Patent Application
20150091689
The present disclosure relates to thermal cut-off devices that provide protection against overheating by interrupting an electrical circuit.
This section provides background information related to the present disclosure which is not necessarily prior art.
Operating temperatures for electrical devices, including appliances, electronics, motors and the like typically have an optimum or preferred range, above which damage can occur to the device or its components, or safely operating the device becomes a concern. Various known devices are capable of protecting against over-temperature conditions by interrupting the electrical current in the device.
One device particularly suitable for over-temperature protection and current interruption is known as a thermal cut-off (TCO) device. A TCO device is typically installed in an electrical application between the current source and electrical components, such that the TCO device is capable of interrupting the circuit continuity in or to a device in the event of an undesirable over-temperature condition. TCO devices are often designed to shut off the flow of electric current to the application in an irreversible manner, without the option of resetting the TCO device current interrupting device.
An exemplary TCO device known in the art is illustrated in
An electric current interruption assembly 114 for actuating the device in response to a high temperature, for example, is generally disposed between the first and second electrical conductors. The current interruption assembly 114 actuates or “trips” to break the continuity of an electric circuit through the TCO device 100. The current interruption assembly includes a moveable, sliding contact member 126 formed of electrically conductive material, such as a metal. The sliding contact member 126 is disposed inside the case 102 and is disposed in peripheral sliding engagement with the internal surface of the case 102 to provide electrical contact therebetween. Moreover, when the TCO device is operating at a temperature that is below its predetermined threshold set-point temperature, the sliding contact member 126 is disposed in electrical contact with the distal end 120 of the second electrical conductor 116.
The current interruption assembly 114 also includes a biasing means. The biasing means biases the sliding contact member 126 against the distal end 120 of the second electrical conductor 116 to establish electrical contact in a first operating condition where operating temperatures are below the threshold set-point temperature of the TCO device. As shown in the Figures, the biasing means includes first and second compression springs 128, 130, each having a different spring rate, which are respectively disposed on opposite sides of the sliding contact member 126. Two disk members 131, 133 are disposed on opposite sides of the first compression spring 128. The disk members act to substantially evenly distribute the bias of the first compression spring 128.
Also included in the current interruption assembly 114 is a thermally responsive member 132 which, when in a solid physical state, can take the form of a pellet. The solid thermally responsive member 132 is disposed in the case 102 and occupies a volume at the first end 106. The first compression spring 128 of the current interruption assembly 114 is disposed between the thermally responsive member 132 and the sliding contact member 126 and biases the sliding contact member 126 toward engagement with the second electrical conductor 116. The second compression spring 130 is disposed between the sliding contact member 126 and the isolation bushing 108 and biases the sliding contact member 126 away from engagement with the second electrical conductor 116. Because the first compression spring 128 has a greater bias than the second compression spring 130, a net force acts against the sliding contact member 126 to urge the sliding contact member 126 into contact and electrical engagement with the enlarged distal end 120 of the second electrical conductor 116. In this manner, an electrical circuit is established through the TCO device by the first electrical conductor 104, through the electrically conductive case 102, to the sliding contact member 126, and to the second electrical conductor 116.
The thermally responsive member 132 has a reliably stable solid phase at a first operating condition where the operating temperature of the device in which the TCO device is incorporated or the temperature of the surrounding environment, for example, is below a predetermined threshold set-point temperature. The solid thermally responsive member 132, however, reliably transitions to a different physical state when the operating temperature meets or exceeds the threshold set-point temperature in a second operating condition. Under such conditions, the thermally responsive member, e.g., melts, liquefies, softens, volatilizes, or otherwise transitions to a different physical state such that it cannot oppose the force of the biasing means.
With further reference to
Under conditions where the operating or ambient temperature meets or exceeds the TCO device's threshold set-point temperature, the thermal pellet transitions to a different physical state such that it no longer occupies the volume at the first end 106 of the case 102. As such, the first compression spring 128 expands to occupy the space formerly occupied by the thermal pellet 132. In doing so, the first compression spring 128 no longer biases the sliding contact member 126 into engagement with the second electrical conductor 116 with enough force to overcome the bias of the second compression spring 130. Consequently, the bias of the second compression spring 130 forces the sliding contact member 126 out of engagement with the second electrical conductor 116, thereby interrupting the electric circuit in the TCO device.
