CN115800631A

CN115800631A - Rotor and motor

Info

Publication number: CN115800631A
Application number: CN202211542543.7A
Authority: CN
Inventors: 包黎明; 陈飞龙; 吴友弟; 张哲�; 谢亮
Original assignee: Gree Electric Appliances Inc of Zhuhai; Zhuhai Kaibang Motor Manufacture Co Ltd
Current assignee: Gree Electric Appliances Inc of Zhuhai; Zhuhai Kaibang Motor Manufacture Co Ltd
Priority date: 2022-12-02
Filing date: 2022-12-02
Publication date: 2023-03-14

Abstract

The application relates to a rotor and a motor, the rotor is used for the motor, the rotor includes: rotor core has the lateral wall, the lateral wall is equipped with the depressed part of a plurality of interval arrangements, follows rotor core's axial direction, the depressed part runs through the lateral wall, wherein, the extending direction of depressed part is relative axial direction has preset inclination, so that when rotor core rotated, follow axial direction, the depressed part can be followed fluid rotor core one side direction opposite side. The rotor and the motor can have a good heat dissipation effect while not additionally occupying the motor space, and the problems of performance reduction and service life shortening of the motor due to overhigh temperature of the motor are avoided.

Description

Rotor and motor

Technical Field

The application relates to the technical field of motors, in particular to a rotor and a motor.

Background

The motor has high running precision and adjustable and controllable speed, and is a driving device for converting voltage signals into torque and rotating speed. The motor can produce a large amount of heats in the long-time operation of in-service use occasion, and the heat can influence the working property of motor, especially can cause great influence to the encoder, makes its precision reduce. Current servo motor relies on natural air cooling to dispel the heat, and because servo motor's application scenario restriction leads to its mounted position space narrow and small, outside air flow poor moreover, when outside air flow is not enough, the heat that servo motor produced can't in time outwards conduct, and the heat gathers on servo motor, causes great influence to the encoder, influences servo motor's precision, has reduced servo motor's life.

Disclosure of Invention

An object of this application is to provide a rotor and motor, has better radiating effect when additionally not taking up motor space, avoids the problem that the motor leads to because of self high temperature performance degradation, life-span shorten.

To this end, in a first aspect, embodiments of the present application provide a rotor for an electric machine, the rotor including:

a rotor core having an outer sidewall, the outer sidewall being provided with a plurality of recesses arranged at intervals, the recesses penetrating the outer sidewall in an axial direction of the rotor core,

the extending direction of the concave part is opposite to the axial direction, a preset inclined angle is formed between the extending direction of the concave part and the axial direction, so that when the rotor core rotates, along the axial direction, the concave part can guide fluid from one side of the rotor core to the other side.

In one possible realization, the recess profile of any cross-section is the same along the extension direction,

the depressed part is relative the first recess of rotor core's axis inside sunken, first recess has the maximum depth of predetermineeing.

In one possible implementation, the preset maximum depth is not greater than 1.5mm.

In a possible implementation manner, the first groove is an arc-shaped groove, and along the axial direction, the concave portion forms a first concave track and a second concave track at two ends of the outer side wall respectively, the first concave track has a first axis, the second concave track has a second axis,

the included angle between the connecting line between the first axis and the second axis and the axis of the rotor core is the preset inclination angle, and the preset inclination angle is not more than 30 degrees.

In one possible implementation, the preset inclination angle ranges between 20 ° and 28 °.

In a possible implementation manner, the outer sidewall is provided with a plurality of second grooves arranged at intervals, the second grooves penetrate through the outer sidewall along the axial direction, and the second grooves and the concave parts are arranged in a staggered manner.

In a possible implementation manner, the minimum distance between the edge of the recess and the edge of the adjacent second groove along the circumferential direction of the outer side wall is greater than 5mm.

In one possible implementation, the rotor core is further provided with a plurality of through holes arranged circumferentially, the through holes being opposed to the second grooves in a radial direction of the rotor core,

the minimum thickness between the through hole and the adjacent concave part is not less than 10mm.

