WO2019151956A1

WO2019151956A1 - Integrated gap retention element for electric motor

Info

Publication number: WO2019151956A1
Application number: PCT/SI2018/000004
Authority: WO
Inventors: Gašper ŠUŠTERŠIČ; Gorazd GOTOVAC
Original assignee: Elaphe Propulsion Technologies Ltd.
Priority date: 2018-02-05
Filing date: 2018-02-05
Publication date: 2019-08-08

Abstract

An electric motor with an integrated electromagnetic air gap retention element for use in direct drive applications, especially in-wheel applications, is proposed. The proposed invention combines a function of electromagnetic air gap retention, and heat removal from the stator by positioning said gap retention element appropriately, and giving it the correct technical functionality to perform both functions. The form and material properties of the feature are such that they provide high increase of heat transfer, and at the same time the form and material of the feature enable the gap retention element to perform its mechanical function - sliding and/or impact contact when and if needed, to prevent contact in the electromagnetic air-gap.

Description

Integrated gap retention element for electric motor

Technical Field

The present invention relates to the field of electrical machines, focusing on mechanical solution for an in-wheel electric motor, embodied in an integrated part that combines high heat transfer capability with the function of the retention of the electromagnetic air gap between the rotor and the stator.

Background Art

Technical problem lies within the need for the electromagnetic air gap, between the active electromagnetic components of the stator and the rotor in electric machines, to always be present (to enable rotation), and at the same time to be uniform and relatively small with respect to the diameter of the electromagnetic air-gap. Multiple physical influences are imposed upon the electric machine in ways that reduce the already thin electromagnetic air gap. For example, electromagnetic influences, like magnetic attraction between the stator and rotor components during offset from coaxiality and offset from axial symmetry. Thermomechanical influences on individual components, where differences in temperatures and in thermal expansion coefficients of individual components of the electric machine influence the size of the electromagnetic air gap substantially. Effect of conservation of momentum and conservation of angular momentum - inertial effects including precession and nutation, where angular momentum of the rotor component assembly can be substantial, and the impulsive nature of the movement of the rotor components assembly can influence the electromagnetic air gap. Also, there can be certain chemical influences on individual components of electric motor assembly, where an increased swelling of rubber and similar materials, under presence of liquids or fluids, is indirectly affecting the electromagnetic air gap. Some physical influences imposed upon the electric machine can act to increase the electromagnetic air gap, centrifugal accelerations and thermomechanical influences of/on individual components possess such characteristics. The electromagnetic air gap dimension variation, caused by road-induced impacts, forces during cornering, and also thermal expansion, is a safety critical event in an in-wheel motor. The retention of this dimension is key to ensure that damage to the functional parts of the motor does not occur during an operation, and that high severity events, like rotor locking, do not occur. The electromagnetic air gap dimension stability depends mostly on stiffness of elements, vibrational modes and scenario of use. In a high torque density motor, one of the design goals is low mass, which can severely limit the stiffness of elements, while due to use in the wheel of a vehicle, the accelerations and forces that the motor should endure are high, when compared to conventional use of electric motor in electric cars.

On the other hand, the core of the electric motor is designed to produce mechanical power from electric power. In the process of this energy conversion, heat is generated and the motor needs to be cooled to sustain the mechanical power. The heat is mainly generated in the winding and stator core, and needs to be either stored, or removed from the motor in order for the motor temperatures to stay within the desired range. In high torque density motors heat management of the motor is even more important, since heat capacity is low due to low relative mass of the motor when compared to the mechanical output power and the generated heat power and due to mostly low specific heat capacity of used materials. The temperatures of thermally sensitive parts, like the insulation and permanent magnets, can thus rise very rapidly if the heat is not immediately removed. While in liquid cooled motors, the heat from conductors in slot is directly removed to the iron blade stack, the end-winding portion of the winding is cooled through the in-slot portion of the winding, and to limited extent through the impregnating material to the cooled housing. Although this impregnating material can have a thermal conductivity around 1 W/mK, the thermal path to cool the end-winding portions is not as good as the one of the in- slot portions, thus hotspots occur in this region.

