Abstract:
An electric motor is disclosed and which includes a braking element which is biasingly urged into a braking relationship relative to the electric motor, when the electric motor is deenergized, but which is further moved out of braking engagement when the electric motor is energized. The invention finds particular usefulness when used on hand tools, such as angle grinders.

Description:
RELATED PATENT DATA 
       [0001]    The present patent application claims priority from German Application Serial No. 10 2009 014 783.7, and which was filed on Mar. 25, 2009, PCT/EP2010/001655, and which was filed on Mar. 17, 2010. 
         [0002]    The present invention relates to an electric motor with a brake driven by an electric motor, and in particular to an electric motor for use with angle grinders, which have a stator, and which further have at least one pole pair, with at least two poles each having a pole shoe which points inwardly towards a rotor, and wherein each pole shoe is surrounded by a winding. In this invention, the stator accommodates at least one braking device for braking the rotor, said braking device comprising a braking element. In this invention, it is possible for the braking element to be moved towards the outer circumference of the rotor for braking purposes and to be moved counter to a braking force by virtue of the magnetic field in the energized state of the electric motor, with the result that the braking device is released in the energized state of the electric motor. 
         [0003]    Electric motors having an electromagnetic brake are already known from the prior art. 
         [0004]    For example, GB 920,485 describes an electric motor with a housing, and poles, which are fixed in the housing, and a rotor and a magnetic braking element, which is arranged within the poles. The braking element brings about braking of the rotor when it is possible for a braking force to be applied to said braking element, or when it is not lifted off from the rotor by a force counteracting the braking force. The document describes a purely radially acting braking device, with the result that the maximum achievable braking effect is comparatively low. 
         [0005]    A further electric motor with a brake is known from DE 847 933, and wherein the brake consists of a rocker-like arm of the stator. Still another electric motor in the form of a split-pole motor with a braking device is shown in DE 538 902; and wherein the brake lever of said the braking device is fastened on the movable stator part of the motor in a rather complicated manner and is shifted thereby. In addition, a brake drum is provided, which is braked from the inside. 
         [0006]    The reference EP 1 149 458 B1 describes an electric motor with an electromagnetic brake, and in particular to a device for use with an angle grinder, of the generic type, and wherein in this arrangement the braking device is accommodated in a pole and the braking element is in the form of a rocker, which is mounted movably about a pivot axis within a free space of the pole, and wherein the braking element is a multiple-armed lever with a braking arm, and a disengaging arm, and where it is possible for a braking force to be applied to the braking arm and a disengaging force, which is in the opposite direction to the braking force, being exerted on the braking element by the disengaging arm when the winding is energized. A similar motor with a braking device within a pole shoe is also disclosed in WO 2004/088821. 
         [0007]    The object of the present invention is to provide an alternative configuration of an electric motor, in which a high braking force can be made available, and which can be produced efficiently and is reliable. 
         [0008]    The present invention achieves this object by providing an electric motor having the features of claim  1 , and wherein the braking device is arranged in the stator and located between two poles in the circumferential direction, and wherein the braking element is in the form of a single-armed brake lever, which is capable of being pivoted at one of its ends about a fulcrum in the stator, with the result that there is no need for a bearing arrangement of the brake lever by virtue of the provision of a shaft or spindle. 
         [0009]    The bearing arrangement of the disclosed braking element is therefore provided not by way of a rotary spindle, but instead the braking element is mounted such that it can pivot about a fulcrum in the housing of the stator. In this case, the guidance takes place not by way of a rotary spindle, but advantageously by way of a slotted-link guide. The slotted link is formed by the stator housing. By virtue of the provision of a slotted-link guide in the yoke of the stator, and a single-armed brake lever, it is possible to dispense with a bearing arrangement provided by means of a shaft. 
         [0010]    In the present invention, the fulcrum is provided at one end of the brake lever, and wherein the brake lever is guided with this end in the aforementioned slotted link in the stator, and can thus be pivoted. 
