Patent Publication Number: US-7215233-B2

Title: Inductive coupler for power line communications

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   The present application is a divisional of U.S. patent application Ser. No. 11/133,671, filed May 20, 2005. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to power line communications, and more particularly, to a configuration of a data coupler for power line communications. 
   2. Description of the Related Art 
   Power line communications (PLC), also known as broadband over power line (BPL), is a technology that encompasses transmission of data at high frequencies through existing electric power lines, i.e., conductors used for carrying a power current. A data coupler for power line communications couples a data signal between a power line and a communication device such as a modem. 
   An example of such a data coupler is an inductive coupler that includes a set of cores, and a winding wound around a portion of the cores. The inductive coupler operates as a transformer, where the cores are situated on a power line such that the power line serves as a primary winding of the transformer, and the winding of the inductive coupler is a secondary winding of the transformer. 
   The cores are typically constructed with magnetic materials, such as ferrites, powdered metal, or nano-crystalline material. The cores are electrified by contact with the power line and require insulation from the secondary winding. Typically, insulation is provided between the cores and secondary winding by embedding both the cores and the secondary winding in electrically insulating material, such as epoxy. During a molding process, the electrically insulating material reaches an elevated temperature. As the electrically insulating material, in a liquid state, flows around the cores, it begins to cool and contract. The thermal coefficient of expansion of the electrically insulating material is typically much higher than that of the core, and consequently, stress cracking of the electrically insulating material may occur during a transition from liquid to solid state. 
   In field operation, stiffly held magnetic cores made of brittle material may crack due to vibration or thermal expansion. There is a need for an inductive coupler configured to avoid such cracking. 
   SUMMARY OF THE INVENTION 
   There is provided an inductive coupler for coupling a signal to a conductor. The inductive coupler includes (a) a magnetic core having an aperture through which the conductor is routed, (b) a winding wound around a portion of the magnetic core, where the signal is coupled between the winding and the conductor via the magnetic core, (c) an electrically insulating material situated between the winding and the magnetic core, having a hardness of between about 10 and about 100 on a hardness type shore A durometer scale. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a front and cross-sectional view of an inductive coupler on a power line. 
       FIG. 2  is a cross-sectional view of an upper and a lower core of an inductive coupler, with compressible layers around the cores. 
       FIG. 3  is a cross-sectional view of an upper and a lower core of an inductive coupler with compressible layers that expose the core faces. 
       FIG. 4  is a cross-sectional view of an upper and a lower core of an inductive coupler, with insulation serving as a compressible layer around the cores. 
   