In order to ensure a stable dimension of the electromagnetic air gap in in-wheel motors, one identifies at least three possible approaches. Firstly, on ensuring a large enough stiffness of the bearing as well as the machine assembly, which means that the bearing as well as the machine assembly is grossly over dimensioned for a vast majority of events in its lifetime. Overall increase of assembly weight is usually direct result from this approach. Secondly, on using at least two bearing axial locations, one on each axial side of the in-wheel motor, creating a very stable situation, assuming rotor or stator possess enough mechanical strength. This approach creates additional costs. Finally, on creating an additional feature for electromagnetic air gap retention, usually with low friction materials or rolling elements, to smoothly intercept the low occurrence impacts. The final approach does not impose a high additional cost, complexity or limit other in-wheel motor functions, due to a feature that only acts occasionally.

In US8664818 patent the proposed technical solution focuses on the electromagnetic air gap retention feature, namely“the touchdown ring”, which performs its function exclusively in the radial sense. Additionally, this solution provides no relief to the thermomechanical influences to the electric machine assembly. Actually, the influence to the function of the machine assembly is detrimental when geometric tolerances of roundness are taken into account (rotor locking can occur at “touchdown” due to thermomechanical effects). Similarly, in US20090243301 the electromagnetic air gap retention feature does not address the thermomechanical influences to the electric machine assembly. In US20080012347 where wind generator system is using attractive magnetic forces to reduce the load on the bearings, the reduction of thermomechanical influences is absent from the electromagnetic air gap retention feature. Within this solution the electromagnetic air gap retention feature presented is always engaged, which means that increase of mechanical or electromagnetic losses are always present. US8373299 focuses on axial electromagnetic air gap rotating electrical machine with the air gap retention feature embodied as sets of gap-maintaining rolling supports. In this solution the electromagnetic air gap retention feature is preloaded and always in function, thus increasing mechanical losses during all operating conditions, also it does not provide reduction of thermomechanical influences. US20060091761A1 describes compact high-power alternator, wherein the electromagnetic air gap retention feature is in the form of a“Mechanisms to prevent the rotor magnets from clashing with the stator by minimizing rotor displacement, and absorbing unacceptable rotor displacement”. Again, the electromagnetic air gap retention feature in this patent provides no relief to the thermomechanical influences to the electric machine assembly.

Electric motors usually use end-winding encapsulation in order to ensure a better thermal conductivity from end-winding conductor portions to the housing. This method is better than normal convective heat removal, but still creates a hotspot in the motor. Therefore, when the motor temperature stabilization is crucial, solutions in the form of an additional cooling body next to the end-winding exist, like presented in US8093770 Bl . In some cases, the additional cooling body is hollow, whereas a cooling liquid is supplied within the hollowed part. Such solutions usually require complex cooling systems with numerous assembly parts that occupy additional space and add to overall weight, which is, at least in in-wheel motors, highly undesirable.

Summary of the Invention

An electric motor with an integrated electromagnetic air gap retention element for use in direct drive applications, especially in-wheel applications, is proposed.

An exemplary embodiment of the present invention is illustrated by way of example in the accompanying drawings in which:

Figure 1 is an axonometric exploded representation of the electric machine assembly;

Figure 2 is a sectional representation of the gap retention element embodiment within the electric machine assembly;

Figure 3 is an axonometric partial representation of the stator with a cut-out area view of winding and its encapsulation material;

Figure 4 is a sectional representation of one of the gap retention element alternative embodiment within the electric machine assembly;

Figure 5a is a sectional schematic representation of the size relation between electromagnetic air gap and axial air gap;

Figure 5b is a sectional schematic representation of the rotor movement under the influence of the external forces (figure 5a and 5b are collectively referred to as Fig.5).

Identical or corresponding elements have the same reference signs throughout the description.

