Rotor for an eddy current braking system and eddy current braking system

Abstract

The invention relates to an optimized eddy current brake (ECB) rotor for an eddy current brake system, comprising: a central region suitable for connection to a shaft of a machine or vehicle; an intermediate region surrounding the central region; and a periphery surrounding the intermediate region, wherein the rotor has a variable thickness in its radial direction, an average thickness in the periphery being smaller than an average thickness in the central region or in the intermediate region.

Description

The present invention relates to a rotor for an eddy current brake system (ECB) and an eddy current brake system with such a rotor, in particular to an optimized design of the rotor which aims to improve the torque of the brake and at the same time to reduce the inertial mass.Prior ArtDE 10 2013 225 093 A1 relates to a rotor for an electric machine.Eddy current brakes (ECB), also referred to as magnetic brakes or non-contact brake systems, use electromagnetic forces for braking without requiring a physical connection between a brake pad and a rotor. Eddy currents are generated by applying a magnetic field to a rotor. These currents then generate a magnetic field that counteracts the originally applied field. This produces a resistance force which can brake the rotor and thus ensures the necessary braking action.In many prior art ECB systems, the rotor is constructed in the same manner. Such a configuration is intentionally chosen to ensure optimum magnetic field engagement and minimize waste of energy that does not contribute to the braking force. However, the design of the rotor, in particular its diameter and its thickness, is decisive for its moment of inertia. Indeed, the radius of the rotor, which quadratically contributes to its moment of inertia, is a critical property that determines its resistance to changes in rotational speed. The larger this moment of inertia, the larger the torque required to change the rotational speed of the rotor, and therefore, the larger the energy required for braking.The traditional rotor design poses some challenges. An enlarged diameter rotor inherently has a greater moment of inertia. This increased moment of inertia requires a higher torque to accelerate or decelerate the rotor, whereby the braking operation is not only less efficient, but also consumes more energy. Conversely, downsizing the rotor may reduce its moment of inertia so that it responds better (less torque is required to accelerate or decelerate the rotor). However, this is expensive because a smaller rotor can also result in a lower braking torque, since the magnetic field is less strongly applied. Such a restriction is particularly problematic in applications requiring a strong braking force.Moreover, the thermal properties of the rotor are also closely related to its thickness and material distribution. A rotor of greater thickness can absorb and conduct away more heat, but at the same time increases the inertia. Adequate thermal management is essential for ECBs. Overheating not only limits the efficiency of the brake, but can also lead to possible malfunctions.In view of these multi-layer challenges, there is an urgent need for a rotor design that accommodates inertia, torque, and thermal management. An optimized rotor design tailored to maximize braking torque while still accounting for inertia and thermal characteristics can dramatically increase the efficiency and reliability of ECB systems.Disclosure of the InventionThe invention includes a rotor for an eddy current brake system according to claim 1 and an eddy current brake system including such a rotor according to claim 8 configured to increase the eddy current brake torque by increasing the size of the rotor while moderately increasing or not increasing the inertia of the rotor, or alternatively to maintain the torque and thermal mass constant while significantly decreasing the inertia mass. In short, it is a matter of finding a shape for the rotor that enables a de-correlation between the increase in torque and thermal mass and the increase in inertial mass.Since the invention aims to increase the braking torque by enlarging the rotor, it proposes a rotor whose diameter can be significantly larger than that of the stator, since such an enlarged diameter directly amplifies the braking torque and ensures a stronger braking action.Preferred refinements are the subject matter of the dependent claims.Advantages of the InventionHowever, the great advantage of the present invention is that when the rotor diameter is increased to increase the braking torque, the rotor has been reconfigured to mitigate the increase in the moment of inertia typically associated therewith. This means that the rotor, although larger, does not suffer from the inertia that a traditional large diameter rotor would have. Instead, the newly designed rotor provides both forceful braking and agillating reaction.Another aspect of the invention is the interaction of the rotor with the coils, in particular its overlap. By determining the ideal degree of overlap with the coils, it is ensured that the braking torque is maximized. It is emphasized that this torque has a saturation point. Starting from a certain overlap size, a further increase in the overlap no longer brings about a higher torque.Therefore, optimizing this overlap is critical to ensuring efficient and strong braking without excess material or design features.Another aspect is the thickness of the rotor. Although it influences the torque, there is a limit, the so-called "minimum thickness", beyond which an increase in the rotor thickness does not substantially increase the torque. Beyond this point, the focus shifts from electromagnetics to thermals. It must be ensured that the rotor can effectively absorb and dissipate the heat generated during braking. To this end, the invention introduces a rotor whose outer layers are thinner compared to the inner layers. This innovative design provides for a consistent thermal mass and a lower moment of inertia. In this way, the rotor can effectively handle the heat while ensuring that its agility and responsiveness during braking are not impaired.Brief Description of the DrawingsFIG. 