Abstract:
A low current motor protector ( 10, 10 ′) has an oblong metal housing ( 12 ) defining a switch chamber and formed with an open end. The housing has opposing flange portions extending transversely from the open end. Spaced apart lid portions ( 18   a,    18   b;    18   a′,    18   b ′) are received on and clamped to the flange portions with an electrically insulating gasket ( 16 ) having a central opening interposed between the housing and the lid parts. The lid parts are formed with a recessed ceramic heater seat disposed in the switch chamber having spaced apart contact shelves ( 18   f ) which receives a ceramic heater ( 20 ) which is maintained in place and provided with contact force by a spring clip ( 22, 22 ′) attached to one of the lid parts. A thermostatic switch is mounted in the housing and has a movable electrical contact movable into and out of engagement with a stationary electrical contact on one of the lid parts.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates generally to motor protectors and more particularly to low current protectors for electrical devices such as compressors, transformers and small motors against overload and locked rotor.  
       BACKGROUND OF THE INVENTION  
       [0002]     It is well known to provide reliable and inexpensive motor protectors that comprise a small housing in which is disposed a small current carrying thermostatic switch having a bimetal disc adapted upon the occurrence of certain thermal conditions to snap into and out of engagement with a stationary contact to respectively close and open an electrical circuit.  
         [0003]     In order to make such protectors quickly responsive to very small current levels, it is also known to provide a supplemental heater mounted in heat transfer relation with the disc. An example of this type of protector is shown and described in U.S. Pat. No. 4,476,452 and comprises a metallic housing having an open end with a flange formed around the open end and a gasket and lid received on and clamped to the housing. A heat responsive electrical switch is disposed in the housing and is adapted to electrically connect and disconnect a current path through the housing and lid upon the occurrence of selected thermal conditions. The lid comprises two discrete, spaced apart portions, one portion having an elongated part extending therefrom to serve as a terminal and the other portion mounting a portion of the switch. A coil heater is electrically and mechanically connected between the spaced apart portions of the lid providing a protector particularly useful for fractional horsepower motors.  
       SUMMARY OF THE INVENTION  
       [0004]     Although motor protectors made according to the above referenced patent are suitable for low current applications, there is a need to provide low cost protectors useful for low current applications having even more current sensitivity yet one which is mechanically robust and one which has increased reset times required for certain applications, such as protecting compressor motors.  
         [0005]     It is therefore an object of the present invention to provide a low current motor protector which overcomes the above noted limitations of the prior art. Another object of the invention is the provision of a low current motor protector which has improved current sensitivity, yet is mechanically robust regarding handling and vibration. Yet another object of the invention is the provision of a motor protector of the low current type which can be easily and accurately adapted for use with different electrical devices, such as compressors, transformers and small motors. Yet another object of the invention is the provision of a low current motor protector which is particularly conducive to low cost assembly techniques. Still another object of the invention is the provision of a low current motor protector which has an end of life, open circuit condition.  
         [0006]     Briefly, in accordance with the invention, a low current motor protector comprises a generally parallelepiped shaped metal housing defining a switch chamber which has an open end formed with an outwardly, laterally extending flange and in which a thermostatic switch is mounted. A window shaped gasket is received on the flange and first and second spaced apart lid parts are received on and clamped to the flange through the gasket electrically separated from the housing. The lid parts are each formed with a recessed contact shelf in alignment with and facing each other and adapted to receive end portions of a ceramic substrate. A thick film heater is disposed on the lower face surface of the ceramic substrate with contact portions disposed at opposite ends for receipt on the contact shelves of the lid parts. The recessed positioning of the ceramic heater into the switch chamber results in placement of the ceramic heater in optimum heat transfer coupling with the thermostatic switch. According to a feature of the invention, the ceramic substrate increases the thermal mass of the heater to provide an extended reset time for the thermostatic switch. The thermostatic switch has a movable contact which is movable into and out of engagement with a stationary contact mounted on the lower or inside surface of one of the lid portions so that upon selected heating of the thermostatic switch by the ceramic heater the switch will cause the movable contact to move from a contacts engaged or closed position to a contacts disengaged or open position.  
