Stator cooling system for cooling of the stator in an electrical motor

Abstract

The invention relates to a stator cooling system for cooling of the stator in an electrical motor, said cooling system comprising an electrical motor; a stator; windings and one or more aluminium cooling pipes inside the stator of the motor, characterized in that the aluminium cooling pipes are insulated to the bores of the stator of the motor by an anodized oxide layer for preventing currents, caused by alternating magnetic flux in the stator. This makes it possible to arrange the cooling pipes proximal to the windings, in alternating magnetic fields near the windings, without additional losses, which would be caused by inducted currents. The smaller distance of the cooling pipes to the windings decreases the winding temperature, respectively the equivalent motor power and the equivalent winding temperature can be achieved by a smaller motor.

Description

[0001] Stator cooling system for cooling of the stator in an electrical motor

[0002] FIELD OF THE INVENTION

[0003] The invention relates to a stator cooling system and a method for cooling of the stator in an electrical motor.

[0004] BACKGROUND OF THE INVENTION

[0005] By improving the cooling of an electrical motor of a vertical mill drive, the motor volume can be decreased without increasing the motor temperature at the equivalent motor torque. Liquid cooling fluids have a significant bigger heat capacity than gas fluids. But the cooling channels of liquid fluids preferably are tightened by pipes. Metallic pipes in the stator core are a kind of winding, if those are electrical connected to the core sheets and currents would be inducted into those by the alternating magnetic field of polyphase motors. The thermal resistance of electrically non-conducting and non-metallic pipes is too high, for an effective cooling.

[0006] Reducing the volume and the mass of electrical motors will decrease the costs of those machines and decreases the necessary space of installation of those machines. The axial dimension of those machines will be decreased also, which is relevant for the arrangement of a vertical roller mill in 0. Decreasing the weight of electrical polyphase machines having a smaller volume provides the consumer with a useful or commercial choice.

[0007] The prior state is to arrange the metallic pipes of cooling fluids in the back iron of the stator, where the alternating magnetic flux is smaller than near the stator windings. The stator core near the winding is the yoke where the magnetic flux is conducted between the stator teeth. Only if this area is saturated, a part of the magnetic flux is conducted at the stator back iron, where are the metallic pipes in accordance with prior art are arranged, as in US 2023 / 02008255 Al. Unfortunately, dynamo-electric steel sheets have a lower heat conductivity than constructional steel, and the distance between the area near the windings and the back iron of the stator causes a significant temperature difference and causes higher winding temperatures. OBJECT OF THE INVENTION

[0008] It is an object of the present invention to overcome or at least alleviate one or more of the above problems of the prior art and / or provide the consumer with a useful or commercial choice.

[0009] It is an object of the present invention to provide a stator cooling system and a method for cooling of stators in an electrical motor.

[0010] It is an object of the present invention to arrange metallic pipes in the stator core of electrical motors near the windings with a high electrical resistance between the metallic pipes and the core sheets of the stator but having a low thermal resistance between those. It is an object of the present invention to fix the pressure of the stator core package by the insulated cooling pipes. Because the cooling pipes can be arranged more centric, nearer to the stator teeth than in the arrangement in the back-region of the stator, which is the prior art (e.g., 2023 / 02008255 Al). By this the critical area of the stator teeth has a higher axial pressure and the risk of broken teeth of the thin stator core sheets decreases, which could be caused by dynamic magnetic forces.

[0011] For preventing a damage of the heavy-duty gearbox in case of liquid leakage by water into the oil circuit, the object is to use the lubrication of gearbox oil as cooling fluid. The objective is to achieve swirling oil flow for a better heat transfer between the oil and the wall of the cooling pipe, because a sufficient convection at the pipe wall caused by a turbulent oil flow is hard to achieve, without such swirling. Different to water as cooling fluid, the kinematic viscosity of the lubrication oil is more than factor 500 bigger.

[0012] It is an object of the present invention to decrease volume of the electrical motor of a vertical roller mill. The torque of motors is proportional to the current of the stator windings, and the resistive losses in the windings increasing quadratic to the current. The limit of the torque and of the current is where the temperature, which is caused by the resistive losses, exceeds the temperature limit of the winding insulation. For equivalent losses, the winding temperature decreases by the improvement of the cooling, and by this the motor volume can be decreased.

[0013] It is a further object of the present invention to provide an alternative to the prior art.

