Abstract:
An electric motor with a thermal safety feature has a rotor having a rotor winding, a brush for a current transmission to the rotor winding, a case which receives the brush, a switch-off contact which is arranged relative to the case so that it is contacted in response to a thermally caused position change of the case.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an electric motor which is provided with a thermal safety feature. 
     Electric motors of the above mentioned general type are known in the art. One of such electric motors is disclosed for example in U.S. Pat. No. 5,089,735. The known electric motor has a rotor with a rotor winding. The known rotor is provided with a brush for a current transmission from a stationary current terminal of the electric motor to a rotatably supported rotor winding. The brush is received movably in a case and is spring-loaded against a collector of the rotor. The known electric motor is also a collector motor. The present invention is however not limited to collector motors, but instead can be used for example for sliding ring motors or in general for electric motors in which a current transmission is performed to the rotor winding through a brush or a similar sliding body. 
     In order to provide the protection from thermal overloading, the known electric motors have a thermal safety means. The thermal safety means include a bimetal-interruption contact which is arranged closely on the brush of the electric motor. During heating of the brush and thereby the bimetal-interruption contact, the bimetal interruption contact opens by a switch-off temperature. Thereby the current supply to the rotor winding is interrupted, and a further thermal heating of the electric motor is prevented. 
     Another electric motor provided with a thermal safety feature is disclosed for example in the European patent document EP-A 171 446. This electric motor has metal spring tongs composed of a shape memory elements, on which the brushes of the electric motor are mounted. The spring tongs press the brushes against a collector of the electric motor. The shape memory elements change their shape abruptly during exceeding of a threshold temperature. The spring tongs composed of the shape memory elements in the known electric motors are shaped and arranged so that in the event of exceeding the threshold temperature, the brushes are lifted from the collector and thereby the current supply to the rotor winding is interrupted. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide an electric motor of the above mentioned general type, which avoids the disadvantages of the prior art. 
     In keeping with these objects and with others which will become apparent hereinafter, one feature of present invention resides, briefly stated, in an electric motor which has a switch-off contact which is arranged relative to a case so that it is contacted during a thermally-caused length change of the case. 
     The thermally dependent position change can be a temperature expansion of the case. Also, it is possible that the case is connected with a synthetic plastic part which in response to an undesirably intense heating softens and melts so that the position of the case changes. 
     The switch-off contact is connected to ground, so that a short circuiting is released when the case supplied with a potential comes in contact with the switch-off contact due to the thermally caused position change. Thereby a melting safety element is burnt through and a current supply to the electric motor is interrupted. 
     It is possible to lift the case from a counter contact by the thermally caused change of the switch-off contact, and thereby to interrupt the current supply to the electric motor. 
     The contacting of the switch-off contact can be reversible. Thereby the electric motor after cooling can return back to operation. On the other hand, in the invention the contacting or decontacting of the switch-off contact can be irreversible. Thereby the electric motor can not be brought back into operation after a thermal overloading. 
     The present invention has the advantage that it provides for a simple and price-favorable thermal safety of the electric motor. The switchoff contact can be formed for example as a tong which is formed of one piece with a housing part of the electric motor and produced by punching. In the predetermined position it is bent relative to the case. Thereby no additional parts or additional working steps are needed during assembly of the electric motor, and the manufacture is not more expensive than the manufacture of an electric motor without the inventive thermal safety feature. The thermal safety is released during a thermal overloading of the electric motor in a reliable manner, and there is no danger of burning. 
     The inventive electric motor is provided for operation of a secondary air pump (impeller), for oxidation of exhaust gasses of an internal combustion engine. In order to oxidate non-burnt or incompletely burnt hydrocarbons and carbon monoxide secondary air is supplied to hot exhaust gasses of internal combustion engines with such a secondary air pump. The secondary air can be supplied in the internal combustion engines both with and without a catalyst. 
     The novel features which are considered as characteristic for the present invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a view which schematically shows an electric motor in a half section, in accordance with the present invention; and 
     FIG. 2 is a view showing a collector arrangement of the electric motor of FIG. 1 on an enlarged scale. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     An electric motor as shown in FIG. 1 is identified as a whole with reference numeral  10 . The electric motor has a cup-shaped housing  12  which is produced by deep-drawing of a metal sheet. Permanent magnets  14  are arranged on an inner side of the housing and form a stator. 
     A rotor  16  with a motor shaft  18  and a rotor winding  20  is rotatably supported in the motor housing  12 . The motor shaft  18  carries a collector (commutator)  22 . Spring-loaded carbon brushes  24  abut against the periphery of the collector  22 . A bearing block  26  is arranged on an inner side. The bearing block  26  is a disk-shaped part which is produced by punching and deep-drawing of a metal sheet. It closes an open end side of the cup-shaped motor housing  12 . The bearing shield  26  is a part of the motor housing. Together with the motor housing  12  it is connected to a ground potential. A ball bearing  30  is pressed in a hollow cylindrical depression  28  of the bearing shield  26 . It is rotatably connected with the motor shaft  18  in the bearing shield  26 . 
