US 3670283 A
A motor overload protector is disclosed which includes a cantilever mounted bimetal snap element provided with a mobile contact at its free end. The snap element moves the mobile contact into and out of engagement with the fixed contact carried by the body. A central stop post is provided by the body located to engage the central portion of the snap element when the contacts are open. A pull-in calibration screw engages the opposite side of the bimetal when the contacts are closed to calibrate the temperatures on opening. The snap travel of the installed snap element is reduced to limit the stresses imposed on the bimetal and reduce the tendancy to fatigue or crack. Contact life is improved because the contacts reliably snap both open and closed.
Description (OCR text may contain errors)
United States Patent Holden MOTOR OVERLOAD PROTECTOR Ronald L. Holden, Mansfield, Ohio Inventor:
US. Cl ..337/354, 200/ l 66 M Int. Cl. ..H01h 37/52 Field of Search ..337/354, 343-348,
References Cited UNITED STATES PATENTS 8/1955 3/1. 969 Epstein ..337/365 X Schwaneke et a1. ..337/348 Mertler ..337/348 1 June 13, 1972 Primary Examiner-C. L. Albn'tton Attamey-McNenny, Farrington, Peame andv Gordon ABSTRACT A motor overload protector is disclosed which includes a cantilever mounted bimetal snap element provided with a mobile contact at its free end. The snap element moves the mobile contact into and out of engagement with the fixed contact carried by the body. A central stop post is provided by the body located to engage the central portion of the snap element when the contacts are open. A pull-in calibration screw engages the opposite side of the bimetal when the contacts are closed to calibrate the temperatures on opening. The snap travel of the installed snap element is reduced to limit the stresses imposed on the bimetal and reduce the tendancy to fatigue or crack. Contact life is improved because the contacts reliably snap both open and closed.
11 Claims, 6 Drawing Figures PATENTEDJum m2 3.670.283
sum ear 2 INVENTOR.
pan/410 A. #04 064/ 1 MOTOR OVERLOAD PROTECTOR BACKGROUND OF THE INVENTION This invention relates generally to thermal responsive switching devices and more particularly to a novel and improved thermostatic switch for use as a motor overload protector or the like.
PRIOR ART Various types of thermal overload protection devices for electric motors are known. One type is mounted in or on the motor housing to sense the temperature of the motor components and the current load of the motor. Such devices are connected in the power circuit of the motor so that the operation of the overload shuts the motor off before the temperature in the motor become high enough to cause damage or create a fire hazard. Examples of such prior art overload devices are illustrated in the US. Pat. No. 3,064,100, dated Nov. 13, 1962, and No. 3,288,967, dated Nov. 29, 1966. The former of these patents illustrates an automatic reset-type overload device, and the latter patent illustrates a manual reset-type device which remains open until it is manually reset.
Both of these illustrated devices provide a switch which includes a fixed contact and a cantilever mounted bimetallic snap arm shaped to move with snap action. A mobile contact is mounted on the free end of the snap arm and is carried into and out of engagement with the fixed contact when the bimetal reaches its operating temperature.
The automatic reset-type motor protector is intended to protect the motor under two substantially different types of overload conditions. In one condition, the motor is running, but the running conditions are such that the operating temperatures in the motor reach undesirable values. In this condition, the overload protector senses high current or high motor temperatures or combinations of both and shuts the motor off. After the temperature in the motor drops to predetermined safe levels, the protector recloses and the motor restarts. The
other overload condition is known as a locked rotor condition and occurs when the motor rotor does not start to rotate. Under locked rotor conditions the motor current is very high, usually many times the normal running current of the motor. In this instance, the overload protector operates primarily as a current sensing device and shuts the motor off before damaging motor temperatures are reached. Here again the protector of the automatic type recloses after the protector cools. The on-off time of the protector is arranged to prevent damaging or hazardous motor temperatures from being reached.
In practice, the automatic protector must be capable of withstanding a large number of on-off cycles, which can occur over an extended period of time under locked rotor conditions, even though the protector is operating to open and close under high current conditions.
Many of the prior art automatic overload protectors have failed under cycling conditions either because the contacts deteriorate and fail to function, or because the bimetal cracks or breaks.
SUMMARY OF THE INVENTION The present invention has several important aspects. In accordance with one aspect of this invention, a device incorporating the invention is capable of operating through greater numbers of cycles when compared to similar prior art devices without failure of either of the contacts or the bimetal arm. In the illustrated overload device, the cantilever mounted bimetal switch arm is arranged so that the movement of the mobile contact between the open and close position is small when compared to similar prior art overload devices, and the arm is not subjected to as much strain. Consequently, the tendency of the bimetal to'crack or break is materially reduced.
