US2949518A - Thermostatic time delay relay - Google Patents

Description

Aug. 16, 1960 M. l. NYSTUEN THERMOSTATIC TIME DELAY RELAY Filed April 24, 19

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ullll INVENTOR Marcus 1. Nysruen ATTORNE United States Patent THERMOSTATIC TIME DELAY RELAY Marcus I. Nystuen, St. Paul, Minn., assignor to Economics Laboratory, Inc., St. Paul, Minn., a corporation of Delaware Filed Apr. 24, 1959, Ser. No. 808,814

6 Claims. (Cl. 200122) This invention relates to electric relay devices, especially of the thermostatic time delay type.

The relay in accordance with the invention is simple in construction, reliable in operation, and inexpensive to manufacture. The nature of the invention and its several advantages will beunderstood from the following description considered in connection with the accompanying drawing, in which:

Fig. 1 is a side view in elevation of the relay according to the invention;

Fig. 2 is a plan view of the relay; and

Fig. 3 is a circuit diagram of connections by which the relay is operable in the manner below described.

Referring to Figs. 1 and 2, the relay according to the invention comprises a suitable base 1 of insulating material to which the component parts are secured. A bimetal, thermostatic strip 2 is affixed at its mid-section to a bracket 3 of U-shaped metal, and this bracket is fastened by a screw, or otherwise, to the base 1. Thus, the respective ends of the strip are free to flex independently. As shown, the bracket is oriented so that the channel thereof is in alignment with the longitudinal axis of the strip 2. This permits circulation of heated air by convection from the lower end to the upper end of the strip, especially when the device is mounted vertically as illustrated in the drawing. If the bracket is of different configuration, it can be perforated or otherwise arranged to permit the circulation of air.

The upper end of the bi-metal strip 2 carries two opposed contacts 4, 5 for the purpose of opening or closing one or more external or load circuits. Movable contact 4 is in contacting alignment with fixed contact 6, and is here shown to be normally closed. Fixed contact 7 is in contacting alignment with movable contact 5 and is normally open. Hence, these contacts open and close,

respectively, upon actuation of the relay after a predetermined delay period. The movement of the upper end of strip 2 with its contacts 4, 5 occurs in response to the respective heating and cooling thereof.

The lower section of strip 2 carries at its end a movable contact 8 which is normally closed against fixed contact 9. When this lower section becomes heated these contacts open, and close again upon its cooling. The lower section is heated by an electric heating coil 10. Obviously, the required characteristics of this coil will depend upon several variables, including the desired rapidity of operation, the physical size of the component parts, the thermal characteristics of the bi-metal strip, and the voltage of the current to be applied. In the embodiment here illustrated Nichrome resistance Wire of 2.5 mils diameter, having a total resistance of approximately 110 ohms, was wound with spaced turns around the strip as shown. A suitable insulating layer 17 is wrapped around the strip before the wire is wound thereon.

A covering of similar or equivalent material may advantageously be applied outside the wire to protect it Patented Aug. 16, 1960 and to conserve the heat, although none is shown in the drawing.

To terminate the ends of the resistance wire of coil 10 and to provide means for connecting the wire in an appropriate circuit, terminal members 11 are employed. These comprise two short lengths of solid copper wire 12 and 12:: which pass through the base 1 and extend near the heater coil. A metal sleeve 13, 13a is dropped over the top of each of the wires 12, 12a, respectively,

and one end of the resistance wire is placed in the space between the inside of the sleeve and the solid terminal wire which it encloses. Then each sleeve is crimped with a crimping tool, thus mechanically and electrically attaching the resistance wire to the terminal wire. Terminal wire 12 is here shown to be electrically connected to fixed contact 9 so that the circuit to the heater is normally closed. If the heater is to be controlled externally of the relay, terminal wires 12 and 12a would both be connected to the external circuit. Connections can be made at terminals 14, 15, and 16, for controlling external load circuits.

