Thermally responsive electrical switch

Field of the Invention

This invention relates generally to thermally responsive electrical switches and more particularly to small single phase hermetic motor protector switches for use inside air conditioning and refrigeration compressors.

Background of the Invention

It is known to provide thermally responsive switches for making and breaking an electrical circuit by moving an electrical contact into and out of engagement with a stationary electrical contact in response to selected changes in the temperature of the thermostatic disc caused by heating and cooling of the disc. Such switches have been placed in enclosed compressor housings in air conditioning and refrigeration systems and arranged to protect the motor and system components therein against over heating and over current conditions. An example of a thermally responsive switch of this type is shown in U.S. Patent No. 3,959,762 that shows a one pin protector in which a fully formed thermostatic disc is attached at a first end to a heater by means of a welded slug. A movable contact is mounted on the second opposite end of the disc and is arranged to move into and out of engagement with a stationary contact mounted on the single pin that extends into the switch chamber of the switch. The device is calibrated by deforming the top of the housing against the first end of the disc. A limitation of this type of protector having a fully formed disc is that cycle life is limited due to stress failure that occurs in the disc in front of the slug. Further, the size of the movable contact is limited in such a device in order to minimize adverse effects on the operational characteristics of the formed disc, i.e., temperature settings, thereby limiting the current capability of the protector.

Another example of a thermally responsive switch of this type is U.S. Patent No. 5,015,985. This patent shows a device having two terminal pins, one pin connected to an electrical resistance heater and a dome shaped housing, the other pin connected to a stationary contact. An oval or rectangular, fully formed thermally responsive snap acting element has one end welded to a metal support plate that is in turn welded to the metal housing and the other end of the snap acting element has a contact welded thereto and movable into and out of engagement with the stationary contact. As in the 3,959,762 patent referenced above, the disc is calibrated by deforming the housing at the location of the fixed end of the disc. Another example of such a single phase motor protector is know from the document EP 1 411 536.

Summary of the Invention

It is an object of the present invention to provide a motor protector having an envelope that is reduced in size yet has enhanced current capability and life expectancy. Another object of the invention is the provision of a thermally responsive switch useful as a motor protector in air conditioning and refrigerator systems particularly subjected to line voltage variations. Yet another object of the invention is the provision of a motor protector that overcomes the above discussed prior art limitations.

Briefly, in accordance with the preferred embodiment of the invention, a motor protector comprises a features according to claim 1.

The motor protector is calibrated by deforming the rigid flat bottom surface of the calibration rill rotationally pivoting the mount of the disc and moving the calibration ridge at the longitudinal end of the rill and disposed over the ring shaped dished portion of the disc against the deformed portion of the disc with the contacts in the engaged position. According to a feature of the invention, an electrical and thermal insulating layer is positioned between the calibration rill and the deformed portion of the disc to protect the ring shaped dished portion of the disc and to extend the off time of the disc.

Brief Description of the Drawings

The accompanying drawings, which are incorporated in and constitute part of the specification, illustrate a preferred embodiment of the invention and, together with the description, serve to explain the objects, advantages and principles of the invention. Dimensions of certain of the parts may have been altered for the purpose of illustration and orientations mentioned in the specification and claims refer to the drawings as shown. In the drawings:

Fig. 1 is a perspective view of a single phase motor protector made in accordance with the preferred embodiment of the invention;

Fig. 2 is a bottom plan view of a first main assembly of the protector comprising the housing of the Fig. 1 protector and a thermostatic disc and associated components mounted therein;

Fig. 3 Fig. 3 is a cross sectional view taken on line 3-3 of Fig. 2;

Fig. 4 is a cross sectional view taken on line 4-4 of Fig. 2;

Fig. 5 is a perspective view looking down at a second main assembly comprising a header, a ceramic insulator plate, a heater, a stationary electrical contact and a terminal pin;

Fig. 6 is a front elevational view of the Fig. 5 assembly;

Fig. 7 is a top plan view of the Fig. 6 structure;

Fig. 8 is a cross sectional view taken on lines 8-8 of Fig. 6;

Fig. 9 is a front elevational view of the Fig. 1 motor protector, with a broken away portion in cross section;

Fig. 10 is a top plan view of the Fig. 1 motor protector; and

Fig. 11 is a cross sectional view looking from the right side of the Fig. 9 motor protector taken through the electrical contacts.

