Abstract:
A motor start capacitor assembly for providing an enclosure for a motor start capacitor and a thermistor electrically connected in series. The enclosure is constructed of a low temperature plastic case and cap with a high temperature plastic core for supporting the thermistor within the low temperature plastic case. The core holds the thermistor in a longitudinal orientation. The core and thermistor and related contacts are assembled without rivets or other fasteners.

Description:
FIELD OF THE INVENTION 
     This invention relates to a start capacitor assembly for an AC motor, and more specifically relates to an enclosure for a capacitor and a thermistor electrically connected in series with the capacitor. The enclosure, including a case and a cap, is fabricated from a relatively low temperature plastic with a high-temperature plastic core for holding the thermistor. 
     BACKGROUND OF THE INVENTION 
     A single-phase permanent split-capacitor (PSC) induction motor is often used to drive a compressor for an air conditioning system. A PSC motor has two windings, a main winding and an auxiliary start winding. A capacitor, whose value is chosen as a compromise between start and run performance, is placed in series with the start winding. This capacitor also affects phase shift of the run windings and thus increases both starting and run torque, though it is not optimized for either. When power is applied, both windings contribute, aided by the phase shift of the capacitor, to the necessary torque to cause initial torque and acceleration. As the motor achieves operating speed, both windings continue to contribute to the motor&#39;s torque although the run winding now contributes to a far greater degree due to its lower impedance. 
     A conventional starting device for a PSC motor includes a capacitor in series with a positive temperature coefficient (PTC) resistor or thermistor. The resistance of this thermistor increases with rising temperature. The starting device, comprising a capacitor which is optimized for providing starting torque and a thermistor, is connected in parallel with the existing capacitor of the PSC motor. When power is connected to the motor, the ac current is connected to the windings on the motor through both the existing and the extra start capacitor (through the thermistor which has a low resistance when cold). As current flows to the windings through the capacitor and the thermistor, the thermistor heats up, and its resistance increases. The increased resistance of the heated thermistor reduces the current through the start capacitor. At some point, the temperature and current through the thermistor reach steady state, with only a very small current connected through the start capacitor to maintain the temperature of the thermistor. 
     The starting device of a capacitor and thermistor is generally packaged as a single unit with two wire leads for connection to the start winding of the motor. Because the thermistor remains hot during the normal operation of the motor, the enclosure for the capacitor and thermistor must be able to withstand the heat generated by the thermistor. 
     U.S. Pat. No. 4,118,753 discloses an enclosure for housing a starting device including a capacitor and thermistor. The enclosure includes an elongated cylindrical case which is closed at one end and closed by a cap at the opposite end. The enclosure has two transverse intermediate walls of heat insulating material that divide the enclosure into three sections along the longitudinal dimension of the enclosure. Each intermediate wall has a spring contact that extends into the center section between the intermediate walls. The thermistor is located between and engages the spring contacts. The thermistor is disk shaped with opposite sides being the electrical contacts for the thermistor. The thermistor is oriented with its axis extending parallel to the longitudinal dimension of the case. In order to hold the thermistor between the intermediate walls in engagement with the contact springs, a combination of standoffs and rivets are employed to maintain the spacing between the intermediate walls against the force exerted by the spring contacts. Thus the center section with the thermistor is separately fabricated with the standoffs and rivets prior to insertion into the case. The case within the center section has a vent hole for dissipation of heat. The vent hole will also allow access by moisture, dust, insects, and the like to the spring contacts and the thermistor. There is no specific disclosure concerning the plastic used for the enclosure other than the material of the case and the cap are the same material, and the intermediate walls are heat insulative. 
