US2707738A

US2707738A - Midget thermostatic switch

Info

Publication number: US2707738A
Application number: US389534A
Authority: US
Inventors: Weissheimer Herbert; Roseeu Rolf
Original assignee: Siemens AG
Current assignee: Siemens Schuckertwerke AG; Siemens AG
Priority date: 1952-11-14
Filing date: 1953-11-02
Publication date: 1955-05-03
Anticipated expiration: 1972-05-03
Also published as: CH319008A

Description

y 1955 H. WEISSHEIMER ETAL MIDGET THERMOSTATIC SWITCH Filed Nov. 2, 1953 estates O NHDGET THERMQSTATIC SWITCH Herbert Weissheimer Rolf Roseeu, Nurnherg, Germany, assignors to SiemensSchnekertwerke Aktiengesellschait, Erlangen, Germany, a German corpora- Lion Application November 2, @535, Serial No. 389,534 Claims priority, application Germany November 14, 1952 4 Gaines. (Cl. 20%-138) Our invention relates to midget-size thermostatic switches whose contact device comprises one or more stationary contact elements and an arched snap plate that consists of a bimetal spring and electrically cooperates with the stationary contact elements. More particularly,

the invention relates to thermostatic switches whose contact device is enclosed in a housing or capsule composed of a base portion and a cover portion.

Such switches are used for the protection of electric and other machinery and must satisfy, inter alia, the following conditions: The dimensions of the switch must be so small that it can readily be inserted between the windin turns of a machine or apparatus so that the temperature of the winding can directly act upon the switch. The time constant, i. e. the delay with which the switch responds to changes in temperature, must be small and the switch must represent a substantially accurate replica of the winding temperature so that the transmission of temperature to the heat-sensitive elements of the switch is rapid and without appreciable losses.

As regards these conditions, the known thermostatic switches leave much to be desired. A known thermostatic switch for the protection of electric motors, though satisfactorily small and suitable for mounting between the motor windings, does not operate with a sufliciently small time delay because the transfer of heat to the heatresponsive plate member of the switch is limited either to a small zone in the center of the plate or to the rim of the plate. Since the housing of this switch consists entirely of metal, there is an increased temperature drop at the mounting point of the switch as compared with normal operation without a built-in switch. This may result in appreciable inaccuracies. With a highly cooled electric machine of small size, for instance, it may happen that the actual temperature of the windings to be protected is considerably above the temperature to which the switch will respond. Another disadvantage lies in the fact that the heat-responsive diaphragm plate of the switch is freely movable within the switch housing. As a consequence, only one-half of the deflective travel of the plate is utilized for controlling the electric circuit so that the current-carrying duty of the switch is rather limited.

It is an object of our invention to provide a thermostatic switch, particularly a switch of midget size, that is suitable for the above-mentioned protective purposes but avoids the shortcomings of the known devices.

To this end, and in accordance with a feature of our invention, the arcuate snap plate of the switch, when in its position of rest, is seated along its entire or nearly entire surface on the base portion of the switch housing or capsule, and this base portion consists of a good heatconducting material and has a face of arcuate shape engageable by the bimetal spring plate.

According to another feature of the invention, the center point of the snap plate is fastened to, or securely held against, the inwardly arched base portion of the capsule. The attachment may be fixed, or it may be effected by means of a pressure bolt that can be adall ice

justed to set the switch for different critical temperatures. According to still another feature of the invention, an especially favorable design is obtained by making the cover portion of the capsule of a thermally and electrically insulating material and mounting the stationary contact or contacts on that cover portion.

The foregoing and more specific features of the invention will be apparent from the embodiments illustrated on the drawing, in which: Figs. 1 and 2 are crosssectional views of one and the same midget thermostatic switch in the circuit-opening position (position of rest) and in the circuit-closing position (heat-responsive position) respectively, while Fig. 3 shows a cross section of another midget-type switch Whose temperature of respouse is adjustable. The same reference numerals are applied to both embodiments for designating similar elements respectively.

According to Figs. 1 and 2, the thermostatic switch has a capsule composed of a cover portion 1 and a base portion 2. The cover portion 1 consists of thermally and electrically insulating material, for instance, of pressuremolded plastic. The base portion 2 consists of a good heat-conducting material, for instance, copper. Each of the two portions is generally cup-shaped. The side walls 3 and 4 of

respective portions

1 and 2 are telescoped into one another and are firmly joined by turning the rim of wall 4 over an annular shoulder of portion 1. An arched snap plate 6 of bimetal and two contact rivets '7 are disposed in the capsule space 5 enclosed by

portions

1 and 2. The snap plate 6 consists of a bimetal spring and has its center fastened by a rivet 6' to the base portion 2 of the capsule. The plate 6 is strip shaped, but may also be given the shape of a star or circular disc. The base portion has an arched cross-sectional shape bulging toward the interior of the capsule space 5. The curvature of the snap plate and of the adjacent face of the base portion are adapted to each other so that the snap plate 6, when in the normal position of rest, is seated in the base portion 2 along the entire surface area of the plate. The rivets 7 are secured to the cover portion 1 of the capsule in the vicinity of the rim zone of plate 6 and are conductively connected by terminal members 3 with current leads 9 on the outside of the cover portion. While only two contact members are shown, it will be understood that three contact rivets may be provided to cooperate with respective three arms of the snap member or with a circular snap plate.