1 is a schematic front view of an ECB rotor showing the various areas and current paths. FIG. 2 is a schematic illustration of the relationship between torque and rotor overhang / overlap at the stator poles. FIG. 3 is a graph showing the relationship between the torque and the thickness of the rotor. FIG. 4 is a schematic illustration of various shapes of the rotor according to an embodiment of the present invention. FIG. 5 is a schematic illustration of the effects of optimizing the rotor in an embodiment of the present invention.DETAILED DESCRIPTION OF THE INVENTIONHereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.FIG. 1 is a schematic front view of an ECB rotor showing the various areas and current paths.The ECB rotor is a disk that consists of different regions with different functions that lead to specific current paths.First, the rotor has a central portion or hub 3 which is connected to the shaft of the engine or vehicle. The eddy currents in this region are normally minimal because the central region is farther from the magnetic poles of the stator. Secondly, the rotor has an intermediate region or disc body 1. this is the major part of the rotor in which the greatest eddy currents flow and which makes the greatest contribution to the torque. As the rotor rotates and enters the magnetic field generated by the stator, eddy currents are induced in this region. The flux of these currents is circular and is perpendicular to the magnetic field. The interaction between these eddy currents and the magnetic field generates a force which counteracts the movement of the rotor and thus generates the desired braking effect. Finally, the rotor has a periphery or outer edge. Due to its distance from the center, this region experiences the highest linear velocity. The eddy currents in this region are also affected by the changing magnetic field as different parts of the rotor move to and remove from the proximity of the magnetic poles of the stator. The currents in this region also flow in a circular pattern, but depending on the design and arrangement of the magnetic poles of the stator, may have a different density and strength than in the central region.A further segmentation of the surface of the rotor can be carried out. The rotor has active regions 5 in which the rotor comes under the direct influence of the magnetic field generated by the stator. As the rotor rotates, the alternating magnetic field in this region induces eddy currents 6. The interaction between the eddy currents and the magnetic field generates a force which counteracts the movement of the rotor and thus brings about the desired braking effect. In contrast, the passive regions 4 are typically located further from the magnetic poles of the stator or very close to the shaft of the machine or vehicle, often near the outer edge or the periphery of the rotor. This part of the rotor is less active in the braking action. Therefore, the eddy currents in this region are much weaker.FIG. 2 is a schematic illustration of the relationship between torque and rotor overhang / overlap at the stator poles.The overhang or overlap refers to how far the rotor protrudes beyond the edges of the stator poles when viewed in cross-section or plan view as can be seen on the left side of FIG. 2, comparing the two designs (the upper with 0 mm overlap and the lower with 25 mm). The relationship is such that as the overhang of the rotor across the stator poles increases, the braking torque initially increases. This is because a larger overlap allows a larger area in which eddy currents can be induced, resulting in a stronger interaction between the rotor and the magnetic field of the stator and thus intensifying the braking effect.However, the increase in torque due to the rotor overhang reaches a saturation point. Starting from a certain overlap, any further increase in the rotor overhang no longer leads to a significant increase in the torque. In essence, there is an optimum overlap size beyond which the yield decreases with respect to torque increase. In the case depicted, this value is 15.6119 mm, from this value all curves overlap considerably. The reason for this saturation is the strength and distribution of the magnetic field. If the overlap becomes too large, the areas of the rotor furthest from the stator poles may no longer be affected as much by the magnetic field of the stator, leading to weaker eddy currents and thus to a plateau in the torque increase.FIG. 3 is a graph showing the relationship between the torque and the thickness of the rotor.The thickness of the rotor plays a decisive role in determining the torque generated in an ECB brake. First, as the thickness of the rotor increases, the braking torque also increases. The greater thickness provides a greater volume of conductive material into which the magnetic field of the stator may penetrate, resulting in a stronger induction of eddy currents. These amplified eddy currents produce a higher braking torque in interaction with the magnetic field (explaining that the curves in the graph of FIG. 3 have a higher torque as the thickness is increased from 5 mm to 15.6 mm).However, there is a certain threshold for rotor thickness, referred to as "minimum thickness", at which the torque increase flattens. Beyond this point, an increase in the rotor thickness no longer leads to a substantial