AU2018200255B2

AU2018200255B2 - Slip ring asynchronous electric motor

Info

Publication number: AU2018200255B2
Application number: AU2018200255A
Authority: AU
Inventors: Albert Kohen
Original assignee: Gino Aka
Current assignee: Gino Aka
Priority date: 2017-01-12
Filing date: 2018-01-12
Publication date: 2022-07-14
Anticipated expiration: 2038-01-12
Also published as: FR3061820A1; AU2018200255A1; FR3061820B1; ZA201800174B

Abstract

Slip ring asynchronous electric motor" ABSTRACT Slip ring asynchronous electric motor comprising, on the rotor, for each phase, a variable starting assistance resistance consisting of an electrolyte in which are placed two electrodes (13, 14), of which at least one (14) is mobile, characterized in that each of the electrodes (13, 14) comprises relief elements (15, 16; 17, 18) of triangular section. Figure for the abstract: Figure 4 3/3 29 24 33 42, ~36 '3 Ch -I-o

Description

3/3

29

24 33

42, ~36 '3

Ch -I-o

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"Slip ring asynchronous electric motor"

FIELD OF THE INVENTION The invention relates to a slip ring asynchronous electric motor, of use in particular in the mining or cementing industry. This industry uses, in particular for ball mills, motors of high power (several MW). To ensure the starting thereof, these motors require the insertion of a three-phase resistance on the rotor. The initial value of this resistance determines the starting torque. The starting sequence lasts a few tens of seconds during which the resistance on the rotor is progressively reduced to a final value close to zero, before being short-circuited. PRIOR ART Any reference herein to known prior art does not, unless the contrary indication appears, constitute an admission that such prior art is commonly known by those skilled in the art to which the invention relates, at the priority date of this application. The patent GB 131 590 describes a control device for a lifting apparatus motor, using electrolyte-based variable resistances. A first resistance consists of an electrolyte placed in a cylinder, a fixed electrode constituting all or part of the cylinder, the other electrode being cylindrical and displaced in said cylinder. A second resistance consists of an electrolyte placed in a cylinder, a fixed electrode being placed at the bottom of the cylinder, the other electrode being displaced in said cylinder. At the end of movement, the resistance must be short-circuited and the electrodes are brought into contact. For the contact to be more effective, provision is made for the electrodes to be conformed with bearing surface elements of conical form. The patent US 2 719 256 describes a device for controlling the starting of a slip ring asynchronous electric motor for the mining industry, with, between the rotor and the ground, variable resistances. For each phase of the rotor, the variable resistance consists of an electrolyte placed in a vertical cylinder, between a fixed electrode at the bottom of the cylinder, and a mobile electrode. The mobile electrode is displaced

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between an initial position at the top of the cylinder and a final position at the bottom of the cylinder, close to the fixed electrode. In this document, the electrodes consist of substantially identical discs. The resistance R between the electrodes is approximated by the formula: R = p.d/S p is the resistivity of the electrolyte, d the distance between the electrodes and S the facing surface area of the electrodes. To ensure the starting of the motor, the initial resistance Ri has to be great, and the mobile electrode is placed at the top of the cylinder. At the end of starting, the resistance Rf has to be as low as possible and the mobile electrode is brought closer to the fixed electrode, at the bottom of the cylinder. To improve the Ri/Rf ratio, the manufacturers have used not only the variation of d, but also the variation of S. To this end, the fixed and mobile electrodes have been each constructed in the form of coaxial cylinders. Thus, at the end of the descent of the mobile electrode, the cylinders of the mobile electrode are inserted between the cylinders of the fixed electrode and the surface area S to be taken into account for the evaluation of the final resistance Rf is the facing surface area of the electrodes. In this embodiment, where the final distance between the cylinders of the two electrodes is approximately 15 mm, it is difficult to achieve the value of 100 for the ratio Ri/Rf. AIMS AND SUMMARY One aim of the invention is to propose a slip ring asynchronous electric motor for the mining industry, equipped with electrolyte-based variable resistances, exhibiting a ratio Ri/Rf significantly higher than 100. Another aim of the invention is to propose a slip ring asynchronous electric motor for the mining industry, equipped with electrolyte-based variable resistances, comprising provisions for augmenting the value of the ratio Ri/Rf and the operational safety of the motor starting control device. The subject of the invention is a slip ring asynchronous electric motor comprising, on the rotor, for each phase, a variable starting assistance resistance consisting of an electrolyte in which are placed two

