DC motor with radiation electromagnetic noise suppression mechanism

Abstract

To reduce electromagnetic noise radiated from a motor to the outside by suppressing spatial conduction of radiation emissions generated from a contact between a rectifier of a DC motor and a carbon brush.SOLUTION: A DC motor comprises a radiation electromagnetic noise suppression mechanism which is provided in a shield cap of the motor and suppresses electromagnetic noise generated in a slide contact part between a rectifier and a pair of brushes. The mechanism includes a shield cover and an antenna. The shield cover covers a power supply line, a choke coil, an EMC filter circuit and a power supply terminal for feeding. The antenna is fixed to a brush insulator near the slide contact part between the rectifier and the brush and connected to a shield cap which is functioned as a ground terminal.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to a DC motor having a function of suppressing radiated electromagnetic noise, and particularly to a mechanism for suppressing radiated electromagnetic noise for a small DC motor suitable for an electric actuator of an automobile or the like.

Background Art

[0002] A small DC motor equipped with a commutator has a carbon brush and a commutator as an electrical contact and a mechanical contact. Therefore, a spark may occur at the contact point between the commutator and the brush at the timing when the brush and the commutator are switched, which causes electrical noise. Conventionally, various measures have been taken for the purpose of removing this spark discharge, in other words, surge.

[0003] For example, Patent Document 1 discloses an example in which two parallel paths are provided to discharge the electrical noise generated by the switching of the contact portion between the brush and the commutator to the ground from the armature shaft. One path is to discharge it to the ground through the bearing of this shaft, and the other path is to discharge it to the ground through the ground terminal and the contact plate from the bearing of the shaft. The electrical noise transmitted through the armature shaft is discharged to the ground through the two bearings, so that the electrical noise is prevented from being emitted from the electric motor.

[0004] Further, Patent Document 2 discloses an invention in which a metal shielding member is inserted near the fitting portion of the reduction housing. This shielding member is provided outside the periphery of the contact portion between the brush and the commutator, which is the source of electromagnetic noise, and is provided so as to be continuous in the axial direction to the yoke housing. When the motor is rotationally driven, electromagnetic noise is generated between the brush and the commutator, and most of the electromagnetic noise scattered from the source to the reduction housing side is shielded by the shielding member. Therefore, the leakage of electromagnetic noise from the reduction housing side is reduced.

[0005] In addition, Patent Document 3 discloses an invention in which a grounding member that contacts a bearing and a yoke is fixed to a ground terminal held by a brush holder. When a power supply brush makes sliding contact with a commutator, electrical noise is generated, but the propagation of the electrical noise to the circuit side is suppressed by a capacitor and a choke coil. Further, although the electrical noise also propagates to the rotating shaft, since the bearing that pivotally supports the rotating shaft is connected to the yoke as a ground via the grounding member, the radiation of the electrical noise is suppressed.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0007] By providing an EMC filter circuit in the middle of the carbon brush of the DC motor and the power supply line, the radiation emission generated from the contact point between the commutator and the carbon brush is considerably reduced. However, according to the experiments of the inventor, for example, the radiation emission generated from the contact point between the commutator and the carbon brush may conduct spatially to a location where the conductive part of the power supply line is exposed, is not properly guided to the EMC filter circuit, and there is a possibility that electromagnetic noise is radiated from the motor to the outside.

[0008] An object of the present invention is to suppress the spatial conduction of radiation emission generated from the contact point between the commutator and the carbon brush and reduce the electromagnetic noise radiated from the motor to the outside.

Means for Solving the Problems

[0009] According to one aspect of the present invention, there is provided a DC motor including a commutator fixed to a rotating shaft and a pair of brushes in sliding contact with the commutator, wherein the rotating shaft is rotatably supported by a motor case and a shield cap. In the brush insulator provided in the metal shield cap, a pair of brush holders are provided, and the pair of brushes are connected to a pair of power supply terminals for power supply via a power line, a choke coil, and an EMC filter circuit. It is provided with a radiated electromagnetic noise suppression mechanism in the shield cap for suppressing electromagnetic noise generated at the sliding contact portion between the commutator and the pair of brushes. The radiated electromagnetic noise suppression mechanism includes a shield cover and an antenna. The shield cover is made of a conductive material and covers the power line, the choke coil, the EMC filter circuit, and the power supply terminals for power supply. The antenna is fixed to the brush insulator and is connected to the shield cap that functions as a ground terminal.

[0010] According to the present invention, it is possible to suppress the spatial conduction of radiated emissions generated from the contact point between the commutator and the carbon brush, and reduce the electromagnetic noise radiated from the motor to the outside.