         [0011]    The arrangement of the brake lever in the circumferential direction between the poles, and the pole shoes, opens up further degrees of freedom in terms of the construction since better access to the outer circumference of the rotor is provided there thereby. 
         [0012]    Furthermore, it has proven to be advantageous when the fulcrum of the brake lever is arranged on the exit side with respect to a brake lining of the brake lever and in the direction of rotation of the rotor. The rotation of the rotor in the braking mode, that is, in the de-energized state of the electric motor, is then such that the braking effect of the brake lever is reinforced by the rotor running onto the brake lining. 
         [0013]    Provision is furthermore made for the braking element to consist of a magnetically conductive material in order to effect the opening of the brake during energization of the field by virtue of the magnetic force counteracting the braking force which presses the braking element against the rotor and thus lifts off the braking element from the rotor. 
         [0014]    Furthermore, provision can be made for the braking element to have a brake lining on its side facing the rotor. The brake lining can be formed from a nonconductive material, which thus prevents a force from being applied to the brake lining when the field is energized in the direction of the rotor. 
         [0015]    In this invention, provision can be made for the brake lining to have such a thickness in the radial direction that it bridges the gap between the stator and the rotor. Even in the state in which the brake lining is lifted off from the rotor, there is preferably only a small gap between the brake lining and the rotor. In the lifted-off state, the gap can correspond to the gap between the pole shoe and the rotor. Thus, it is only necessary for the braking element to travel a short distance when disengaging from the braking position. 
         [0016]    Furthermore, provision can preferably be made for the braking force to be applied by a spring. In this case, the spring can extend substantially in the radial direction, as is provided in the prior art in accordance with EP 1 149 458 B1, for example. Furthermore, provision can be made for an axially extending spring which acts on the braking element with a braking force in the direction of the rotor. In this arrangement, it is possible for the spring to be in the form of a bar spring. Such a provision of a bar spring provides advantages over the use of a radial spring, as is described in the prior art, because it is no longer necessary for a plurality of spring elements to be provided. Alternatively, however, other springs or biasing elements can also be provided, for example radially acting springs, may be employed. 
         [0017]    In the disclosure that is provided, it should be appreciated that the axial direction should always be understood to mean the axial direction of a motor shaft. 
         [0018]    In this invention, more than one braking device may be provided. For example, in the case of a two-pole electric motor, provision can be made for braking devices to be provided on two mutually opposite sides and which are located between the poles. The braking devices are then generally rotationally symmetrical. In principle, provision can also be made for more than two poles to be provided, in which case one or more braking devices can be arranged between the poles. In this arrangement, the braking devices which are arranged between the poles are not in any event surrounded by the field windings and which surround the pole shoe. 
         [0019]    In one form of the invention, the face of the braking element to which the brake lining is applied, and which points in the direction of the rotor to continue the contour of the stator which surrounds the braking element. That is to say that, in this case, only the brake lining bridges the gap between the stator and the rotor. For this purpose, the brake lining needs to have a sufficient thickness in the radial direction. 
         [0020]    Furthermore, provision can be made for at least one squirrel-cage windings to be provided on the brake lever. 
         [0021]    In the aforementioned arrangement, the squirrel-cage winding serves the purpose of preventing the lever and therefore the brake from wobbling during AC operation. 
         [0022]    In one form of the invention, the braking device can be arranged on the entry side, for example, with respect to the direction of rotation of the rotor. In this arrangement, it has a self-energizing effect. In the reverse arrangement with respect to the direction of rotation, the braking force is only determined by the spring force. However, in this arrangement it is easier to monitor the opening and closing of the braking device. 
         [0023]    In one form of the invention, the braking element can extend in the axial direction over the entire length of the stator or only over part thereof. In particular, provision can be made for it to extend over approximately the entire axial length and for there to be no braking element provided at the two ends of the stator, and in the axial direction for reasons of stability. 
