Patent Application
20070085651
This invention relates generally to thermostat metal, such as bimetal, actuated devices and more specifically to a method for providing ambient temperature compensation for a series of devices having different current ratings.
Presently, thermally compensated thermostat metal actuated electrical devices use a thermostat metal, such as bimetal, compensator to provide relatively constant levels of hold and trip currents as a function of ambient temperature. However, in providing suitable thermostat metal compensation for a series of devices having different current ratings the level of compensation must be changed appropriately. To provide for a range of compensation that may be needed as one goes from one ampere rating to the next, different thickness thermostat metals and different types metals for the thermostat metals are used to obtain varying levels of bimetal activity (movement per degree Fahrenheit). The formula for thermostat metal movement is shown below:
B(thermostat metal movement)=0.53F(ΔT)L2/t
As noted above, it is known to use different thickness thermostat metals to obtain different levels of compensation, i.e., different amounts of movement per degree of temperature, for example 0.023, 0.026, 0.028, 0.030 inch thickness. However, this approach for changing compensation levels has several disadvantages. The first disadvantage is that this approach is relatively expensive to provide because manufacturing different thicknesses requires the use of heavy rolling mills and the like that produce large quantities of material while only small quantities are needed for each rating of compensation members thereby resulting in excessively large amounts of inventory. Further, the mass of the compensators and associated latches for higher ratings increase along with thickness making the circuit breakers more sensitive to shock and vibration.
Changing the length of the thermostat metal compensator is impractical because of packaging constrains. That is, designers of equipment with which the devices are to be used, such as aircraft, typically are not able to accommodate device packages of different sizes.
It is therefore, an object of the present invention to provide ambient temperature compensation for a series of thermostat metal actuated electrical devices that is not subject to the prior art limitations noted above.
Another object of the invention is the provision of a method for ambient temperature compensating electrical aircraft circuit breakers for a series of different current ratings that is relatively inexpensive while at the same time providing such breakers that are generally insensitive to vibration and shock resistance from one device rating to another.
Briefly, in accordance with the invention, the effective length of a thermostat metal compensator element is changed, while keeping the actual length unchanged, by forming various selected dimple or rib configurations in the element. Preferably, a compensator element is formed from thinner material than any of the presently used compensators and then ribbed to effectively reduce the active or effective length of the element without changing the overall length of the element. One compensator element thickness, e.g., 0.018 inch, with various dimple patterns can be used for an entire family of circuit breakers, or other thermostat metal actuated devices, providing the most active to the least active compensation by increasing the deformations in a controlled manner. Compensation elements used in accordance with the invention are less costly and provide improved shock and vibration resistance, particularly in higher current ratings that have the trip latch attached to the compensator element, since the trip latch for higher current rating breakers made in accordance with the invention will have less mass than circuit breakers with conventional thicker compensator elements. Thus, in combination with lower friction latches, lower actuation forces are utilized thereby minimizing concerns of shock and vibration issues. Another advantage is that this approach also allows the use of low force piezo-resistive actuators to work more effectively with associated latches in arc fault or similar applications.
Other objects, features and advantages of the present invention will appear from the following detailed description of a preferred embodiment taken together with the accompanying drawings.
With reference to
The inner end of latch plunger 20 is formed into a yoke for pivotably mounting a bell crank latch 24 on pin 25 extending between the opposed legs 20d of the yoke (one leg being shown in
Bell crank latch 24 is formed with a latch part 24b adapted to be received on a catch surface 28c of catch member 28, to be discussed. Bell rank latch 24 also has a downwardly extending leg 24a formed with an aperture 24c that serves as a connection location for a coil spring 30 also connected to anchor plate 32. Anchor plate 32 is fixedly mounted relative to latch plunger 20 so that a bias is applied to bell crank latch 24 urging it in a counter clockwise direction, as viewed in
A leaf spring bent back on itself serves as a movable contact arm 32 and carries bridging movable contacts 32a. Arm 32 has one end 32b mounted in a notch in leg 24a of bell crank latch 24 while hook 24d of the latch engages an outer face of movable arm 32 on the opposite side of the bend limiting movement of the arm and serving as a motion transfer member in opening of the contacts when catch surface 28c moves out of engagement with latch part 24b.
First and second, spaced apart and electrically separated stationary electrical contacts 34a(one being shown) are mounted in the housing with terminal TI mounting one stationary contact 34a and the other stationary contact being electrically connected by a pig tail connector (not shown), or the like, to one leg of current carrying, generally U-shaped thermostat metal trip arm 36. The other leg of trip arm 36 is mounted on and electrically connected to terminal T2 so that current passes through T2, thermostat metal trip arm 36 to the stationary contact hidden behind contact 34a shown in
As best seen in
A horizontally slidable motion transfer member 42 is disposed between the bight or upper ends of thermostat metal trip arm 36 and thermostat metal ambient compensation member 38 to transfer motion from trip member 36 to the compensation/catch assembly.
Trip arm 36 reacts both to ambient temperature and to the heat generated by current passing through the arm and upon heating, the upper portion of the trip arm will bend to the left, as seen in
For a more detailed description of the operation of the circuit breaker, reference may be had to U.S. Pat. No. 3,361,882, issued Oct. 24, 1965 to the assignee of the present invention which shows and describes this type of aircraft circuit breaker.
As noted above, thermostat metal movement can be determined by the following formula:
B(thermostat metal movement)=0.53F(ΔT)L2/t where
F is flexivity (10−7/degree Fahrenheit); T is degrees Fahrenheit; thermostat metal movement B, length L and thickness t are in inches.
As discussed above, the present practice in providing ambient compensation is to change the thickness of the thermostat metal used for the ambient compensation member in order to match the thermostat metal movement of the trip arm at different current ratings due to the impracticality of changing the other variables in the formula. However, in accordance with the present invention, the effective length of the thermostat metal member is changed through the use of metal deformations such as grooves, dimples or the like. A compensator member is formed from the thinnest thermostat metal used in the existing compensators presently used or, if desired, thinner than any presently used, forming controlled ribs to effectively reduce the effective length and thereby make the member less active. Thus, one basic temperature compensator thickness, e.g., 0.018 inch thick, can be modified with various dimple patterns for use in an entire family of circuit breakers providing compensators ranging from the most active (with relatively few, if any, deformations) to the least active with the greatest amount of deformations. Compensators made in accordance with the invention have the additional advantage of being lighter in weight (along with lighter associated latches), as well as being less costly and provide significantly improved shock and vibration resistance in ambient compensated devices in which the compensation members are attached to the trip latch. In combination with low friction latches, lower actuation forces are needed (without fear of shock and vibration issues) for use with low force piezo-resistive actuators to work more effectively with the latches in arc fault or similar application.
Thus, with reference to
The depth, width and the length of the deformations each has an affect on the effective length so that tooling for the deformations needs to be tailored to obtain desired specific thermostat metal movement for given current ratings.
Although the invention has been described for use in a circuit breaker, it will be appreciated that the invention can be applied to any device that utilizes an ambient temperature compensation member.
It should be understood that although a particular preferred embodiment has been described by way of illustrating the invention, other embodiments are possible. It is intended that the invention includes all modifications and equivalents of the disclosed embodiment that fits within the scope of the claims.