     Another prior art starting device is shown in FIGS. 1 and 2. The starting device  1  includes a plastic encased capacitor  2  topped by a plastic enclosure  3 . The capacitor  2  has an outer cylindrical shell  14  and electric terminals  15  and  16 . The plastic enclosure  3  comprises an outer cylindrical shell  4 , an internal support structure  5 , and a cap  6 . A thermistor  7  is supported within recess  8  of the internal support structure  5  of the enclosure  3 . The thermistor  7  is sandwiched between contacts  9  and  10  which are in turn connected to wires  11  and  12  respectively. The contacts  9  and  10  have spring elements and are held in contact with the thermistor  7  by protrusions  19  extending from the inside of cap  6 . Thus the springs in the contacts  9  and  10  tend to force the cap  6  loose from the shell  4  of the enclosure  3 . Therefore, during assembly, the cap  6  must be held against the spring force onto the shell  4  of the enclosure  3  during welding or gluing of the cap  6  to the shell  4 . Wire  11  is connected to terminal  16  of capacitor  2 , and wire  13  is connected to terminal  15  of capacitor  2 . The enclosure  3 , including the cylindrical shell  4 , the internal support structure  5 , and the cap  6 , is made of high temperature plastic to withstand the heat generated by the thermistor  7 . On the other hand the outer plastic shell  14  of the capacitor  2  is made of a low temperature plastic based on cost considerations. Because the shell  14  of the capacitor  2  and the enclosure  3  are made of different plastic materials, the capacitor shell  14  and the enclosure shell  4  cannot be reliably connected to each other by welding or gluing. Consequently, for the prior art starting device  1 , the shell  14  of the capacitor  2  and the shell  4  of the enclosure  3  for the thermistor  7  are mechanically connected by a series of metal clips  17 . The metal clips  17  engage the support structure  5  of the enclosure  3  by means of a tab  18 . The clips  17  then frictionally engage the capacitor shell  14  by means of a series of sharp barbs along the length of each clip  17  which dig into the shell  14  and resist separation of the enclosure  3  from the capacitor  2 . Because the attachment is mechanical, any leakage by the capacitor around its terminals will also leak out of the enclosure  3 . In addition, because the enclosure fits over the shell  14  of the capacitor  2 , a protruding flange  19  is created which is aesthetically objectionable and which makes the resulting assembly larger in diameter than the capacitor shell alone would be without the cap  6 . 
     SUMMARY OF THE INVENTION 
     The present invention solves the above problems by providing an enclosure for a starting device comprising an outer case and cap both constructed of a low cost, low temperature plastic for enclosing the capacitor and the thermistor of the starting device. The case is cylindrical to accommodate the cylindrical shape of a conventional metal can starting capacitor. The case has a closed end and an open end to which the cap is fitted and substantially sealed. Because the cap and the case are made of the same low temperature plastic, attaching the cap to the case can be reliably accomplished by welding or gluing. 
     The thermistor is supported within the case by means of a core assembly. The core assembly consists of a high temperature molded plastic core which includes at least one chamber for accommodating the thermistor, and first and second electric contacts for engaging the terminals of the thermistor. The contacts are frictionally fitted on the sides of the chamber. The chamber in the molded plastic core is dimensioned so that when the disk shaped thermistor is inserted into the chamber, the thermistor is oriented with it thickness dimension (and axis) extending in the transverse dimension of the case, and the thermistor&#39;s diameter is oriented in the longitudinal direction of the case. Because the thermistor is oriented in the longitudinal direction of the case, the contact pressure between the contacts in the chamber and the thermistor is contained by the sides of the chamber. No pressure from the electric contacts is exerted on the case or cap of the enclosure. 
     Therefore, it is an object of the present invention to provide an enclosure for a starting device including a capacitor and thermistor connected in series. 
     More particularly it is an object of the present invention to provide a starting device enclosure with an outside case and cap that is constructed primarily of a low cost, low temperature plastic that may be glued or welded together. 
     It is further an object of the present invention to provide an enclosure with a core assembly for holding the thermistor that does not require any fasteners or the like to hold the thermistor in place. 
     It is also an object of the present invention to provide an enclosure with a core assembly for holding the thermistor in which the electric contact with the thermistor does not exert any pressure on the outside case or cap. 
    
    
     Further objects, features and advantages will become apparent upon consideration of the following detailed description of the invention when taken in conjunction with the drawing and the appended claims. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a prior art motor start capacitor assembly. 
     FIG. 2 is a cross section view of the prior art motor start capacitor assembly of FIG.  1 . 
     FIG. 3 is a perspective view of a motor start capacitor assembly in accordance with the present invention. 
     FIG. 4 is a front elevation view, partially in cross section, of a first embodiment of the motor start capacitor assembly in accordance with the present invention. 
     FIG. 5 is a front elevation view, partially in cross section and similar to FIG. 4, of the first embodiment of the motor start capacitor assembly in accordance with the present invention. 
     FIG. 6 is a top plan view of a core assembly for the first embodiment of the motor start capacitor assembly in accordance with the present invention. 
