US 3284597 A
Description (OCR text may contain errors)
Nov. 8, 1966 A. J. HOLLIS 3,284,597
ELECTRICAL CONTROL DEVICE OF THE THERMAL DELAY TYPE Filed April 30, 196
ARTHUR J. HOL
INVENT ATTORNEY United States Patent 3,284,597 ELECTRICAL CONTROL DEVICE OF THE THERMAL DELAY TYPE Arthur J. Hollis, Danvers, Mass., assignor to Sylvania Electric Products Inc., a corporation of Delaware Filed Apr. 30, 1965, Ser. No. 452,047 3 Claims. (Cl. 200-122) This invention relates to electrical control devices and more particularly to those of the thermal delay type.
Most thermal delay relays employing a combination of bimetals with a heater either have a slow make and break or a snap make and break contact action. The slow make and break contact action is generally limited to resistive type loads or it requires the use of a slave relay to handle the load. If the unit is ambient-compensated, it ordinarily becomes quite complicated and expensive. Because of their nature, snap make and break bimetallic devices cannot be ambient-compensated. The control of cycle times is difiicult because a snap bimetallic blade inherently has a temperature differential between snap open and snap closed. This temperature differential also presents a difficult problem with respect to fast cycle times.
In view of the foregoing, one of the principal objects of this invention is to provide an essentially ambientcompensated, self-recycling thermal time delay device with contact ratings to handle the controlled load directly.
Essentially the control device of this invention is an automatic recycling thermal timer with normally closed load contacts. By incorporating a variable resistance, the timing cycle can be adjusted. The bimetal elements are designed to give the load control contacts a good positive make and break action. The bimetal elements also are designed to make the device ambient-compensated throughout a broad range of temperatures. In addition, these bimetal elements can be designed to be an eifective current limiter or circuit protector upon the conditions of either unusual circuit load or faults in the heater circuit. Also, it is possible to adjust the timing cycle to an unusually short period for a thermally actuated device. The device may also have optional time delay adjustment. Further, the bimetal elements of the device may be easily varied to provide a line of current-rated, time delay devices which incorpo rate their own overcurrent protection, thus eliminating the need for a fuse or circuit breaker. As used herein, the term bimetal refers to thermostat metal and is not limited to two metals but may, for example, include three, i.e., trimetal.
In the specific embodiment of the invention illustrated in the accompanying drawing:
FIGURE 1 is a plan view of a thermal delay device embodying the principles of my invention.
FIGURE 2 is a sectional view of the device of FIG. 1 taken along the line 22 thereof.
FIGURES 3 and 4 are schematic diagrams illustrating by way of example two circuits with which the thermal delay device of this invention may be employed.
As shown in FIGS. 1 and 2, the thermal delay device is mounted on a base of insulating material. Essentially, the device comprises a primary bimetal member 12, a secondary bimetal member 14 and a heater coil 16. The primary bimetal member 12 is substantially U-shaped with one leg 12a being longer than the other leg 12b. The outside face of the U-shaped primary bimetal member 12 is the high expansion side and the inside face is the low expansion side. The secondary bimetal member 14 is narrower in width than the primary bimetal member 12 and is a flat piece. As seen in FIG. 2, the upper face of the secondary bimetal member 14 is the high expansion side and the lower face is the low expansion side. The secondary bimetal member 14 is welded at one end thereof to the end of the short leg 12b of the primary bimetal member 12, with the low expansion side of the secondary bimetal member 14 engaging the high expansion side of the U-shaped primary bimetal member 12. A movable electrical contact 18 is attached to the other end of the secondary bimetal member 14 on the low expansion side thereof.
The base 10 is provided with a pair of terminal posts 20 and 22. A fixed electrical contact 24 is mounted on top of terminal post 22 and is normally closed with respect to the movable electrical contact 18. The long leg 12a of the primary bimetal member 12 is welded at its end to terminal post 20. Terminal post 20 is connected through base 10 to electrical contact pin 26 and terminal post 22 is similarly connected to a similar pin 28. The ends of the heater coil 16 are connected to contact pins 30 and 32.
The particular arrangement of the bimetal system (members 12, 14 and 16) disclosed in the accompanying drawing and described above is characterized by a high degree of mechanical stability, and more particularly by improved shock resistance. The fact that the heavier cross section of the bimetal is at the fixed end where it is welded to the mounting stud contributes significantly to the attainment of this high degree of mechanical stability. It also assists in the thermal conduction desired in the new system. The heater coil 16 is preferably located on the longer leg 12a of the substantially U- shaped primary bimetal member 12 adjacent to the welded connection of the leg 12a to the terminal post 20. This positions the heater coil so as to give maximum motion to the contacts for the heat energy transfer to the bimetal. It also places this auxiliary heated section, leg 12a of the primary bimetal member 12. close to the relatively massive terminal post 20 which can act both as a heat sink and a heat conducting path to the cooler external wiring. Thus this better segment of the primary bimetal member can be cooled more rapidly to provide a shorter open circuit delay.