In a second aspect, an embodiment of the present application provides an electric machine, including:

the shell is provided with an inner cavity and comprises a first end cover and a second end cover, the first end cover is provided with an air inlet communicated with the inner cavity, and the second end cover is provided with an air outlet communicated with the inner cavity;

a drive assembly disposed in the inner cavity, the drive assembly including a rotor shaft; and

as described in the above, the rotor is fitted to the outer peripheral side of the rotor shaft.

In a possible implementation manner, the air inlet and the air outlet are respectively provided in a plurality, and along the axial direction, the air inlet and the air outlet are respectively located at two sides of the rotor,

the axis of the air inlet is parallel to the axis of the rotor, and a preset included angle is formed between the axis of the air outlet and the axis of the rotor.

In one possible implementation, the preset included angle ranges from 30 ° to 60 °.

In one possible implementation, the fluid in the inner cavity between the rotor and the first end cap has a first pressure p1, the fluid in the inner cavity between the rotor and the second end cap has a second pressure p2, and the preset inclination angle and the preset maximum depth of the recess are calculated according to the following formula:

W＝(p2-p1)/ρ+(c22-c12)/2；

EK＝mv2/2；

wherein W is a total kinetic energy of the rotor, ρ is a fluid density, c1 is an absolute velocity of the fluid at the air outlet at a first temperature, c2 is an absolute velocity of the fluid at the air outlet at a second temperature, the second temperature is higher than the first temperature, EK is a change amount of the kinetic energy, m is a mass of the rotor, and v is a rotation speed of the rotor.

In a possible implementation manner, the air conditioner further comprises a dust screen, and the dust screen is respectively arranged at the position corresponding to the air inlet and the air outlet.

According to the rotor and the motor provided by the embodiment of the application, the outer side wall of the rotor core is provided with a plurality of concave parts with preset inclination angles, so that the plurality of concave parts are combined on the outer side wall of the rotor to form a spiral fan-shaped chute structure. Therefore, when the rotor works in the motor and rotates, local vacuum can be formed around the rotor core, air around the rotor core is sucked into the concave part, and the air flowing effect is generated. The recessed portion is provided so that the direction of the airflow is parallel to the axial direction, and the airflow can flow into and out of the rotor core in the axial direction. Therefore, through the depressed part that sets up at rotor core, can have better radiating effect when not additionally occupying motor space, avoid the motor because of performance degradation, the life-span shortened problem that self high temperature leads to.

Drawings

In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the drawings needed to be used in the description of the embodiments or the prior art will be briefly described below, and it is obvious that the drawings in the following description are some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative efforts. In addition, in the drawings, like parts are denoted by like reference numerals, and the drawings are not drawn to actual scale.

Fig. 1 is a cross-sectional front view of an electric machine provided in an embodiment of the present application, in which a direction indicated by an arrow H is an axial direction;

FIG. 2 is a schematic view of a rotor structure provided in an embodiment of the present application;

fig. 3 is a schematic structural diagram illustrating a second end cap of an electric machine according to an embodiment of the present application;

fig. 4 shows a schematic structural diagram of a first end cover in a motor provided by an embodiment of the present application;

fig. 5 shows a schematic view of an air flow direction in an electric machine provided by an embodiment of the present application.

Description of the reference numerals:

100. a motor; 1. a second end cap; 11. an air outlet; 111. an air outlet channel; 2. a first end cap; 21. an air inlet; 211. an air inlet channel; 3. an inner cavity; 4. a drive assembly; 41. a rotor shaft; 42. a rotor; 42a, a rotor core; 421. a recessed portion; 422. a second groove; 423. a through hole; 5. a dust screen.

Detailed Description

In order to make the objects, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application, and it is obvious that the described embodiments are some embodiments of the present application, but not all embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present application.

Fig. 1 is a cross-sectional front view of a motor provided in an embodiment of the present application, in which a direction indicated by an arrow H is an axial direction. Fig. 2 shows a schematic structural diagram of a rotor provided in an embodiment of the present application.