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electrodes, of which at least one is mobile, characterized in that each of the electrodes comprises relief elements of triangular section. Preferably, the triangular section of the relief elements is an isosceles triangle whose vertex angle lies between 3 and 150. Advantageously, the relief elements of the electrodes have interpenetrating forms. According to an embodiment, the relief elements of the electrodes are prisms. According to an embodiment, the relief elements of the electrodes are coaxial rings. Advantageously, the radii of the coaxial rings of each electrode differ by a constant pitch. Advantageously, the radii of the coaxial rings of the mobile electrode differ from the radii of the coaxial rings of the fixed electrode by a half-pitch. According to an embodiment, the fixed and mobile electrodes are arranged in a cylindrical insulating tank of a diameter close to that of the electrodes. Preferably, the insulating tank is vertical, the fixed electrode is at the bottom of the insulating tank and the mobile electrode is displaced in the insulating tank from a high position in which its lateral surface is partly covered by the insulating tank. According to an embodiment, a pump ensures the forced circulation of the electrolyte in the insulating tank. Advantageously, the height of the insulating tank is around 750 mm. Advantageously, the travel of the mobile electrode is around 600 mm. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a simplified representation of an electrolyte-based resistance for an asynchronous motor of the prior art; Figure 2 is a representation in schematic cross section of cylinder electrodes of an asynchronous motor of the prior art, after reciprocal insertion;

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Figure 3 is a schematic view of electrodes according to an embodiment of the invention, in the close position; Figure 4 is a schematic view of the electrodes of Figure 3 during reciprocal insertion; Figure 5 is a view in radial cross section of the fixed electrode of an asynchronous motor according to an embodiment of the invention; Figure 6 is a plan view of the electrode of Figure 5; Figure 7 is a view in radial cross section of two electrodes in the final position of insertion according to an embodiment of the invention; Figure 8 is a schematic view in cross section of an electrolyte based resistance according to an embodiment of the invention; Figure 9 is a perspective view of two electrodes according to an embodiment of the invention, before insertion; Figure 10 is a perspective view of two electrodes according to another embodiment of the invention. DETAILED DESCRIPTION In a slip ring asynchronous motor, for the mining or cementing industry, it is common practice to use, for each phase of the rotor, a variable resistance to ensure the starting of the motor. Normally, this resistance consists of a tank of electrolyte in which are arranged a fixed electrode and a mobile electrode. In Figure 1, the tank 1 is filled with electrolyte 2. The fixed electrode 3, arranged at the bottom of the tank 1, is linked to the rotor of the motor. The mobile electrode 4, that can be displaced vertically, is linked to the ground. During the starting sequence, the mobile electrode 4, initially at the top of the tank 1, is lowered and brought closer to the fixed electrode 3, without coming into contact. According to a known embodiment (Figure 2), the fixed electrode 3 comprises two coaxial cylinders 5, 6, and the mobile electrode 4 comprises an axis 7 and a cylinder 8 that are coaxial. When the mobile electrode 4 is lowered into the vicinity of the fixed electrode 3, the axis 7 is inserted into the axis of the internal cylinder 5, and the cylinder 8 is inserted between the cylinders 5 and 6. In the case of Figure 1, the value of the resistance is a function of the distance d between the two electrodes 3 and 4, and of the surface

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area of each of the disc-shaped electrodes. Between the initial position in which the mobile electrode is at the top of the tank, and the final position, in which it is at the bottom just facing the fixed electrode, the resistance varies from Ri to Rf as a function of the only distance d between the two electrodes. In the case of Figure 2, when the mobile electrode 4 is at the top of the tank, its apparent surface area, seen from the fixed electrode 3, is the base surface area of the cylinder 8, substantially equal to the surface area of the electrode 4 of Figure 1. The distance between the two electrodes is the distance between the free edges of the cylinders 8 and 6. When the mobile electrode 4 is inserted into the fixed electrode 3 (Figure 2), the distance between the electrodes 3 and 4 is the radial distance between the cylinders 8 and 6, or 8 and 5, or between the axis 7 and the cylinder 5. The surface area to be taken into account is then the sum of the facing surface areas of the axis 7 and of the cylinder 5 (inner face), of the cylinder 5 (outer face) and of the cylinder 8 (inner face), and of the cylinder 8 (outer face) and of the cylinder 6 (inner face). The greater this surface area, the lower the resistance Rf. With this coaxial cylindrical ring arrangement of the fixed and mobile electrodes, the values of the ratio Ri/Rf usually lie between 80 and 100. According to the invention, in Figure 3, the electrodes consist of rings with triangular section. The fixed electrode 13 comprises two coaxial rings 15, 16. The mobile electrode 14 comprises a central cone 17 and a ring 18. In an axial plane, the section of the rings 15, 16, 18, and of the cone 17, is an isosceles triangle, the vertex angle of which preferably lies between 3 and 100. In the close position (Figure 3), the distance d between the electrodes 13 and 14 is the distance between the vertices of the triangles corresponding to the rings 16 and 18 for example. This distance is of the order of 16 mm. When the mobile electrode 14 is inserted into the fixed electrode 13 (Figure 4), the distance between the electrodes, which is the distance between two facing faces of the rings with triangular section, decreases to be of the order of 3 mm, because of the triangular form of the section of the rings. The facing surface areas of the electrodes are substantially of the same