Fig. 3 is a circuit diagram illustrating the connections by which the time delay relay of the invention can be operated to control one or more external load circuits. As shown diagrammatically, the heating coil 10 is wrapped around the lower (left-hand) section of strip 2. One end, 12, of the coil is connected to fixed contact 9, and the other end to a control switch 18. This switch, which may comprise a relay, or equivalent, is connected through a source 19 of electric voltage, here represented as a battery, which, in turn, is connected to strip 2 to complete the circuit. Since contacts 8, 9 are normally closed, the coil 10 will become heated when switch 18 is closed. If a DC. voltage source such as a battery is used, it would be desirable to connect a large condenser across contacts 8, 9 to prevent arcing, as is well known in the art. However, for many reasons it is advantageous to employ alternating current, in which case the condenser would not be necessary. Alternating current at a convenient voltage, such as 24 volts, is readily obtainable from a suitable step-down transformer.

Load circuit 20 is, as shown, connected between terminals 15 and 16 and, therefore, will be closed upon actuation of the relay. Load circuit 21 is connected between terminal 15 and battery 19, and its circuit is opened when the relay is actuated. In this arrangement the voltage source 19 is available to energize load circuit 21. Load circuit 20 would, of course, also be provided with actuating voltage if required.

The delayed operation of the relay according to the invention is an important aspect and, therefore, requires further description. The lower section of bi-metal strip 2, which is directly heated by coil 10, will flex shortly after the circuit is closed by switch 18. This opens contacts 8, 9 and de-energizes the heating circuit, whereupon that section of the strip will cool, and contacts 8 and 9 will close again, resuming the heating. This cyclic operation will continue indefinitely and will assume a uniform rate, once the strip has reached its operating temperature. The rate of cycling can be adjusted by adjusting the height of contact 9 and by varying the current through the heating coil.

Although the re-establishment of connection between contacts 8 and 9 involves cooling of the hotter end of the bi-metal strip, some of the heat generated during the successive heating periods will travel along strip 2 by conduction in the metal and thus will heat the upper section of strip 2, which in Fig. 3 is to the right. Addiitonally, when the relay is positioned vertically, as shown in Figs. 1 and 2, the air in the neighborhood of coil 10 which is heated thereby will flow upwardly and supple ment the heat carried through the bimetal strip itself.

,the heated air through its channel.

As a result of this heating, the upper section of the strip will flex upwardly breaking contacts 4, 6 and closing contacts 5, 7. This change in connections constitutes actuation of the relay.

From the foregoing, it will be understood that the period' of time required to actuate the relaydepends upon several factors. One controlling factor is the effect of bracket 3. This bracket, being of metal as described, has a certain degree of thermal capacity, viz., it absorbs heat from the strip and to this extent effectively comprises a heat barrier between the heated and unheated ends of the strip. Furthermore, the heat-radiating surfaces of bracket 3 tend to dissipate the heat which it absorbs and thus prevent such heat from passing to the upper end of the bi-rnetal strip. Hence, the thermal characteristics of bracket 3 have an important bearing on the time delay in actuation of the relay. It has been found that the heated air from coil 10 tends to bypass the barrier effect of the bracket, especially if the bracket is constructed, as shown, so as to permit free flow of Thus, by changing the angular position of bracket 3 with respect to the axis of strip 2, it is possible to adjust, to some extent, the delay in the time of actuation of the relay after the heating circuit has been initially closed.

Other expedients can also be employed to control the rate of actuation of the relay. For example, the actuation time will be shorter if more heat is retained within the structure by enclosing the sides of the strip. This is especially effective if the relay is mounted horizontally.

Introducing thermal insulation between the strip 2 and bracket 3 will also increase the actuating rate.

The bi-metal strip 2 is usually a continuous piece. However, two strips can be clamped together at the bracket 3, either abutting or overlapping. The rate of heat conduction between the extreme ends of the strip can be adjusted by changing the separation or overlapping of the sections at the joint or by interposing therein materials of different heat conductivity. The delay in actuation will be increased by thus decreasing the rate trated and described but of dimensions smaller than those in the drawing, it was found that an optimum operating temperature for the bi-metal strip 2 was approximately 275 F. This is high enough to make changes in ambient temperature unimportant in their effect on the thermostatic strip. Such a temperature was reached after approximately 60 seconds of heating of the coil; and with the described adjustments the delay before cycling commences could be adjusted between approximately 5 seconds and 1 /2 minutes. This range can be extended in either direction by appropriate changes in the above-mentioned variables.