TCO devices are known to have an element of self-heating (I2R heating) when they carry electrical current. A reduction in this self-heating would improve the TCO device's operating by allowing it to run at a cooler temperature away from the TCO device's threshold set-point temperature and the phase transition temperature of the thermal pellet. Also, the continued evolution of the TCO device's design requires changes in its construction, such as material options, plating thicknesses, contact systems, etc. In several instances, prior attempts to change these features to improve the TCO device have resulted in unfavorable shifts in the performance of the TCO device.
This section provides a general summary of the disclosure, and is not a comprehensive disclosure of its full scope or all of its features.
In one aspect, the present disclosure provides a thermal cut-off device including a case extending along a longitudinal axis from a first end to a second end. A first electrically conductive member is disposed at the first end of the case and extends from the case along the longitudinal axis. A thermally responsive member comprises a non-electrically conductive material that transitions from a solid physical state to a non-solid physical state at or above a threshold temperature. A second electrically conductive member is disposed at the second end of the case and extends from the case along the longitudinal axis. The second electrically conductive member includes a contact surface at a distal end. An electrically conductive, moveable contact is disposed between the thermally responsive member and the second electrically conductive member. A first biasing member is disposed between the thermally responsive member and the moveable contact. The first biasing member biases the moveable contact in a direction along the longitudinal axis toward the second electrically conductive member. A second biasing member is disposed between the moveable contact and the second end of the case. The second biasing member biases the moveable contact along the longitudinal axis away from the second electrically conductive member. The distal end of the second electrically conductive member has a concave portion and the contact surface has a generally flat, annular portion that encircles a periphery of the distal end. Below the threshold temperature, the annular portion of the contact surface directly engages the moveable contact.
In another aspect, the concave portion can be located near a central portion of the distal end of the second electrically conductive member and the second electrically conductive member does not engage the moveable contact at the central portion. Further, the distal end can include a shoulder portion opposite the contact surface. The diameter of the shoulder portion can be greater than a diameter of the contact surface. In another aspect, the diameter of the shoulder portion can be less than a diameter of the contact surface.
The thermally responsive member can be an organic compound, and below the threshold temperature it can be a solid in the form of a pellet. As a solid it can oppose the bias of the first biasing member and of the second biasing member such that the movable contact is biased into engagement with the second current conducting member. Above the threshold temperature, the thermally responsive member can be a liquid or a gas and no longer opposes the bias of the first biasing member and the second biasing member. As such, the moveable contact is biased out of engagement and moves away from the second current conducting member.
In still another aspect of the disclosure, the thermal cut-off device includes a first disk disposed between the thermally responsive member and the first biasing member, and a second disk disposed between the first biasing member and the moveable contact.
In yet another aspect of the disclosure, a thermal cut-off device for interrupting an electric circuit at a threshold temperature has a case, first and second leads and a current interruption assembly. The current interruption assembly includes a movable, electrically conductive contact engaging an interior wall of the case and being biased against a contact surface of the second lead at a temperature below the threshold temperature. The distal end of the second lead can include a concave portion and the contact surface can include a generally flat portion about a perimeter of the distal end of the second lead. The concave portion can be located near a central portion of the distal end of the second lead. The contact surface does not engage the contact at the central portion.
In still another aspect of the disclosure a thermal cut-off device has a case, first and second leads and a current interruption assembly. The contact surface comprises a convex portion and the contact comprises a concave portion. The convex portion and the concave portion have substantially the same radius of curvature so that the convex portion and the concave portion can closely correspond to one another. The convex portion and the concave portion can engage one another in a nesting relationship. A disk member located adjacent to the contact can include a second concave portion and the contact can further include a second convex portion opposite to the concave portion. The second concave portion can closely correspond to the second convex portion.
Further areas of applicability will become apparent from the description provided herein. The description and specific examples in this summary are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.