     FIG. 7 is a front elevation view, partially in cross section, of a second embodiment of the motor start capacitor assembly in accordance with the present invention. 
     FIG. 8 is a front elevation view, partially in cross section and similar to FIG. 7, of the second embodiment of the motor start capacitor assembly in accordance with the present invention. 
     FIG. 9 is a top plan view of a core assembly for the second embodiment of the motor start capacitor assembly in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings, in which like reference numerals represent like parts throughout the several views, FIG. 3 discloses a motor start capacitor assembly  30 . With reference to FIGS. 3,  4 ,  5 , and  6 , the motor start capacitor assembly  30  includes an enclosure  32 , a core assembly  34 , a capacitor  36 , external wires  58  and  60 , and, for the first embodiment of the present invention, two thermistors  38  and  40 . 
     The enclosure  32  constitutes a cylindrical case  42  with a closed end  44  and an open end  46 . A cap  48  is fitted to the open end  46  to close the case  42 . The enclosure  32  further has a longitudinal dimension  50  and a transverse dimension  52  (FIG.  3 ). The case  42  and the cap  48  are molded from a low temperature, low cost plastic. Because the case  42  and the cap  48  are molded form the same plastic material, the cap can readily be jointed to the case by welding or gluing. 
     A plastic material that is suitable for molding the case  42  and the cap  48  is ABS (Acrylonitrile Butadiene Styrene). ABS is a general purpose plastic and possesses a specific gravity of 1.06, a tensile yield strength of 5,800 psi, and a heat deflection temperature of 190 degrees F. (at 264 psi). Consequently, ABS will deform readily with any significant loading above 190 degrees F. ABS has the further desirable property of being very easy to ultrasonically weld. Ultrasonic welding may be used to attach the ABS cap  48  to the ABS case  42 . ABS is also easy to mold and costs approximately $0.32/lb at the time of filing this application. Other suitable low temperature plastics for the case  42  and cap  48  include polyolefins such as polyproplene or polyethylene. 
     The capacitor  36  is a conventional metal can capacitor used in motor starting devices. The capacitor  36  is cylindrical in shape, has a first end  62  and a second end  64 , is housed in a metal can, and has terminals  54  and  56  located at the first end  62 . The capacitor  36  fits within the case  42  with its second end  64  resting on the closed end  44  of the case  42 . 
     The core assembly  34  supports the thermistors  38  and  40  inside of the case  42  and provides for electrical interconnection between the capacitor  36 , the thermistors  38  and  40 , and the external wire  58 . The core assembly  34  includes a molded core  66  comprising a plurality of interconnected ribs  68  (FIG.  6 ). With continuing reference to FIG. 6, the ribs  68  form two chambers  70  and  72 . U-shaped contacts  74  and  76  are press fit into the chambers  70  and  72  to provide electrical contact to the thermistors  38  and  40  within the chambers  70  and  72 . The contact  74  includes a female connector  75 , and the contact  76  includes a male connector  77 . As best seen in FIGS. 4 and 6, the contact  74  is connected to the terminal  54  of the capacitor  36  by means of female connector  75 . The contact  74  then contacts the right hand terminal of thermistor  40  and the left hand terminal of thermistor  38  as oriented in FIG.  6 . The contact  76 , on the other hand, contacts the left hand terminal of thermistor  40  and the right hand terminal of thermistor  38  as oriented in FIG.  6 . The contact  76  is then connected to the wire  58  by means of male connector  77 . The thermistors  38  and  40  are oriented in the chambers  70  and  72  so that the thermistors are electrically connected in parallel between the capacitor terminal  54  and the external wire  58 . A channel  78  is also provide through the core  66  to accommodate connection of the wire  60  directly to the terminal  56  of the capacitor  36 . 
     The chambers  70  and  72  are formed with a short dimension extending in the transverse direction  52  (FIG.  3 ). Consequently, when the thermistors  38  and  40  are installed in the chambers  70  and  72  respectively, the thickness of the thermistors is also in the transverse direction  52 . The diameter of the disk shaped thermistors is oriented in the longitudinal direction  50  (FIG.  3 ). 