Brief Description of the Drawings

[0011]

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Mode for Carrying Out the Invention

[0012] Hereinafter, embodiments of a DC motor provided with a radiated electromagnetic noise suppression mechanism according to the present invention will be described with reference to the drawings. Figure 1 is a longitudinal sectional view of a DC motor equipped with a radiated electromagnetic noise suppression mechanism according to the first embodiment of the present invention. The motor case 11 of the DC motor 10 is a cup-shaped metal case such as iron with one end open. A magnet 12 that constitutes the stator of the DC motor is fixed to the motor case 11. A laminated iron core 14 and an armature winding 15 that constitute the rotor of the DC motor are integrally formed with the rotating shaft 13, and this rotating shaft is rotatably supported by the metal motor case 11 and an aluminum alloy shield cap 33 via a pair of bearings. The laminated iron core 14 is polarized into multiple poles in the circumferential direction, and the armature windings 15 are wound around the respective iron cores, and their ends are connected to a commutator 16. A pair of carbon brushes 17 held by a brush holder 18 and connected to a power supply terminal 23 for power supply are in sliding contact with the commutator 16. 28 is a shield cover made of a conductive material that forms part of the radiated electromagnetic noise suppression mechanism.

[0013] Figure 2 is a longitudinal sectional view taken along line A-A of Figure 1. Figure 3 is a perspective view of the shield cap 33 shown with a part of the shield cover 28 transparent. Further, Figure 4 is a perspective view of the shield cap 33 of Figure 3 as seen from the back side. The brush insulator 20 provided in the shield cap 33 is provided with a pair of brush holders 18 and a pair of coil springs 19 that apply pressure to the pair of carbon brushes 17. The carbon brushes 17 in sliding contact with the commutator 16 are each connected to a pair of power supply terminals 23 for power supply via a pigtail 21, a choke coil 25, and an EMC filter circuit 27 that constitute a pair of power lines. Note that a jumper terminal 24 is provided between the pigtail 21 and the choke coil 25 in one of the power lines. An antenna 30, which is part of the radiated electromagnetic noise suppression mechanism, is fixed to the shield cap 33 made of a fixed metal and functioning as a ground terminal at the middle of the pair of coil springs 19 on the brush insulator 20. More specifically, the antenna 30 is disposed in the space between the sliding contact portion of the commutator 16, which is a noise source, and the pair of brushes 17 and the pair of coil springs 19. As shown in FIG. 2, one jumper terminal 24, a pair of choke coils 25, a pair of EMC filter circuits 27, and a pair of power supply terminals 23 that constitute a power line are covered by one shield cover 28 made of a conductive material. As shown in FIG. 4, the shield cover 28 is connected to the metal part of the motor case 11 that functions as a ground terminal via a wire 29 made of a conductive material. Further, this shield cover 28 is fixed to the brush insulator 20. Note that a part of the pair of pigtails 21 is also covered by the shield cover 28.

[0014] FIG. 5 is a perspective view showing a state in which the shield cover 28 of the shield cap 33 in FIG. 2 is removed. The EMC filter circuit 27 has a structure in which it is covered by a shield cover 28 made of a conductive material connected to the metal part of the motor case 11 in order to be electromagnetically isolated from the contact part of the brush 17 and the commutator 16, which are noise sources, for the purpose of improving electromagnetic shielding characteristics. Thereby, electromagnetic wave noise generated from the noise source can be short-circuited to the ground terminal, and spatial conduction to the EMC filter circuit 27 can be suppressed. Further, the power supply terminal 23 and the jumper terminal 24 also have a structure in which they are covered by the shield cover 28 in order to be electromagnetically isolated from the noise source.

[0015] As shown in FIG. 6, the antenna 30 is a monopole antenna with low directivity, and has an antenna part 30a and a plate-shaped fixing part 30b. The antenna 30 is made of a springy metal, for example, beryllium copper, and is fixed to the brush insulator 20 by the fixing part 30b, and the tip of the fixing part 30b is pressed against the shield cap 33 by a spring force and is in a contact state. That is, the antenna 30 is configured as a grounded monopole antenna. Since this antenna 30 needs to be short-circuited to the metal part of the motor case 11 that functions as a ground terminal at the shortest distance, it has a structure that directly short-circuits to the metal shield cap 33 on the back surface of the brush insulator 20. That is, although electromagnetic noise may be conducted in space to the exposed part of the power supply terminal, by providing a monopole antenna near the noise source, such electromagnetic wave noise can be short-circuited to the metal part of the motor case at the shortest distance, reducing the spatial conduction of electromagnetic noise. In addition, the shield cap 33 can further enhance the electromagnetic wave shielding characteristics by using an iron-aluminum alloy with increased conductivity instead of the zinc-plated steel sheet commonly used in the past.

