Abstract:
A self-regulating electric heater includes a casing made of ceramic and a heating unit housed in the casing. The heating unit includes a PTC body for generating heat when electric current is supplied, heat dissipating plates essentially made of brass and coupled to the PTC body in such a manner as to sandwich the PTC body between the heat dissipating plates and terminals connected to each of the heat dissipating plates for the external electrical connection. The self-regulating electric heater further includes lid member and sealing member for sealing the casing hermetically.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a heater and more particularly to a heater employing a positive temperature coefficient (PTC) body. 
     Generally, the above described type of heater is employed in a compressor, used in, e.g., refrigerator or air-conditioning device, for maintaining the lubricant contained in the compressor housing above a predetermined temperature. In conventional refrigeration components, a refrigerant, such as those sold under the trademark &#34;Freon&#34; by E. I. du Pont de Nemours &amp; Co., may, in liquid form, migrate from the condenser into the compressor lubricant. Upon start-up of the compressor, the sudden reduction in crankcase pressure may cause the refrigerant to boil, thus causing the lubricant to form with consequent loss of lubrication to other mechanical parts of the compressor. It has been conventional to employ a crankcase heater to maintain the compressor crankcase at a temperature above that of the rest of the refrigeration system so as to prevent the migration of refrigerant into the crankcase lubricant. 
     Formerly, fixed constant-resistance heaters were used for heating the crankcase. These heaters, however, were not self-regulating and thus required further temperature controls to limit the heat output of the heater so as to prevent damage to the lubricating oil. These constant-resistance heaters and their associated temperature controls were complicated and expensive. 
     Then, in order to eliminate such disadvantages, a heater made of ceramic material having a positive temperature coefficient (PTC) of resistivity has been proposed. Such a heater has a relatively low resistance at usual ambient temperatures, but after initial energization by a source of electrical power will self-heat and increase their temperature and resistance. Heat will be generated and the resistance will increase rapidly above a threshold or anomaly temperature until the heat generated balances the heat dissipated at which time the temperature and resistance stabilize with the resistance many times the initial value. Thus, these heaters are self-regulating at a temperature that will not exceed a safe value. 
     One prior art device employing the PTC element is shown in FIG. 1 includes potting compounds 1 to electrically insulate the PTC heater 2 from the heater case 3 to provide increased heat transfer from the heater to the case and to locate the heater within the case. However, it has been found that certain potting compounds, such as epoxy resin materials or the gases emitted therefrom, deleteriously affect the PTC heater when it is operated at high temperature. 
     In view of this, an electric heater having a barrier that separates the PTC body and the potting compounds is proposed and is disclosed in detail in U.S. Pat. No. 3,940,591 to Ting, issued on Feb. 24, 1976. However, according to this patent, no consideration is given to the efficiency of heat transmission from the outer surface of the PTC body to the outer surface of the casing. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is a primary object of the present invention to provide a self-regulating electric heater which can efficiently transmit heat produced from the PTC body to an outer surface of a casing provided for the PTC body. 
     It is a further object of the present invention to provide a self-regulating electric heater which is simple in construction and can readily be manufactured at low cost. 