The electric motor presented comprises at least a bearing (3), a rotor (2), and a stator (1). The bearing is positioned between the rotor and the stator and has the function of an interface between the rotating and the static part of the motor. The rotor and the stator in general include groups of components that are involved in torque generation, referred to as the electromagnetic components, and components that have the main function of transferring the torque to the wheel (or other driven component) and to the car (or other component providing resistance against rotation), referred to as the housing components. Both groups have other important functions as well. Two functions of the motor housing components affected by the present invention are, the prevention of excessive reduction of clearance between the rotor and the stator electromagnetic part (4, 5), and the heat dissipation. In order for the electromagnetic component assembly to perform its function efficiently, the distance between the rotor electromagnetic part (4) and the stator electromagnetic part (5) has to be small. On the other hand, the use of the motor in the wheel of a car induces the loads on the motor that are of a variable and possibly impulsive nature. They can possibly lead to the change of electromagnetic air gap (6 - Figure 2) dimension, and finally also to the interference between the rotor (2) and stator (1) components, not resistant to repetitive sliding, or other type of contact / interference. This interference increases the probability of occurrence of events related to safety critical failure modes, such as rotor locking, high voltage insulation damage, permanent magnet damage, and others. The function of the motor housing components and the bearing is to prevent the possible contact between the two electromagnetic parts under all possible loads encountered during use/implementation. Predominant approach is to guard the radial electromagnetic air gap (45 - Figure 5) by radial retention elements, but the origin of the load based air-gap variation is the bending of the bearing, so retention of the axial movement (42 - Figure 5) in the right position can be more effective, and lower tolerances of production are needed. Indeed, it turns out that the axial movement versus the electromagnetic air-gap variation is usually in the order of magnitude of 2: 1 , or even 3: 1 for the motor sizes needed for passenger cars. Due to the motor geometry, and the center of tilt between the rotor (2) and the stator (1), the axial impact surfaces can be further apart and thus preventing possible premature interference of the stator impact surfaces (9) due to thermal expansion of elements or production tolerances. Thus, the air gap (18) formed between rotor and stator impact surfaces (4, 5) can be larger than the electromagnetic air gap (6). This consequentially also allows potentially greater wear of the impact surfaces in an embodiment comprising sacrificial impact surfaces, causing no detrimental effects on functionality of the motor which is not the case in the prior art.

On the other hand, the electromagnetic part of the electromagnetic machine, especially the stator (1), generates a large amount of heat. The function of the motor housing components is therefore also to dissipate this heat, either by being cooled from within, in in-wheel motors usually by liquid cooling, or from outside by air cooling.

Finally, the housing has to stay compact and light, therefore combination of several functions in one part of the housing is desirable. The proposed invention combines a function of electromagnetic air gap retention, and heat removal from the motor, by positioning the gap retention element (7 - Figure 2) part appropriately and giving it the correct technical functionality to perform both functions. The gap retention element is positioned axially adjacent to the end-windings (8 - Figure 3) of the electric motor. The form and material properties of the feature are such that they provide high increase of heat transfer. At the same time the form and material of the feature must enable the gap retention element to perform its mechanical function - sliding and/or impact contact when needed. Due to the intermittent nature of the gap retention element, air gap retention function, its impact surface (9 - Figure 2) properties need not be significantly different from the base material properties. However, low friction and increased hardness of impact surface, result in reliable function and overall quality product in various working conditions. The sporadic nature of potential impact or sliding occurrences for which gap retention element air gap retentor function is meant, allows non-removable and heat conductive resin encapsulated assembly part, which is not meant to be replaceable in maintenance process, or in other words, there is no need for replacing it, since its wear should be in acceptable dimensional limits allowed by the axial direction of the contact. The gap retention element can be only partially present on the circumference of the stator, since the potentially harmful deformations occur only in predictable directions close to the axis perpendicular to the road surface. This allows usage of usually not more than two pad-like gap retention elements, appropriately dimensioned and distributed along stator circumference, as a simpler and much cheaper alternative to a ring-shaped gap retention element.

But if one would prefer removable and serviceable air gap retention element (7), for motor usage in more demanding environment with much greater impact probability, or for other various reasons, then a two-part gap retention element solution with removable part (72 - Figure 4), embodying alternative stator impact surfaces (91 - Figure 4), is proposed. Such solution allows non-treated and non-coated gap retention element impact surfaces, but at the same time requires said impact surfaces to act as sacrificial surfaces. This replaceable part of the gap retention element should thus be made out of softer material in comparison to material of rotor impact surfaces, should still have relatively smooth surfaces, but must not have substantially lower heat conductivity than the base part (71 - Figure 4) material.

