Abstract:
A device to attenuate EMI between a source and a load is provided. The device includes a first cable to electrically couple the source and the load and a second cable positioned adjacent to the first cable and configured to attenuate a common-mode current.

Description:
BACKGROUND 
       [0001]    The invention relates generally to electromagnetic interference and in particular, to reduction of common-mode noise. 
         [0002]    Electronic devices may experience serious operating difficulties when subjected to unintended electromagnetic noise. Electromagnetic noise that interferes with the normal operation of a device, is generally known as electromagnetic interference (EMI). In order to ensure the reliable operation of electronic devices it is desirable that EMI be reduced to a minimum. 
         [0003]    The manner in which EMI is suppressed is dependent on the nature of the interference. There are two ways undesirable noise can propagate in signal transmission paths: one is differential-mode interference, and the other is common-mode interference. Differential-mode interference causes the potential on one side of a signal transmission path to be changed with respect to another side. With this type of interference, the interference current path is wholly in the signal transmission path. 
         [0004]    Common-mode interference appears between two signal transmission paths and a common reference plane (ground), and causes the potential of both sides of the transmission path to be changed simultaneously and by the same amount relative to the reference plane. Common-mode noise may be caused by an electric (capacitive) or magnetic (inductive) field when interference is induced in both signal transmission paths equally. Noise voltages developed may be the same in both transmission paths. 
         [0005]    Common-mode filtering typically uses multiple filter assemblies in series to achieve the desired filtering wherein additional multiple filter assemblies contribute to additional cost, increase size and weight of the total filter assembly which is especially disadvantageous for volume and weight constrained applications. Further, working environments wherein aerospace applications require substantial attenuation of common-mode noise in multiple conductors with reduced weight and size. The common-mode performance of the filters is not sufficient. 
         [0006]    Therefore, it is desirable to provide an apparatus that is capable of attenuating common-mode noise with decreased weight and minimal environmental impact. 
       BRIEF DESCRIPTION 
       [0007]    Briefly, a device to attenuate EMI between a source and a load is provided. The device includes a first cable to electrically couple the source and the load and a second cable positioned adjacent to the first cable and configured to attenuate a common-mode current. 
         [0008]    In one embodiment, a device to attenuate EMI between a source and a load is provided. The device includes an un-shielded cable to electrically couple the source and the load. An attenuation cable is positioned adjacent to the first cable and a clamp-on core is coupled to the un-shielded cable and the attenuation cable. The device further includes a common-mode filter coupled to the un-shielded cable wherein at least one of the attenuation cable, the clamp-on core and the common-mode filter is configured to mitigate a common-mode noise. 
         [0009]    In one embodiment, a system to mitigate electromagnetic interference is presented. The system includes a source and a load coupled via a first cable to carry a load current, a second cable coupled between the source and the load and disposed adjacent to the first cable. The second cable is configured to provide common-mode noise attenuation by providing an alternate path for a common-mode current to flow. 
         [0010]    In one embodiment, a system to mitigate electromagnetic interference is provided. The system includes a source and a load coupled via a first cable to carry a current, a second cable coupled between the source and the load and disposed adjacent to the first cable, a common-mode filter coupled to the first cable, and a clamp-on core coupled to the first cable and the second cable. At least one of the second cable, the common-mode choke and the clamp-on core is configured to provide common-mode noise attenuation by providing an alternate conduction path to a common-mode current. 
         [0011]    In one embodiment, a method to attenuate EMI between a source and a load is proposed. The method includes providing an alternate path for a common-mode noise, providing an attenuation electrical wire(s) or cable to eliminate a shield, coupling a common-mode filter between the source and the load, disposing a clamp-on core on the attenuation electrical wire(s) or cable and attenuating the common-mode noise. 
     
    
     
       DRAWINGS 
         [0012]    These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
           [0013]      FIG. 1  illustrates simplified equivalent circuit of a power system having a three phase inverter supplying power to a motor load; 
           [0014]      FIG. 2  illustrates a common-mode equivalent circuit of  FIG. 1 ; 
           [0015]      FIG. 3  illustrates a shielded cable system of  FIG. 2 ; 
           [0016]      FIG. 4  illustrates a common-mode equivalent circuit implementing an alternate path for common mode current; 
           [0017]      FIG. 5  illustrates a power system implementing the alternate path for common mode current of  FIG. 4 ; 
           [0018]      FIG. 6  illustrates an experimental setup implementing an alternate path for common-mode currents; 
           [0019]      FIG. 7  illustrates an exemplary attenuation profile; 
           [0020]      FIG. 8  illustrates an exemplary attenuation profile; and 
           [0021]      FIG. 9  illustrates an exemplary attenuation profile with the common-mode filter. 
