WO2007026047A1

WO2007026047A1 - Cooling system of an aggregate

Info

Publication number: WO2007026047A1
Application number: PCT/FI2006/000290
Authority: WO
Inventors: Ari Piispanen; Asko Parviainen
Original assignee: Axco-Motors
Priority date: 2005-08-31
Filing date: 2006-08-31
Publication date: 2007-03-08
Also published as: EP1920523A1; FI20050866A0

Abstract

The invention relates to a cooling system for an aggregate set, in which the aggregate set comprises an axial flux electric machine (1) with a disc-like rotor (2) and a disc-like stator (3) , and a screw compressor (4) with cooling. In accordance with the invention the electric machine (1) essentially comprises an airtight sealed housing (5) , inside which the rotor (2) and the stator (3) are placed. In addition, the housing is also arranged with a cooling duct (6) , the fluid circulation of which is common with the cooling of the screw compressor, and the cooling system comprises a heat exchanger (7) after the compressor to reduce the fluid temperature before it returns to the cooling duct of the housing of the electric machine.

Description

COOLING SYSTEM OF AN AGGREGATE FIELD OF THE INVENTION

The invention relates to a cooling system for an aggregate set as defined in the preamble of claim 1.

BACKGROUND OF THE INVENTION

Compressed air is nowadays extensively used in almost any industrial operation. Light, powerful, and easy-to-use pneumatic tools and actuators have been developed almost for all purposes. In industry, also a large proportion of various assembly lines and production processes operates either partly or completely on compressed air. Compressed air has several benefits compared for instance to electricity. It is fire-safe, it improves the air quality in the working environment, and it is almost risk-free compared to life-threatening electric shocks, and it is also easy and simple to install and operate. The only problems in the use of compressed air are caused by the compressor required in the generation of the compressed air. In large plants, compressed air can even be generated by compressors situated in separate buildings, thereby preventing problems. In smaller units and in individual targets instead, the compressors are relatively close to the working places. In that case, the amount of noise caused both .by the compressor and the electric motor driving it is far too large for a normal working environment. Therefore, at least efficient hearing protectors are required.

Noise problems can be reduced by enclosing both the compressor and the motor. However, together with the increased housing size, also the space requirement increases; in addition, the more massive is the housing, the more cooling is required, which in turn further increases the need for space. Consequently, silent enough air compressors with an electric motor drive are disproportionately large and expensive compared to their power output .

OBJECT OF THE INVENTION

It is an object of the present invention to eliminate the aforementioned drawbacks.

The object of the invention in particular is to introduce a novel aggregate set that generates compressed air, in other words, a combination or a set of an air compressor and an electric motor drive, which is compact and operates at as low noise level as possible.

SUMMARY OF THE INVENTION

The cooling system of a compressed air generator according to the invention is characterized in what will be presented in claim 1. The power unit of the compressed air aggregate set of the invention is an axial flux electric machine, equipped with a disc-like rotor and a disc-like stator. This way, an electric machine is achieved that is short in axial direction and also as a whole requires little room. Furthermore, the compressed air aggregate set has a screw compressor with cooling, which, depending on the purpose and the required characteristics, may be either dry-running or lubricated. According to the invention the electric machine essentially comprises an airtight sealed housing, in which the rotor and stator are placed. In addition, the housing is provided with a cooling duct encircling in the housing, arranged with a fluid circulation that is common with the cooling fluid circulation of the screw compressor. Further, according to the invention the cooling system includes a heat exchanger after the compressor to reduce the fluid temperature before it returns to the cooling duct of the housing of the electric machine.

The fluid circulation of the cooling duct is preferably arranged in series with the cooling of the screw compressor. The cooling duct is appropriately placed first in the flow direction, because the heat amount produced by the electric machine is clearly smaller than the heat amount produced by the compressor. Depending on the required fluid flow rates it is also possible that a part of the fluid is led past the cooling duct directly to the cooling of the compressor. In an embodiment of the invention the cooling duct and the cooling of the screw compressor are connected in parallel.

The rotor of the electric motor and the rotor of the screw compressor are coupled on the common shaft to rotate simultaneously. They thereby form a rigid unit, and thus the rotation speed of the electric motor is always the same as the rotation speed of the rotor of the screw compressor. In that case the housing of the electric motor and the outer casing of the compressor are preferably attached to each other and they form a single and uniform unit, in which inside the same casing and on the common shaft the electric machine and the compressor are operating, being both however placed in rooms tightly sealed from each other. It is however also possible that the electric motor and the screw compressor have separate shafts with a coupling between them to enable connecting the shafts to rotate simultaneously.