[0016] Fig. 7 shows a configuration example of the EMC filter circuit 27 in the first embodiment of the present invention. This EMC filter circuit 27 is a circuit in which elements such as capacitors C1, C2, C3, and TVS (Transient Voltage Suppressor) that make up the IC filter circuit are mounted on a substrate. Capacitor C1 and choke coils (L1, L2) function as countermeasures for differential-mode noise, and capacitors C2 and C3 function as countermeasures for common-mode noise. TVS is a voltage clamp type surge protection element that absorbs and reduces surge voltage.

[0017] Next, with reference to Figs. 8 to 10, the operation and effects of the radiated electromagnetic noise suppression mechanism of the present invention will be described. First, Fig. 8 is a perspective view of the shield cap of a conventional small DC motor (hereinafter referred to as the DC motor of the comparative example) equipped with a commutator and brushes, which has an EMC filter circuit in the middle of the power line but does not have a shield cover or an antenna. In the DC motor of the comparative example, the radiated noise of the electromagnetic wave generated at the contact part of the brush 17 and the commutator 16, which are the noise sources, directly reaches the jumper terminal 24 and the power supply terminal 23 of the power line, and the EMC filter circuit 27 cannot sufficiently process the radiated electromagnetic noise, which is radiated to the outside of the motor as electromagnetic wave noise.

[0018] On the one hand, as shown in FIG. 9, in the shield cap provided with the radiated electromagnetic noise suppression mechanism according to the first embodiment of the present invention, the power supply terminal 23 and the jumper terminal 24 are covered with the shield cover 28. Therefore, the radiated noise of the electromagnetic wave is short-circuited to the ground terminal, and it is suppressed from directly reaching the power supply terminal 23 and the jumper terminal 24 by space conduction. Furthermore, a part of the radiated noise of the electromagnetic wave is received by the antenna 30 and led to the shield cap 33 that functions as a ground terminal. Thereby, the noise to be processed by the EMC filter circuit 27 is reduced. As a result, the noise radiated to the outside of the motor can be reduced.

[0019] In addition, in order to make the grounded antenna 30 function as an ideal grounded λ / 4 monopole antenna, when the frequency of the radiated electromagnetic noise is 2.4 GHz, λ = about 12 cm, and a length corresponding to λ / 4 = about 3 cm is required. Also, the frequency of the radiated electromagnetic noise generated at the contact portion between the brush and the commutator contains many components with frequencies lower than 2.4 GHz. However, as for DC motors suitable for automotive electric actuators and the like, for example, many small motors with an outer diameter of the motor case 11 of about 25 mm to 60 mm are adopted, and there is a limit to the height (axial length) of the shield cap 33, and the length of the monopole antenna 30 cannot be made sufficiently long. Therefore, in these small motors, for example, when the outer diameter D of the motor case 11 is 25 mm, the length of the monopole antenna is 10 mm, when the outer diameter D is 40 mm, the length of the monopole antenna is 15 mm, and when the outer diameter D is 60 mm, the length of the monopole antenna is about 20 mm, which is practical. According to the inventor's experiments, it was confirmed that even such a short monopole antenna is effective for reducing high-frequency noise in a wide range of the GHz order.

[0020] FIG. 10 is an explanatory diagram of the radiation emission state of the DC motor according to the first embodiment of the present invention. The electromagnetic wave radiation noise generated at the contact portion between the commutator 16 and the brush 17 reaches the EMC filter circuit 27 inside the motor via the power line, where noise reduction processing is performed. The power line is further connected to an ECU such as a vehicle control system outside the motor 10. The exposed portions of the power terminal 23 and the jumper terminal 24 of the power line inserted inside the shield cap 33 are considered to function as an antenna and receive the electromagnetic noise generated at the sliding contact portion between the commutator 16 and the pair of brushes 17 as radiated electromagnetic noise. When the power terminal 23 or the like acts as an antenna and receives radiated electromagnetic noise, it is considered that noise is induced in the common mode. Also, the exposed portion of the EMC filter circuit 27 is considered to receive radiated electromagnetic noise. Since the electromagnetic noise generated from the motor 10 may lead to malfunction of an external ECU, it is necessary to make it difficult for the electromagnetic noise from the motor to be transmitted to the ECU. According to the present invention, noise reduction processing is performed in the EMC filter circuit 27 inside the motor, and further, by providing the shield cover 28, the antenna 30, and the shield cap 33, the emission of electromagnetic noise to the outside of the motor is prevented. Therefore, the transmission of noise to the external ECU is significantly reduced.

[0021] As described above, according to the radiated electromagnetic noise suppression mechanism according to the first embodiment of the present invention, by covering the power terminal 23, the jumper terminal 24, and the EMC filter circuit 27 with the shield cover 28 and providing the monopole antenna 30, the electromagnetic noise that reaches the EMC filter circuit 27 directly from the noise source without passing through the power line can be reduced. As a result, the noise to be processed by the EMC filter circuit 27 can be decreased, and thus the noise radiated to the outside of the motor can be reduced.

