Hermetic single phase motor protector

[0002] This application claims priority under 35 USC Section 119 (e) (1) of provisional application No. 60/251,138 filed Dec. 4, 2000.

[0003] This invention relates generally to thermally responsive electrical switches and more particularly to such switches which are useful as protectors for electrical motors such as those used for hermetic compressors.

[0004] Electrical motors which operate compressors in refrigerators and air conditioners, and the like, are typically enclosed in shells together with selected coolant fluids. Such motors are normally provided with motor protector devices, sealed to exclude the coolant fluids, fitted on or near the motor windings to optimize their thermal response to heat generated by the windings. A thermally responsive member is arranged within the typical motor protector device to respond to the winding temperature for moving electrical contact means to open a circuit when the thermally responsive member is heated to a selected temperature. Usually a heater is disposed in heat-transfer relation to the thermally responsive member to be connected in series with the motor for promptly heating the thermally responsive member to open the circuit to the motor when an overload current occurs in the motor's windings. An example of such a motor protector is shown and described in U.S. Pat. No. 4,376,926 assigned to the assignee of the present invention. In motor protectors of this type, a bimetallic, snap-acting disc is cantilever mounted on a heater which in turn is mounted on a terminal pin extending through a header plate electrically isolated from the plate which may be provided with electrically insulated material surrounding the pin to provide a suitable dielectric distance between the pin and the header. An electrically conductive tube shaped housing having a closed end is hermetically attached to the header and is provided with a stationary electrical contact attached to the housing wall in the vicinity of its closed end. A movable electrical contact is mounted on the distal end portion of the disc with the disc adapted to move between a normally contacts engaged, dished shape disc having a concave configuration facing the stationary contact to a contacts opened, convex configuration facing the stationary contact upon being heated to a preselected temperature. The preselected temperature is chosen to prevent overheating of the motor.

[0005] In making the above described motor protector, calibration is typically effected by deforming the housing to move the stationary contact to obtain the preselected opening temperature, i.e., the actuation temperature of the disc.

[0006] It is an object of the present invention to provide an improved, lower cost yet reliable hermetic motor protector for use with hermetic compressors, such as 240 VAC, 50/60 Hz compressors. Another object is the provision of a motor protector capable of being mounted on or near a motor winding and having up to 90 amps locked rotor capacity. Yet another object of the invention is the provision of a hermetic motor protector for use with compressor motors which require no temperature calibration after assembly of the protector for meeting motor protection requirements.

[0007] Briefly, in accordance with the invention, an elongated header plate is formed with first and second pin receiving apertures through which extend glass sealed, respective first and second terminal pins. A stationary contact is attached to the first terminal pin and a resistive heater element formed in the shape of a loop has one end attached to the second terminal pin and a second end attached to the header plate. A bracket is mounted to the header plate and is formed with a disc seat spaced above the header plate. The disc seat is provided with a vertically upstanding tab which is received in an aperture of a disc, to be discussed below. The bracket is also formed with a generally U-shaped portion having a first leg disposed on the header plate and a second leg spaced above the header plate and extending in a direction toward the stationary contact. A bimetallic, dished shaped circular disc having a centrally located aperture therethrough is disposed on the disc seat with the upstanding tab of the disc seat received through the aperture in the disc and with an edge of the disc received under the distal free end of the second leg of the U-shaped bracket portion. A movable spring contact arm, also preferably provided with an aperture, has one end cantilever mounted to the top of the second leg of the U-shaped bracket portion with the aperture in the arm aligned with the upstanding tab of the disc seat. A movable contact is mounted on the bottom side of the spring contact arm and is normally biased into electrical engagement with the stationary contact. The spring contact arm is preferably provided with a downwardly extending motion transfer projection such as by deforming a portion of the spring contact arm into a bump, which is aligned with the edge portion of the disc diametrically opposite the second leg of the U-shaped bracket portion. When the disc snaps from a normal downwardly facing, i.e., toward the header plate, concave configuration at ambient temperature with the disc completely unrestrained to an oppositely shaped downwardly facing convex configuration, the disc moves against the second leg and pivots on the disc seat with the disc performing as a lever so that the outer peripheral portion of the disc which is diametrically opposite to the second leg engages the motion transfer projection and moves the spring contact arm upwardly and concomitantly the movable contact out of electrical engagement with the stationary contact. The top of the disc seat against which the disc pivots as a fulcrum comprises the bight portion of another U-shaped portion of the bracket with the bight positioned within a selected window of vertical distances, relative to the face of the header plate, from the bottom of the motion transfer projection of the spring contact arm. The contacts closed contact force can be adjusted by applying a downward force on the second leg. Assembly of the protector is completed by hermetically welding an elliptical dome shaped housing with a portion of the housing preferably being deformed at a location aligned with the movable contact to serve as a motion limiting stop.