The housing of the electric machine is preferably cylindrical and it comprises round end casings and a peripheral casing interconnecting the end casings from their outer edges. Although the cooling ducts may be located at different sides of the housing, it is yet preferable that the cooling duct is either fully or at least to a significant degree located in the peripheral casing. In that case the peripheral casing may be a two-layer structure inside which the cooling duct is arranged. In order to efficiently transfer the heat generated by the electric machine to the fluid circulation, the inner surface of the housing, preferably the peripheral casing, can be provided with finning or other surface profile that increases the heat-exchange surface.

As an alternative to the cooling duct and the finning, the heat exchange can be arranged also by separate heat exchange tubes or heat exchange ducts placed inside the housing. They may be attached, such as welded to the inner surfaces of the housing, or they may be placed on appropriate support members in the space delimited by the housing at a sufficient, safe distance from rotating parts.

When the rotor and stator are essentially in an airtight sealed housing, they are surrounded by air to which the heat from the rotor and stator transfers. The rotating rotor itself causes air flow and thus intensifies the heat transfer through the housing to the fluid or to the fluid in the ducts situated in the housing. However, fins or other air guides that form a centrifugal blower are preferably arranged on the surface of the disc-like rotor surface. This way air is efficiently made to circulate inside the housing. The fins are preferably placed on the rotor surface that is against the stator so that a strong centrifugal air flow in the gap between the stator and rotor cools them both simultaneously and equally efficiently.

The cooling fluid to be used can always be selected according to the purpose and needs. The fluid is preferably oil, but also air, water, and various other liquids are possible. In particular when using oil as the fluid, an oil -lubricated compressor can be used, and the cooling can be implemented for the rotor of the compressor with same oil by oil mist cooling. This way, a lower noise level is achieved, because a lubricated compressor is known to be quieter due to the sealing tightness and high efficiency. However, it is also possible to use a dry-running compressor in the invention, in which case the cooling fluid circulation can be arranged inside the rotor and/or in the casing enclosing the rotor.

In an embodiment of the invention, a frequency converter is arranged in the same cooling fluid circulation between the heat exchanger and the electric machine. With the frequency converter, the rotation speed of the electric machine can be regulated. Thus the component that generates the least amount of heat is placed first in the flow direction in the cooling circulation and the component generating the most heat is placed last.

In an embodiment of the invention the housing is cylindrical, in which case it comprises end casings and a peripheral casing. Consequently, the cooling duct for the implementation of the cooling fluid circulation can be arranged as a tube system encircling the inner surface of the peripheral casing.

The aggregate set comprising an electric machine and an air compressor according to the invention has significant benefits compared to the known technology. The aggregate set can be made a very small-size and compact unit, the noise level of which is in a class of its own compared to other known corresponding set-ups. Hence the invention enables a more flexible and easier use of compressed air in various targets without the need of other noise control structures and equipment, such as hearing protectors. BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in greater detail by means of an example with reference to the attached drawing, in which an aggregate set of the invention is presented both diagrammatically and partly as a cut-away drawing.

DETAILED DESCRIPTION OF THE INVENTION

The aggregate set of the invention illustrated in the drawing comprises an axial flux electric motor 1 operating as a power unit, which is arranged to rotate the screw compressor 4 to generate compressed air. The screw compressor comprises, in a way known per se, a rotor 8 that is provided with helical slotting and placed inside the casing 10. The rotor is supported to the power shaft 9, which is common with the electric motor 1. In addition, the compressor comprises an air inlet 16, a compressed air outlet 17, and a shot lubrication 18, which lubricates the compressor rotor, simultaneously sealing its mating surfaces against the other corresponding rotor (not shown in the figure) .

The electric motor 1 comprises a cylindrical and essentially airtight sealed housing 5, which is limited from the sides by end casings 11 and from the periphery by a peripheral casing 12. The peripheral casing is a two-layer structure so that inside the peripheral casing, there is a cooling duct 6 enclosing the motor. On the inner surface of the peripheral casing, there is also a heat exchange finning 13. The motor comprises inside the housing a disc-like stationary stator 3 supported to the housing 5 and the also disc-like rotating rotor 2, which is supported to the same shaft 9 with the rotor 8 of the compressor 4 to rotate simultaneously. The drawing illustrates in the inner surface of the lower part of the peripheral casing 12 a group of cooling ducts, that is, fin tubes 22, by which the cooling duct 6 and the heat exchange finning 13 can be replaced. This way, the cooling fluid circulation can be implemented simply and easily inside the housing. Further, the housing structure can be made simpler and thus also cheaper to manufacture.