The drawings described herein are for illustrative purposes only of selected embodiments and not all possible implementations, and are not intended to limit the scope of the present disclosure.
Corresponding reference numerals indicate corresponding parts throughout the several views of the drawings.
Example embodiments will now be described more fully with reference to the accompanying drawings. The example embodiments are provided so that the disclosure thoroughly conveys the scope to those who are skilled in the art. Numerous specific details are set forth such as examples of specific components, devices, and methods, to provide a thorough understanding of embodiments of the present disclosure. It will be apparent to those skilled in the art that specific details need not be employed, that example embodiments can be embodied in many different forms and that neither should be construed to limit the scope of the disclosure. In some example embodiments, well-known processes, well-known device structures, and well-known technologies are not described in detail.
Referring to
The TCO device 1 provides protection against overheating by interrupting the electric circuit between the first electrical conductor 4 and the second electrical conductor 16 when the TCO device 1 experiences a temperature that meets or exceeds a threshold cut-off temperature, such as a predetermined operating temperature. When the temperature of the TCO device 1 meets or exceeds the threshold cut-off temperature, the electric current interrupter assembly 14 actuates and breaks the continuity of the electric circuit. The threshold cut-off temperature for the TCO device 1 can be based on the physical properties of the thermal pellet 32, the spring rates and the relaxed lengths of the first and second compression members 28, 30, and the spacing and tolerance stack-up between the several components of the TCO device 1.
The increased surface area provides performance improvements and manufacturing benefits not available in prior known TCO device designs. For example, the TCO device's manufacturability and assembly process is improved by the relatively large annular portion 42 of the contact surface 38 (e.g., instead of the minimal contact achieved in prior known devices) that engages the sliding contact member 26 and supports and stabilizes the sliding contact member 26 during assembly of the TCO device 1. The increased contact surface area also decreases the current density in the circuit at the contact area between the sliding contact member 26 and the second electrical conductor 16. This reduces the resistance in the electric circuit at the contact surface 38 and across the TCO device, generally. For example, a reduction in the resistance across the TCO device on the order of 10-15% can be achieved. The reduction in resistance improves the aging performance of the TCO device 1. Moreover, the reduction in resistance at the contact surface 38 enables the case 2 of the TCO device 1, which forms part of the electric circuit through the TCO device 1, to be manufactured from a material having a lower copper content than that used in prior known TCO devices, which results in a significant reduction in the material costs for the TCO device 1.
As illustrated in
Alternative embodiments of a second electrical conductor 16′ and 16″ are shown in
As illustrated in
As illustrated in
Referring now to
The sliding contact member 326 can include on first side 333 a concave portion 344 that correspondingly engages with the convex contact surface 338 of the second electrical conductor 316. In this regard, the concave portion 344 and the convex contact surface portion 338 can have substantially the same radius of curvature R so that the respective mating surfaces closely correspond to one another so that the second electrical conductor 316 at its contact surface portion 338, and the sliding contact member 326 at its concave portion 344, nest together in close contact over a large surface area. The concave portion 344 together with the convex contact surface 338 increase the area of direct surface contact between the sliding contact member 326 and the second electrical conductor 316 and provide performance and manufacture benefits not available in prior know TCO devices.
Optionally, the disk member 335 can also include a concave indentation 337 that correspondingly engages a convex portion 339 of a second side 341 of the sliding contact member 326, which is located opposite to the concave portion 344. The concave indentation 337 of the disk member 335 can also have a radius of curvature that is substantially the same as the radius of curvature of the concave portion 344 and the convex contact surface portion 338. This optional configuration for the disk member 335 could improve the manufacturability of the TCO device 300 and, in particular, the current interruption assembly 314. Of course, the disk member 335 can also be configured as shown in the TCO 1 of
The foregoing description of the embodiments has been provided for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but, where applicable, are interchangeable and can be used in a selected embodiment, even if not specifically shown or described. The same can also be varied in many ways. Such variations are not to be regarded as a departure from the disclosure, and all such modifications are intended to be included within the scope of the disclosure.