The structure is also arranged to insure that the contacts do not creep, either open or close, but rather move with snap action both in opening and closing. Still further, the structure is arranged so that substantial amounts of kinetic energy are available to break the contacts apart in the event that contact welding occurs.
In accordance with another aspect of this invention, adjusting means are provided to pull-in" the snap element so that the device operates at a desired predetermined operating temperature. Consequently, manufacturing variations do not cause variations in operating temperatures between similar units. In the illustrated device the snap element is shaped so that the upper operating temperature, at which the snap element would operate if free of restraint, is reduced by the adjusting screw to the predetermined operating temperature desired. This insures that all units open at the same operating temperature, and further insures that the contacts will open with snap action.
BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a plan view of an automatic motor overload protector incorporating this invention;
FIG. 2 is a plan view similar to FIG. 1 with the cover removed to illustrate the switch mechanism;
FIG. 3 is an enlarged cross section taken along 3-3 of FIG. 2 illustrating the switch mechanism in the contact closed condition;
FIG. 3a is a view partially in longitudinal section of the bimetallic snap arm illustrating the arm in the various position it would assume if it were unrestrained in its movement;
FIG. 4 is an enlarged view similar to FIG. 3 illustrating the position of the elements immediately before opening under a condition in which contact welding or the like resists contact separation; and
FIG. 5 is a view similar to FIGS. 3 and 4 illustrating the switch mechanism in the open position.
DETAILED DESCRIPTION OF THE DRAWINGS Referring to the drawings the overload switching device 10 includes a two-piece body consisting of a base member 11 and a cover member 12. A pair of terminal assemblies 13 and 14 are secured to the body 11 by pairs of rivets l6 and 17, respectively.
The terminal assembly 13 includes a cross plate 18, a bimetallic snap element 19, and a weld plate 21 which are welded together at 22. The bimetallic snap element 19 is elongated and is cantilever mounted at one end between the two plates 18 and 21 and is provided with a mobile contact 23 at its other end. The central portion of the snap element 19 is bumped to a shallow dished shape at 24 to provide the bimetal with a snap disc characteristic.
The terminal assembly 14 is a two-piece assembly including the main terminal member and a fixed contact 26 supported thereon. The main tenninal member includes a terminal portion 27 connected to one end of a serpentine heater portion 28. The heater portion 28 is connected at its other end to a contact support portion 29. The various elements are proportioned so that the operation of the snap element 19 causes the mobile contacts 23 to move in and out of engagement with the fixed or stationary contact 26. When the contacts are closed, a circuit is provided connecting the cross plate 18 to the terminal portion 27 through the bimetallic snap element 19 and the serpentine heater 28. When the contacts separate, however, such connection is broken.
In some overload devices of this type, the heater 28 is eliminated and the fixed contact 26 is mounted directly to a cross portion of the terminal 14. Such overload devices, without an internal heater, are generally used on larger motors where current sensitivity need not be as great. Even in such devices without heaters, heating of the bimetal occurs due to the current flow therethrough. However, in the illustrated embodiment wherein the heater 28 is provided, the temperature of the bimetal is a function of the current through the bimetal and environmental temperature.
the heater 28, as well as the FIG. 3a illustrates the manner in which the bimetallic snap element snaps in the free state in which its travel is not limited. The full line position is one snap position, and is the position of the snap element immediately prior to snapping in the upward direction. When the temperature is increasing and the snap temperature is reached, the element moves with snap action from the full line position A to the phantom position B. Such free snap temperature is determined by the shape formed in the disc portion during manufacture.
If the temperature of the bimetal continues to increase, the free snap element moves to higher positions than the phantom position B. However, on decreasing temperature from the upper snap temperature, the snap element slowly moves with creep-type movement until the second free snap position at C is reached when the bimetal reaches the lower operating temperature. Snap action then occurs to the phantom position D which is beyond the free snap position at A. If the bimetal temperature decreased further, the element moves downward beyond the position D, but as the bimetal again starts to increase in temperature, it moves with creep action back to the first free snap position at A.
Because the movement in the range between the two free snap positions A and C is the only zone of movement which is snap-type movement in both directions, the device is considered to have a snap range equal to the spacing between positions A and C. This free snap range in the illustrated device is in the order of 0.050 inches. The zones between B and C and between A and B are zones of overtravel and, in the illustrated unit, this movement is in the order of 0. l inches.