The thermostatic switch may be mounted into the winding of an electric machine or apparatus to be protected from overheating. When so mounted, the temperature of the winding is directly transmitted to the good heatconducting base portion 2 of the capsule. When a given excessive temperature is exceeded, the bimetal disc 6 snaps from the position of rest (Fig. l) to the active position (Fig. 2) in which it touches the contact rivets 7. This closes a control circuit which actuates a relay to re lease a circuit breaker or to issue a danger signal.

For making the switch adjustable to a desired temperature at which it will respond, the snap plate, instead of being riveted or otherwise firmly joined with the base portion 2, may be held against the base portion by means of a set screw 10 that passes through the cover portion 1 as is illustrated in Fig. 3. The force with which the snap plate has its center area urged against the base portion 2 can thus be varied by changing the setting of screw 10. In this manner, the sensitivity of response of the snap plate and hence, the critical temperature can be adjusted within certain limits.

If a thermostatic switch according to the invention is to be used for the protection of high-tension machines, the base portion of the capsule is preferably coated with a heat conducting but electrically insulating layer.

zyz'ovgrss By virtue of the complete or nearly complete engagement of the snap plate with the heat-supplying arcuate face of the base portion, the transmission of heat by conductance is greatly improved thrus minimizing the time delay in the response of the switch. Besides, the possible maximum of switch travel is fully utilized due to the fact that part of the deflectively movable plate is kept in a fixed position relative to the capsule. Due to the friction-free mounting of the snap plate, the switch maintains the desired critical temperature with a maximum degree of accuracy and constancy. Since the snap plate is fixed at its center point and the end or marginal Zone perform the elastic snap movement, two or more stationary contacts can readily be accommodated with the ad vantage of keeping the fastening means of the snap plate free of current-carrying duty. When using heat insulat ing material for the cover portion of the capsule, the heat dissipation at the mounting point of the switch is only slight, thus preventing the actual temperature of the protected machine winding from excessively increasing beyond the temperature effective upon the switch. No additional insulation of the stationary contacts is needed at the points where they pass through the capsule. Besides, by coating the base portion of the capsule with an electrically insulating material, the switch is also made suitable, in a simple manner and without appreciable expenditure, for use with machines operating at high tenston.

It will be obvious to those skilled in the art upon a study of this disclosure that thermostatic switches according to our invention may be modified in various respects and may be embodied in devices other than those specifically illustrated and described, without departing from the essential features of the invention and within the scope of the claims annexed hereto.

We claim:

1. A thermostatic switch, comprising a capsule having a base portion of good heat-conducting material and a cover portion of thermally and electrically insulating material, an electric contact device disposed within said capsule and having stationary contact means mounted on said insulating cover portion and an arcuate snap plate lconsisting of a bimetal spring and engageable with said contact means when hot, said base portion having a convex face facing the interior of said capsule, said plate being mounted on said base portion at the center of the plate, and said plate, when cold, having substantially its entire surface seated upon said convex face.

2. A miget thermostatic switch, cornprisin a capsule having a base portion and a cover portion, an electric contact device mounted in said capsule and having stationary contact means and an arcuate snap plate of springy bimetal, said base portion consisting of a good heat-conducting material and having an arcuate inner face adjacent to said snap plate, said snap plate, when cold, having substantially its entire surface seated upon said arcuate face, and pressure means engaging said plate at the center thereof for holding said plate against said face, said pressure means being adjustable for varying the hold ing pressure to thereby vary the response temperature of the switch.

3. In a switch according to claim 2, said pressure means comprising a screw mounted on said base portion and accessible from the outside of said capsule for adjusting said temperature.

4. A midget thermostatic switch for use in temperature control of high tension machines, comprising a capsule having a base portion and a cover portion, said base portion having a coating consisting of a heat conducting but electrically insulating material, an electric contact device mounted in said capsule and having stationary contact means and an arched snap plate of springy bimetal, said base portion consisting of a good heat-conducting material and having an arched inner face adjacent to said snap plate, and said snap plate, when cold, having substantially its entire surface seat upon said face.

References Cited in the tile of this patent UNiTED STATES PATENTS 2,447,649 Harrison Aug. 24, 1948 2,455,011 Ingels Nov. 30, 1948 2,490,103 Stilwell, Ir. Dec. 6, 1949