[0014] SUMMARY OF THE INVENTION

[0015] In a first aspect, the invention relates to a stator cooling system for cooling of a stator in an electrical motor. The cooling system comprises an electrical motor; a stator; windings, and one or more aluminium cooling pipes inside the stator of the motor. The aluminium cooling pipes are insulated to the bores of the stator of the motor by an anodized oxide layer for preventing currents, caused by alternating magnetic flux in the stator, so that the cooling pipes can be placed proximal to the windings, which causes a smaller heat resistance between winding and cooling pipe, and by this lower winding temperatures, and the structural axial fixation of the stator core sheets at the critical region of the stator teeth is improved.

[0016] The invention relates to metallic cooling pipes for liquid fluids inside the stator of the electrical motor of a vertical roller mill, which have a low thermal resistance to the stator core sheets, and which have a significant high electrical resistance to the stator core sheets. Additionally, the cooling pipes are electrically insulated to each other and to the supply channels of the liquid cooling fluid. The current inside the liquid cooling fluid is prevented by using non-conductive oil or deionized water.

[0017] The axial fixing of the laminated stator core is improved, especially in the region of the stator teeth, compared to the prior art, where such pipes must be arranged in the back-iron of the stator.

[0018] The helix channels in the cooling fluid channels enable the use of the high viscosity lubrication oil of the gearbox as cooling fluid, still having a sufficient low heat transfer resistance between the wall of the cooling pipe and the cooling fluid.

[0019] An airgap between the cooling pipe and stator core, which is necessary for inserting the cooling pipe into the stator bore, is preferably eliminated by radial plastic deformation of the cooling pipe by a hydraulic pressure inside the fluid channel of an incompressible liquid fluid, to eliminate the enormous heat resistance of such an air gap.

[0020] In another embodiment of the present invention are tools for tightening the hydraulic pressure during assembly which are fixed to each other by a threaded rod and a hydraulic nut pressing the laminated stator core before it is fixed by the radial plastic deformation of the cooling pipes, to achieve a structural stiffness for the avoidance of the vibration of the thin core sheets, caused by dynamic magnetic forces, especially at the stator teeth. The cooling pipes are preferably connected to a fluid inlet and fluid outlet by an adapter plate. The adapter plate may be insulated by the anodized oxide layer, preventing that the cooling pipes are connected to a short circuit winding.

[0021] A cooling fluid flow of the fluid inlet of a first cooling pipe may be rerouted to a second pipe. The second pipe is preferably connected to the fluid outlet, by the rerouting element.

[0022] In another embodiment of the present invention, a double helix element inside the fluid channel is preferably made of an electrical non-conductive thermoplastic, causes a swirled flow of high viscose oil, which is electrically non-conductive, and which causes a velocity component orthogonal to cooling fins at the inner wall of the cooling pipe, for decreasing the thermal resistance between the cooling fluid and the wall of the cooling pipe. By this measure, electrically non-conductive oil can be used as cooling fluid, which has a significant higher viscosity than water, achieving turbulences in the oil, which occurs in water without this measure.

[0023] In a second aspect, the invention relates to a vertical roller mill comprising, one or more rollers and a rotating grinding table, a gearbox with an integrated electrical motor and further comprising a stator cooling system according to any of the above-mentioned embodiments.

[0024] In a third aspect, the invention relates to a method for cooling a stator in an electrical motor comprising a stator and windings. The method comprises the steps of arranging one or more aluminium cooling pipes inside the stator of the motor and arranging the cooling pipes to be insulated to a bore of the stator of the motor by an anodized oxide layer for preventing currents, caused by the alternating magnetic flux in the stator.

[0025] In a fourth aspect, the invention relates to the use of a stator cooling system, according to any of the above-mentioned embodiments, in a mill.

[0026] The first, second, third and fourth aspects of the invention may be combined.

[0027] BRIEF DESCRIPTION OF THE FIGURES

[0028] The figures show one way of implementing the present invention and is not to be construed as being limiting to other possible embodiments falling within the scope of the attached claim set.

[0029] Figure 1 schematically illustrates a vertical roller mill with gearbox and integrated electrical motor inside the gearbox. Figure 2 schematically illustrates an anodized oxide layer 1 between the aluminium cooling pipe 2 and the dynamo-electric stator core sheet 3. 5 is the fluid channel. The anodized oxide layer 1 is drawn in non-scale, because the real thickness of typically 15 pm would be the thickness of the drawing lines.