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order of magnitude as in the case of the cylindrical rings of the prior art, but, because of the decreased distance between the electrodes, the value of the final resistance is lower and the ratio Ri/Rf is practically doubled compared to the prior art. Furthermore, the coaxial structure with several rings of triangular cross section ensures an increase of the facing surface areas in the close position of the electrodes. In the practical exemplary embodiment of Figures 5 and 6, the fixed electrode 24 comprises a cross-shaped base 25 onto which are fixed coaxial rings 26, with a section in the general form of an isosceles triangle, separated by spaces 27 allowing the circulation of the electrolyte. The centre 28 of the electrode is assigned to the electrical connection with the rotor of the motor. In this exemplary embodiment, the fixed electrode 24 comprises five coaxial rings 26. The corresponding mobile electrode 29 comprises four rings. The relative arrangement of the electrodes as they are brought closer together is represented in Figure 9. At the end of insertion, the two fixed 24 and mobile 29 electrodes according to another embodiment are represented in Figure 7. In the embodiment of Figure 8, the assembly of the fixed 31 and mobile 32 electrodes and the electrolyte 33 is housed in an outer reservoir 34. Inside the reservoir 34, there is a cylindrical insulating tank 35 of vertical axis and of a diameter close to that of the electrodes 31, 32. The height of the tank 35 is around 750 mm. The mobile electrode 32 is displaced in the tank 35 between a high position, in which its lateral surface is partly covered by the insulating tank 35, and a low position in which it is inserted into the fixed electrode 31. The insulating tank 35 ensures a channelling of the current lines between the electrodes. Below the fixed electrode 31, there is a pump 36 which ensures a forced circulation of the electrolyte 33 in the tank 35 to extract the power dissipated between the electrodes, particularly when they are brought closer together. In this exemplary embodiment, the travel of the mobile electrode 32 is around 600 mm, whereas, in the prior art, this travel is around 300 mm.

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Thus, with electrodes with triangular profile, a high insulating tank and a long travel of the mobile electrode, the ratio Ri/Rf reaches a value of approximately 300. In the embodiment of Figure 10, the fixed 41 and mobile 42 electrodes are not of circular base, but rectangular. The rings are replaced by prisms of triangular section, arranged parallel to one another. The reciprocal insertion of the electrodes proceeds in the same way as in the preceding embodiments. In another embodiment, the movement of the electrodes is organized according to an axis that is not vertical but horizontal. The bringing closer together of the electrodes is performed either between a fixed electrode and a mobile electrode, or between two mobile electrodes. The amplitude of the relative movement of the electrodes can be greater, of the order of 1000 mm, and the electrolyte reservoir still has a smaller depth. Where ever it is used, the word "comprising" is to be understood in its "open" sense, that is, in the sense of "including", and thus not limited to its "closed" sense, that is the sense of "consisting only of". A corresponding meaning is to be attributed to the corresponding words "comprise", "comprised" and "comprises" where they appear.

Claims

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1- A slip ring asynchronous electric motor comprising, on the rotor, for each phase, a variable starting assistance resistance consisting of an electrolyte in which are placed two electrodes, of which at least one is mobile, wherein each of the electrodes comprises relief elements of triangular section. 2- A motor according to claim 1, wherein the triangular section of the relief elements is an isosceles triangle whose vertex angle lies between 3 and 150. 3- A motor according to claim 1 or 2, wherein the relief elements of the electrodes have interpenetrating forms. 4- A motor according to any one of claims 1 to 3, wherein the relief elements of the electrodes are prisms. 5- A motor according to any one of claims 1 to 3, wherein the relief elements of the electrodes are coaxial rings. 6- A motor according to claim 5, wherein the radii of the coaxial rings of each electrode differ by a constant pitch. 7- A motor according to claim 6, wherein the radii of the coaxial rings of the mobile electrode differ from the radii of the coaxial rings of the fixed electrode by a half-pitch. 8- A motor according to any one of claims 1 to 7, wherein the fixed and mobile electrodes are arranged in a cylindrical insulating tank of a diameter close to that of the electrodes. 9- A motor according to claim 8, wherein the insulating tank is vertical, the fixed electrode is at the bottom of the insulating tank and the mobile electrode is displaced in the insulating tank from a high position in which its lateral surface is partly covered by the insulating tank. 10- A motor according to claim 8 or 9, wherein a pump ensures the forced circulation of the electrolyte in the insulating tank. 11- A motor according to any one of claims 8 to 10, wherein the height of the insulating tank is around 750 mm.

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12- A motor according to any one of claims 8 to 11, wherein the travel of the mobile electrode is around 600 mm.