     In accomplishing these and other objects, a self-regulating electric heater according to the present invention comprises a casing made of electrically non-conductive and thermally high conductive ceramic, and having a cylindrical outer configuration and a chamber formed therein. The chamber has a rectangular cross-section and is opened at one end of the cylindrical casing and is closed at the other end opposing the opened end. A heating unit is housed in the casing in such a manner as to establish a thermal contact with the casing. The heating unit comprises a PTC body for generating heat when an electric current is supplied therethrough. The PTC body has first and second substantially parallel surfaces spaced from one another, said first and second surfaces being deposited with first and second electrodes, respectively, for forming ohmic contact surfaces. The heating unit further comprises first and second heat dissipating plates each essentially made of brass and having first and second flat surfaces, said first and second heat dissipating plates sandwiching the PTC body such that the first flat surface of the first heat dissipating plate is held in contact with the first electrode and the first flat surface of the second heat dissipating plate is held in contact with the second electrode, and first and second terminal means connected to the first and second heat dissipating plates, respectively. The self-regulating electric heater further comprises a biasing means provided in the casing for biasing the heating unit to hold the heating unit in contact with an inner wall of the casing, a lid member mounted on the casing for closing the opening, said lid member having two removed sections for allowing the first and second terminal means to pass therethrough for the external electric connection, and a sealing member deposited around the lid member for sealing the casing hermetically. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects and features of the present invention will become apparent from the following description taken in conjunction with preferred embodiments thereof with reference to the accompanying drawings, throughout which like parts are designated by like reference numerals, and in which: 
     FIG. 1 is a cross-sectional view of a self-regulating electric heater according to the prior art; 
     FIG. 2 is an exploded view of a self-regulating electric heater according to a first embodiment of the present invention; 
     FIG. 3 is a cross-sectional view of a self-regulating electric heater according to the first embodiment of the present invention; 
     FIG. 4 is a perspective view particularly showing a lid member of the second embodiment of the present invention; 
     FIG. 5 is an exploded view of a self-regulating electric heater according to a second embodiment of the present invention; 
     FIG. 6 is a cross-sectional view of a self-regulating electric heater according to the second embodiment of the present invention; 
     FIG. 7 is a top plan view of a lid member; 
     FIG. 8 is a perspective view of a modified terminal member to be employed in the self-regulating electric heater of either one of the first and second embodiments; 
     FIG. 9 is an exploded view of a self-regulating electric heater according to a third embodiment of the present invention; 
     FIG. 10 is a cross-sectional view of a self-regulating electric heater according to the third embodiment of the present invention; 
     FIG. 11 is a circuit diagram of the self-regulating electric heater of the third embodiment; 
     FIG. 12 is a perspective view of a modified terminal member to be employed in the self-regulating electric heater of the third embodiment; and 
     FIG. 13 is a graph showing a relationship between the thickness of a heat dissipating plate and power consumed in the self-regulating electric heater. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIGS. 2 and 3, a self-regulating electric heater according to the first embodiment of the present invention comprises a cylindrical casing 11 made of ceramics having high heat conductivity, such as an alumina (Al 2  O 3 ), steatite or mullite, and having a cylindrical outer configuration and a chamber 11a formed therein which is opened at one end to form a chamber mouth. The chamber 11a has a rectangular cross-section and extends along the axis of the cylindrical casing 11. The other end of the chamber remote from the chamber mouth is closed by a bottom wall 11b of the casing 11. 
     Accommodated in the chamber 11a is a heating unit H comprising a PTC element 12, two heat dissipating plates 14 and 15 and two terminal plates 16 and 17. The PTC element 12 has a rectangular configuration with first and second substantially parallel surfaces, respectively, spaced from one another by the thickness of the element. These surfaces are applied with electrode layers 13a and 13b of electrically conductive material for forming an ohmic contact surfaces. Each of the heat dissipating plates 14 and 15 made of an electrically and thermally conductive material, preferably brass, has approximately the same configuration as that of the PTC element 12 with a predetermined thickness. The heat dissipating plates 14 and 15 are held in contact with the electrodes 13a and 13b, respectively, in such a manner as to sandwich the PTC element 12 between the heat dissipating plates 14 and 15 for the electrical-contact and heat-transfer. The terminal plate 16 is made of electrically and thermally conductive material, such as brass or stainless steel, and has a configuration similar to the PTC element 12. The terminal plate 16 is positioned next to the heat dissipating plate 14 so that one face of the terminal plate 16 is in electrical-contact and heat-transfer relation with a face of the heat dissipating plate 14 and the other face of the terminal plate 16 is in heat-transfer relation with a first inside surface of the casing 11. The terminal plate 16 has a projection 16p extending upwardly, when viewed in FIG. 2, for the electrical connection with a lead wire 18a provided with an insulator. 