Detailed Description of Preferred Embodiment

Fig. 1 shows a perspective view of a hermetic, single phase motor protector 10 made in accordance with the preferred embodiment of the invention comprising a first main assembly of a thermostatic disc 16 and associated components mounted on housing 12 and shown in Figs. 2-4 and a second main assembly of a header 14, insulating plate 32, heater 26 and terminal pin 28 shown in Figs. 5-8.

With respect to Figs. 2-4, housing 12 of the first main assembly is made of suitable electrically conductive metal such as steel drawn into an elongated cup shaped configuration having a top wall 12a, a side wall 12b extending downwardly around the periphery of the top wall and joined thereto by a rounded junction 12c, the walls forming a switch chamber 12d. Housing 12 preferably is suitably coated for corrosion resistance.

A channel shaped calibration rill 12e is formed, as by stamping, into top wall 12a that extends along longitudinal axis 2 of the housing from a first housing end 12f to a calibration ridge 12h intermediate to housing ends 12f and 12g. Calibration rill 12e is formed through rounded junction 12c at housing side 12f and has side walls 12k angled down to a flat bottom wall 12m that is rigid due to the generally narrow width of wall 12m and particularly the angled side walls. A weld projection 12n is formed in calibration rill along the longitudinal axis generally midway between side 12f and calibration ridge 12h that extends downwardly into the switch chamber for welding attachment of thermostatic disc 16 to be discussed.

Elongated thermostatic disc 16 of suitable material, such as bimetal, has a weld slug 18 of suitable material, such as steel, at one end 16b of the disc and a movable electrical contact 20 having a highly electrically conductive facing, such as a silver alloy face, mounted on the same side of disc 16 at the opposite end 16c. Disc 16 is placed along the inside of top wall 12a and end 16b is welded to weld projection 12n of the calibration rill as shown at 12p, weld slug 18 and calibration rill 12e sandwiching the disc so that the disc lies in a plane generally parallel to the plane in which flat bottom wall 12m of calibration rill 12e lies. Top wall 12a may be formed with a downwardly extending dimple 12t to serve as a positive stop for the disc. As seen in Fig. 4, a selected clearance 12r is provided between disc 16 and the front and back side wall 12b.

A ring shaped dished deformation 16a is formed in thermostatic disc 16 generally in the center thereof to impart snap action between oppositely dished configurations in response to selected temperature conditions leaving opposite ends 16b, 16c unformed.

A layer 22 of electrically and preferably thermally insulating material, such as Kapton, is disposed on the inside surface of calibration rill 12e along the deformed portion of the disc up to and preferably slightly beyond the calibration ridge 12h. Insulation layer 22 electrically insulates housing 12 from the deformed portion 16a during assembly welding in order to prevent any adverse effect on the deformed area of the disc which could cause changes in the temperature settings of the disc. Further more, layer 22 thermally insulates the formed area of disc 16 from housing 12 during operation of the motor protector thereby increasing the off time of the protector so that the protector does not cycle too rapidly in an application.

Calibration ridge 12h is aligned with ring deformation 16a and preferably is offset slightly short of the center of the ring deformation for optimum disc performance in the protector providing proper throw of the disc and proper close snap gaps between the electrical contacts. Optimization of these disc functions extends the life of the protector.