The special features of the proposed invention are described below. The stator electromagnetic part (5) includes at least a winding (10 - Figure 3), and encapsulation material (11) impregnating the winding. Usually the stator electromagnetic part also includes a ferromagnetic core with slots (12) where the winding is inserted. In general, the end- winding (8) is a part of the winding which is not substantially perpendicular to the direction of rotation of rotor (2), and in the case of the slotted ferromagnetic core (12) the end- winding is the part of the winding protruding out of the slots. The stator housing (13) includes a main portion (14), which transfers the torque from the electromagnetic part of the stator. Usually this main portion has a radial interface surface (15 - Figure 2) with the electromagnetic part. As a novel feature, the stator housing also includes a second portion, namely protruding portion (16 - Figure 1), protruding in the radial direction towards the rotor (2), axially adjacent to end-windings at least on one axial side, and with at least one stator impact surface (9). The protruding portion has firstly a substantially good thermal interface with the encapsulating material (11) surrounding the end- winding, and secondly, a substantially good thermal interface with the main portion ( 14) of the stator housing. The positioning of the protruding portion ( 16) is ideal for the stator impact surface (9), therefore on the opposite side of the stator impact surface, the rotor impact surface (17) is proposed, together forming a sufficiently thin air-gap to prevent further change of the electromagnetic air- gap (18 - Figure 2), between the rotor and the stator electromagnetic part (4, 5) upon a higher than usual load. In order to minimize the impact effect, wear of the impact surfaces, and to reduce the noise and vibrations, at least one of the impact surfaces is selectively surface treated or coated with appropriate material.

In one embodiment either of the impact surfaces opposite to the low friction impact surfaces, are anodized so that they have a higher hardness. In another embodiment these impact surfaces can be chemically nickel plated, powder coated or even PTFE enameled. In one such embodiment the impact surfaces (9, 17) are positioned on the axial side of the motor, where the rotor is mounted to the bearing (3). In another such embodiment the impact surfaces (9, 17) are positioned on the opposite axial side of the motor side, where the rotor is mounted to the bearing. In another embodiment the impact surfaces form an air-gap, which has a special shape which protects it from external water jets. The surfaces thus can perform an additional function of a labyrinth seal.

In the preferred embodiment, the gap retention element (7) feature counteracts the axial displacements of the electric machine rotor (2), and its consequent radial displacement. The gap retention element is integrated in stator housing (13) either by encapsulating material (11) impregnating end-windings (8), or by direct fixation, using screws, or both if necessary. Encapsulated part of the gap retention element is axially adjacent to the end-windings, while its axially, towards rotor protruding part, embodies stator impact surfaces (9). On axially opposite side, rotor impact surfaces (17) form a sufficiently thin air-gap with said stator impact surfaces. At least one of the impact surfaces (9, 17) is selectively surface treated or coated with appropriate material for reducing surface friction or increasing surface hardness. The other feature of the gap retention element is its capability of high heat conductivity. It reduces the thermomechanical effects of the stator (1) by reducing the stators temperature, namely by increasing heat transfer away from the stator, having substantially good thermal interface with the encapsulating material (11 - Figure 3). Alternatively, the radially outward protruding portion can further comprise a heat transfer element, where sufficient pressure applied on said heat transfer element is necessary for having substantially good thermal interface with the encapsulating material.

In the preferred alternative embodiment, the gap retention element (7) feature is two-part assembly comprising of base part (71 - Figure 4) and removable part (72 - Figure 4) embodying alternative stator impact surfaces (91 - Figure 4). Said gap retention element feature counteracts the axial displacements of the electric machine rotor (2), and its consequent radial displacement. The base part (71) is integrated in stator housing (13) either by encapsulating material (1 1) impregnating end-windings (8), or by direct fixation, using screws, or both if necessary. The removable part (72) of the gap retention element is fixated onto the base part by direct fixation using screws or by other methods of fixation which is able to withstand the forces and thermal loads generated by friction of the rotor and stator. Encapsulated part of the gap retention element is axially adjacent to the end-windings, while its axially, towards rotor protruding removable part (72), embodies alternative stator impact surfaces (91). On axially opposite side, rotor impact surfaces (17) form a sufficiently thin air-gap with said stator impact surfaces. Said alternative stator impact surfaces (91) are acting as sacrificial surfaces, while removable part (72) is being made out of softer material in comparison to material of rotor impact surfaces, having low friction surfaces, and reducing the thermomechanical effects of the stator (1) by reducing the stators temperature, namely by increasing heat transfer away from the stator, having substantially good thermal interface with the base part (71), which also has substantially good thermal interface with the encapsulating material (1 1 - Figure 3).

As those of ordinary skill in the art will understand, various features of the embodiments illustrated and described with reference to any one of the figures, may be combined with features illustrated in one or more other figures, to produce alternative embodiments that are not explicitly illustrated or described. The combinations of features illustrated provide representative embodiments for typical applications. However, various combinations and modifications of the features, consistent with the teachings of the present disclosure, may be desired for particular applications or implementations.