Referring to fig. 1 and 2, the embodiment of the present application provides a motor, and the motor 100 may be, for example, a servo motor, which will not be separately emphasized later. The servo motor is an engine for controlling the motion of a mechanical element in a servo system, is an auxiliary motor indirect speed changing device, and can be applied to robots, motors and the like, and the specific application scene is not particularly limited herein.

The present application further provides a rotor, wherein the motor 100 comprises a housing having an inner cavity 3, the housing comprises a first end cap 2 and a second end cap 1 connected to each other, the first end cap 2 and the second end cap 1 arranged along the axial direction H are detachably connected to each other, and the inner cavity 3 is formed after the first end cap and the second end cap are connected to each other. The inner cavity 3 is used for arranging a driving assembly 4, the driving assembly 4 comprises a rotor shaft 41, a coaxial rotor 42 is arranged on the rotor shaft 41, and the rotor 42 is sleeved on the outer peripheral side of the rotor shaft 41. It will be appreciated that the drive assembly 4 may also include other mating components, such as a stator, that mate with the rotor 42, and will not be described in detail herein.

First end cover 2 is equipped with the air intake 21 with inner chamber 3 intercommunication, and second end cover 1 is equipped with the air outlet 11 with inner chamber 3 intercommunication, and rotor 42 includes rotor core 42a, has the lateral wall, and the lateral wall is equipped with the depressed part 421 of a plurality of interval arrangements, along rotor core 42 a's axial direction H, and the depressed part 421 runs through the lateral wall. The air inlet 21 and the air outlet 11 corresponding to the recess 421 are disposed on the first end cap 2 and the second end cap 1, respectively, and the recess 421 is disposed on the rotor 42, so as to dissipate heat inside the motor 100, without additionally disposing a corresponding heat dissipation device in the motor 100, and without additionally occupying space.

Among the plurality of concave portions 421 provided on the outer side wall of the rotor core 42a, the extending direction of the concave portion 421 has a predetermined inclination angle with respect to the axial direction H, so that when the rotor core 42a rotates, the concave portion 421 can guide the fluid from one side of the rotor core 42a to the other side along the axial direction H. A plurality of concave portions 421 with a preset inclination angle are arranged on the outer side wall of the rotor core 42a, so that the plurality of concave portions 421 are combined on the outer side wall of the rotor 42 to form a spiral fan-shaped skewed slot structure. When the rotor 42 rotates during the operation of the rotor 42 in the motor 100, a partial vacuum is formed around the rotor core 42a, and air around the rotor core 42a is sucked into the recess 421, thereby generating an air flow effect. The recessed portion 421 is provided so that the direction of the airflow is parallel to the axial direction H, and the airflow can flow into and out of the rotor core 42a in the axial direction H. Therefore, the recess 421 provided in the rotor core 42a can have a good heat dissipation effect while not occupying extra space of the motor 100, and avoid the problems of performance degradation and life shortening of the motor 100 due to over-high temperature.

In an alternative embodiment, referring to fig. 2, the recessed portions 421 of any cross section are the same in shape along the extending direction, and the recessed portions 421 are first grooves recessed inward with respect to the axis of the rotor core 42a, and the first grooves have a predetermined maximum depth. That is, the recessed tracks formed by the recessed portions 421 of the respective cross sections have the same shape and size along the extending direction of the recessed portions 421, and the maximum depth of the recess of each portion with respect to the rotor core 42a is the same. Through the design, the depth of the recess 421 relative to the recess of the rotor core 42a is limited, so that when the rotor core 42a can rotate through the recess 421, the recess 421 rotates along with the rotor core to form a vortex, air flowing inside the motor 100 is realized, and the heat dissipation inside the motor 100 is realized by matching the air inlets and the air outlets arranged on two sides.

It is understood that the fluid may be air, or other gas or liquid capable of flowing, and is not limited in particular. In the embodiment of the application, the fluid refers to air, and the subsequent emphasis is not separately provided.

Optionally, the preset maximum depth is not greater than 1.5mm. It is understood that the preset maximum depth is a preferred embodiment, and the preset maximum depth is adjusted accordingly when the motors 100 of different models and specifications are used, and the positions, sizes, and the like of the inlet and the outlet, and is not limited in this respect.