         [0007]     The ceramic heater is received on the recessed seat formed by the contact shelves and is held in place by means of a mechanical clip extending across one of the lid portions which applies suitable force on the outer surface of the substrate against the contact shelves for good electrical engagement of the ceramic heater contacts therewith. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]     Other objects, advantages and details of the novel and improved electrical motor protector of this invention appear in the following detailed description referring to the drawings in which:  
         [0009]      FIG. 1  is a top plan view of a motor protector made according to the prior art;  
         [0010]      FIG. 2  is a cross sectional view taken through line  2 - 2  of  FIG. 1 ;  
         [0011]      FIG. 3  is a blown apart perspective of a motor protector made in accordance with a preferred embodiment of the invention; and  
         [0012]      FIG. 4  is similar to  FIG. 3  of a modified embodiment of the invention but shown without the thermostatic switch. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0013]     With reference to  FIGS. 1 and 2 , a prior art low current motor protector comprises an oblong metallic housing  1  having a bottom wall  1   a , sidewall  1   b  and a laterally, outwardly extending flange  1   c  at a free end of the sidewall. A gasket  2  of electrically insulating material is received on flange  1   c  and a lid  3  formed of spaced apart parts  3   a ,  3   b  are received on gasket  2 . An extended portion  1   d  of flange  1   c  on opposed sides of the housing are bent over to clamp the lid parts  3   a ,  3   b , through the gasket.  
         [0014]     Gasket  2  is formed with a window  2   a  aligned with a switch chamber defined by the sidewall  1   b  of the housing and a thermostatic switch comprising a snap-acting bimetallic member  4  has one end fixedly mounted on the bottom wall  1   a  of the housing and a free distal end mounting a movable electrical contact  4   a  movable into and out of engagement with stationary contact  3   c  welded to lid part  3   a.    
         [0015]     A supplemental coil heater  5  has one end welded to lid part  3   b  and an opposite end welded to lid part  3   a . Lid part  3   b  is formed with a terminal portion  3   e  and housing  1  is formed with a terminal  1   e.    
         [0016]     Movable contact  4   a  is normally in electrical engagement with stationary electrical contact  3   c  thereby forming a current path between the terminals through bimetal  4  and coil heater  5 ; however, upon being heated to a selected temperature, for example, due to an overload current, disc  4  will snap to its dashed line configuration to open the circuit.  
         [0017]     Although the prior art motor protector described above is effective for certain applications, a protector having even more current sensitivity is desired in order to be useful in a wider market range. This requires increased resistance of the heater which could be obtained by decreasing the cross sectional area of the coil heater; however, this results in heater elements which are too fragile for normal handling. Further, in order to be useful in certain markets such as compressors having positive temperature coefficient (PTC) starting devices, a longer off or reset time is needed to allow appropriate cooling of the PTC starting device.  
         [0018]     These limitations are overcome by a protector made in accordance with the present invention. As shown in  FIG. 3 , a motor protector  10  comprises a metallic oblong housing  12  having a bottom wall  12   a , sidewalls  12   b  extending away from the bottom wall and having a flange  12   c  extending laterally and outwardly from the free end of the sidewall.  
         [0019]     A thermostatic switch  14  is received in a switch chamber  12   d  defined by sidewalls  12   b . Switch  14  comprises a bimetallic, snap acting disc  14   a , known in the art, having one end  14   b  cantilever attached to the bottom wall  12   a  of the housing, preferably at an inwardly extending platform  12   e , as by welding thereto using welding slug  14   c . A movable electrical contact  14   d  is mounted at the free end  14   e  of the disc on the side thereof facing away from the bottom wall of the housing.  
         [0020]     An electrically insulating gasket  16 , generally in a shape of a window frame  16   a , is received on and covers flange  12   c  of the housing. The gasket has an extended portion  16   b  along two elongated opposite sides which are folded back toward the center of the window frame configuration into a generally V-shape in order to sandwich two opposed flange portions of the housing between layers  16   a  and  16   b . Preferably, an additional portion  16   c  extends from extended portion  16   b  for placement along the sidewalls  12   b  of the housing to ensure electrical isolation between lid parts, to be discussed, and the housing.  
         [0021]     A lid  18  comprises first and second parts  18   a ,  18   b , respectively. Each lid part has a flat support portion  18   c ,  18   d , respectively, lying in a plane, for reception on the frame gasket portion  16   a  on flange  12   c  and opposed tabs  18   e  bent back toward the center of the lid part forming a generally a V configuration with the support portion. Tabs  18   e  on lid part  18   b  are formed with a cut-out on the curved portion of the bend of the tabs to define catch surfaces  18   k  lying in the plane of support portion  18   d  extending into the cut-out for a purpose to be described.  
         [0022]     Each lid part is formed with a heater seat in the form of a contact shelf  18   f  spaced from the plane in which the respective support portion  18   c ,  18   d , lie on the side of the lid parts facing the switch chamber so that the shelves are disposed within the switch chamber  12   d  when the lids are placed on the housing. Respective side and back walls  18   g ,  18   h  are joined to the shelves to ensure a robust seat for maintaining a selected location of a heater element. Shelves  18   f  are aligned and face each other and are spaced from each other a selected amount to provide direct, close, radiational heat coupling of a heat element received on the shelves with snap acting thermostatic disc  14   a.    