Claims

CLAIMS:

1. An electric machine comprising:

a rotor (2), wherein the rotor comprises rotor housing (19) with at least one rotor impact surface (17), wherein said rotor housing comprises rotor electromagnetic part (4); a bearing (3), wherein the bearing being any part or assembly enabling rotor and stator (1) rotational engagement, thus interfacing motor rotating and static portions; and a stator, wherein the stator comprises stator housing (13), wherein said stator housing comprises a main portion (14) for transferring the torque from the electromagnetic part of the stator (5), wherein said main portion comprises a winding (10) with an encapsulation material (1 1) impregnating said winding, wherein said stator housing further comprises a radially outward protruding portion (7) axially adjacent to end windings (8) at least on one axial side and comprising at least one stator impact surface (9), said radially outward protruding portion having a high heat dissipation capability, and a good thermal contact with the encapsulation material of the main portion, wherein said stator impact surface being axially separated from the rotor impact surface by sufficiently small air gap (18) able to compensate axial to radial movement variations ratio of up to 3: 1, enabling engagement of impact surfaces before the critical reduction of magnetic air gap (6), wherein at least one surface forming the air gap being low friction surface.

2. An electric machine according to claim 1, wherein the radially outward protruding portion (7) comprises at least one stator impact surface (9) in axial and radial direction, forming an air gap (18) with at least one rotor impact surface (17) in axial and radial direction, thus enabling electromagnetic air gap retention in both axial and radial direction.

3. An electric machine according to claim 1, wherein the radially outward protruding portion (7) having high heat conductivity capability and being partly encapsulated, reduces the thermomechanical effects of the stator (1) by reducing the stators temperature, namely by increasing heat transfer within the stator, having substantially good thermal interface with the encapsulating material (11).

4. An electric machine according to claim 1, wherein the radially outward protruding portion (7) having high heat conductivity capability and being partly encapsulated, further comprising a heat transfer element, reduces the thermomechanical effects of the stator (1) by reducing the stators temperature, namely by increasing heat transfer from the stator, having substantially good thermal interface with the encapsulating material (11), wherein sufficient pressure applied on said heat transfer element is necessary.

5. An electric machine according to claim 1, wherein the radially outward protruding portion (7) comprises base part (71) and removable part (72), said removable part being fixated onto the base part directly using screws or other fixation methods, comprising alternative stator impact surfaces (91), acting as sacrificial surfaces.

6. An electric machine according to claim 1 and 5, wherein removable part (72) being made of softer material compared to rotor impact surfaces (17) material, having low friction coefficient surfaces.

7. An electric machine according to claim 1, wherein either rotor impact surfaces (17) or stator impact surfaces (9) or all of them, being selectively treated or coated, thus achieving increased hardness, low friction structure and wear resistance on said surfaces, regardless of rotor and stator base material.

8. An electric machine according to claim 1 and 7, wherein selective treatment or selective coating of rotor or stator impact surfaces (17, 9) being either anodizing treatment, chemical nickel plating, powder coating, PTFE enameling, or any other process increasing surface hardness or decreasing surface friction.

9. An electric machine according to claim 1 and 8, wherein rotor (2) and stator (1) base material being lightweight metal, said metal being also non-ferromagnetic, well thermally conductive and hard.

10. An electric machine according to claim 1 and 2, wherein the radially outward protruding portion (7) comprises not more than two pad-like elements appropriately distributed along stator (1) circumference, wherein said pad-like elements comprise stator impact surfaces (9).

11. An electric machine according to claim 1 and 2, wherein the rotor and stator impact surfaces (17, 9) being positioned on the axial side of the motor where the rotor (2) is mounted to the bearing (3).

12. An electric machine according to claim 1 and 2, wherein the rotor and stator impact surfaces (17, 9) being positioned opposite from the axial side of the motor where the rotor (2) is mounted to the bearing (3).

13. An electric machine according to claim 1 and 2, wherein the shape of the air gap (18) between rotor and stator impact surfaces (17, 9) serves as a simple labyrinth seal.

14. An electric machine according to any of the above claims, wherein electric machine being an in- wheel electric motor, wherein said in- wheel electric motor's radius dimension being substantially larger than its axial width.

15. An electric machine according to any of the above claims, wherein radially outward protruding portion being a gap retention element.