5 designates a central breakthrough. The first 2 and the second electrode 3 are separated by a first isolation path 6. The third electrode 4 is separated from the first electrode 2 and the second electrode 3 by second isolation paths 7. In the present embodiment the first 6 and second isolation paths 7 exhibit the form of a "Y" . However, it has to be understood that the electrodes 2, 3, 4 may be designed in any suitable geometrical form. For example, it is possible that the third electrode 4 forms a ring surrounding the first 2 and the second electrode 3.

The first 2 and the second electrode 3 form in the shown arrangement a C _x capacitor. The first 2 and the third electrode 4 form a C _y capacitor, and the second 3 and the third electrode 4 form a further C _y capacitor. A capacity of the C _x capacitor may be increased by providing a first SMD capacitor 8 which is switched between the first 2 and the second electrode 3. In a similar way second SMD capacitors 9 may be switched between the first electrode 2 and the third electrode 4 and between the second electrode 3 and the third electrode 4 in order to increase the capacity of the C _y capacitors. Besides the shown the SMD capacitors 8, 9 there may be provided further elements like resistors, varistors, NTC-resistors, VDR-devices, diodes and the like in order to set or adapt the electric properties of the disk-shaped element 1 with respect to the respective noise suppression requirements .

Fig. 2A shows a sectional view through a first device. The first electrode 2 extends from an upper surface around an outer circumference to a lower surface of the disk-shaped element 1. A first supporting element 10 is fixed by a first clamp 11 on the upper surface of the disk-shaped element 1. The first clamp 11 comprises at its first end El a hook 12

which forms a first terminal to be contacted with a power source (not shown here) .

The first clamp 11 may be casted into the first supporting element 10 so that its second end E2 is accessible for connecting it with a first connector 13 of a first coil 14. The first coil 14 is wound on the first supporting element 10. A second connector 15 of the first coil 14 is contacted with a first brush arm 16. The first brush arm 16 is made from a flexible electrically conductive metal strip. The first brush arm 16 is provided at its free end (not shown here) with a first brush.

The first connector 13 may be electrically connected with the second end E2 of the first clamp 11 by clamping, soldering or the like. In a similar way the second connector 15 may be electrically connected with the first brush arm 16 by clamping, soldering or the like.

The first supporting element 10 is made preferably by a polymer. Said polymer may be a conventional plastic material or resin which may be formed for example by injection molding. The inductivity of the first coil 14 may be set by manufacturing the first supporting element 10 from a polymer which contains one or more materials selected from the following group: MeOFe ₂ O ₃ , MeFe ₂ O ₄ , wherein Me is one of the following ions: Mn ²⁺ , Fe ²⁺ , Co ²⁺ , Ni ²⁺ , Zn ²⁺ , Mg ²⁺ , Cu ²⁺ or mixtures thereof; Fe-; Co-; Ni- and rare earth powders. The inductivity of the first supporting element 10 being combined with the first coil 14 may be set by a composition of the aforementioned material.

Fig. 2B shows a sectional view through the first device realizing a feedthrough capacitor. From the supporting element 10 there extends through a breakthrough provided in

the disk-shaped element 1 a connection pin 11a the first end El of which is in the form of a hook 12. The connection pin 11a may be fixed at the first electrode 2, e. g. by soldering or the like. A first brush being mounted at first brush arm 16 is designated with the reference sign 24.

Preferably the second electrode 3 is provided with a similar or identical arrangement shown in Fig. 2A or 2B.

Fig. 3 shows a plan view of an embodiment of a supporting arrangement. The supporting arrangement comprises here a first 10 and a second supporting element 17 which are connected by a bridge element 18. The first supporting element 10, the bridge element 18 and the second supporting element 17 are preferably cast or injection moulded in one part. The supporting elements 10, 17 are structured in a similar way as shown in Fig. 2. Each supporting element 10, 17 comprises a coil 14, 19 which is wound around the supporting element 10, 17. A third connector 20 of the second coil 19 extends around the outer circumference of the disk- shaped element 1 to the lower surface thereof . A fourth connector 21 of the second coil 19 is electrically connected with a second brush arm 22. The second brush arm 22 is provided with a second brush 23. Reference sign 24 designates the first brush being fixed at a free end of the first brush arm 16.

Fig. 4 shows the supporting arrangement of Fig. 3 being mounted on the disk-shaped element 1. As can be seen from Fig. 4 there is provided a second clamp 25 by means of which the second supporting element 17 is fixed on the upper surface of the disk-shaped element 1 and by which at the same time the first end El of the third connector 20 is pressed against the second electrode 3 which extends from the upper surface circumference to the lower surface of the disk-shaped

element 1. In a similar way the first supporting element 10 is fixed and contacted with the first electrode 2 (not shown here) .