Between the disc-like rotor and stator there is an air slot 19, into which the fins 14 of the centrifugal blower are arranged attached to the surface of the rotor 2. In addition, the equipment includes an oil circulation 20, in which the oil circulates from the housing 5 of the electric motor to the compressor 4 and from there further to the oil cooler 7, from which the oil circulates back through the frequency converter 15 to the electric motor.

The equipment operates as follows. The frequency converter 15 controls the electric motor 1 at a desired rotation speed. The control causes slight generation of heat in the frequency converter; the heat is transferred from the frequency converter by the oil circulation 20. As the electric motor 1 rotates, thermal losses occur also in the motor both in the rotor and in the stator. These losses are transferred to the ambient air. This heat transfer is intensified by the fins 14 on the rotor surface, which rotate the air strongly around the rotor and stator in the manner of a centrifugal blower. The heated air flows on the inner surface of the peripheral casing 12 around the finning 13 , and thus the heat transfers via the finning and through the peripheral casing to the cooling duct 6 that encircles inside the casing; in the cooling duct, there flows oil that is only slightly warmed up in the frequency converter.

The circulating oil that has further warmed up is led from the cooling duct 6 through the connecting duct 21 to the compressor 4. In the compressor the oil is led by shot lubrication 18 onto the surface of the rotor 8 of the compressor, the compressor rotor rotating on the same shaft 9 with the rotor 2 of the electric motor. The oil heated up in the compressor is separated from the compressed air flow and led further to the heat exchanger 7 , in which it is cooled. Essential for the equipment is that it is a compact and encapsulated structure, in which the heat flows are in control. The electric motor being completely encapsulated, it does not cause air flows outwards from the structure, and the structure is therefore almost completely silent. Furthermore, the sealed and uniform structure based on the common shaft reduces the need for bearings and the bearing noise.

The invention is not restricted to the embodiments described above as examples, but many variations are possible within the scope of the inventive idea defined by the claims.

Claims

1. A cooling system for the aggregate set comprising an axial flux electric machine (1) , with a disc-like rotor (2) and a disc-like stator (3) , and a screw compressor (4) with cooling, c h a r a c t e r i z e d in that the axial flux electric machine (1) comprises an essentially airtight sealed housing (5) , inside which the rotor (2) and the stator

(3) are placed; the housing is arranged with a cooling duct (6) , the fluid circulation of which is common with the cooling of the screw compressor; and that the cooling system comprises a heat exchanger (7) placed after the compressor to reduce the fluid temperature before it returns to the cooling duct of the housing of the electric machine.

2. A cooling system as defined in claim 1, charact eri zed in that the fluid circulation of the cooling duct (6) and the cooling of the screw compressor are arranged in series, the cooling duct (6) being placed first in the flow direction.

3. A cooling system as defined in claim 2, charac t e ri z ed in that a by-pass duct has been arranged past the cooling duct (6) so that only a part of the fluid circulation passes through the cooling duct.

4. A cooling system as defined in claim 1, charac teri zed in that the fluid circulation of the cooling duct (6) and the cooling of the screw compressor are arranged in parallel .

5. A cooling system as defined in claim 1, charac t eri zed in that the rotor (2) of the electric machine (1) and the rotor (8) of the compressor (4) are coupled on the same shaft to rotate simultaneously.

6. A cooling system as defined in claim 1, charac teri zed in that the housing (5) of the electric machine (1) and the casing (10) of the compressor (4) are connected together to form a uniform unit.

7. A cooling system as defined in any of claims 1-6, charact eri zed in that the housing (5) is cylindrical and comprises end casings (11) and a peripheral casing (12) , inside which the cooling duct (6) has been arranged.

8. A cooling system as defined in claim 7, charact eri zed in that the inner surface of the peripheral casing (12) has a finning (13) to intensify the heat transfer between the air surrounding the rotor (2) and the stator (3) and the fluid circulating in the cooling duct.

9. A cooling system as defined in any of claims 1-8, charac t eri zed in that the rotor

(2) comprises fins (14) or corresponding air guides to arrange the centrifugal air flow in the housing (5) .

10. A cooling system as defined in any of claims 1-9, charact eri zed in that the fluid is oil.

11. A cooling system as defined in claim 10, charact eri zed in that the oil cooling has been arranged as oil spray to the rotor (8) of the compressor (4) .

12. A cooling system as defined in any of claims 1-11, characteri zed in that the fluid circulation comprises a frequency converter (15) between the heat exchanger (7) and the electric machine (1) .

13. A cooling system as defined in any of claims 1-6, characteri zed in that the housing (5) is cylindrical and it comprises end casings (11) and a peripheral casing (12) , in which case the cooling duct (22) has been arranged as a tube system encircling the inner surface of the peripheral casing.