In an alternative embodiment, the first groove is an arc-shaped groove, and along the axial direction H, the recessed portion 421 forms a first recessed track and a second recessed track at two end positions of the outer sidewall respectively, the first recessed track has a first axis, the second recessed track has a second axis, and an included angle between a connection line between the first axis and the second axis and an axis of the rotor core 42a is a preset inclination angle. So that the first groove is formed as a diagonal groove having an outer wall inclined at a certain angle with respect to the axial direction H, and the plurality of diagonal grooves form a spiral groove on the outer wall, so that when the rotor core 42a rotates, the rotation of the plurality of diagonal grooves forms a vacuum around the rotor core 42a, and the surrounding air can form a vortex, thereby achieving a heat dissipation effect.

It is understood that the first groove may be a square groove, an elliptical groove, or the like, as long as the requirements of the inclination angle and the depth mentioned above can be satisfied, and is not limited specifically herein.

Optionally, the preset inclination angle is not greater than 30 °. Preferably, the predetermined tilt angle is in the range of 20 ° to 28 °. In the embodiment of the present application, the preset inclination angle is set to 25 °, and is not specifically limited herein.

The following describes in detail a specific structure of a rotor provided in an embodiment of the present application with reference to the drawings.

Referring to fig. 2, the present embodiment provides a rotor, on the basis of a cylindrical rotor core 42a, an outer side wall of which is provided with a plurality of second grooves 422 arranged at intervals, the second grooves 422 penetrate through the outer side wall in an axial direction H, and the second grooves 422 and the recessed portions 421 are arranged in a staggered manner. The second recess is used as an air hole formed in the rotor 42, and when the recess 421 is formed in the outer side wall of the rotor core 42a, the recess 421 and the second recess need to be arranged to be away from each other, so that interference with the second recess is avoided, and a certain distance is formed between the recess 421 and the second recess.

Optionally, a minimum distance between an edge of the recess 421 and an edge of the adjacent second groove 422 in a circumferential direction of the outer sidewall is greater than 5mm.

In an alternative embodiment, the rotor core 42a is further provided with a plurality of through holes 423 arranged circumferentially, the through holes 423 being opposite to the second groove 422 in the radial direction of the rotor core 42a. The through hole 423 is used for installing a stator, and will not be described in detail here.

When the plurality of recessed portions 421 are provided, the minimum thickness between the through hole 423 and the adjacent recessed portion 421 is not less than 10mm in order to avoid that the distance between the provided recessed portion 421 and the through hole 423 is too short, which causes local weakness of the rotor core 42a and further affects the strength of the rotor core 42a itself. It is understood that, for different models of the motor 100, the rotor core 42a may be disposed differently, and the minimum thickness may be different according to different requirements, and may be adaptively adjusted according to actual conditions, which is not specifically limited herein.

Alternatively, the number of the concave portions 421 provided on the rotor core 42a may be adaptively selected according to actual conditions such as the maximum depth and the inclination angle of the concave portions 421 and the model of the motor 100, and is not particularly limited herein. In the embodiment of the present application, the number of the recess 421 is 10.

The motor 100 is provided with the recess 421 and then is matched with the air inlet 21 and the air outlet 11, so that the heat dissipation inside the four motors 100 can be realized, and the damage of the encoder is avoided. The recess 421 of the rotor 42 can be adapted according to the above definition in combination with the specific model of the motor 100, and will not be described in detail here. Alternatively, the recessed portion 421 provided in the rotor 42 may be obtained by calculation, specifically as follows:

in a specific embodiment, referring to fig. 3 to 5, the fluid in the inner cavity 3 between the rotor 42 and the first end cap 2 has a first pressure p1, the fluid in the inner cavity 3 between the rotor 42 and the second end cap 1 has a second pressure p2, and the predetermined inclination angle and the predetermined maximum depth of the recess 421 are calculated according to the following formulas:

W＝(p2-p1)/ρ+(c22-c12)/2；

EK＝mv2/2；

wherein W is a total kinetic energy of the rotor, ρ is a density of the fluid, c1 is an absolute velocity of the fluid at the air outlet 11 at the first temperature, c2 is an absolute velocity of the fluid at the air outlet 11 at the second temperature, the second temperature is higher than the first temperature, EK is a change amount of the kinetic energy, m is a mass of the rotor 42, and v is a rotation speed of the rotor 42.