In the illustrated device, the snap element is mounted so as to eliminate all of the overtravel between A and D and most of the overtravel between C and B. Further, the device is mounted so that most of the snap range is eliminated. Consequently, the stresses on the bimetal created during snap movement are materially reduced and fatigue, cracking or breaking of the bimetal material is virtually eliminated.
Referring to FIG. 3 maximum range of movement of the mobile contact end of the snap element is between the position of H6. 3 wherein the mobile contact engages the fixed contact 26 and the position of FIG. 5 wherein the upper side of the snap element engages a body stop 31 formed in the cover 12. This range of movement, in the illustrated device, is in the order of 0.020 inches which is a distance which is substantially less than the free snap range of the element.
Mounted in the cover 12 above the center of the snap disc portion 24 is an adjusting screw 32 positioned to engage the center portion of the disc 24 and limit upward movement thereof to a position determined by the adjusted position of the screw The base 1 1 is formed with a center post providing a stop surface 33 aligned with the screw 32 and positioned for engagement with the center portion of the disc in the manner described in detail below. The surface 33, however, is spaced from the bimetal when the switch is closed and the bimetal engages the screw 22.
The bimetal element is formed with a shallow lateral bend at 34 immediately inward from the mobile contact 23. This bend 34 provides stiffening of the bimetal at the free end zone and strengthens the bimetal in the zone to prevent fatigue failures in the free end portion. The mounting end of the snap element is also bent downward at 36 by a laterally extending bend, as best illustrated in FIG. 3a, to bias the mobile contact 23 toward the fixed contact 26. i
The various elements are proportioned so that the first installed or opening snap position is detennined by the location of the fixed contact 26 and is above and substantially spaced from the corresponding free snap position A of FIG. 30. Similarly, the other installed or closing snap position is determined by the location of the stop surface 33 and'is also above the corresponding free snap position C.
Preferably, the structure is arranged so that the free end of the snap element creeps away from the stop 31 slightly while remaining in contact with the stop surface 33 as the snap element approaches its closing snap temperature and its closing snap position. With such a structure, the disc is not pulled-in on closing and the closing temperature is substantially the same as the corresponding free temperature formed in the snap element.
On the other hand, the opening operating temperature is changed by the pull-in of the screw 32 from the corresponding free operating temperature determined by the form of the disc. In practice, the snap element is formed with a free operating temperature higher than the desired opening temperature of the device and the screw 32 is adjusted to pull-in the disc and cause it to snap open at the desired calibration temperature. After calibration, the screw is covered by a plate 35 to prevent change in calibration. With such a calibration screw, accuracy of the opening temperature, the most important temperature in the device, is obtained without requiring extremely close manufacturing tolerances.
The operation of the switch can best be understood by referring to FIGS. 3, 4, and 5. Assuming that the switch is closed and the temperature of the bimetal is increasing, the elements remain in the position of FIG. 3 as the temperature increases until the calibrated upper operating temperature of the disc element is reached. As soon as the calibrated upper temperature is reached, the disc begins to snap toward the position of FIG. 5. During this snap movement, the center of snap portion 24 of the element moves through the central plane from its upward dished shape of FIG. 3 to its lower dished shape of FIG. 5. It is during this movement that the mobile contact 23 snaps away from the fixed contact 26.
The bend 36 produces a low rate spring force or biasing force tending to urge the mobile contact toward the fixed contact. This biasing force resulting from the bend 36 tends to maintain the two contacts in engagement and tends to overcome the inertia forces resulting from the acceleration of the snapping mass. Such biasing force, therefore, insures that the disc is in snap movement when the contacts open. Further, such biasing force tends to prevent contact bounce resulting from vibration as the calibrated operating temperature is approached.
If contact welding has occurred to resist contact separation, the contacts may remain in engagement as the snap action takes place. If the weld is sufficiently strong, a condition illustrated in FIG. 4 occurs in which the snap portion 24 engages the stop surface 33 before the contacts separate. Such a condition occurs before the snap action of the portion 24 is completed. Consequently, the kinetic energy of motion of the central portion 24 produces a high reactive force on the snap element at the instant the stop surface 33 is engaged. This reactive force is sufficient to break any welds which may exist and insures that the contact will open properly. The various elements then move to the position of FIG. 5 wherein the free end engages the body stop 31. The body stop 31 limits overtravel in an upward direction.