improvement in the braking torque. In FIG. 3, this corresponds to the value of 15.6 mm. From this point on, the focus shifts on the thermal properties of the rotor. Thicker rotors have the advantage that they can absorb and dissipate more thermal energy resulting from the resistance to eddy currents. When the thickness of the rotor exceeds the minimum thickness, its ability to manage this thermal energy becomes more and more important. In short, a thicker rotor can absorb more thermal energy without reaching excessive temperatures. This means that it can maintain its integrity and function without overheating, even if the additional thickness does not substantially increase its braking torque.FIG. 4 shows various forms of rotor for an ECB brake to optimize the rotor in connection with limiting its inertia while ensuring an adequate mass for torque and thermal performance.The essence of the present invention is to optimize the shape of the rotor in order to establish a balance between achieving an optimal braking torque and the heat dissipation on the one hand and minimizing the moment of inertia of the rotor on the other hand. The optimum braking torque and the dissipation of heat are of greatest importance for the function of the brake. The reduced moment of inertia results in the rotor responding better to changes, ultimately making the ECB system more efficient and effective.To achieve this balance, the rotor has a variable thickness. The outer portions of the rotor are thinner than the inner portions that are closer to the center. This design choice ensures that the parts of the rotor that are most strongly influenced by the magnetic field of the stator often maintain the inner areas, a thickness that generates an optimum braking torque. At the same time, the overall moment of inertia of the rotor is reduced by the smaller thickness toward the outer edges, as a result of which the rotor responds more quickly.Certain design constraints may be advantageously incorporated. For example, in the areas directly facing the stator poles, the rotor should have a certain "minimum thickness" in order to optimize the electromagnetic function that ensures the desired braking torque.From these ideas, several forms can be derived, as shown in the various embodiments in FIG. 4. It should be noted that although the rotor and the stator are shown as having the same diameter, this is only for illustrative purposes. As indicated above, in advantageous embodiments, the stator and rotor should overlap.Embodiment (a) shows a prior art ECB brake 10 consisting of a stator 11 to which a plurality of poles 12 are fixed. A plurality of coils 13 are wound around their respective poles 12, respectively. A rotor 14 is mounted around the shaft 15 of an engine or a vehicle. In the embodiment (a), the thickness of the rotor 14 (in the longitudinal direction of the shaft 15) is constant.Conversely, in embodiments (b), (c) and (d), the rotor is optimized according to the present invention. In embodiment (b), the optimization is performed by forming the rotor from two parts: a first part 14 aand a second part 14 b. The first part 14a of the rotor has the same diameter as the prior art rotor 14 shown in embodiment (a), but is thinner. The second part 14b of the rotor has a smaller diameter than the first part 14a. In illustrative embodiments, the cumulative diameter of the first part 14 aand the second part 14 bmay be larger than the diameter of the conventional rotor 14, so that the total mass of the rotor can be maintained while the moment of inertia is significantly reduced in embodiment (a).FIG. 5 is a schematic illustration of the effects of optimizing the rotor in an embodiment of the present invention.FIG. 5 shows such an approach in which the mass of the rotors in both embodiments (a) and (b) is maintained at 5 kg, while the moment of inertia of the rotor in the second embodiment is reduced from 275 g m2 (embodiment (a)) to 190 g m2. In the embodiment (c), the optimization is achieved by making the rotor of three parts having the same concept as in the embodiment (b): a first part 14a, a second part 14b and a third part 14c, each having a decreasing diameter. The embodiment (d) is different from the embodiments (b) and (c) in a part 14 dthat decreases in diameter in the radial direction toward the periphery of the rotor. In the figure, the decrease is constant, but this is not necessary, the decrease may be variable. For example, the decrease could be greater near the center of the rotor.In these embodiments (b) to (d), the main feature is that the total thickness of the rotor decreases from the central portion where the rotor is fixed to the shaft of the vehicle or the engine toward the periphery of the rotor. This decrease can be effected stepwise or continuously and / or correspond to a polynomial function. Some local increases in thickness are possible. However, the average thickness of the rotor in its radial direction toward the periphery should have a markedly decreasing tendency. It should be noted that in the embodiments in which the variation in thickness is stepwise, the rotor does not need to be manufactured in multiple parts (for example, a first rotor corresponding to the first part 14 aand a second rotor corresponding to the second part 14 b). The rotor can be manufactured directly in one piece without the need to assemble the various parts corresponding to the different thicknesses.References included in the specificationThis list of documents cited by the applicant has been produced in an automated manner and is only included for the better information of the reader. The list is not part of the German patent application or utility model application. The DPMA does not take any adhesion for any faults or omissions.Patent Literature citedDE 10 2013 225 093 A1