       
    
    
     DETAILED DESCRIPTION 
       [0022]      FIG. 1  illustrates a simplified schematic of a power system having an inverter supplying power to a load. In an exemplary embodiment, a power system  10  includes a direct current (DC) source  12  coupled to an inverter  14  having multiple switching devices  16  coupled between DC bus  18  and  20 . Examples of switching devices include MOSFET, FET, insulated bipolar junction transistor (IGBT), and the like. The inverter  14  supplies a load current  22  to an AC motor  24 . Capacitors  26 ,  28  are coupled respectively between DC bus  18 ,  20  and a first ground node  30 . The motor  24  includes motor windings  32  coupled together at a neutral point  34 . The neutral point  34  is coupled to a second ground node  36  through a parasitic capacitance  38  that may exist between the motor windings  32  and the environment as illustrated by reference numeral  38 . Generally the parasitic capacitance  38  is coupled between the second ground node  36  and the motor neutral  34 . It may be appreciated that an equivalent parasitic inductance  40  may be coupled between the first ground node  30  and second ground node  38 . Physical geometry and positioning of the inverter  14 , motor  24 , and surroundings may substantially influence the value of such parasitic elements  38 ,  40 . 
         [0023]      FIG. 2  illustrates a common-mode equivalent circuit of  FIG. 1 . A common-mode output voltage  42  of the inverter  14  is represented by a voltage source  44 . The common-mode output voltage  42  of the inverter  14  is an average of the inverter&#39;s individual phase voltages. In one embodiment, inductance of the motor windings  32 , line inductance  46  (See  FIG. 1 ), and parasitic capacitance  38  may be represented as a common-mode load impedance  48 . The common-mode equivalent circuit  50  includes a wire/cable  52  coupling the common-mode voltage source  44  and the common-mode load impedance  48 . The cable  52  may include a single conductor of a given diameter, wherein the current carrying requirement dictates the diameter. In one embodiment, the wire  52  may include multiple strands of electrical conductors bundled together. The voltage source  44  is coupled to the first ground node  30 . Similarly, the load impedance  48  is coupled to the second ground node  36 . Parasitic elements such as parasitic inductance  40  may exist between the first ground node  30  and the second ground node  36 . A path of minimum impedance for a common-mode current  54  may exist between the ground nodes  30 ,  36 . During an operation, the voltage source  44  directs the common-mode current  54  to the load  48  via the wire  52 . As will be appreciated by one skilled in the art, common-mode current  55  between the ground nodes  30 ,  36  may create an unwanted potential difference (noise) between ground nodes  30  and  36 . It is desirable to minimize common-mode current  55  returning along the grounding network between ground nodes  30 ,  36 , thereby minimizing EMI emission and susceptibility. 
         [0024]      FIG. 3  illustrates a shielded cable system of  FIG. 2 . In the shielded cable system  56 , to minimize the common-mode noise between ground nodes  30  and  36  as described above, a shielded cable  58  is coupled between the common-mode voltage source  44  and the common-mode load impedance  48 . The cable  58  includes an inner conductor  60  and a cable shield  62 . The cable shield  62  is electrically coupled to the first ground node  30  on the source side, and to the second ground node  36  on the load side. During an operation, the common-mode current  54  flows through the inner conductor  60  between the common-mode voltage source  44  and the common-mode load impedance  48 . The common mode current  55  returns through the minimum impedance return path between the ground nodes  30  and  36  via the cable shield  62 . Such an arrangement prevents the common-mode current  55  from flowing along the grounding network path, thus eliminating the unwanted noise (potential difference) between ground nodes  30  and  36  reducing EMI emission and susceptibility. 
         [0025]    However, implementing such cable shield connection in power systems may be difficult to practice. For example, in aerospace applications, wherein the shielded cable between various components may contribute to significant weight of the overall electrical circuitry, or limit heat transfer from inside the cabling to the ambient environment. Further, due to constantly changing environment within an aircraft, such as change in temperature and pressure over different altitude, shielded cable may degrade faster, augmenting the maintenance economics of the aircraft apart from the environmental concerns that may arise due to moisture entrapment within the shielded cable. Other embodiments of the present invention are intended to overcome the disadvantages of the cable shield by introducing an alternate path for the common-mode currents. 