Reclosing of the switch on decreasing temperature occurs in the following manner. As the temperature of the disc drops and approaches the lower snap temperature, the disc moves with creep movement toward a closing snap position in which the free end is spaced slightly from the body stop 31. Subsequently, the mobile contact 23 moves with snap action downwardly into engagement with a fixed contact 26 and'the snap portion 24 returns to the position of FIG. 3 in engagement with the calibration screw 32.
With the'present invention the overload device provides a relatively long trouble-free service life since fatigue failures in the bimetal are virtually eliminated because the strains applied to the bimetal are substantially reduced,and because the bimetal is strengthened by the bend 34 in the zone wherein failures tend to occur. Further, failure of the device created by contact welding or the like is virtually eliminated since the contacts open and close with snap action and since proper opening of the contacts occurs even under a contact weld condition. Further, the operating temperature of the device is accurately maintained by the calibration of the screw 32.
Although a preferred embodiment of this invention is illustrated, it is to be understood that various modifications and rearrangement of parts may be resorted to without departing from the scope of the invention disclosed and claimed herein.
What is claimed is:
1 An overload protector or the like comprising body means, a fixed contact mounted on said body means, elongated bimetallic snap means mounted at one end on said body means and providing a mobile contact adjacent its free end movable with snap action into and out of engagement with said fixed contact, said body means providing a first fixed stop surface engagable with said snap means intermediate its ends on the same side thereof as said fixed contact and a second fixed stop surface engagable with said snap means intermediate its ends on the side thereof opposite said fixed contact, said first stop surface and fixed contact being located so that said snap means engages said first stop surface when said contacts are separated and is spaced from said first stop surface when said contacts are closed, said second stop surface and fixed contact being located so that said snap means engages said second stop surface when said contacts are closed and is spaced from said second stop surface when said contacts are open.
2. An overload protector as set forth in claim 1 wherein said body means includes a body stop preventing substantially all overtravel of said snap means when said contacts open.
3. An overload protector as set forth in claim 2 wherein said snap means are spaced from said body stop before said snap means to a contact closed position.
4. An overload protector as set forth in claim 1 wherein said snap means are provided with stiffening means which strengthen said snap means adjacent to said mobile contact.
5. An overload protector as set forth in claim 4 wherein said stiffening means is provided by a lateral bend in the bimetal adjacent to said mobile contact.
6. An overload protector as set forth in claim 1 wherein said snap means is formed so that when its movement is unrestrained it has first and second free snap positions and a predetermined snap range equal to the distance between said free snap positions, the location of said fixed contact deter mining one installed snap position and the location of said first stop surface substantially determining the other installed snap position, both of said installed snap positions being displaced from their corresponding free snap positions, said first stop surface and said fixed contact being located so that said snap means has an installed snap range which is equal to the distance between said installed snap positions and is substantially less than said free snap range.
7. An overload protector as set forth in claim 6 wherein said snap means is resiliently biased to maintain said snap means in engagement with said first stop surface when said snap means is in said other installed snap position.
8. An overload protector as set forth in claim 1 wherein said second stop surface operates when said contacts are closed to apply force to said snap means intermediate its ends urging said snap means toward its contact closed position.
9. An overload protector as set forth in claim 8 wherein said second stop surface is adjustable to control the opening temperature of said snap means.
10. An overload protector as set forth in claim 9 wherein said second stop surface is provided by a screw carried by said body means.
11. An overload protector as set forth in claim 10 wherein said screw reduces the operating temperature of said snap means on opening of said contacts when compared to the corresponding operating temperature of said snap means when unrestrained in its movement.
UNITED STATES I PATENT OFFICE gCiE- TIFIQATE: ]QF COR EC I N ir wehorfi v is.lcertifiied'fth at "efforspp ears in i he.)abeve ident'ified patent, r
and tha1;' said Letters Pi-tgntl are hrebyporreicted as shown below:
. Claim '3, line 3 0, "after "'f-mearisl- .insert -"-snaps' Sig ned and sealed. this zn d ay'- of-Janfiary 1973. Q
EDWARD M. FLQETCH ER,*JR. Attesting Officer 1 v ROBERT GOTTSCHALK Commissioner of- Patents v UNITED STATES PATENT OFFICE Patent rthfit $15,501; za pp-egirs-gin ifhe. abev-idntified patent and fthat; si'd; Lettrs P a te n t are hereby correi'c ted as shown below:
. Claim '3', line-"30 "a-fter""'--'mear 1s"'- insert "-Snaps' s gned and sealedfC-hiS 2nd dayo f Janfia ry 1973.1
' EDWARD M'. PLET'CHER;JRL ROBERT GOTTSCHALK Attesting Officer- Commissioner of Patents