[0008] Other objects, advantages and specific features of the novel and improved hermetic motor protector of the invention appear in the following detailed description of the preferred embodiment of the invention, the detailed description referring to the drawings in which:

[0009] FIG. 1 is a top plan view of a hermetic motor protector made in accordance with the invention shown with the cover cut away;

[0010] FIG. 2 is a cross sectional view taken on line 2—2 of FIG. 1;

[0011] FIG. 3 is a side elevational view shown with the cover in cross section: and

[0012] FIGS. 4a-4dshow perspective views of a partially assembled hermetic motor protector at different stages of assembly.

[0013] A hermetic motor protector 10 made in accordance with the invention comprises a header plate 12 of suitable electrically conductive material, such as cold rolled steel, generally oval in shape when viewed from above and preferably provided with an orientation feature such as corner portion 12a. First and second bores 12b, 12care formed through header plate 12 for receipt therethrough of respective first and second terminal pins 14, 16 electrically isolated from the header plate by suitable glass material 18. An electrical contact 20 is suitably mounted on the butt end of terminal pin 14, as by welding, spaced above floor surface 12dto conform with the position of other protector components to be described and to provide a suitable dielectric distance between the header plate and stationary contact 20.

[0014] A bracket 22 formed of suitable electrically conductive material such as plated cold rolled steel, is mounted on floor 12d, as by welding and has a first generally U-shaped portion comprising a disc seating bight 22aspaced above floor 12dby legs 22b. A tab 22cwhose function will be discussed below projects vertically upward from bight portion 22a, as seen in FIG. 2. A second generally U-shaped portion comprises first and second legs 22d, 22ejoined by bight portion 22f. Leg 22dis disposed on floor surface 12dwhile leg 22eextends generally parallel to and above the floor surface by bight portion 22f.

[0015] A heater element 24 composed of suitable material based on the particular motor application for which the protector is intended, e.g., steel, alloy 52, etc., has one end butt welded to the end of terminal pin 16 and its opposite end formed into an L-shaped portion for spacing the heater element above the header plate, welded to header plate 12. Heater element 24 is preferably configured as a loop extending at least part way around legs 22band disc seating bight 22a, see FIG. 4b.

[0016] A heat responsive, non-current carrying, bimetallic disc 26 is disposed on disc seat 22awith tab 22creceived through a centrally located aperture 26ain disc 26. Disc 26 is dished shape to provide snap-action between oppositely dished-shaped configurations when heated to a preselected actuation temperature. The disc is placed on disc seat 22awith an outer peripheral portion thereof received under leg 22eof bracket 22 and is preferably circular in shape making it insensitive to angular orientation. As noted in FIG. 2, disc 26 is arranged such that its normal ambient temperature, concave dished configuration faces header plate 12.