Alternatively, the first temperature and the second temperature are two different temperatures of the motor 100 to obtain corresponding measurements of the temperature difference therebetween, for example, the second temperature may be 60 °, the first temperature may be 40 °, and detailed description thereof is omitted.

It is understood that the calculation formula in the above can be obtained based on bernoulli principle and euler equation as theoretical basis, and the detailed derivation process is not described in detail here. And W = EK is set according to the law of conservation of energy to obtain the mass of the rotor core 42a, so that the number of the arranged recesses 421 and the depth, shape, etc. of the recesses 421 are designed according to the change of the mass in combination with the actual situation, which will not be described in detail herein.

The inclination angle of the recess 421 can be selected adaptively according to the working principle of the electric fan, and the specific derivation and selection process thereof are not described in detail herein.

No matter what way the rotor 42 with the recess 421 is obtained, when the rotor 42 is disposed in the inner cavity 3 of the motor 100, the specific arrangement of the air inlet 21 and the air outlet 11 correspondingly disposed in the first end cap 2 and the second end cap 1 can be adjusted.

In an alternative embodiment, the air inlet 21 and the air outlet 11 are respectively provided in a plurality of numbers, along the axial direction H, the air inlet 21 and the air outlet 11 are respectively located at two sides of the rotor 42, an axis of the air inlet 21 is parallel to an axis of the rotor 42, and an axis of the air outlet 11 and the axis of the rotor 42 have a predetermined included angle therebetween. The air inlet 21 and the air outlet 11 which are arranged can be matched with the concave part 421 of the rotor 42, and in the process that air flows by the rotation of the concave part 421, the air inlet 21 and the air outlet 11 are arranged to ensure that the air speed of inlet air and outlet air is high, so that a good heat dissipation effect is ensured.

Optionally, the preset included angle is in the range of 30-60 °. Preferably, the preset included angle of the air outlet 11 is 45 °.

It is understood that the number of the air inlets 21 and the number of the air outlets 11 disposed in the first end cap 2 and the second end cap 1 may be the same or different, and is not limited in detail.

In an alternative embodiment, the euler equation may be used to correspondingly calculate the corresponding air inlet and outlet amount according to the concave portion 421 provided in the rotor 42, and then adaptively select the air inlet 21 and the air outlet 11 with appropriate diameters and numbers according to the air inlet and outlet amount. Or, on the basis, the number of the air ports is adaptively increased or reduced by combining the selected power of the motor 100. Preferably, the number of the air inlets 21 is 6, and the number of the air outlets 11 is 10.

Optionally, the axis of the rotor core 42a is taken as a center, the maximum diameter of the enclosure of the plurality of air inlets 21 is a first diameter, the minimum diameter of the enclosure of the plurality of air outlets 11 is a second diameter, the second diameter is larger than the first diameter, and the first diameter of the air inlets 21 matches or is close to the minimum diameter of the enclosure of the plurality of recesses 421, which is not limited specifically herein.

In an alternative embodiment, the air inlet 21 forms an air inlet channel 211 extending along the axial direction H on the first end cap 2, and the air outlet 11 forms an air outlet channel 111 inclined outward with a predetermined included angle relative to the axial direction H on the second end cap 1. The diameters of the positions in the air inlet channel 211 and the air outlet channel 111 may be the same or different, and the minimum diameters thereof are respectively used as the air inlet 21 and the air outlet 11, which is not specifically limited herein.

Optionally, the air conditioner further comprises a dust screen 5, and the dust screen 5 is respectively disposed at positions corresponding to the air inlet 21 and the air outlet 11. That is, the dust-proof net 5 may be disposed in the corresponding air inlet channel 211 and air outlet channel 111, or may be disposed outside the air inlet channel and the air outlet channel, as long as the dust-proof filtering function is achieved, and is not specifically limited herein. It is understood that other structures capable of having the dustproof filtering effect may be provided, and are not limited to the dustproof mesh 5. Alternatively, dust-proof and water-proof filters may be directly disposed outside the first end cap 2 and the second end cap 1, which will not be described in detail herein.