     The terminal plate 17 made of electrically and thermally conductive material with high resiliency, such as, brass or stainless steel, has a cut in a shape of U by any known means, such as punching, to form a tongue portion which is corrugated to provide a leaf spring 17a in a central portion of the terminal plate 17, and frame portion 17b around perimeter of the terminal plate 17. The frame portion 17b has an outer configuration similar to that of the PTC element 12. The terminal plate 17 is positioned next to the heat dissipating plate 15 in such a manner that the frame portion 17b is in electrical-contact and heat-transfer relation with a face of the heat dissipating plate 15 and the leaf spring 17a is heat-transfer relation with a second inside surface, which is opposing the first inside surface, of the casing 11. Accordingly, the biasing force produced by the leaf spring 17a is exerted on the heating unit H to tightly hold the PTC element 12 and plates 14, 15, 16 and 17 together and to bias the terminal plate 16 towards and into close heat-transfer relationship with the first inside surface of the casing. The terminal plate 17 has a projection 17p  extending upwardly, when viewed in FIG. 2, for the electrical connection with a lead wire 18b provided with an insulator. 
     Since the depth of the chamber 11 is greater than the height of the heating unit H, the heating unit H can be completely inserted into the chamber 11, and yet leaving a predetermined space above the heating unit H. After the heating unit H has been inserted into the chamber 11a until the bottom of the heating unit H contacts the bottom wall 11b of the casing 11, it is hermetically sealed within the chamber 11 by a lid member 19 and sealing material 20 which closes the mouth of the chamber. The lid member 19 has a size approximately equal to the chamber mouth and at least two removed portions for receiving the projections 16p and 17p, and is placed on top of the heating unit H prior to or subsequent to the connection of lead wires 18a and 18b with the projections 16p and 17p, respectively. According to the first embodiment, the removed portions of the lid member 19 are formed by a recess 19a and a rectangular opening 19b, as best shown in FIG. 4. The connection of the lead wires 18a and 18b to the projections 16p and 17p can be carried out by means of spot welding, soldering or clamping, solely, or otherwise in combination of spot welding and clamping. The sealing material 20 made of high heat resistance material, such as silicone resin is injected into the chamber 11a above the lid member 19 to positively seal the lead wires 18a and 18b and hermetically close the mouth of the chamber effectively. By the employment of the lid member 19, the sealing material can be prevented from coming into contact with the PTC element 12, thus avoiding the deterioration of the PTC element. 
     When in use, the self-regulating electric heater according to the present invention is fittingly inserted into a recess formed in a crankcase CK, as best shown in FIG. 3. 
     According to the self-regulating electric heater of the present invention, the thickness, width and/or length of each of the heat dissipating plates 14 and 15 can be varied to adjust the coefficient of heat emission from the heat dissipating plates. Accordingly, the heat to be supplied to the crankcase CK can be adjusted. 
     Since the self-regulating electric heater of the present invention employs heat dissipating plates 14 and 15, the wall of the casing 11 defining the chamber 11a is relatively thin when compared with the self-regulating heaters having no heat dissipating plates, provided that the outer configuration of the heater is the same. In other words, it can be said that a part of wall of the casing 11 is replaced by the heat dissipating plates 14 and 15. Since the heat dissipating plates 14 and 15, which are made of brass, has a thermal conductivity of 74 to 79 kcal./m·hr·°C. which is much greater than the thermal conductivity of the ceramic, which has 17 to 22 kcal.m·hr·°C., the heat emitted from the surface of the PTC element can be transferred to the outer surface of the casing 11 with high efficiency. This is further clarified from the following description. 