         [0024]    Furthermore, the invention relates to an electric handtool device, and in particular an angle grinder, having an electric motor of the abovementioned type. 
     
    
     
         [0025]    Further advantages and features will be understood from the rest of the present application. The invention will be further explained, in more detail, below, by reference to the attached drawing, and wherein: 
           [0026]      FIG. 1  a partial, transverse vertical sectional view taken through an electric motor of the present invention. 
           [0027]      FIG. 2  is a fragmentary plan view taken in the axial direction of an electric motor of the present invention. 
           [0028]      FIG. 3  is a perspective side elevation view of the an electric motor of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0029]    In  FIG. 1 , an electric motor is denoted by the reference symbol  10 , and wherein only the left-hand half (in the illustration) is shown of the electric motor  10 . The right-hand half of the electric motor  10  is understood to be rotationally symmetrical. The electric motor in this case comprises a stator  12 , and a rotor  14 . The stator  12  comprises a pole pair, of which a first pole is denoted by the reference symbol  16  and a second pole is denoted by the reference symbol  18 . The poles  16 ,  18  are diametrically opposite one another. The poles  16 ,  18  in this case comprise pole shoes  17 ,  19 , which are supported and extend inwardly from a yoke  20  and in the direction of the rotor. Each pole has have two pole tips  22 , with only one pole tip  22  being shown per pole shoe  17 ,  19  in  FIG. 1 . Windings  24 , are located in an internal space formed by the gap which exists between the pole tip  22  and the yoke  20 . The windings do not protrude beyond the ends of the pole tips  22  in the circumferential direction, and are provided between the pole tips  22  and the yoke  20 . 
         [0030]    In this case, the rotor  14  preferably rotates in the clockwise direction. However, provision can also be made for there to be a reverse direction of rotation. 
         [0031]    The respective windings  24  therefore surround the respective pole shoes  17 ,  19 , respectively. 
         [0032]    A braking device  26 , which comprises a braking element  28  and which is depicted in the form of a single-armed brake lever  29 , is provided in a circumferential location and is oriented between the two poles  16 ,  18 . In this case, the brake lever  29  is mounted such that it can rotate about a fulcrum  30  in the stator  12 . The fulcrum  30  is provided at one end of the single-armed brake lever  29 , and wherein the guidance of the brake lever  29  in the stator  12  is realized via a slotted-link guide. In this invention, it is no longer necessary for the brake lever  29  to be mounted by means of a shaft within the stator  12 . 
         [0033]    The brake lever  29  mounts a brake lining  32 , which is lifted off or slightly spaced from the rotor  14  in the state illustrated, with the result that there is a slight gap between the brake lining  32  and the rotor  14 . The gap between the brake lining  32  and the rotor  14  corresponds or is similar to the gap provided between the rotor  14  and the pole shoes  17 ,  19 . The brake lining  32  is arranged on that side  38  of the brake lever  29  which points towards the rotor  14 . 
         [0034]    In this case, the brake lining  32  is arranged in such a way that it is capable of applying a radially directed braking force to the rotor  14 . It is located centrally on the brake lever  29 , and does not extend over the entire length of the lever arm of the brake lever  29 . 
         [0035]    The material forming the brake lining  32  is fabricated from a substantially magnetically, nonconductive material. On the other hand, the material forming the brake lever  29  is fabricated from a magnetically conductive material. In this arrangement, the brake lever  29  forms two contact faces  34  and  36  with the stator  12 . If pivoting of the brake lever  29  about the fulcrum  30  takes place, the contact faces  34  and  36  are lifted off from the associated faces of the stator  12 , with the result that a slight gap is produced by this movement. As a result of this slight movement, the brake lining  32  comes to bear a radial force against the rotor  14 . 
         [0036]    Furthermore, the brake  29  has a face  38 , which points in the direction of the rotor  14 . The face  38  generally follows the contour or exterior shape of the stator  12 . 