[0002]

Claims

A rotor (14, 14a, 14b, 14c, 14d) for an eddy current brake system, ECB, comprising: a central region (3) suitable for connection to a shaft (15) of an engine or a vehicle; an intermediate region (1) surrounding the central region; and a periphery (2) surrounding the intermediate region, the rotor having a variable thickness in its radial direction, wherein an average thickness in the periphery (2) is less than an average thickness in the central region (3) or in the intermediate region (2).The rotor (14, 14a, 14b, 14c, 14d) of claim 1, wherein a variation in the variable thickness of the rotor in its radial direction towards the periphery (2) has a substantially decreasing tendency, regardless of local thickness variations.The rotor (14, 14a, 14b, 14c, 14d) according to claim 1 or 2, wherein the variable thickness gradually decreases along the radial direction of the rotor.The rotor (14, 14a, 14b, 14c, 14d) of claim 3, wherein the rotor comprises a plurality of parts (14a, 14b), each part having a certain diameter and each subsequent part having a smaller diameter than the previous part in the radial direction towards the periphery.The rotor (14, 14a, 14b, 14c, 14d) of claim 4, wherein the parts are manufactured as an integral piece.The rotor (14, 14a, 14b, 14c, 14d) according to any one of the preceding claims, wherein the decrease in thickness towards the periphery (2) of the rotor is achieved by a continuous, polynomial or variable function, or wherein the decrease in thickness of at least a part of the rotor towards its periphery (2) is achieved by a continuous, polynomial or variable function.The rotor (14, 14a, 14b, 14c, 14d) according to any of the preceding claims, wherein a predetermined minimum thickness is maintained in the intermediate region (1) to optimize electromagnetic interactions and to generate a desired braking torque.An eddy current brake system, ECB, (10) comprising: a stator (11) equipped with magnetic poles (12) each comprising a coil (13); and a rotor (14, 14a, 14b, 14c, 14d) according to any one of claims 1 to 7.The eddy current brake system (10) of claim 8, wherein a diameter of the rotor (14, 14a, 14b, 14c, 14d) is greater than a diameter of the stator (11).Eddy current brake system (10) according to Claim 8 or 9, wherein an overhang between the rotor and the stator (11) is of the same size as the magnetic poles (12) with coils (13) of the stator (11).

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