[0017] A movable spring contact arm 28 is cantilever mounted on the top surface of leg 22eat one end 28athrough welding slug 32 and mounts a movable electrical contact 30 on the distal free end portion 28bof the contact arm. Contact arm 28 is adapted to move into and out of electrical engagement with stationary contact 20 and is normally biased into engagement therewith. Contact arm 28 is formed with an aperture 28cwhich receives tab portion 22ctherethrough thereby preventing dislocation of disc 26. A welding slug 32, of suitable material such as cold rolled steel, has a plate portion 32afor welding attachment to end 28aof the spring contact arm and a welding projection 32bfor welding attachment to leg 22e. Spring contact arm 28 is formed of suitable electrically conductive material having good spring characteristics, such as beryllium copper and is formed with a downwardly extending motion transfer projection or bump 28ddisposed diametrically opposite to leg 22eand aligned with the outer peripheral portion of disc 26. Laterally extending back and front edge portions 28eof contact arm 28 are bent upwardly to stiffen the outer portion of the arm, i.e., that portion of the arm outboard of and including the motion transfer bump 28d.

[0018] An oval domed shaped cover 34 of suitable material such as steel is received on header plate 12 and hermetically welded thereto forming a switch chamber for the several components of the protector. The cover is preferably deformed at 34ain alignment with the movement of movable contact 30 to provide an over-travel stop for movable contact arm 28.

[0019] The current path of the protector extends from terminal pin 16 through heater element 24, header plate 12, bracket 22, slug 32, movable spring contact arm 28, movable contact 30, stationary contact 20 to terminal pin 14. The terminal pins are serially connected to a motor circuit so that upon an overcurrent condition heat generated by heater element 24 is radiated to disc 26, along with heat conducted by the motor windings through the protector housing, raising the temperature of the disc to its actuation temperature when the disc snaps to an oppositely dished configuration, i.e., a downwardly facing convex configuration (not shown in the drawing). Movement of that portion of disc 26 under leg 22eis constrained causing the disc to pivot on disc seat or bight 22aso that the outer peripheral portion of the disc diametrically opposite to leg 22eengages motion transfer bump 28dand pushes movable contact arm 28 upwardly with leveraged movement thereby moving movable contact 30 out of contact engagement with stationary contact 20. When the temperature of the motor reaches a safe level which allows the disc to cool to its reset temperature the protector will automatically reset.

[0020] During assembly of the protector the vertical distance, relative to header plate surface 12d, between the top of disc seat 22aand the bottom of motion transfer bump 28dis maintained within a preselected window of distances. Further, a force can be applied through welding slug 32 in order to obtain a selected contact force between the contacts in the normally contacts engaged position.

[0021] According to the invention, the protector's opening temperature is the same as the disc free snap open temperature with switching being effected by positioning motion transfer bump 28dof movable contact arm 28 within the dimensional window relative to disc 26 on disc seat 22a. Such positioning does not require temperature calibration and results in a larger dimensional window than provided in prior art hermetic compressors resulting in improved product yields. Manufacturing costs for making protectors according to the invention are reduced by eliminating temperature calibration and by utilizing a common disc with common dimensional set-up. High cycle life is achieved due to a reduction in contact arm fatigue stress existing in prior art devices. The invention results in tighter temperature and improved ultimate trip capability over cycle life associated with non-current carrying disc designs.

[0022] It should be understood that although a particular embodiment of the motor protector of this invention has been described, various modifications can be made which come within the purview of the invention. For example, although bracket 22 is described as being electrically conductive, the disc seat portion need not be conductive since disc 26 is non-current carrying. With regard to the heater, it should be noted that the heater element can be formed to serve as a fusible link to ensure that the device fails in an open state. Although disc 26 is advantageously mounted on bracket 22 through tab 22cto provide leveraged movement of movable arm 28, it will be appreciated that, if desired, the disc could also be supported by its outer periphery with a fulcrum disposed in a location below the disc corresponding to bight 22aand with a motion limiting member extending over the outer periphery of the disc at a location diametrically opposite to the motion transfer bump of the movable contact arm. It is the intention that the invention include all modifications and equivalents of the disclosed embodiment falling within the scope of the appended claims.