Patent Application
20120019348
This invention relates generally to motor starting devices and more particularly to positive temperature coefficient (PTC) thermistor devices with improved failure control.
Most small compressors for refrigeration applications require a PTC thermistor in line with the compressor start/auxiliary motor winding in order to provide a phase shift between the start/auxiliary motor winding and the main motor winding as well as a means for reducing the start/auxiliary winding current flow as the compressor motor reaches its operating rotational speed. It is possible for the thermistor (commonly referred to as a pill) to enter a state called “thermal runaway,” a failure mode during which the thermistor resistance significantly exceeds its typical steady-state (self-regulated) resistance and passes its maximum resistance (RMAX) value, after which its resistance decreases and the thermistor heats up at an uncontrolled rate due to the increasing resultant current. One possible thermistor failure mode is fracture, which occurs due to thermal stress.
U.S. Pat. No. 6,172,593, assigned to Murata Manufacturing Co., Ltd., describes a prior art device with spring contacts and non-conductive support posts which maintain contact with the thermistor throughout the normal usable life of the thermistor. Because thermistors used for starting refrigerator compressors typically self-regulate at 160° C.-170° C. during the switched (high-resistance) state, materials in intimate contact with the thermistor electrodes have to have a higher relative temperature index (RTI) which are more expensive. In addition, this embodiment places portions of the thermistor in tension and compression on opposing sides of the pill with those forces reversed in direction near the other end of the thermistor. Unfortunately, under some circumstances if the pill doesn't crack in a manner in which current flow is stopped, fractured portions of the pill (also referred to as rubble) remain in electrical contact with internal terminals which can cause arcing and overheating.
In refrigerator applications there is a need for a more reliable, lower cost design which minimizes electrical arcing, current flow and subsequent overheating upon failure of the pill. Conventional thermistor devices do not always fail in a manner which eliminates excessive overheating and electrical arcing upon failure of the thermistor.
Conventional thermistor devices do not sufficiently eliminate overheating and arcing problems upon failure of internal components because some of the rubble remains wedged between conductive components.
In one embodiment of the present invention, an electronic device includes an electronic element having opposite sides with first and second electrodes located on the opposite sides of the electronic element, the electronic element having sidewall portions connecting the opposite sides, the surface of the sidewall portions defining an outer periphery of the electronic element; a first elastic support including a first contact section in contact with the first electrode; and a second elastic support including a second contact section in contact with the first electrode at a different position from the first contact section. The device further includes a third support disposed closer to the center of the electronic element than the first contact section and the second contact section, and including a third contact section in contact with the second electrode; and a pair of offset posts disposed on the same side of the electronic element as the third support and each having an end with a tip, and each tip end is closer to an axis, through the centroid of the electronic element and approximately perpendicular to the electronic element, than the first and second contact sections. Such a design advantageously provides better positioned dynamic forces and fulcrum positions to minimize undesired electrical contact after failure.
In a certain embodiment, the first and second elastic supports and corresponding first and second contact sections apply a first and second force, respectively, on the electronic element which is opposed by a third force applied by the third support and third contact section, and the third force individually is greater than either the first force or the second force. This distribution of forces improves the distribution of rubble under failure conditions.
Other embodiments provide a resilient third support, offset posts having slanted cut-away sections or rounded sections, and offset posts spaced apart from the electronic element. Such features can provide a better and more effective distribution of conductive rubble away from electrical contacts thus minimizing arcing and overheating.
In another embodiment, the first and second elastic supports each comprise a cantilever spring. By using cantilever springs additional force can be applied to distribute fragments of a fractured pill further from the electrical contact sections. In yet another aspect of the invention, the third support includes a fusible link and at least one withdrawal spring adapted to remove the third contact section from electrical contact with the second electrode in response to electronic element failure.
Another aspect of the present invention is a method for directing fractured portions of a fractured electronic element such that electrical contact to the electronic element is removed and electrical arcing among fractured portions and contact sections is minimized which includes the steps of providing a housing and a pair of first and second spring contacts disposed in the housing on one side of the electronic element, providing in the housing a pair of offset posts on an opposite side of the electronic element and a third contact disposed therebetween and elastically supporting the electronic element before fracture, with first, second and third spring contacts. Upon fracture of the electronic element, the technique further includes forcing contact of the electronic element with the pair of offset posts and distributing the fractured portions away from the third contact. The embodiments disclosed herein, may be employed in devices such as those manufactured by Sensata Technologies, Inc. of Attleboro, Mass., U.S.A.
The foregoing will be apparent from the following description of particular embodiments disclosed herein, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles disclosed herein.
Embodiments of the invention disclosed herein provide for improved PTC thermistor devices to minimize arcing and overheating upon component failure. Embodiments include supports, contacts and offset posts configured to assist the fracturing of failed thermistor “pills” and to distribute the fragments of the fractured pills into compartment away from electrically conductive contacts in order to minimize arcing and overheating.