         [0023]     A heater element in the form of a ceramic substrate  20  has opposed first and second face surfaces  20   a ,  20   b  and first and second ends  20   c ,  20   d , respectively. An electrical contact layer  20   e  of suitable material, such as a silver containing material, preferably formed with external contact bumps, extend across each end  20   c ,  20   d  on first face surface  20   a  and an electrical resistive thick film layer  20   f  covered by a glass layer is disposed on the first face surface  20   a  extending between and in electrical connection with the contact layers. The contact layers of the ceramic substrate are adapted to be received on ledges  18   f  with the ceramic element closely fitting in the recessed seat and with the heater surface facing thermostatic disc  14   a.    
         [0024]     A stationary electrical contact  21  is mounted preferably on a platform formed in support portion  18   c  of lid part  18   a  on the side of the lid part having shelf  18   f . Movable contact  14   d  is adapted to move into and out of engagement with stationary contact  21  in dependence upon the dished configuration of the thermostatic disc  14   a.    
         [0025]     A spring clip  22  is formed of suitable material such as stainless steel and generally has an elongated body portion to extend across the width of housing  12  with opposite end portions  22   a  bent back on themselves to form a generally V configuration with the body portion and a locking tab  22   b  is struck out from each bent over portion with the free end  22   c  of the tab extending away from the free end of each locking tab portion  22   b . A force application portion in the form of a projection  22   e  extends away from the body portion of clip  22  on the same side of the clip that end portions  22   a  are bent to extend.  
         [0026]     One terminal  12   f  extends from housing  12  and another terminal  18   m  extends from lid part  18   b.    
         [0027]     Once thermostatic switch  14  is mounted in switch chamber  12   d , gasket  16  is slipped onto flange  12   c  followed by lid parts  18   a ,  18   b  with V-shaped tabs  18   e  slipped over gasket  16 , including portion  16   b . The lid parts are spaced from one another a selected distance sufficient to ensure electrical separation and with ledges  18   f  property spaced from each other to receive ceramic substrate  20  thereon with the contact surfaces  20   e  received on respective shelves  18   f . Tabs  18   e  are then bent inwardly to clamp the lid parts in their selected positions. The ceramic substrate is then inserted and clip  22  is placed over lid portion  18   b  so that end portions  22   a  are received over tabs  18   e  and with struck out locking tab  22   b  received under respective catch surfaces  18   k  and with force application portion  22   e  placing a force on face  20   b  of ceramic substrate  20 .  
         [0028]      FIG. 4  shows a modified embodiment  10 ′ in which catch surfaces  18   k  are formed in lid portion  18   a ′ and clip  22 ′ is formed with a leg portion  22   f  for positioning force application projection  22   e ′ so that it will be aligned with the center of ceramic substrate  20  when clip  22 ′ is attached to lid part  18   a ′. Leg  22   f  may be bent upwardly, as shown in the drawing, for example, along with dashed line  18   g , to provide a suitable bias to the ceramic substrate.  
         [0029]     Motor protector  10 ,  10 ′ made in accordance with the preferred embodiments offer a number of advantages over the prior art. The cross section of the heater material is decreased to provide increased resistance making the protector more current sensitive but without loosing robustness. The ceramic substrate adds thermal mass to the heater element to increase the reset time of the thermostatic switch, a feature which is important for certain applications, for example, those with compressors which require an extended cool down time for a PTC starter. Placement of the heater in a recess formed in the switch chamber of the housing provides optimum thermal coupling with the thermostatic switch as well as providing a seat for the heater protected from accidental dislodgement during handling, vibration and the like.  
         [0030]     The thick film heater provides a fail safe end of life, i.e., burn out of the heater material or breaking of the ceramic substrate results in an open circuit. Use of the thick film heater also provides an advantage in that the heater film can be trimmed to provide accurate resistance values resulting in accurate time behavior. Further, laser trimming allows more flexibility in defining the nominal resistance value and can be used with the wider range of values than a corresponding coil heater and hence can be used in a wider range of applications.  
         [0031]     The use of the spring clip to maintain the ceramic heater in its seat ensures optimum electrical and mechanical connection while avoiding welding or soldering operations.  
         [0032]     While the invention has been described in combination with a specific preferred embodiment thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art in view of the foregoing description. It is intended that the invention include all modifications and equivalents of the disclosed embodiment falling within the scope of the appended claims.