         [0026]      FIG. 4  illustrates a common-mode equivalent circuit implementing an alternate path for common mode current. As discussed above, in applications wherein the cable shield is not practical to implement, an alternate embodiment is provided by way of an added wire. The common-mode equivalent circuit  64  illustrates a common-mode voltage source  44  coupled to the common-mode load impedance  48  through an unshielded cable  52 . In an exemplary embodiment, the common-mode voltage source  44  is coupled to the first ground node  30  and the common-mode load impedance  48  is coupled to the second ground node  36 . An added second wire  66  is coupled between the ground nodes  30 ,  36 . The added second wire  66  may comprise a single conductor or a multi-strand conductor or multiple conductors. It may be noted that the current carrying capability of the second wire  66  may be lower than the first wire  52  that carries the bulk of the load current. In operation, the common-mode voltage source  44  gives rise to common mode load current  54  that flows to the common-mode load impedance  48  via the first wire  52 . As discussed earlier, if the minimum impedance return path is along the grounding network path, potential differences may be developed due to the presence of parasitic elements, creating a potential difference between the first ground node  30  and the second ground node  36 . Ideally, a zero potential difference is desired between the ground nodes  30 ,  36 . However, the return common-mode current  55  may find a path of least impedance through the second wire  66 . In one embodiment, alternate path of least impedance is provided for the common-mode current  55  by re-routing the current  55  through second wire  66 . The alternate path eliminates the need for a cable shield. However, without the cable shield or the second wire, the common-mode current would flow through the voltage source  44  via parasitic elements causing a noise along with the flow of load current  22 . The terms “first wire,” “second wire,” and “added wire,” as the terms are used herein, are intended to denote an electrical coupling between various components of the power system to perform the tasks of the invention. The term “wire” is intended to denote any electrical cable or multiple electrical conductors capable of conducting current during an operation of the power system. 
         [0027]      FIG. 5  illustrates a power system implementing the alternate path for common mode current. The illustrated schematic of power system  68  implements the alternate path for common-mode current as discussed in the common-mode equivalent circuit of  FIG. 4 . The power system  68  includes a power source  12  supplying power to an inverter  14 . Multiple switches  16  are arranged between the DC bus  18  and  20  and configured to switch in an organized scheme. The output of the inverter  14  may include three phase AC voltage supplying a load current  22 , for example, to a motor load  24 . The power system  68  includes a first ground node  30  on the inverter input side and a second ground node  36  coupled to the motor neutral  34  through parasitic capacitance  38 . Typically, a potential difference may exist between the first ground node  30  and the second ground node  36  due to common-mode current that may flow between the ground nodes  30 ,  36  via parasitic elements and interfere with the operation of the inverter  14 . However, in one embodiment, an alternate path for the returning common-mode current  55 , is implemented via a fourth wire  66  coupled between the first ground node  30  and the second ground node  36 . The fourth wire  66  provides a least impedance path to the returning common-mode current  55 . Such added wire provides advantages of eliminating the need for a cable shield in three phase cable system. Further, common-mode filters may be implemented within the system in conjunction with the added wire to increase the efficiency of common-mode mitigation within the power system  68 . An experimental setup having an exemplary scheme of implementing the added wire in the power system is discussed in  FIG. 6   
         [0028]      FIG. 6  illustrates an experimental setup implementing an alternate path for common-mode currents. The experimental setup  70  includes a base generally made of an electrically conductive sheet  72  (e.g. aluminum) that represents the grounding network. Insulating posts  74  and  76  are mounted on the aluminum sheet  72  to support routing of connecting cables. A first wire  78  is bonded on to the aluminum sheet  72  at  80 . A high frequency amplifier  85  is coupled to the first wire  78 , wherein the amplifier  85  represents a common-mode noise source. The first wire  78  may include a single conductor of a given diameter, wherein the current carrying requirement dictates the diameter. In one embodiment, the first wire  78  may include multiple strands of electrical conductors bundled together. In an exemplary embodiment, the high frequency amplifier  85  is configured to inject high frequency current in the range of about 100 kHz to about 30 MHz. The first wire  78  is bonded the aluminum sheet  72  at the far end  82 . Further, a second wire  84  is disposed adjacent to the first wire  78  and bonded on to the aluminum sheet  72  at  86  and  88 . A current sensor  90  (or a current transformer) is coupled to the first wire  78  and the second wire  82  and a spectrum analyzer  92  is coupled to the current sensor  90  to measure a frequency response. In one embodiment, clamp-on ferrite cores  94  are disposed onto the first wire  78  and the second wire  84 . The placement of ferrite core  94  creates magnetic coupling. Such magnetic coupling provides an alternative path for the common-mode current to return through the second wire  84  instead of the undesirable path through  72 . In another embodiment, a common-mode filter  96  is coupled to the first wire  78 . In an exemplary embodiment, the common mode filter  96  is an inductor having a circular magnetic core with one set of windings disposed on each half of the magnetic core. The magnetic core may include, for example, materials such as ferrite or silicon steel or amorphous material or nano-crystalline material. In one embodiment, the windings may include a bifilar winding around the magnetic core wherein the two wires are twisted between them and wound consistently to spread across the magnetic core. The advantages of bifilar windings are illustrated in  FIG. 9 . The common-mode filter  96  adds common-mode impedance to the first wire  78 , thereby reducing the common-mode current via the first wire  78 . In may be noted that coupling the common-mode filter to either a single wire or both of the wires may yield varying results depending on the power system parameters such as operating voltages, currents, actual length of the cables used, for example. In may be prudent to use the common-mode filter coupling in a manner best suited for a given power system. 