   DESCRIPTION OF THE INVENTION 
   In a PLC system, power current is typically transmitted through a power line at a frequency in the range of 50–60 hertz (Hz). In a low voltage line, power current is transmitted with a voltage between about 90 to 600 volts, and in a medium voltage line, power current is transmitted with a voltage between about 2,400 volts to 35,000 volts. The frequency of the data signals is greater than or equal to about 1 megahertz (MHz), and the voltage of the data signal ranges from a fraction of a volt to a few tens of volts. 
     FIG. 1  is an illustration of a front view with internal components visible with dashed lines, and a cross-section view, of an inductive coupler  100  on a conductor, i.e., power line  110 . Inductive coupler  100  has a split magnetic core configured of an upper core  120  and a lower core  125  that are shaped such that when they are placed adjacent to one another, they provide an aperture  105  through which power line  110  is routed. Inductive coupler  100  also has a winding  130  wound around a portion of lower core  125 . Winding  130  is for connection with a modem or other communications equipment (not shown). In  FIG. 1 , winding  130  is shown as being wound once around lower core  125 , but in practice, winding  130  may be wound two or more times. A data signal is coupled between winding  130  and power line  110  via the split magnetic core. 
   Upper core  120  is enveloped by a compressible material, configured as an inward layer  140 B, an outward layer  140 A, an end layer  140 C and an end layer  140 D. A layer  150  of an electrically insulating material is disposed over outward layer  140 A, end layer  140 C and end layer  140 D. 
   Lower core  125  is enveloped by a compressible material, configured as an inward layer  145 B, an outward layer  145 A, an end layer  145 C and an end layer  145 D. A layer  155  of an electrically insulating material is molded into a three-dimensional shape and disposed over inward layer  145 B, outward layer  145 A, end layer  140 C and end layer  140 D. Layer  155  also envelopes the portion of winding  130  that is wound around core  125 . In cross-sectional views of  FIGS. 1–3 , layer  155  is represented as having a portion  155 A disposed over outward layer  145 A, and a portion  155 B disposed over inward layer  145 B 
   The compressible material of inward layer  140 B, outward layer  140 A, end layer  140 C and end layer  140 D has a hardness that is less than that of the electrically insulating material of layer  150 . Outward layer  140 A, end layer  140 C and end layer  140 D compress as layer  150  cures, cools and contracts during a molding process. Such compression obviates cracking of layer  150  during a cooling phase. Furthermore, outward layer  140 A, inward layer  140 B, end layer  140 C and end layer  140 D provide an environmental seal for upper core  120 . 
   The compressible material of outward layer  145 A, inward layer  145 B, end layer  145 C and end layer  145 D has a hardness that is less than that of the electrically insulating material of layer  155 . Outward layer  145 A, inward layer  145 B, end layer  145 C and end layer  145 D compress as layer  155  cures, cools and contracts during a molding process. Such compression obviates cracking of layer  155  during a cooling phase. Outward layer  145 A, inward layer  145 B, end layer  145 C and end layer  145 D also provide an environmental seal for lower core  125 . 
   The compressible material of outward layers  140 A and  145 A, inward layers  140 B and  145 B, and end layers  140 C,  140 D,  145 C and  145 D preferably has a hardness of between about 10 and about 100 on a hardness type shore A durometer scale. An example of such a material is Ethylene Propylene Diene Monomer (EPDM). Hardness testing procedures are provided by the American Society for Testing &amp; Materials, ASTM D2240-03. 
   In practical operation, inductive coupler  100  may be subjected to a variety of temperatures and environmental conditions, for example, summer heat, winter cold, rain, snow and ice. Because of a difference between thermal coefficients of expansion of upper core  120  and layer  150 , a gap may tend to develop between upper core  120  and layer  150 . Water could accumulate in the gap, thereafter freezing and expanding, i.e., frost heave, further aggravating the gap, and resulting in cracks in both upper core  120  and layer  150 . Such gaps and cracks in inductive coupler  100  could lead to electric discharge, causing radio frequency noise, which is detrimental to the operation of a power line communications system. Electric discharge may also cause a deterioration of the electrically insulating material of layer  150 , over time, and may lead to insulation failure. Outward layer  140 A, end layer  140 C and end layer  140 D seal such gaps and cracks, and thus reduce opportunities for discharges to occur. Additionally, outward layer  140 A, inward layer  140 B, end layer  140 C and end layer  140 D absorb physical shock and vibration that could damage upper core  120 . Outward layer  145 A, inward layer  145 B, end layer  145 C and end layer  145 D provide similar benefits with regard to layer  155  and lower core  125 . 
   The compressible material of outward layers  140 A and  145 A, inward layers  140 B and  145 B, and end layers  140 C,  140 D,  145 C and  145 D, also, preferably, has a semi-conductive electrical property. Thus, each of outward layers  140 A and  145 A, and inward layers  140 B and  145 B, when subjected to an electric charge, distribute the electrical charge over their respective volumes, and provide an equipotential volume. In a preferred implementation, a bulk resistivity of outward layers  140 A and  145 A, and inward layers  140 B and  145 B is between about 5 and about 1000 ohm-cm so that a voltage difference between upper core  120  and lower core  125  will not exceed 2% of a voltage on power line  110 . 
   Outward layer  140 A, inward layer  140 B, end layer  140 C and end layer  140 D, are in physical and electrical contact with outward layer  145 A, inward layer  145 B, end layer  145 C and end layer  145 D. Upper core  120  and lower core  125  are thus connected to one another and are at a common electrical potential as one another, minimizing any potential difference that might cause an electrical discharge between upper core  120  and lower core  125 . Outward layers  140 A and  145 A, inward layers  140 B and  145 B, and end layers  140 C,  140 D,  145 C and  145 D collectively form a semi-conducting sheath that minimizes partial discharge or corona in inductive coupler  100 . 
     FIG. 2  is a cross-sectional view of an upper and a lower core of an inductive coupler, with compressible layers around the cores. Upper core  120  has a pole face  200  and lower core  125  has a pole face  205 . Pole face  200  and pole face  205  are spaced apart from one another by an air gap  210 . 
   The term “air gap” is a term of art that refers to a region, between magnetic cores, having non-magnetic material therein. Air gaps improve magnetic characteristics of a magnetic circuit at a high current level. 
   Outward layer  140 A, inward layer  140 B, and end layers  140 C and  140 D (not shown in  FIG. 2 ) converge with one another and cover pole face  200 . Outward layer  145 A, inward layer  145 B, and end layers  145 C and  145 D (not shown in  FIG. 2 ) converge with one another and cover pole face  205 . Thus, the layers provide a fill of non-magnetic material for air gap  210 . This configuration of material in air gap  210  also cushions pole faces  200  and  205  from physical shock and vibration, to reduce an opportunity for fracturing of upper core  120  and lower core  125 . 
     FIG. 3  is a cross-sectional view of an upper and a lower core of an inductive coupler with compressible layers that expose the core faces. Outward layers  140 A and  145 A, inward layers  140 B and  145 B, end layers  140 C,  140 D,  145 C and  145 D (not shown in  FIG. 3 ) terminate at, and do not cover, pole face  220  and  225 . However, outward layers  140 A and  145 A, inward layers  140 B and  145 B, and end layers  140 C,  145 C,  140 D and  145 D are in contact with one another. Thus, there is electrical continuity between outward layers  140 A and  145 A, inward layers  140 B and  145 B, and end layers  140 C,  145 C,  140 D and  145 D. 
     FIG. 4  is a cross-sectional view of an upper and a lower core of an inductive coupler, with insulation serving as a compressible layer around the cores. A layer  400  is disposed on an outward surface of upper core  120 , and disposed on an inward surface and an outward surface of lower core  125 . Layer  400  is composed of a material that is both insulating and compressible. That is, layer  400  is an insulator and is also compressible for over-molding of upper core  120  and lower core  125 . Preferably, layer  400  has a hardness of between about 10 and about 100 on a hardness type shore A durometer scale. Layer  400  can be composed of a silicone, for example. 
   The techniques described herein are exemplary, and should not be construed as implying any particular limitation on the present invention. It should be understood that various alternatives, combinations and modifications could be devised by those skilled in the art. The present invention is intended to embrace all such alternatives, modifications and variances that fall within the scope of the appended claims.