     The region of the collector  22  and the carbon brushes  24  is shown in FIG. 2 on an enlarged scale and can be clearly understood from it. The carbon brushes  24  of the electric motor  10 , as shown in FIG. 2, are displaceably supported in a case  32 . The case  32  is produced from a metal sheet. It has a rectangular cross-section such as the carbon brushes  24 . The case  32  is arranged radially to the motor shaft  18 . The brushes  24  are displaceable radially to the motor shaft  18 . A helical pressure spring is arranged as a brush spring  34  in the case  32 . It abuts against inwardly bent metal plate tongs  26  of the case  32 . The carbon brushes  24  are pressed in a springy manner against the collector  22 . A side wall of the case  32  is provided with a slot  38  which extends radially to the motor shaft  18 . A brush cable  40  passes through the slot  38  and is mounted on the carbon brush  24 . A current supply is provided through the brush cable  40  via the carbon brushes  24  and the collector  22  to the rotor winding  20  which is not shown in FIG.  2 . 
     The case  32  is provided with two tong pairs  42 ,  44  on its side wall. The case  32  is mounted with the tong pairs on a brush holder  46  which is produced by injection molding from a thermal plastic. The brush holder  46  is mounted on an inner side of the bearing shield  26 . It electrically insulates the case  32  on the inner side of the bearing shield  26 . The brush holder  46  has two slot pairs  48  through which the tong pairs  42 ,  44  of the case  32  pass. The tong pairs  42 ,  44  are bent away from one another at the side of the brush carrier  46  which faces away from the case  32 . Thereby the tong pairs engage the brush holder  46  and hold the case  32  on the brush holder  56 . 
     The slot pair of the brush carrier  46 , through which the motor shaft  18  passes near the tong pair  42  of the case  32 , has a length which corresponds to a width of this tong pair  42 . This slot pair is thereby coextensive with the tong pair  42  and not visible in the drawings. The tong pair  42  located near the motor shaft  18  is non displaceable in the slot pair through which it extends. It forms a fixed bearing  42  of the case  32 . 
     Another tong pair  44  of the case  32  which is remote from the motor shaft  18 , passes through the slot of the slot pair  48 . It has a greater length than that corresponding to a width of the tong pair  44 . The tong pair  44  of the case  32  which is remote from the motor shaft  18  is thereby displaceable radially from the motor shaft  18 . It forms a releasable bearing  44  of the case  32 . 
     A tong which forms a switch-off contact  50  is bent inwardly on the bearing shield  26 . The switch-off contact  50  is located at a distance from the case  32  at the side which faces away from the motor shaft  18 . The switch-off contact  50  overlaps an outer edge  52  of the case  32 . A gap between the switch-off contact  50  and the outer edge  52  is seen in FIG.  2  and identified with arrow F. 
     A brush arrangement shown in FIG. 2 has a thermal safety element  4  for the electric motor  10 . When the electric motor is thermally overloaded due to an excessively high or excessively long current, its carbon brush  24  is heated. The carbon brush  24  heats the case  32  with the tong pairs  42 ,  44 . The case  32  and the tong pair  42 ,  44  deviate from the brush holder  46  which is composed of a thermoplast so that the force of the brush spring  44  displaces the case  32  outwardly radially from the motor shaft  18 . During the displacement, the tong pair  42  of the deviating brush holder  46  near the motor shaft  18  which is formed in a fixed bearing is deformed, and the tong pair  44  which forms a releasable bearing is displaceable. The outer edge  52  of the case  32  is in contact with the switch-off contact  50 . While the switch-off contact  50  which is of one piece with the bearing shield  26  is connected to ground similarly to the bearing shield  26  and the total motor housing  12 , the case  32  has an electrical potential which is different from the ground through the brush cable  40  and the brush  24 . The contacting of the case  32  with the switch-off contact  50  provides a short circuiting, with which a not shown melting safety element is burnt through and thereby the current supply of the electric motor is interrupted. 
     The short circuiting is irreversible, and the electric motor can not be brought back into operation. The softening temperature of the brush holder  46  is selected so that the brush holder  46  is softened by a brush temperature, at which no inflamation temperature in the electric motor  10  takes place and also no burning danger is generated. It abuts for example against the motor shaft  18  near the tong pair  42 , since the brush  46  at the abutting against the collector  22  is heated more than at a distance from the collector  22 . For example, the softening temperature of the burst carrier  46  is reached, when a brush temperature is increased to approximately 320-360° C. 
     It will be understood that each of the elements described above, or two or more together, may also find a useful application in other types of constructions differing from the types described above. 
     While the invention has been illustrated and described as embodied in electric motor with thermal safety, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention. 
     Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can, by applying current knowledge, readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention.