Electric motor

Abstract

An electric motor (100), in particular a series-wound motor, for a driven electric hand-held power tool, comprising a rotor (110) with a driven rotor shaft extending along a longitudinal axis (L), and a rotor body (112) non-rotatably connected to the rotor shaft, which in cross-section has a number of tooth heads (12), wherein a space (12a) for winding a winding space (20) of the rotor body with coil wire is defined between each pair of tooth heads, characterized in that each tooth head (12) has, in cross-section, a surface-enhancing structure (14) on its outer circumference with a number of crenellated projections (16) and / or recesses (18) in cross-section, which are arranged next to each other at substantially regular intervals in the circumferential direction (U) with respect to the respective tooth head (12).wherein the spaces (16a) between two adjacent projections (16) and / or the recesses (18) of the surface-enlarging structure (14) are dimensioned so small in the circumferential direction (U) that the surface-enlarging structure (14) is able to swirl a cooling airflow in the air gap between the rotor body (112) and adjacent components of the electric motor (100), in particular a stator that at least partially surrounds the rotor body (112).

Description

The invention relates to an electric motor, in particular a series-wound motor, for a driven electric hand-held power tool, comprising a rotor with a driven rotor shaft extending along a longitudinal axis, and a rotor body connected to the rotor shaft in a rotationally fixed manner, which in cross-section has a number of tooth heads, wherein between two tooth heads an intermediate space for winding a winding space of the rotor body with coil wire is defined. In powered electric hand tools, such as angle grinders, the electric motors are often operated at comparatively very high speeds. At the same time, the installation space in which the electric motor is housed is relatively small to ensure easy handling of the powered power tool. A well-known problem with electric motors, especially those used in powered power tools, is therefore the high heat generation of the motor, which significantly impairs its performance and lifespan. For this reason, solutions are already known in the art that are intended to enable improved heat dissipation of the heat generated by the electric motor. For example, EP 2 495 847 A1 describes an electric motor in which the rotor has an air guide groove on its outer circumferential surface at each tooth tip, extending essentially from one end face of the rotor to the other. These air guide grooves serve to further support an airflow between the rotor and the stator in the direction of the rotor's longitudinal axis, thereby providing improved heat dissipation. In one embodiment, these air guide grooves are slightly inclined relative to the rotor's longitudinal axis to take the rotor's direction of rotation into account during operation. From DE 10 2004 026 501 A1, a rotor body design is further disclosed in which a rotor encapsulation in the construction of a high-speed generator is intended to reduce air friction losses. In this context, the document describes how axial flow in the air gap between the rotor and stator, in the case of air-cooled high-speed drives and generators, leads to a significant increase in air friction losses and thus a reduction in the cooling effect. As a solution to this problem, the document proposes an outer encapsulation of the rotor with an outer circumferential surface featuring a multitude of surface deformations. In all illustrated embodiments, these are formed by axially spaced annular grooves that circumferentially around the outside of the encapsulation, essentially perpendicular to the flow direction. This promotes local air turbulence and thus improves heat transfer in the axial direction. A similar solution is also known in JP S60 194734 A. An alternative solution is known from publication JP 2009-159763 A and, similar in design, from DE 1 939 184 A. These two documents describe solutions in which cooling grooves or cooling channels extending radially inwards from the outer surface of the rotor are formed on the rotor. Finally, GB 221 811 A describes another alternative solution to an alternative problem, namely current losses due to eddy currents, in which an AC motor has a rotor whose surface has a series of circumferential grooves, each formed around the outer circumference and running perpendicular to the longitudinal direction of the rotor. These grooves are intended to reduce losses due to eddy currents occurring between the rotor and stator. The circumferential grooves increase the electrical resistance at the surface of the rotor, thereby reducing eddy currents occurring at its surface and the associated losses. The present invention aims to provide an improved solution for an electric motor, in which heat losses can be reduced and at the same time an extended service life of the electric motor is achieved. Accordingly, the invention proposes an electric motor with the features of claim 1. This motor is characterized in particular by the fact that each tooth head, viewed in cross-section on its outer circumference, has a surface-enlarging structure with a number of crenellated projections and / or recesses, wherein the spaces between two adjacent projections and / or the recesses of the surface-enlarging structure are dimensioned so small in the circumferential direction that the surface-enlarging structure is able to swirl a cooling airflow in the air gap between the rotor body and adjacent components of the electric motor, in particular a stator that surrounds the rotor body at least partially. The crenellated projections, viewed in cross-section, are arranged side by side at essentially regular intervals in the circumferential direction relative to the respective tooth head. "Viewed in cross-section" refers to a section through the rotor that runs perpendicular to the rotor's longitudinal axis. According to the claim, each tooth tip has several projections and / or recesses on its outer circumference. The special feature of the present invention is that the spaces between two adjacent projections and / or the recesses are not intended, as