The second main assembly, Figs. 5-8, include header 14 comprising a plate of suitable material, such as steel, formed with an aperture 14a therethrough for reception of a copper cored terminal pin 28. Pin 28 is electrically isolated from header 14 by electrically insulative material, such as an annulus 30 of sealing glass. A flat electrical insulator plate 32, preferably of ceramic material, is disposed on header 14 and attached thereto, as with suitable epoxy. Insulator plate 32 is formed with an aperture 32a with terminal pin 28 protruding through the aperture. Header 14 may be formed with a guide protrusion 14b for receipt in a guide recess 32b formed in the bottom surface of insulator plate 32. Aperture 32a of the insulator plate is preferably expanded on the face surface of the plate received on header 14 around pin 28, as shown at 32c in Fig. 9, to allow for the meniscus of glass annulus 30 so that the plate will lie evenly on the top surface of the header.

Heater 26 is made up of a choice of different materials selected on the basis of specific applications for which the motor protector is to be used. Heater 26 has a first end 26a formed with a pin circumference conforming configuration 26b to serve as a location feature. The heater extends from end 26a along a first segment 26c in a direction lying in a plane generally parallel to a plane in which header 14 lies and continues in a second segment 26d bent to extend toward header 14 to a third segment 26e which is bent to extend in a plane generally parallel to the plane in which header 14 lies. A suitable electrical contact, such as a silver based alloy contact 34 is mounted on the third segment 26e, as by welding with the stepped profile allowing contact 34 to sit flat on the face of insulator plate 32 while maintaining segment 26c in close optimum radiant heat transfer relation to disc 16, as seen in Fig. 9. The stepped up portion, segment 26c, can be tailored to different dimensions to affect the amount of radiant heating, depending on the application. End 26a of the heater is then welded to the side of terminal pin 28 protruding out beyond ceramic insulator plate 32 with the contact on third segment 26f sitting flat on the insulator plate.

If desired, header 14 can be formed with an orientation feature to facilitate assembly and handling, as by generally squaring off a corner 14b of the header as shown, for example, in Fig. 1.

With reference to Figs. 9-11, housing 12, whose side wall 12b is preferably flared at the free end 12s thereof to facilitate welding, is placed on header plate 14 such that contacts 20, 34 mate. The assembly is welded around the perimeter of the housing forming, along with glass annulus 30, a hermetic seal inside switch chamber 12f. The internal atmosphere in the switch chamber is controlled for both gas mixture and pressure to optimize performance of the motor protector.

Motor protector 10 is calibrated to a specific operating temperature by rotationally deflecting calibration rill 12e, as by deforming the housing with a probe at the longitudinal end of the rill, as shown by dashed line 4 of Fig. 1. This changes the angle in which flat bottom wall 12m lies and in turn, the disc assembly, that is, the disc mount, through deformation of the rigid flat bottom wall 12m of the calibration rill 12e, so that the angle of the plane in which the rigid flat bottom wall 12m lies is changed, the flat wall in effect rotating about rounded portion 12c at end 12f of the housing. It should be noted that the entire length of the flat 12m is deformed angularly, without changing the flatness of wall 12m, in order to provide the desired protector function. Calibration is effected by deforming the housing with a probe engaging the housing along dashed line 4 and deforming the housing at the longitudinal end of calibration rill 12e in a localized area that includes calibration ridge 12h.

Among the advantages provided by the invention, the single pin configuration allows for a smaller overall device size than a two pin configuration. The ring form disc, as used in the invention with calibration ridge 12h applying a force to the ring shaped deformed area 16a of the disc through insulation layer 22, has the advantage of increased cycle life due to reduced stress in the disc because calibration occurs at the center of the disc rather than pivoting about a slug. Due to the ring form, a larger electrical contact can be mounted on the unformed end of the disc without adversely effecting the temperature settings of the deformed area of the disc thereby allowing the possibility of increased current capacity within a small device envelope. This type of disc and calibration method also provides excellent temperature stability over life. The heater and disc configuration allows for quicker trip time at low currents in comparison to prior art devices in which the disc is connected electrically to the heater and terminal pin. Quicker trip times at lower currents are particularly advantageous for applications which require protection at lower currents due to line voltage fluctuations.