[0022] Next, with reference to FIGS. 11 to 14, the effects of the present invention will be described based on measurement data. In each case, the outer diameter D of the motor case 11 is 25 mm. Also, the length of the monopole antenna in the embodiment of the present invention is 15 mm. First, FIG. 11 is a diagram showing an example of measurement data of horizontal radiation emission in a frequency band of 1 GHz to 2.5 GHz in a DC motor of a comparative example. On the other hand, FIG. 12 is a diagram showing an example of measurement data of horizontal radiation emission in a frequency band of 1 GHz to 2.5 GHz in a DC motor of an embodiment of the present invention provided with a shield cover 28 and an antenna 30. In the DC motor of the comparative example in FIG. 11, the radiation emission shows a value centered around 30 dBμV / m in the frequency band of 1 GHz to 2.5 GHz. In contrast, in the DC motor of the present invention in FIG. 12, the radiation emission shows a value centered around 20 dBμV / m in the frequency band of 1 GHz to 2.5 GHz. Thus, in the DC motor of the embodiment of the present invention, it is recognized that the value of the horizontal radiation emission is significantly reduced with respect to the DC motor of the comparative example, that is, there is a noise suppression effect.

[0023] Next, FIG. 13 is a diagram showing an example of measurement data of vertical radiation emission in a frequency band of 1 GHz to 2.5 GHz in a DC motor of a comparative example. On the other hand, FIG. 14 is a diagram showing an example of measurement data of vertical radiation emission in a frequency band of 1 GHz to 2.5 GHz in a DC motor of an embodiment of the present invention. In the DC motor of the comparative example in FIG. 13, the radiation emission shows a high Field Strength value in the frequency bands around 1.2 GHz and 2.1 GHz. In contrast, in the DC motor of the present invention in FIG. 14, the radiation emission shows a lower Field Strength value in the high-frequency band of 1 GHz to 1.4 GHz centered around 1.2 GHz and in the frequency band centered around 2.1 GHz. Thus, in the DC motor of the embodiment of the present invention, it is recognized that the value of the vertical radiation emission is also significantly reduced with respect to the DC motor of the comparative example, that is, there is a noise suppression effect.

[0024] Next, with reference to FIG. 15, a radiated electromagnetic noise suppression mechanism according to a second embodiment of the present invention will be described. In this embodiment, one jumper terminal 24, one choke coil 25, an EMC filter circuit 27, and one power supply terminal 23 for power supply that constitute a power line are covered with a shield cover 280. Also, a part of one pigtail 21 is covered with a shield cover 28. An antenna 30 is fixed to a shield cap 33 made of metal and functioning as a ground terminal in the middle of a pair of coil springs 19. This second embodiment is suitable for adoption when the outer diameter D of the motor case 11 is small and the surface area of the brush insulator 20 is narrow. That is, even if the outer diameter D of the motor case 11 is small, there is a limit to reducing the size of each member such as the brush holder and the carbon brush. Therefore, it is suitable when it is difficult to widely cover the surface of the brush insulator 20 with the shield cover 280. Compared with the first embodiment, the noise suppression effect is considerably reduced. However, the inventor has confirmed that there is still a noise suppression effect compared with the comparative example.

Description of Signs

[0025] 10 DC motor 11 Motor case 11 12 Magnet 13 Rotating shaft 14 Laminated iron core 15 Armature winding 16 Commutator 17 Carbon brush 18 Brush holder 19 Coil spring 20 Brush insulator 21 Pigtail 23 Power supply terminal 24 Jumper terminal 25 Choke coil 27 EMC filter circuit 28 Shield cover 30 Antenna 33 Shield cap

Claims

1. A DC motor comprising a commutator fixed to a rotating shaft and a pair of brushes in sliding contact with the commutator, wherein the rotating shaft is rotatably supported by a motor case and a shield cap. A brush insulator provided in the metal shield cap is provided with a pair of brush holders, and the pair of brushes are connected to a pair of power supply terminals for power supply through a power line, a choke coil, and an EMC filter circuit. It is provided in the shield cap and includes a radiated electromagnetic noise suppression mechanism for suppressing electromagnetic noise generated at the sliding contact portion between the commutator and the pair of brushes. The radiated electromagnetic noise suppression mechanism includes a shield cover and an antenna. The shield cover is made of a conductive material and covers the power line, the choke coil, the EMC filter circuit, and the power supply terminals for power supply. The antenna is fixed to the brush insulator and is connected to the shield cap that functions as a ground terminal. A DC motor characterized by this.

2. The DC motor according to claim 1, wherein the antenna is a monopole antenna.

3. In claim 2, It has a pair of coil springs provided in the shield cap and applying pressure to the pair of brushes. The pole antenna is disposed between the sliding contact portion between the commutator and the pair of brushes and the pair of coil springs. A DC motor characterized by this.

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