[0030] Figure 3 schematically illustrates a cooling pipe after inserting it into the core sheets, with a gap 4 between the core and the pipe. The gap 4 between the pipe and the core sheet is drawn in nonscale.

[0031] Technical approved is a difference between the diameter of the bore and of the pipe of 0.1 mm, for manufacturer tolerances reasons.

[0032] Figure 4 schematically illustrates dimensions of the cross section of an example of 2.

[0033] Figure 5 schematically illustrates Tools (6, 7) for tightening the aluminium pipes 2 during the plastic deformation of 2 by hydraulic pressure.

[0034] Figure 6 schematically illustrates inlet 19 and outlet 20 of the cooling fluid.

[0035] Figure 7 schematically illustrates rerouting element 22 and helix channels 24.

[0036] Figure 8 schematically illustrates double helix element 25 made of electrical nonthermoplastic.

[0037] DETAILED DESCRIPTION OF THE INVENTION

[0038] In a preferred embodiment of the present invention, the stator cooling system for cooling of stators in an electrical motor, comprises: an electrical motor 28, a stator 3, windings 18 and one or more aluminium cooling pipes 2 inside the stator 3 of the motor 28.

[0039] The aluminium cooling pipes 2 are insulated to a bore of the stator 3 of the motor 28 by an anodized oxide layer 1 for preventing currents, caused by alternating magnetic flux in the stator, so that the cooling pipes 2 can be placed proximal to the windings 18, which causes a smaller heat resistance between winding and cooling pipe, and by this lower winding temperatures, and the structural axial fixation of the stator core sheets at the critical region of the stator teeth is improved.

[0040] An airgap 4 between pipe and stator core, which is necessary for inserting the cooling pipe into the stator bore, is eliminated by radial plastic deformation of the cooling pipe by a hydraulic pressure inside the fluid channel 5 of an incompressible liquid fluid. Tools 6 and 7 of tightening the hydraulic pressure during assembly are fixed to each other by a threaded rod 12 and a hydraulic nut 13 pressing the laminated stator core before it is fixed by the radial plastic deformation of the cooling pipes 2 in the stator bores.

[0041] The cooling pipes 2 are connected to a fluid inlet 19 and fluid outlet 20 by an adapter plate 21. The adapter plate 21 is insulated by the anodized oxide layer 1, preventing that the cooling pipes are connected to a short circuit winding.

[0042] A cooling fluid flow of the fluid inlet 19 of a first cooling pipe is rerouted to a second pipe. The second pipe is connected to the fluid outlet 20, by the rerouting element 22.

[0043] A double helix element 25 inside the fluid channel 5 is made of an electrical non- conductive thermoplastic, causing a swirled flow of high viscose oil, which is electrically non-conductive, and which causes a velocity component orthogonal to cooling fins 29 at the inner wall of the cooling pipe 2, for decreasing the thermal resistance between the cooling fluid and the wall of the cooling pipe 2.

[0044] The first key factor of the invention is an electrical non-conductive anodized oxide layer 1 at the outer surface of an aluminium pipe 2, which is pressed into the bore of the dynamo-electric stator core sheets 3, see 0. The thickness of the anodized oxide layer 1 (15 pm) is drawn in non-scale in 0 and 0.

[0045] The second key factor is the radial plastic deformation of the aluminium pipe 2, to press it into the bore of the dynamo-electric core sheets 3 by an incompressible liquid fluid inside the fluid channel 3 under high pressure, to eliminate the airgap 4 (0.05 mm drawn in nonscale in 0). The airgap 4 is necessary for inserting the aluminium pipe 2 into the bore, because the bore must have a bigger diameter than the pipes, otherwise the pipe would not fit into the bore, because of the manufacturing tolerances of the core sheets and of the stacking tolerances of the core.

[0046] The airgap 4 would be a non-acceptable heat resistance, caused by convection at the airgap 4.

[0047] The pressure inside the fluid channel 5, which is necessary for the plastic deformation, would destroy the aluminium pipes at sections, which are outside the bore of the stator core 9, without the support of the core material at the outer circumference of the cooling pipe. For this reason, the length of the aluminium cooling pipe 2 is limited to the axial length of the bore of the stator plus the length of the two bores of the two tools 6 and 7 (see 0) which are used to tighten the fluid channel at the two ends of 2 during the hydraulic pressure is supplied into the connection for hydraulic pressure supply 8 in the tool 6.