     Referring to FIG. 13, a graph depicted therein shows a relationship between the thickness (mm) of the heat dissipating block and the power (watt) consumed in the PTC element 12, as obtained through a series of tests carried out by the present inventors. In the tests, a PTC element having a size of 13×20×2.8 (mm) and Curie point of 160° C. is employed, and the voltage supplied to the PTC element is 260 volts (a.c.). When a self-regulating electric heater having no heat dissipating plate is tested, it consumed 24 watts in water (0° C.) and 5.7 watts in air (25° C.). When a self-regulating electric heater having a heat dissipating plate of a size 13×20×1 (mm) is tested, it consumed 29.4 watts in water (0° C.) and 7.7 watts in air (25° C.). And, when a self-regulating heater having a heat dissipating plate of a size 13×20×2 (mm) is tested, it consumed 34 watts in water (0°  C.) and 9.7 watts in air (25° C.). As apparent from the tests, the electric power consumed in the PTC element, thus the heat generated from the PTC element, increases as the increase of the thickness of the heat dissipating plates. It is to be noted that the heat generated from the PTC element also increases as the increase of mass of the PTC element. 
     Referring to FIGS. 5 and 6, there is shown a self-regulating electric heater according to the second embodiment of the present invention. In this embodiment, the heat dissipating block 14 has a pair of recesses 14a and 14b formed in its opposite longitudinal sides which are in parallel relation to the axis of the cylindrical casing. Similarly, the heat dissipating block 15 has a pair of recesses 15a and 15b formed in opposite longitudinal sides. In compliance with the recesses 14a and 14b, the terminal plate 16 is provided with a pair of fins 16x and 16y on opposite long sides and at right angles to the surface of the terminal plate 16 so that when the terminal plate 16 is positioned next to the heat dissipating block 14, the fins 16x and 16y fittingly engages in recesses 14a and 14b, respectively. Such an engagement prevents the terminal plate 16 from being slipped aside in the axial direction particularly when inserting the heating unit H into the casing 11. Likewise, the terminal plate 17 is formed with a pair of fins 17x and 17y which fittingly engage respectively in the recesses 15a and 15b of the heat dissipating plate 15. 
     According to the second embodiment, the mouth of the chamber 11a formed in the casing 11 is somewhat enlarged to form shoulders 11b and 11c within the chamber 11a so as to allow the lid member 19 abut against the shoulders 11b and 11c, as shown in FIG. 6. Furthermore, the mouth of the casing 11 is formed with semi-cylindrical recesses 11d and 11e above the shoulders 11b and 11c, respectively, as best shown in FIG. 6, for facilitating the positioning of lead wires 18a and 18b. Instead of the recess 19a and opening 19b, the lid member 19 has a pair of recesses 19c and 19d, as shown in FIG. 7, for receiving the projections 16p and 17p of the terminal plates 16 and 17, respectively. Instead of forming the shoulders 11b and 11c, it is possible to form projections on the inside surface of the casing 11 for supporting the lid member 19. 
     It is to be noted that the heat dissipating plates 14 and 15 of the second embodiment serves in the same manner as those described above in the first embodiment. 
     It is also to be noted that the engagement effected between the heat dissipating plate and the terminal plate, e.g., between 15 and 17, can also be accomplished by a pair of fins provided on top and bottom sides, when viewed in FIG. 5, of the terminal plate 17. In this case, the heat dissipating plate 15 is not necessarily be formed with recesses for receiving such fins. 
     It is further to be noted that the terminal plate 17 in either of the first and second embodiments can be replaced by a modified terminal plate 17&#39; shown in FIG. 8. Instead of the leaf spring 17a, the modified terminal plate 17&#39; has a semi-cylindrical portion 17&#39;a formed between the opposite long sides. Furthermore, the modified terminal plate 17&#39; has a pair of fins 17&#39;x and 17&#39;y for the engagement with the heat dissipating plate 15. In this case, the projection 17&#39;p for the electrical connection with the lead wire 18b can be provided on the fin 17&#39;y formed at top side of the terminal plate 17&#39;. 