It should be noted that the rotor 42 and the motor 100 provided in the embodiment of the present application are not limited to be applied to the fields of industrial robots and aerospace, but may also be applied to other scenes that require setting of a transmission precision draft, a large speed ratio, a small volume, and a simple structure, for example, the fields of precision equipment manufacturing, and are not described again.

It should be noted that references in the specification to "one embodiment," "an example embodiment," "some embodiments," etc., indicate that the embodiment described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is within the knowledge of one skilled in the art to effect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described.

It should be readily understood that "on … …", "above … …" and "above … …" in this disclosure should be interpreted in the broadest manner such that "on … …" means not only "directly on something", but also "on something" with intermediate features or layers therebetween, and "above … …" or "above … …" includes not only the meaning of "above" or "above" something, but also the meaning of "above" or "above" without intermediate features or layers therebetween (i.e., directly on something).

Furthermore, spatially relative terms, such as "below," "lower," "above," "upper," and the like, may be used herein for ease of description to describe one element or feature's illustrated relationship to another element or feature. Spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. The device may have other orientations (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly as well.

It is noted that, in this document, relational terms such as "first" and "second," and the like, are used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. Also, the terms "comprises," "comprising," or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. Without further limitation, an element defined by the phrase "comprising a … …" does not exclude the presence of another identical element in a process, method, article, or apparatus that comprises the element.

Finally, it should be noted that: the above embodiments are only used for illustrating the technical solutions of the present application, and not for limiting the same; although the present application has been described in detail with reference to the foregoing embodiments, it should be understood by those of ordinary skill in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some or all of the technical features may be equivalently replaced; and the modifications or the substitutions do not make the essence of the corresponding technical solutions depart from the scope of the technical solutions of the embodiments of the present application.

Claims

1. A rotor for an electric machine, the rotor comprising:

2. The rotor as set forth in claim 1 wherein said depressions of either cross-section have the same profile along said direction of extension,

3. A rotor according to claim 2, wherein the preset maximum depth is not more than 1.5mm.

4. The rotor according to claim 2, wherein the first groove is an arc-shaped groove, the recessed portion forms a first recessed track and a second recessed track at two end positions of the outer side wall along the axial direction, the first recessed track has a first axis, the second recessed track has a second axis,

5. The rotor of claim 1, wherein the preset inclination angle ranges between 20 ° -28 °.

6. A rotor according to any one of claims 1-5, characterised in that the outer side wall is provided with a plurality of spaced apart second grooves extending through the outer side wall in the axial direction, the second grooves and the recesses being staggered.

7. The rotor of claim 6, wherein a minimum distance between an edge of the depression and an edge of the adjacent second groove in a circumferential direction of the outer sidewall is greater than 5mm.

8. The rotor according to claim 6, wherein the rotor core is further provided with a plurality of through holes arranged circumferentially, the through holes being opposed to the second grooves in a radial direction of the rotor core,

9. An electric machine, comprising:

the rotor according to any one of claims 1 to 8, which is fitted to an outer peripheral side of the rotor shaft.

10. The motor according to claim 9, wherein the air inlet and the air outlet are respectively provided in plural numbers, and the air inlet and the air outlet are respectively located on both sides of the rotor in the axial direction,

11. The electric machine of claim 10, wherein the predetermined included angle is in the range of 30 ° -60 °.

12. The electric machine of claim 10 wherein the fluid in the cavity between the rotor and the first end cap has a first pressure p1, the fluid in the cavity between the rotor and the second end cap has a second pressure p2, and the predetermined angle of inclination and the predetermined maximum depth of the recess are calculated according to the following equations:

W＝(p2-p1)/ρ+(c22-c12)/2；

EK＝mv2/2；

13. The motor of claim 10, further comprising dust screens, wherein the dust screens are respectively disposed at positions corresponding to the air inlet and the air outlet.