[0048] The tools 6 and 7 in 0 are tightened and fill six aluminium cooling pipes at the same time. The connection 8 for the high-pressure water pump is arranged at the geodetical lowest level. During filling the channels with water, the compressible air is released at the vent screw 15. After the air is vented, the vent screw 15 is closed and the pressure inside the pipes rises. The hydraulic nuts pressing the stator core between the tools 6 and 7 before the aluminium cooling pipes 2 are expanded radial into the stator bores, because the hydraulic nuts are connected to the pressure volume, see 0. The two tools 6 and 7 are linked to each other by three threaded rods 12. The hydraulic nuts 15 are connected to the hydraulic pressure by 16. After the aluminium pipes are expanded into radial direction against the bores of the stator, the pressed stator core is fixed by the aluminium pipes, after the pressure is removed from the hydraulic nuts.

[0049] The cross section of a manufactured example of the aluminium cooling pipe 2 is drawn in 0. A numerical elastic-plastic simulation of subjecting the pipe with an inner pressure below 500 MPa has the result that there is still a gap between the pipe and the bore of the stator after the pressure is removed. An inner pressure above 500 MPa gives a plastic deformation of the cooling pipe plus an elastic deformation of the bores of the stator, which ensure to have no gap between stator and pipe after the inner pressure and the elastic deformation of the pipe is removed. Praxis test with a sample an cutting the sample after the plastic deformation have confirmed this result.

[0050] After the radial plastic deformation, the aluminium cooling pipes are in direct thermal contact with the stator bores and additionally fixing the pressure of the stator core, without a high convective heat resistance of any airgap. The anodized oxide layer 1 prevents electrical current flow from the pipes to the dynamo electric stator core sheets 3. After removing the tools 6 and 7, the cooling pipes are connected to the cooling fluid supply by the arrangement which is illustrated in 0 with the inlet 19 and the outlet 20 of the cooling fluid supply. The anodized oxide layer 1 prevents the current flow to the cooling fluid adapter plate 21 also. This insulation prevents that the cooling pipes become short circuit windings, driven by the alternating magnetic fields between the cooling pipes. On the axially opposite stator side to the inlet 19 and outlet 20, the cooling fluid flow of the cooling pipe of the inlet flow 19 is rerouted to the cooling pipe, which is connected to the outlet 20 by the rerouting element 22, see 0.

[0051] The current flow inside the cooling fluid must be prevented also. If the cooling fluid is water, the water must be non-ionic, otherwise unwanted water electrolysis would be possible. Alternatively to the high effort for the water preparation, oil can be used, which is quasi electrical non-conductive. The kinematic viscosity of the lubrication oil of the gearbox is more than factor 500 bigger than of water. Turbulent flow at the surfaces in the fluid channel 5 would ensure a good heat transfer. The higher the viscosity of the cooling fluid is, the higher the pressure must be to achieve turbulent flow. Higher fluid pressure and volume rate cause a higher consumption of pump energy. To achieve turbulent flow with the lubrication oil of heavy-duty gearboxes of mill drives is not feasible for economical reasons. For these reasons, the sufficient heat transfer is achieved by a different method. The oil flow is swirled in a double helix channel 24 (0). The double helix elements 25 (0) are inserted into the aluminium pipes, and the inner wall of those pipes have cooling fins 29. The tangential velocity component, generated by the helix channels, cause turbulences at the edges of those cooling fins 29. The mixing of the oil and the turbulences increases the heat transfer between the wall of the pipe and the oil. The cooling fins 29 inside the cooling pipes additionally increase the heat transfer surface. The aluminium pipes with those inner cooling fins 29 are manufactured by extrusion moulding.

[0052] The stator cooling system, according to any of the preceding embodiments, may be arranged in a vertical roller mill. The vertical roller mill comprises one or more rollers and a rotating grinding table 26 and a gearbox with an integrated electrical motor 28.

[0053] The invention also relates to a method for cooling a stator in an electrical motor 28 comprising a stator 3 and windings 18. The method comprises the steps of arranging one or more aluminium cooling pipes 2 inside the stator 3 of the motor 28 and arranging the cooling pipes 2 to be insulated to a bore of the stator 3 of the motor 28 by an anodized oxide layer 1 for preventing currents, caused by the alternating magnetic flux in the stator.