Referring now to
The PTC thermistor device 100 further includes a first elastic support 102a including a first contact section 104a in contact with the first electrode 112 and a second elastic support 102b including a second contact section 104b in contact with the first electrode 112 at a different position from the first contact section 104a on the opposite side of the center of the pill. The PTC thermistor device 100 further includes a third support 108 disposed closer to the center of the pill 110 than the first contact section 104a and the second contact section 104b. The third support 108 includes a third contact section 109 in contact with the second electrode 114 and a pair of offset posts 106a and 106b (collectively referred to as offset posts 106) disposed on the same side of the pill 110 as the third support 108. Offset posts 106 each have a tip end 107a and 107b, respectively, and each tip end 107a and 107b is closer to an axis 116, perpendicular to the pill 110 and passing through the centroid of the pill, than either of the first and second contact sections 104a and 104b. Preferably, the offset posts 106 are located on opposite sides of a plane including the axis 116 (i.e., on opposite sides of the contact section 109) and each of the first and second contact sections 104a and 104b are located across from and further from a centroid point 115 of the pill 110 than the corresponding offset posts 106a and 106b. The shape of end of the offset posts 106 includes, but is not limited to, rectangular sections, rounded sections and slanted sections.
In a preferred embodiment, the offset posts 106 are located closer to the outer periphery 113 of the pill 110 than the third support 108. In this configuration the offset posts 106 provide additional leverage to assist in fracturing the pill 110 upon failure.
In normal operation of thermistor device 100, the pill 110, is mechanically supported between two opposing, conductive terminals (not shown in
In a preferred embodiment, the offset posts 106 are not in contact with the pill 110 when it is functioning normally. In this embodiment, both offset posts 106 are spaced apart from the pill 110 and electrically isolated from the second electrode 114 and the third contact section 109. The offset posts are generally less than 2 millimeters from second electrode 114, and preferably between near zero and 0.5 millimeters. Due to the lack of intimate contact during normal operation, less expensive materials may be employed for the offset posts due to the fact that they are not exposed to as high of a temperature as they would be if in direct contact with second electrode 114. Additionally, such materials typically are easier to mold, as well as being less brittle.
Under normal operating conditions, the first and second elastic supports 102 and corresponding first and second contact sections 104a and 104b (collectively referred to as contact sections 104) apply a first and second force, respectively, on the electronic element which is opposed by a third force applied by the third support 108 and third contact section 109. The third force is generally greater than the first force and the second force. The force applied by the third support 108 approximately in the center of the pill 110 increases the probability of fracture along a plane going through the axis 116 of the pill 110 at the time of pill 110 failure.
Referring now to
Referring now to
As shown in
In one embodiment, the first contact section 604a, second contact section 604b and third contact section 609 can each include multiple split fingers 617, for example, three fingers as shown in
The offset posts 606 are located on the side of the second electrode 114, each having a tip end (shown in more detail in
After assembly, the third support is mounted to and electrically connected to the second terminal connector 607 which is substantially parallel to the pill 100 and the third force is directed along the axis substantially perpendicular to the pill 110. The first and second forces have a force component directed substantially perpendicular to the pill 110 and in the embodiment using cantilever or similar springs, a force component is also directed towards the outer periphery of the electronic element. In one embodiment, the elastic supports 602 are welded to the first terminal connector 605 and the third support 608 is welded to the second terminal connector 607. In an alternate embodiment the first elastic support 602a, the second elastic support 602b and first terminal connector 605 can each be an integrated into a one-piece terminal component. Also, the third support 608 and second terminal connector 607 can be an integrated into a one-piece terminal component.
In more specific detail, as seen in
A pair of contact alignment posts 832a and 832b are located adjacent to the third support 608 and on opposite sides of the axis 116. In one embodiment, the offset posts 606 and corresponding contact alignment posts 832a and 832b form compartments 846 therebetween to collect and insulate fractured electronic element rubble from the contact sections 609 and 604. The pair of contact alignment posts 832a and 832b (collectively referred to as alignment posts 832), facilitate, in manufacturing and operation, the alignment, protection and orientation of the third support 608 and the third contact section 609. In one embodiment, the third support 608 is an elastic support and in an alternative embodiment, the third support 608 and the third contact section 609 are integrated as a rigid support, for example a single conductive component terminated at one end with a contact section which does not damage the pill 110 under normal operating conditions but aids in fracturing the pill 110 upon failure. In another embodiment, the third support 608 and the third contact section 609 are implemented as a conductive pedestal. In yet another embodiment, an integrated motor protector (not shown) is included in the housing electrically connected to thermistor device 100.
Before the pill 110 is fractured, it is supported by the elastic contacts 602 forcing the pill 110 against third support 608, which as described above can be a rigid support on an elastic support. Upon pill 110 failure, the fractured pill 110′ is forced to contact the pair of offset posts 606 and as a result fracture portions of the fractured pill 110′ are distributed away from the first, second and third contacts. Under some failure modes the fractured portions are rotated around the outer edges (e.g., the slanted tips) of the pair of offset posts and distributed into compartments 842, 844 and 848.
Although the invention has been described with regards to specific preferred embodiments thereof, variations and modifications will become apparent to those of ordinary skill in the art. It is therefore, the intent that the appended claims be interpreted as broadly as possible in view of the prior art to include such variations and modifications.