in the prior art (for example, in the solution of EP 2 495 847 A1), to guide and thereby support a cooling airflow, for example, in the direction of the rotor body, but on the contrary, are designed in such a way that a cooling airflow in the air gap between the rotor body and adjacent components of the electric motor is additionally turbulently arose. The surface-enhancing structure, as its name suggests, provides a larger surface area for the rotor, allowing it to dissipate heat more effectively to the environment, particularly the surrounding cooling air. The additional turbulence of the cooling air within the air gap further increases heat energy transfer to the cooling air, resulting in improved cooling performance. Initial field tests have demonstrated a cooling improvement of approximately 10 to 20% compared to existing solutions, leading to increased performance and extended service life for the electric motor. Furthermore, it may be provided that the rotor body has a large number of individual laminations stacked in the direction of the longitudinal axis. In particular, it can further be provided that the surface-enhancing structures of two adjacent individual laminations of the rotor body are aligned with each other. This means that the respective projections and / or recesses of the adjacent surface-enhancing structures are aligned with each other. In particular, the crenellated projections can be essentially the same length in the circumferential direction. Furthermore, it may be provided that the length of the crenellated projections in the circumferential direction essentially corresponds to the length of the interstices (i.e., distances) in the circumferential direction between two adjacent crenellated projections of a tooth head. Furthermore, it may be provided that the spaces (i.e., distances) between two adjacent tooth heads in the circumferential direction are larger than the spaces (i.e., distances) between two adjacent projections and / or the depressions of the surface-enhancing structure of a tooth head in the circumferential direction. It can also be provided that the tooth tips of two adjacent individual laminations of the rotor body are aligned with each other. This is particularly useful because the spaces between two tooth tips can be used to hold coil wire. It may also be provided that the spaces between two adjacent projections and / or the depressions of the surface-enlarging structures are less than 2 mm in the circumferential direction, in particular 1 mm or less. Finally, the invention also relates to a rotor body with the features of claim 9. The present invention is described in more detail below with reference to the accompanying figures. The figures show one possible embodiment of the invention, in which individual features of the invention are shown in combination with one another. These features can, of course, also be considered separately and, if necessary, combined into meaningful sub-combinations without requiring any inventive effort from a person skilled in the art. The figures show schematically: Fig. 1 an electric motor according to the invention in isometric view; and Fig. 2 a top view of a single lamination of the rotor according to Fig. 1 . Fig. 1 shows an electric motor 100 according to the invention with a rotor 110 in an isometric view. The rotor body 112 of the rotor 110 extends in the direction of a longitudinal axis L and is composed of a plurality of individual laminations 10. Fig. 2 shows a top view of a single sheet 10 of a rotor body 112 of an electric motor 100 according to the invention. A plurality of such single sheets 10 are usually stacked in the direction of the longitudinal axis L of the rotor 110 and thus form a rotor body 112 according to the invention. The single sheets 10 are aligned with each other such that they are stacked coaxially and the radially outwardly extending tooth tips 12 (Fig. 2) are aligned with each other and are in line with each other (see also Fig. 1). As can be clearly seen in Figures 1 and 2, each tooth head 12 has a surface-enhancing structure 14 formed by a number of projections 16 (four projections in the illustrated embodiment) that extend radially from the outer surface of the rotor body. Alternatively, the surface-enhancing structure 14 could also be described as being formed by a number of recesses 18 (three recesses 18 in the illustrated embodiment) that extend radially inwards from the outer surface of the rotor body. In principle, it would also be conceivable to provide both projections and recesses, whereby such a design variant would differ from the one shown in that the recesses would not necessarily be the same size as the projections, but might be larger or smaller in their radial extent relative to the longitudinal axis L. It is clearly evident in the figures, particularly in Fig. 2, that the distances, i.e., spaces 16a between two projections 16, and the distances, i.e., spaces 18a between two recesses 18, are each the same length as the projections 16 or recesses 18, respectively. The length of the projections 16 or spaces 16a, or of the recesses 18 or spaces 18a, is measured in the circumferential direction U. According to the invention, this length, denoted by l and la in Fig. 1, can be less than 2 mm, in particular 1 mm or less. It is also clearly evident that the distances between two tooth heads 12, i.e. the spaces 12a, for winding the or inserting coil wire into the winding space 20 are significantly larger (in circumferential direction U) than the spaces 16a or 18a. The specific design of the surface-enlarging structure 14, in particular with the small gaps 16a or the recesses 18, ensures that a cooling airflow in the air gap between the rotor body and adjacent components of the electric motor, for example between the rotor body and the stator, is not guided over the surface-enlarging structure 14 in the direction of the longitudinal axis L of the rotor, but is additionally swirled by it. Thus, not only is the surface-enlarging structure 14 designed to ensure better heat dissipation due to its increased surface area, but the cooling air is also able to absorb heat more effectively. Overall, this significantly improves the cooling effect compared to prior art solutions.