     A clip  82  is provided as a convenience for anchoring the motor start capacitor in place while in use. In order to install the clip  82 , the core  66  also has a clip channel  80  to accommodate the clip  82 . The clip  82  has a bent section  84  that engages the side of channel  80 . Once the cap  48  is in place, the clip  82  is secured in the channel  80 . The cap  48  also has a lug  86  which also may be used for anchoring the motor start capacitor in place while in use. 
     The core  66  is molded from a high temperature plastic, such as polybutylene terephthalate (PBT). One such PBT plastic that is useful for the present invention is manufactured by General Electric Company and sold under the trademark Valox. Valox is a polyester based compound. Valox has a specific gravity of 1.5, a tensile yield strength of 17,400 psi, and a heat deflection temperature of 302 degrees F. (at 264 psi). Valox molds at a very high temperature (about 500 degrees F.) and is thus more costly to process than the ABS that is used for the case  42  and cap  48 . In addition Valox has a material cost of 5 to 6 times that of ABS. Another suitable plastic for the core  66  includes a plastic manufactured by General Electric Company and sold under the trademark Noryl. Noryl is a polyphenylene oxide and has similar properties to Valox. 
     The motor start capacitor assembly is simple and economic to manufacture. The case  42 , the cap  48 , and the core  66  are molded. The contacts  74  and  76  are press fit into the chambers  70  and  72 , and the thermistors  38  and  40  are inserted into the chambers  70  and  72  in contact with contacts  74  and  76  to complete the assemble of the core assembly  34 . 
     The core contact  74  of core assembly  34  is then connected to capacitor terminal  54  by means of the female connector  75 . The wire  58  is connected to the male connector  77  of the contact  76 . The wire  60  is connected directly to the terminal  56  of the capacitor  36 . 
     The combination of the core assembly  34 , the capacitor  36 , and the connecting wires  58  and  60  are inserted into the case  42  as a unit. The clip  82  is inserted into the case along with the core assembly  34  and the capacitor  36 . The cap  48  is then welded to the case  42  to complete the assembly. None of the internal connectors or parts exert any pressure on the cap  48  to cause the weld of the cap  48  to the case  42  to fail. 
     A second embodiment of the invention is shown in FIGS. 7,  8 , and  9 . The second embodiment of motor start capacitor assembly  30 ′ differs from the first embodiment of the motor start capacitor assembly  30  in that only a single thermistor  38 ′ is used instead of two thermistors  38  and  40 . The motor start capacitor assembly  30 ′ therefore has a different core assembly  34 ′. 
     The core assembly  34 ′ supports the thermistor  38 ′ inside of a low temperature molded plastic case  42 ′ and provides for electrical interconnection between a capacitor  36 ′, the thermistor  38 ′, and an external wire  58 ′. The core assembly  34 ′ includes a high temperature molded plastic core  66 ′ comprising a plurality of ribs  68 ′ (FIG.  9 ). With continuing reference to FIG. 9, the ribs  68 ′ form a chamber  70 ′. Contacts  74 ′ and  76 ′ are fit into the chamber  70 ′ to provide electrical contact to the thermistor  38 ′. The contact  74 ′ includes a female connector  75 ′, and the contact  76 ′ includes a male connector  77 ′. The contact  74 ′ is connected to the terminal  54 ′ of the capacitor  36 ′ by means of female connector  75 ′. The contact  74 ′ then contacts the right hand terminal of thermistor  38 ′ as oriented in FIG.  9 . The contact  76 ′, on the other hand, contacts the left hand terminal of thermistor  38 ′ as oriented in FIG.  9 . The contact  76 ′ is then connected to the wire  58 ′ by means of male connector  77 ′. A channel  78 ′ is also provide through the core  66 ′ to accommodate connection of wire  60 ′ directly to terminal  56 ′ of capacitor  36 ′. 
     The chamber  70 ′ is formed with a short dimension extending in the transverse direction  52  (as shown in FIG.  3 ). Consequently, when the thermistor  38 ′ is installed in the chamber  70 ′, the thickness of the thermistor is also in the transverse direction  52 . The diameter of the disk shaped thermistor is oriented in the longitudinal direction  50  (FIG.  3 ). The assembly of the motor start capacitor  30 ′ is similar to that described above for the motor start capacitor  30 . 
     While this invention has been described with reference to preferred embodiments thereof, it is to be understood that variations and modifications can be affected within the spirit and scope of the invention as described herein and before and as described in the appended claims.