         [0037]    For the application of a braking force, a spring  40  is provided which runs substantially in the axial direction of the electric motor  10 , with the result that the spring  40  bears against the brake lever  29  over its axial extent and applies a radially directive force inwardly onto said brake lever over substantially the entire length thereof, with the result that the brake lever  29  is pressed against the outer circumference of the rotor  14 . The fitting of the spring  40  is in this case is best seen in  FIGS. 2 and 3 . The spring is in this case a bar or leg spring. 
         [0038]    If the electric motor  10  is now switched on, i.e. the windings  24  are energized, a magnetic flux is induced in the stator  12 , and this magnetic flux results in the contact faces  34  and  38  being drawn close to the associated corresponding faces of the stator  12  and thus coming to bear against said faces. In the process, the contour of the contact faces  34  and  36  precisely follows the contour of the stator  12  in this region. This force counteracts the braking force provided by the spring  40  which exerts a force radially inwardly. As should be understood, the force induced by the magnetic flux is greater in terms of the magnitude of the braking force caused by the spring  40 , with the result that the brake lining  32  comes out of contact with the rotor  14 . For this purpose, the brake lever  29  performs a limited pivoting movement about the fulcrum  30 . This movement is sufficient for lifting or displacing the brake lining  32  off or away from the rotor  14 . In the process described, above, the faces of the brake lever attempt to close the gap between them and the circumferential faces of the stator  12 . 
         [0039]    Furthermore, a at least one squirrel-cage winding  42  is provided and which operates to prevent any vibration of the braking device  26  when the winding  24  is energized, which would result in the brake lining  32  coming into contact temporarily again and again with the rotor  14  and would result in the brake wobbling and noise being developed. 
         [0040]    If the electric motor  10  is switched off, the magnetic effect of the windings  24  diminishes and therefore so do the forces which hold the brake lever  28  with its contact faces  34  and  36  firmly against the corresponding faces of the stator  12 . The braking force applied by the spring  40  then exceeds the aforementioned force which lifts the brake lining  32  off from the rotor  14 , with the result that the brake lining  32  is pressed against the rotor  14  under the spring force of the spring  40 . In this state, there is a slight gap between the contact faces  34  and  36  and the associated faces of the stator as earlier discussed. 
         [0041]    As a result of the release of force the brake lining  32  is pressed against the rotor resulting in the rotor  14  being braked. As can be appreciated, the braking device  26  is automatically triggered when the motor  10  is switched off. 
         [0042]      FIG. 2  shows a fragmentary, plan view of an electric motor  10 , as is illustrated in the perspective side elevation view of  FIG. 3 . The drive shaft of the electric motor is in this case denoted by the. reference symbol  44 . 
         [0043]    As will be appreciated, identical components parts are denoted by identical reference symbols, with the fastening of the spring  40  being shown here. As can be seen, the spring is fixed on a component part  46 , with the result that it exerts a substantially radially, inwardly directed force on the braking element  28 . The arrangement of the spring  40  can also be seen in  FIG. 3 . The spring  40  is fixed in this case in a so-called coil supporting head  50 , and wherein the contact-making elements  52 , which are unused here, are used for fixing or orienting the spring and for other accommodating purposes. 
         [0044]    The abovementioned configuration makes it possible to provide an electric motor  10 , in which a simple bearing arrangement of a braking device  26  and which is located outside of the poles  16 ,  18  is possible, with it being possible to dispense with a bearing arrangement located about a pivot. 
         [0045]    This is particularly advantageous in the case of an angle grinder, in which a cutting disk is intended to be braked as quickly as possible when the motor  10  is switched off in order that there is no notable overrun of the cutting disk once the motor  10  has been switched off since this is associated with a high potential risk of damage to an associated work piece, or injury to a worker. It is therefore possible to ensure in the abovementioned way that a cutting disk of an angle grinder is always stopped as quickly as possible when the motor  10  is switched off.