Claims

An electric motor (100), in particular a series-wound motor, for a driven electric hand-held power tool, comprising a rotor (110) with a driven rotor shaft extending along a longitudinal axis (L), and a rotor body (112) non-rotatably connected to the rotor shaft, which in cross-section has a number of tooth heads (12), wherein a space (12a) for winding a winding space (20) of the rotor body with coil wire is defined between each pair of tooth heads, characterized in that each tooth head (12) has, in cross-section, a surface-enhancing structure (14) on its outer circumference with a number of crenellated projections (16) and / or recesses (18) in cross-section, which are arranged next to each other at substantially regular intervals in the circumferential direction (U) with respect to the respective tooth head (12).wherein the spaces (16a) between two adjacent projections (16) and / or the recesses (18) of the surface-enlarging structure (14) are dimensioned so small in the circumferential direction (U) that the surface-enlarging structure (14) is able to swirl a cooling airflow in the air gap between the rotor body (112) and adjacent components of the electric motor (100), in particular a stator that at least partially surrounds the rotor body (112). Electric motor (100) according to claim 1, characterized in that the rotor body (112) has a plurality of individual laminations (10) stacked in the direction of the longitudinal axis (L). Electric motor (100) according to claim 2, characterized in that the surface-enlarging structures (14) of two adjacent individual laminations (10) of the rotor body (112) are arranged in alignment with each other. Electric motor (100) according to one of the preceding claims, characterized in that the crenellated projections (16) are essentially of the same length in the circumferential direction (U). Electric motor (100) according to one of the preceding claims, characterized in that the length (l) of the crenellated projections (16) in the circumferential direction (U) corresponds substantially to the length (1a) of the spaces (16a) in the circumferential direction (U) between two adjacent crenellated projections (16) of a tooth head (12). Electric motor (100) according to one of the preceding claims, characterized in that the spaces (12a) between the tooth heads (12) for winding the winding space (20) in the circumferential direction (U) are larger than the spaces (16a) between two adjacent projections (16) and / or than the recesses (18) of the surface-enlarging structure (14) of a tooth head (12) in the circumferential direction (U). Electric motor (100) according to one of the preceding claims 2 to 6, characterized in that the tooth heads (12) of two adjacent single laminations (10) of the rotor body (112) are arranged in alignment with each other. Electric motor (100) according to one of the preceding claims, characterized in that the spaces (16a) between two adjacent projections (16) and / or the recesses (18) of the surface-enlarging structure (14) in the circumferential direction (U) are smaller than 2 mm, in particular 1 mm or smaller. Rotor body (112) for an electric motor (100) with the features of claims 1 to 8, in particular with a rotor (110) having a driven rotor shaft extending along a longitudinal axis (L), and a rotor body (112) non-rotatably connected to the rotor shaft, which in cross-section has a number of tooth heads (12), wherein a space (12a) for winding a winding space (20) of the rotor body (112) with coil wire is defined between each pair of tooth heads, characterized in that each tooth head (12) has, in cross-section, a surface-enhancing structure (14) on its outer circumference with a number of crenellated projections (16) and / or recesses (18) in cross-section, which are arranged next to each other at substantially regular intervals in the circumferential direction (U) with respect to the respective tooth head (12).wherein the spaces (16a) between two adjacent projections (16) and / or the recesses (18) of the surface-enlarging structure (14) are dimensioned so small in the circumferential direction (U) that the surface-enlarging structure (14) is able to swirl a cooling airflow in the air gap between the rotor body (112) and adjacent components of the electric motor (100), in particular a stator that at least partially surrounds the rotor body (112).

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