The secondary bimetal member 14'is reversed in its mode of deflection as compared with the primary bimetal member 12, such that it moves to apply increasing contact pressure with self 'heating due to passage of current. As noted above in the description of a specific embodiment of the invention, the secondary bimetal member is of substantially smaller cross sectional area than the primary bimetal member. Thus, when both bimetal members are of substantially the same resistivity, the self-heating of the secondary member produces temperatures much hotter than those generated in the primary member when an electrical current is passed through the system. When both members are of substantially the same bimetallic flexivity, then the motion per unit length of the hotter bimetal is greater. This provides initially increasing contact pressure as a current is applied to the unit. As the beater transfers heat to the section of the lower bimetal, this action eventually overtakes the motion of the secondary bimetal and moves to open the contacts. At the instant of opening, the hotter secondary bimetal moves to accelerate the contact opening since it cools rapidly, principally through the beat-conducting path to the cooler or primary bimetal. Thus this secondary bimetal member provides make and break contact acceleration when the resistivities are properly selected for the electrical current values handled to produce the desired self heating temperatures.
The heater coil 16 does not necessarily have to-be a fixed value and may be made to have a controllable variable resistor, which may be thermally remote, to effect variable wattage such that delay times can be adjusted through a range of times. Also, since the bimetal elements are essentially designed to be under low stresses under normal operating conditions, the bimetal can be operated at relatively high temperatures. This is as opposed to a bimetal formed to provide a snap action which inherently contains high forming stresses. Such bimetal forms are limited to much lower temperatures. In addition the bimetal system is inherently partially ambient compensated and thus reduces the stresses on the bimetals caused strictly by the variation in contact pressures as these bimetals react to the broad range of seasonal ambients. This capability of running at comparatively hot temperatures contributes to make possible an essentially ambient insensitive device as well as a rapid cycling device.
The heater coil 16 is preferably of a material having a radically positive temperature coefficient of resistance. Although nichrome wire of the type normally used for electric heaters gives good results, it has been found that even better results may be obtained in applications involving applied voltage variations by using Wilbur B.
. Drivers Balco which exhibits a 100% change in resistance over approximately 150 C. temperature change. Even further improvements may be obtained by having the external or variable resistance made of the same material.
The use of a high positive coefficient of resistance material enables the attainment of a much narrower operational temperature band through the voltage range. Thus,
if the top temperature limit of the heater wire is kept at the desired design level, then the temperature of the lower end of the band can be raised and the resultant is a higher overall average operational temperature. This makes the unit less ambient sensitive as well as less voltage sensitive. By using the same material in the external variable resistance, one obtains essentially an ambient temperature compensation tending to produce a more consistent relay timing. As the ambient drops, the external resistance decreases and allows more wattage to be developed in the internal heater and vice versa. The resultant is a more uniform delay for a given set point on the rheostat throughout the broad ambient range.
Two circuits wit-h which the electrical control device of this invention may be employed are illustrated by way of example in FIGS. 3 and 4. In these schematic diagrams, parts corresponding to those illustrated in FIGS. 1 and 2 and described above are identified with the same reference numbers. In addition, these schematic diagrams 4 include a variable resistor 34, a battery or power source 36 and a switch 38. The circuit of FIG. 3 essentially requires a driven switch 42 related with the load 40 to make the circuit self-recycling. By reconnecting as in FIG. 4, the device will be self-cycling as long as the external switch is closed.
What I claim is:
1. An electrical control device of the thermal delay type comprising:
a pair of terminal posts;
a fixed electrical contact mounted on one of said terminal posts;
a substantially U-shaped primary bimetal member, the legs thereof being of unequal length, the outside face thereof being the high expansion side and the inside face thereof being the low expansion side, the outside face of the free end of the longer leg being secured to the other of said terminal posts;
a substantially flat secondary bimetal member, narrower in width than said primary bimetal member, the upper face thereof being the high expansion side and the lower face thereof being the low expansion side, said secondary bimetal member being afiixed at one end of the lower face thereof to the outside face of the free end of the shorter leg of said primary bimetal member;
a movable electrical contact mounted on the other end of the lower face of said secondary bimetal memher, said movable electrical contact being normally closed with respect to said fixed electrical contact; and a 'heater coil wrapped about the longer leg of said primary bimetal member.
2. The combination of claim 1 in which the heater coil is of a material having a radically positive temperature coefficient of resistance.
3. The combination of claim 2 and a variable resistor connected in series with said heater coil to effect variable wattage such that delay times can be adjusted through a range of times.
References Cited by the Examiner UNITED STATES PATENTS Harrold 200-113 BERNARD A. GILHEANY, Primary Examiner.