         [0029]    The high frequency amplifier  85  injects high frequency current during an operation of the experimental setup  70 . In an exemplary embodiment, to study the effect of the second wire  84 , the experiment is carried out in a sequential manner by (i) providing the first wire only (ii) providing the second wire adjacent to the first wire (iii) providing clamp-on core on the first wire and the second wire (iv) providing a common-mode filter on the first wire with clamp-on core on the first wire and the second wire and (v) providing a common-mode filter on both the first wire and the second wire with clamp-on core on the first wire and the second wire. The results are illustrated in  FIG. 7 ,  8  showing the change in attenuation as a result of changes in the circuit. 
         [0030]    Turning now to  FIG. 7 , a frequency response plot  100  having an ordinate axis represents attenuation in  102  measured in decibels (dB) and the abscissa axis represents frequency  104  measured in MHz. In the illustrated embodiments, the attenuation profile  106  provides results for (i) only first wire  78  used. The attenuation profile  108  illustrates results for (ii) providing the second wire adjacent to the first wire. It may be noted that, by adding the second wire, the attenuation increased from about 10 dB to about 22 dB at about 1 MHz frequency. Referring to  FIG. 8 , the ordinate axis represents attenuation in  114  measured in decibels (dB) and the abscissa axis represents frequency  116  measured in MHz. In the illustrated embodiments, the attenuation profile  118  illustrates results for (iii) providing clamp-on core on the first wire (only one wire)coupled to the common-mode filter. The attenuation profile  120  illustrates results for (iv) coupling the common-mode filter on the first wire and the second wire with clamp-on core on the first wire and the second wire. The attenuation profile  122  illustrates results for (v) coupling the common-mode filter on the first wire only with clamp-on core on the first wire and the second wire. Attenuation up to about 60 dB may be achieved at about 1 MHz frequency by providing a second wire adjacent to the first wire, clamp-on core on the first wire and the second wire, and a common-mode filter on the first wire only. 
         [0031]    In an exemplary embodiment, the bifilar windings in the common-mode filter have an improved attenuation profile as illustrated in  FIG. 9 . Referring to  FIG. 9 , frequency response plot  124  includes the ordinate axis representing attenuation in  126  measured in decibels (dB) and the abscissa axis representing frequency  129  measured in MHz. In the illustrated embodiments, the attenuation profile  130  illustrates results for a normally wound common-mode filter coupled to the first wire. The attenuation profile  132  illustrates results for a bifilar wound common-mode filter coupled to the first wire. It may be noted from the profiles  130  and  132  that increased attenuation of about 10 dB may be achieved at about 1 MHz frequency by implementing a bifilar winding in the inductor used as the common-mode filter  96  as referenced in  FIG. 6 . 
         [0032]    Advantageously, a simple added wire approach may be used to implement added filtering, or to have the same filtering efficacy as a conventional filter at lighter weight. The added wire approach can be used in conjunction with a common-mode filter to provide added filter attenuation thus eliminating heavy and bulky components. Such added wires are simple in construction and lightweight with the added advantage of increased performance common-mode filter. The added wire also helps eliminate environmental, weight, and reliability concerns. 
         [0033]    While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.