The adjustments herein described make it possible with the structure as shown to cause actuation of the relay, viz., movement of contacts 4, 5, after a delay of between approximately 20 seconds and 2 /2 minutes. These limits apply to the particular embodiment herein described by way of example, but either or both can be extended in either direction by changing the rate of heating of the upper section of strip 2 by means of the expedients previously discussed.

For many applications the effective of ambient or surrounding temperatures is unimportant. However, if the relay is to be subjected to large changes in ambient temperature, the eifect can be compensated for by known expedients, one of which is to employ bi-metallic material in the bracket which supports the strip, or in the bracket on which the fixed contact is mounted.

I claim:

1. A time delay relay of the thermostatic type comprising a base, a mounting bracket secured to said base, a longitudinal bi-metal strip mounted at its midsection on said bracket so that its two ends can flex independently with respect thereto, a first movable contact carried on a first end of said strip, a first fixed contact positioned on the base in contacting alignment with said first movable contact, a heating coil secured to said strip between said first movable contact and said bracket, means connecting one end of said coil to one of said contacts and means adapted to connect the other end of said coil to an electric source, a second movable contact carried on the second end of said strip, and a second fixedcontact positioned on said base in contacting alignment with said second movable contact, said bracket being of heatconductive material and being in heat-conductive contact with said strip, whereby to delay the transfer of heat from said coil into the second end of said strip.

2. A time delay relay according to claim 1 in which said bracket is shaped and disposed so as to be open to the flow of air along said strip between the first and second ends thereof.

3. A time delay relay according to claim 1 in which said bracket is of U-shaped metal and is positioned so that its channel is in alignment with the longitudinal axis of said strip.

4. A time delay relay comprising thermostatically deformable strip means, heat-conductive means rigidly supporting said strip means intermediate its extreme ends, and in heat-conductive contact with said strip means, said ends being independently flexible, a movable contact carried by a first one of said ends, a first fixed contact disposed to make connection with said movable contact when said strip means is unflexed, an electric heating element disposed to heat said first end more than said second end, electric connections including said contacts for energizing said element, a second movable contact carried by the second end of said strip means and a second fixed contact disposed in contacting alignment with said second movable contact, whereby said second contacts are closed or opened in response to heating of said strip means after a time delay in part predetermined by the heat conductivity characteristic of said heat conductive means.

5. A time delay relay comprising a thermostatically deformable strip, a heat-conducting bracket supporting said strip intermediate its ends, the ends of said strip being independently flexible, said bracket having a predetermined thermal capacity and a heat-radiating surface, a first movable contact carried by a first end of said strip, a first fixed contact disposed in contacting alignment with said movable contact, means for connecting an external circuit to said contacts, a second movable contact carried by the second end of said strip, a second fixed contact in contacting alignment with said second movable contact, said second contacts being adjusted to be closed when the second end of said strip is unflexed, an electric heater element disposed to heat the second end of said strip and thereby open said second contacts, and circuit means including said second contacts for energizing said element when said second contacts are closed, the heatconductive path from the second end of said strip through said bracket to the first end of said strip being such as to delay the movement of said first movable contact for a predetermined time interval after the commencement of heating by said element.

6. A time delay relay comprising a thermostatically deformable strip, a heat-conducting bracket supporting said strip intermediate its ends, said bracket having a heat-radiating surface and said ends being independently flexible, a first movable contact carried by a first end of said strip, a first fixed contact disposed in contacting alignment with said movable contact, means for connecting an external circuit to said contacts, a second movable contact carried by the second end of said strip, a second fixed contact in contacting alignment with said second movable contact, said second contacts being adjusted to be closed when the second end of said strip is unfiexed, an electric heater element disposed to heat the second end of said strip and thereby open said second References Cited in the file of this patent contacts, and circuit means including said second con- UNITED STATES PATENTS tacts for energizing said element when said second contacts are closed, whereby the heating and cooling of said 2108964 Freudenberg et a1 1938 second end occurs cyclically, the heat-conductive path 5 2,317,830 Vaughan 1943 from the second end of said strip through said bracket to the first end of said strip being such as to delay the move- FOREIGN PATENTS ment of said first movable contact for a time interval 33,729 France July 31, 1928 measured by a predetermined number of heating and (Addition to No. 638,663)

cooling cycles of said heating element. 10 601,932 Germany Sept. 5, 1934

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