Fig. 5 shows an end cap 26 for a small electric motor (not shown here) . Within the end cap 26 there is mounted a further device of the invention. The device again comprises a disk- shaped element 1 having a central breakthrough 5. The disk- shaped element 1 fits essentially with the form of a inner end face 27 which is surrounded by side walls 28 of the end cap 26. A shaft 29 extends through the central breakthrough 5. For a sake of clarity the shaft 29 is shown here without a commutator and an anchor or armature, respectively. A further supporting assembly is similar to the supporting assembly shown in Fig. 3. It comprises a first 10 and a second supporting element 17 which are formed together in one part. Each supporting element 10, 17 comprises here a cylindrical extension 31 around which there is wound the first 14 and second coil 19, respectively (not shown here) .

In contrast to the embodiment shown in Figs . 3 and 4 the brush arms 16 and 22 do not extend from the, preferably cylindrical formed, extensions 31 supporting the coils 14, 19, but from the bridge member 18 which connects the first supporting element 10 with the second supporting element 17.

The further supporting assembly is in engagement with an opening 32 provided within the side walls 28 of the end cap 26. Opposite to the opening 32 there may be provided at least one further breakthrough within the side wall 28. Such a further breakthrough (not shown here) may be formed in form of a slit. The further breakthrough is formed such that by means of a tool the flexible brush arms 16, 22 may be held apart of each other from the exterior of the end cap 26 when mounting the end cap 26 at a housing of a small electric

motor comprising a shaft 29 with a commutator. Upon removal of such a tool the brushes 23, 24 are pressed against the surface of a commutator by the flexible brush arms 16, 22.

As can be seen from Fig. 5 the disk-shaped element 1 essentially covers the inner end face 27. This has a shielding effect by which an escape of electromagnetic noise in the surrounding of a housing of a small electric motor being closed with the end cap 26 can be counteracted.

Fig. 6 shows a substitute circuit diagram of a circuit for noise suppression of a small electric motor. A C _x capacitor is installed between connection clamps Al, A2 which are used for the connection to a power source. Further, each electrode of the C _x capacitor is connected via an inductivity Ll, L2 with a first brush Bl and a second brush B2 by which a sliding contact to a commutator is realized. Parallel to the C _x capacitor, a varistor V may be installed. Instead or in addition to the varistor V, an ohmic resistor can also be provided. Each of the connection clamps Al, A2 as well as each of the inductivities Ll, L2 are also connected via a C _y capacitor with ground. C _D designates a feedthrough capacitor which may be provided additionally (see Fig. 2B) .

Fig. 7 shows a plot of the conducted noise in dependency of the frequency of a conventional electric motor and the same electric motor being provided with a device for noise suppression in accordance with the invention.

The values measured for the conventional electric motor are signified by "crosses", the values measured for the electric motor being provided with the device for noise suppression in accordance with the invention are signified by "dots". The measurements have been carried out in accordance with the norm CISPR25.

As can be seen from Fig. 7 by the electric motor being provided with the device for noise suppression in accordance with the invention the conducted emissions or noise, i.e. the emissions which are emitted for example via wire, can be reduced remarkably.

Figs. 8 and 9 each show a simplified sectional view of a third and a fourth device. The disk-shaped element 1 may be designed as described above. A third support element 33 is made in the form of a disk the shape of which is congruent with the shape of the disk-shaped element 1. The third support element 33 may be laminated on an upper surface of the disk-shaped element 1. Reference sign 34 designates a ferrite-element which is formed like the third support element 33. This disk-shaped ferrite-element 34 is fixed at a lower surface of the disk-shaped element 1. The lower surface is vis-a-vis the upper surface. The third support element 33 supports together with the disk-shaped ferrite-element 34 the first 14 and the second coils 19.

Fig. 9 shows a fourth device. Here a diameter of the third support element 33 and the disk-shaped ferrite-element 34 is larger than the diameter of the disk-shaped element 1. The first 14 and the second coils 19 are provided in sections projecting beyond the circumference of the disk-shaped element 1.

By providing the disk- shaped ferrite-element 34 it is possible to additionally damp lower frequencies which cannot effectively been damped by the third supporting element 33 which is preferably filled with a powder improving the conductivity of the first 14 and second coils 19.

List of reference signs

1 disk-shaped element

2 first electrode 3 second electrode

4 third electrode

5 central breakthrough

6 first isolation path

7 second isolation path 8 first discrete capacitor

9 second discrete capacitor

10 first supporting element

11 first clamp 11a connection pin 12 hook

13 first connector

14 first coil

15 second connector

16 first brush arm 17 second supporting element

18 bridge element

19 second coil

20 third connector

21 fourth connector 22 second brush arm

23 second brush

24 first brush

25 second clamp

26 end cap 27 inner end face

28 side wall

29 shaft

31 cylindrical extension

32 opening 33 third support element

34 disk-shaped ferrite-element

ALl first connection clamp

AL2 second connection clamp Bl first brush

B2 second brush

El first end

E2 second end

Ll first inductivity L2 second inductivity

V varistor