US3447113A

US3447113A - Positive-acting lower power thermally-responsive bimetallic switch

Info

Publication number: US3447113A
Application number: US599484A
Authority: US
Inventors: Arne Jensen
Original assignee: Danfoss AS
Current assignee: Danfoss AS
Priority date: 1965-12-07
Filing date: 1966-12-06
Publication date: 1969-05-27
Anticipated expiration: 1986-05-27
Also published as: AT264640B; JPS4917344B1; FR1503191A; DK112662B; SE302992B; GB1130660A; NL6616804A; ES334208A1; DE1515630B1

Description

May 27, 1969 A. JENSEN 3, POSITIVEACTING LOW POWER THERMALLY'RESPONSIVE BIM LLIC SWITCH Filed D80. 6, 1966 United States Patent U.S. Cl. 337-1 5 Claims ABSTRACT OF THE DISCLOSURE A compact, sensitive and positive acting thermally-responsive switch having bimetallic contacts normally closed. A flexible resistive film supported on a flexible insulating strip, is positioned between the two bimetallic strip contacts to maintain the open position of the bimetallic contacts, and prevent intermittent closing thereof, with the application of low power.

This invention relates generally to thermally-responsive switches and more particularly to switches using bimetallic contacts.

Thermally-responsive switches having bimetallic strip contacts are known in which a heating resistor, for example a heating coil, is situated next to or envelopes the bimetallic strips and maintains the switch open, after it has once opened, by application of heat. The known thermally-responsive switches, however, require a relatively large power input to be applied thereto since a relatively large amount of heat, and therefore power or potential and current, for maintaining the bimetallic contacts open reliably and positively. In these switches the heating resistor is short circuited as long as the switch is closed. Heating of the heating resistor begins when the switch opens. If an appropriate power source cannot be provided to enable the heating resistor to reach a high temperature quickly the bimetallic strip contacts may cool sufiiciently after having opened the switch so that they tend to move in a direction of closing the switch and even close it and cause it to operate intermittently.

The switch in accordance with the invention overcomes the defect of intermittent operation or chattering of switches of this type and provides for a switch held effectively in an open position with very low power input requirements.

A feature of the thermally operated switch construction in accordance with the invention is the provision of a no lag flexible resistive film supported on a flexible insulating strip and electrically connected between the bimetallic strip contacts of the switch adjacent the free ends of the strip contacts, with a free loop portion allowing the resistive film to extend or assume a draped condition when the contacts are in an open and a closed condition respectively. The resistive film has opposite ends connected to the respective bimetallic strips and making intimate electrical connection therewith. The resistive film has a resistance, which may vary along the length thereof, effective to generate heat under a low application of power. The heat generated is sufficient to keep contact portions of the bimetallic strip contacts spaced apart once the bimetallic strips have deflected due to the heating thereof in response to application of a potential difference between them. The film resistor serves only to quickly make up for heat losses in the bimetallic strips once they have assumed a spaced-apart position due to their bending in response to internal heat generated by current flow therethrough.

While it is known to use, on bimetallic strip contacts,

3,447,] 13 Patented May 27, 1969 an insulating layer and a resistive film on the metal layers having the greatest coefiicient of expansion in practice, however, difficulties have been encountered in connecting the resistive film with appropriate leads. The problem has been that at least one terminal or end portion of the film must move with the bimetallic strips as they deflect or bend. The difficulties heretofore encountered are avoided in the present invention because the ends of the resistive film which correspond to the freely moving ends of the bimetallic strips are connected directly to the bimetallic strips. The intermediate portion of the resistive film forms a loop free of the contacts, near the fixed points of the bimetallic strip contacts. This loop portion is free to extend and allow unimpaired deflection of the bimetallic strips.

The invention is partciularly applicable to bimetallic strip switches in which the switches are normally closed. The construction of the new switch permits very stiff, and small switch assemblies to be constructed which react rapidly due to the large effective length of the bimetallic strips and which are positive in their opening and closing. The switch in accordance with the invention is more positive and sensitive and permits more compact construction than heretofore.