[0054] The stator cooling system, according to any of the preceding embodiments, may be used in a mill. Although the present invention has been described in connection with the specified embodiments, it should not be construed as being in any way limited to the presented examples. It should also be understood that the form of this invention as shown is merely a preferred embodiment. Various changes may be made in the function and arrangement of parts; equivalent means may be substituted for those illustrated and described; and certain features may be used independently from others without departing from the spirit and scope of the invention as defined in the following claims.

[0055] References:

[0056] (1) Anodized oxide layer

[0057] (2) Aluminium pipe

[0058] (3) Dynamo electric stator core sheet

[0059] (4) Airgap between pipe and stator core

[0060] (5) Fluid channel

[0061] (6) Tool for tightening the fluid channel at the first end of the aluminium cooling pipe

[0062] 2, with connection for hydraulic pressure.

[0063] (7) Tool for tightening the fluid channel at the second end of the aluminium cooling pipe 2.

[0064] (8) Connection for hydraulic pressure supply

[0065] (9) Stator core, made of stator core sheets 3.

[0066] (10) Pressure plates of 9.

[0067] (11) Motor housing

[0068] (12) Threaded rod

[0069] (13) Hydraulic nut

[0070] (14) Centring pin

[0071] (15) Vent screw

[0072] (16) Connection of the hydraulic pressure to 13

[0073] (17) Sealing

[0074] (18) Winding coil

[0075] (19) Fluid inlet

[0076] (20) Fluid outlet

[0077] (21) Adapter plate (22) Rerouting element

[0078] (23) Flow of the cooling fluid

[0079] (24) Channels of the double helices’ elements

[0080] (25) Double helix element

[0081] (26) Rollers and rotating grinding table

[0082] (27) Gearbox with an integrated electrical motor

[0083] (28) Electrical motor

[0084] (29) Cooling fins

Claims

Claims:

1. A stator cooling system for cooling of a stator in an electrical motor, said cooling system comprising an electrical motor (28); a stator (3); windings (18) and one or more aluminium cooling pipes (2) inside the stator (3) of the motor(28), characterized in that the aluminium cooling pipes (2) are insulated to a bore of the stator (3) of the motor (28) by an anodized oxide layer (1) for preventing currents, caused by alternating magnetic flux in the stator, so that the cooling pipes (2) can be placed proximal to the windings (18).

2. A stator cooling system according to claim 1, wherein an airgap (4) between pipe and stator core is eliminated by radial plastic deformation of the cooling pipe (2) by a hydraulic pressure inside a channel (5) of an incompressible liquid fluid.

3. A stator cooling system according to claims 1 or 2, wherein tools (6) and (7) of tightening the hydraulic pressure during assembly are fixed to each other by a threaded rod (12) and a hydraulic nut (13) pressing the laminated stator core before it is fixed by the radial plastic deformation of the cooling pipes (2).

4. A stator cooling system according to any of the preceding claims, wherein the cooling pipes (2) are connected to a fluid inlet (19) and fluid outlet (20) by an adapter plate (21), the adapter plate (21) is insulated by the anodized oxide layer (1), preventing that the cooling pipes are connected to a short circuit winding, and the avoidance of losses by electrical currents.

5. A stator cooling system according to claim 4, wherein a cooling fluid flow of the fluid inlet (19) of a first cooling pipe is rerouted to a second pipe, the second pipe is connected to the fluid outlet (20), by the rerouting element (22).

6. A stator cooling system according to claims 2-5, wherein a double helix element (25) inside the fluid channel (5), is made of an electrical non-conductive thermoplastic, causes a swirled flow of high viscose oil, which is electrically non- conductive, and which causes a velocity component orthogonal to cooling fins(29) at the inner wall of the cooling pipe (2), for decreasing the thermal resistance between the cooling fluid and the wall of the cooling pipe (2), for decreasing the winding temperature.

7. A vertical roller mill comprising: one or more rollers and a rotating grinding table (26),a gearbox with an integrated electrical motor (28) and further comprising a stator cooling system according to any of the preceding claims.

8. A method for cooling a stator in an electrical motor (28) comprising a stator (3) and windings (18); said method comprising the steps of arranging one or more aluminium cooling pipes (2) inside the stator (3) of the motor (28), and arranging said cooling pipes (2) to be insulated to the a bore of the stator (3) of the motor (28) by an anodized oxide layer (1) for preventing currents, caused by the alternating magnetic flux in the stator.

9. Use of a stator cooling system, according to any of the preceding claims, in a mill.

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