     Referring to FIGS. 9 and 10, there is shown a self-regulating electric heater according to the third embodiment of the present invention which includes two PTC elements 23 and 24. For preventing the mutual heat transmission between the PTC elements 23 and 24, the PTC elements 23 and 24 are spaced from each other by a suitable spacer means. More specifically, the heating unit H of this embodiment includes a center terminal plate 22, two PTC elements 23 and 24, two heat dissipating plates 14 and 15, and two side terminal plates 16 and 17, all having approximately the same configuration but different thickness. 
     The central terminal plate 22, which is sandwiched between the two PTC elements 23 and 24, has a cut in a shape of U by any known means to form a tongue portion which is corrugated to provide a leaf spring 22a in a central portion of the terminal plate 22, and frame portion 22b around the perimeter of the terminal plate 22. The central plate 22 accordingly serves as terminal means, spring means and spacer means. Each of the PTC elements 23 and 24 has the same configuration as the PTC element 12 described above in the first embodiment and is applied with electrode layer 13a and 13b. The heat dissipating plate 14 is held in contact with the electrode layer 13a of the PTC element 23, and the heat dissipating plate 15 is held in contact with the electrode layer 13a of the PTC element 24. The terminal plates 16 and 17, each made of a plain metallic plate, are held in contact with the heat dissipating plates 14 and 15, respectively. When these elements and plates forming the heating unit H are accommodated in the chamber 11a of the casing 11, the leaf spring 22a pushes the elements and plates away from the central terminal plate 22 so that the terminal plates 16 and 17 are held in contact with opposite inside surfaces of the casing 11 in heat-transfer relation. 
     The central terminal plate 22 has a projection 22p extending upwardly from its top side for the electrical connection with a lead wire 18a provided with an insulator, and each of the side terminal plates 16 and 17 has a projection 16p, 17p extending upwardly from its top side in offset relation with the projection 22p for the electrical connection with a lead wire 18b provided with an insulator. 
     Before connecting the lead wires 18a and 18b, the heating unit H inserted into the chamber 11a is hermetically sealed by a lid member 19&#39; which has recesses 19&#39;a and 19&#39;b for receiving the projections 16p and 17p, and an opening 19&#39;c for receiving the projection 22p. Preferably, the projections 16p and 17p are bent towards each other to facilitate the connection to the lead wire 18b. After placing the lid member 19&#39; and connecting the lead wires 18a and 18b to the respective projections, the cast-in material serving as a sealing member 20&#39; is injected into the chamber 11a above the lid member 19 to positively seal the lead wires 18a and 18b and hermetically close the mouth of the chamber. According to the third embodiment, the PTC elements 23 and 24 are connected parallelly to each other between the lead wires 18a and 18b, as illustrated in a circuit of FIG. 11. 
     It is to be noted that the heat dissipating plates 14 and 15 provided in the self-regulating electric heater of the third embodiment contributes in effective heat transmission from each of the PTC elements 23 and 24 to the outer surface of the cylindrical casing 11 in a manner described above in connection with FIG. 13. 
     Referring to FIG. 12, there is shown a modified central terminal plate 22&#39; which is constituted by an arcuate portion 22&#39;a producing a spring effect and a projection 22&#39;p for the connection with the lead wire 18a. 
     It is to be noted that the terminal plates 16 and 17 may have engaging means with respect to the corresponding heat dissipating plates 14 and 15 in a manner described above in connection with FIGS. 5 and 6 or FIG. 8 for preventing the displacement of the terminal plates 17 and 16, particularly when inserting the heating unit H into the chamber 11a. 
     Although the present invention has been described in connection with several preferred embodiments, many modifications and variations will now be apparent to those skilled in the art, and it is therefore understood that the scope of this invention is limited not by the details of the embodiments described herein, but only by the appended claims.