Another feature of the switch in accordance with the invention is the provision of single insulating flexible strip support for the resistive film which is prefabricated and is simply bonded to the bimetal strips thus mounting the resistive film automatically on the bimetallic strips.

In order to avoid heat build-up or burn-through on the insulative strip section forming the loop between the bimetal strips the conductance of the resistive film can be made high compared with the conductance of the resistive film portions disposed adjacent the bimetal strips physically positioned parallel thereto. Provision is made to vary the conductivity in this portion by providing a current path as a film cover over the loop portion of the resistive film.

Other features and advantages of the switch in accordance with the present invention will be better understood as described in the following specification and appended claims, in conjunction with the following drawings in which:

FIG. 1 is a fragmentary view, partly in section, of bimetallic switch contacts for a switch to be provided with the invention;

FIG. 2 is an enlarged cross section view of a portion of the switch parts illustrated in FIG. 1; and

FIG. 3 is a schematic diagram of a motor provided with a starter switch embodying the invention.

While the switch in accordance with the invention is suitable as a starting switch, and will be so described, for single-phase split-phase motors in which the switch is connected to an auxiliary winding so that the starting current is reduced when the motor is up to speed. Those skilled in the art will understand that the principles are equally applicable to time-delay switches and the like.

As illustrated in FIG. 1 an insulator 1 supports bimetallic strip contacts 2, 3. The bimetallic strips 2, 3 are made of dissimilar metals and have the metallic layers thereof having the greater temperature coefficient of expansion confronting each other and are provided at the free ends thereof with contacts or contact portions 4, 5 normally closed. The bimetallic strips are provided with

end portions

6, 7 fixed in the support 1 and to which electrical connection is made to a switch made in accordance with the invention as later described. When a potential difference is applied to the bimetallic strip contacts the contact portions 4, -5 will assume a spaced apart or open position since the bimetallic strips will bend or deflect outwardly or away from each other in response to the heat generated therein in response to current flowing therethrough.

Provision is made, in a switch construction in accordance with the invention, for keeping the contact portions 4-, 5 spaced apart once the switch is open by a quickresponse element in the form of a resistive layer which replenishes heat losses in the bimetallic strips so that they remain in their bent or deflected position thereby keeping the contact portions 4, 5 open. Thus a flexible resistive film 8 is provided and supported on an insulating strip 9 having its opposite end portions 10, 11 disposed in intimate contact with the layers of the bimetallic strips having the greater temperature coefficient of expansion. A middle or intermediate length or portion v12; of the resistive film forms a loop between the two bimetallic strips 2, 3 which is free of the bimetallic strips and will allow suflicient and unimpeded deflection or bending of the bimetallic strips to assume a spaced apart position when the switch opens.

The resistive film 8 is electrically connected to the bimetallic strips 2, 3 at

connections

13, 14 in the neighborhood of the contact portions 4, 5 and the strip 9 is provided with an adhesive layer 15 only in the region of the portions 10, 11 bonded and held in intimate contact with the bimetallic strips. The center section 12 of the resistive film 8 is provided with a metal layer 16 overlying the film as illustrated in FIG. 2, which fragmentarily illustrates substantially one half of the insulating strip and resistive layer thereon. The metal layer 16 provides a different conductivity value in the intermediate length and avoids excessive heating of the loop or tree portion of the resistive film.

One example of a construction of a switch in accordance with the invention has been constructed in which polytetraflurethylene is used for the insulating strip 9. The resistive film 'formed on the support strip 9 consists of a mixture of epoxy resin and collodial graphite. The mixture is made so that the film has the desired resistance characteristics for the desired switch. Epoxy resin is used as the layer ;15 for bonding the support strip onto the metallic plates or strips and the

connections

13, 14 are made of a mixture of an adhesive and collodial silver. The connections consist of conductive material which is disposed over the ends of the film and supporting strip effecting electrical contact between the resistive film and the bimetallic strips as illustrated in FIG. 1. If necessary, in view of the environment in which the switch will operate, the resistive film 8 can be coated with an insulating layer and a reflecting layer, not shown, in order to avoid heat losses.

FIG. 3 illustrates an application of a switch in accordance with the invention as applied to a switch for motor starting. As illustrated, a

circuit having connections

17, 18 for applying power from a source, not shown, to a single-phase asynchronous motor 19' having a main winding 20 and a starting winding 21 energized when a switch 22 is closed. A starting switch 23 built in accordance with the invention as heretofore described, is connected in series with the starting winding. When the main switch 22 is closed current flow, determined by the winding resistances, flows through the main winding 20 as well as through the auxiliary winding 21. The auxiliary winding current heats the bimetallic strips of the switch 23, which are in series with the starting winding, with a result that the switch opens. Upon opening of the switch the resistive film 8 has instantaneously applied to it the vector difference between the voltages of the main winding 20 and the auxiliary winding 21. This voltage potential dif- :Eerence remains approximately constant even with main supply fluctuations at the terminals 17, 1-8 and produces suflicient power to heat the starter switch sufficiently to maintain it in the open position due to the current flow in the resistive film of the starting switch 23.

Those skilled in the art will understand that the bi- 4 metallic switch 23 in accordance with the invention has the current going through the bimetallic contacts while it is closed so that the resistive film is protected from application of excess current. When the switch opens only a low current flows through the resistive film which has only to restore to, or compensate for, heat losses of the bimetallic contacts to maintain the switch positively open. Since the film is immediately efiective there is no lag and no possibility of the contacts efiecting even momentary closure. Moreover, the switch can be provided with suitable adjusting screws, not shown, on one bimetallic strip for setting the contact pressure between the contacts 4 and 5 so that the positive action of the switch can be further maximized.

While a preferred embodiment of the invention has been shown and described it will be understood that many modifications and changes can be made within the true spirit and scope of the invention.

What -I claim and desire to be secured by Letters Patent is:

1. In a device for opening and closing an electric-a1 circuit having a pair of contacts comprising a pair of electrically conductive bimetallic strips connectable in series in said circuit, each strip comprising layers of dissimilar metals having different temperature coeflicients of expansion and contact portions at free ends thereon normally in electrical contact and automatically operably to a spaced-apart, circuit-opening position in response to heating thereof upon application of a selected potential difference between said bimetallic strips, a lag-free, quick response electrical means connected to and parallel with said bimetallic strips in said device and said circuit to continue to apply heat to said bimetallic strips substantially immediately upon said bimetallic strips assuming said open-circuit position and heat effective to maintain said bimetallic strips heated and in said open-circuit position, said quick-response electrical means comprising a flexible resistive film, connection means connecting opposite ends of said resistive film to said bimetallic strips, and said resistive film having an intermediate loop portion free of the bimetallic strips.

2. In a device according to claim 1, in which said opposite ends of said resistive film are connected to said bimetallic strips adjacent said contact portions, and each of said bimetallic strips having a metal layer thereof having the greater temperature coefficient of expansion connected to respective ones of said connection means.

3. In a device according to claim 1, including a flexible strip of insulating material on which said resistive film is supported, said strip of insulating material having portions bonded to said bimetallic strips and an intermediate loop portion supporting said loop portion of said resistive film.

4. In a device according to claim '1, in which said resistive film comprises a film having different conductance along different lengths thereof, thereby to preclude overheating and rupture of said film.

5. In a device according to claim 1, including a metallic layer overlying said resistive film along a length corresponding to at least a part of said intermediate loop portion and said length thereby having greater conductivity than the remainder of said film.

References Cited UNITED STATES PATENTS 2,403,803 7/ 1946 Ke'arsley 2l9-5l1 BERNARD A. GILHEANY, Primary Examiner.

R. L. COHRS, Assistant Examiner.

US. Cl. X.R.