Abstract:
A filter device reduces reflections on power lines from the motor drive to AC motors by providing a differential mode choke in series with a common mode choke both shunted by resistances tailored to the characteristic impedance of the power cable for differential mode and common mode reflections respectively. By treating both common mode based and differential mode based reflections, superior transient control and motor drive performance may be obtained.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
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     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
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     BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to three-phase AC motor drives and particularly to a filter device for power lines communicating between such drives and a connected motor. 
         [0002]    Common AC induction motors use three-phase electrical power connected to stator windings of the motor. Each stator winding receives a different conductor of a three-phase power transmission line, where each conductor communicates an AC power waveform shifted with respect to the other conductors by plus and minus 120°. 
         [0003]    Referring to  FIG. 1 , the motor  22  may connect directly to a power grid or, as shown, connect to a solid-state motor drive  10 . In this latter case, the motor drive  10  will receive three-phase power  12  from the power grid at a rectifier  14  and the rectifier  14  will convert the received three-phase AC power into DC power on a DC link  16 . The rectifier  14  uses an active or passive rectification system of a type known in the art. 
         [0004]    The DC power on the DC link  16  is received by an inverter  18  which synthesizes new three-phase power  20 , for example, having a different frequency than the three-phase power  12  to provide for motor speed control or control of other motor parameters. 
         [0005]    The synthesized three-phase power  20  may be communicated to the motor  22  by means of a power cable  24 . Such power cable  24  may extend for hundreds or even thousands of feet allowing the motor drive  10  to be located at a substantial distance from the motor  22 . Such power cables  24  normally include three conductors (one for each power phase) and one or more ground conductors within a conductive shield, the latter to reduce the transmission of electrical interference to surrounding equipment. 
         [0006]    The distributed inductance and capacitance of the power cable  24  can create electrical reflections along the power cable  24  resulting generally from an impedance mismatch between the characteristic impedance of the power cable  24  and the motor  22 , at one end of the power cable  24 , and between the characteristic impedance of the power cable  24  and the inverter  18 , at the other end of the power cable  24 . These reflections produce voltage and current surges  28  that can damage conductor insulation, cause arcing across motor bearings, and boost the voltage on the DC link  16  damaging the inverter  18  or causing the inverter  18  to shut down to avoid damage. 
         [0007]    U.S. Pat. No. 5,990,654 assigned to the assignee of the present invention and hereby incorporated by reference describes a filter device  32  receiving the three-phase power  20  from the inverter  18  on one side through a relatively short cable where reflections are not a problem and attaching to the power cable  24  on the other side. The filter device  32  operates to reduce surges  28  by reducing reflections on the power line  24 . 
         [0008]    Referring now to  FIG. 2 , for this purpose, the filter device  32  may include a common mode choke  34  having three inductors  36 , one attached to each conductor of each different phase of the synthesized three-phase power  20 . Each of these inductors  36  is connected in series with one corresponding inductor  38  of three inductors  38  of a differential mode choke  40 . 
         [0009]    As is generally understood in the art, and as shown in  FIG. 3 , the common mode choke  34  forms each inductor  36  as a separate coil wound around a common core  46 , with a well known geometry such as a toroidal core or a U-I core. In this way, the flux path passes through each inductor  36  in series so that each of the inductors  36  presents a relatively high impedance to common mode current components (those having a phase alignment) and a relatively low impedance to differential mode current components (those not aligned in phase). 
         [0010]    In contrast, as shown in  FIG. 4 , the differential mode choke  40  forms each inductor  38  as a separate coil wound on one leg of an E-core  50  whose legs are bridged by an I-core  52  to provide a parallel rather than series flux path through each inductor  38 . In contrast to the common mode choke  34 , the differential mode choke  40  presents a relatively high impedance to differential mode current components and a relatively low impedance to common mode current components. 
         [0011]    The common mode choke  34  and differential mode choke  38  can also be wound on a single magnetic structure  41 , as shown in  FIG. 5 . The differential mode portion of this choke is formed by each inductor  38  as a separate coil wound on one leg of an E-core  50  whose legs are bridged by an I-core  52 . The common mode portion of this choke is formed by each inductor  36  as a separate coil wound around the common U-core  46  bridged by the I-core  52 , which is shared with the differential mode portion of the choke. Each of the common mode inductors  36  is connected in series with one corresponding differential mode inductor  38 . This series combination provides high impedance to both common mode and differential mode current components. 
         [0012]    Referring still to  FIG. 2 , each of the inductors  38  of the differential mode choke  40  is shunted by a resistor  42  connected in parallel with the individual inductor  38 . These resistors  42  have impedance selected to provide a matching to a characteristic impedance of the power cable  24  to thereby reduce reflections on the power cable  24  at the interface between the filter device  32  and the power cable  24 . 
         [0013]    While the filter device  32  may significantly reduce reflections on the power line  24 , significant surges  28  may still occur in certain cases. 
       BRIEF SUMMARY OF THE INVENTION 
       [0014]    The present inventors have recognized that there are, in fact, two different reflection modes associated with the power cable, a reflection mode related to the power cable&#39;s common mode impedance and a reflection mode related to the power cable&#39;s differential mode impedance. The prior art filter device reduced differential mode reflection (and hence transients) while still permitting common mode reflections and transients. Common mode voltage and current transients can cause premature bearing failure by creating a condition for arcing across the bearings, pitting the finely machined bearing surfaces. 
         [0015]    Generally, the present invention works by providing separate impedance matching for the common mode impedance and differential mode impedance of the power line. In particular, the common mode choke and differential mode choke of the prior art are used to apply the appropriate resistance to the appropriate reflection mode thereby addressing both common mode and differential mode impedance mismatch. For a common type of power cable, the resistor values used for these different impedance matching requirements may be close to each other making it possible to use the same resistor (with an increase in power rating) for the suppression of both common mode and differential mode reflections. 
         [0016]    Specifically, the present invention provides an apparatus for reducing electrical transients on a supply cable having three conductors and providing three-phase power to an AC motor from a solid-state motor drive. The apparatus includes a common mode choke having three inductors and a differential mode choke having three inductors, each of the latter inductors connected in series with a corresponding inductor of the common mode choke. Each of the series connected inductors of the differential mode choke and common mode choke are connected in series with one conductor of the power cable. A resistor is placed in parallel with each of the inductors of the common mode choke. 
         [0017]    It is thus an object of one embodiment of the invention to address reflections caused by mismatch with the common mode characteristic impedance of the power cable thereby reducing common mode surge voltages or currents. Placing the resistors in parallel with the common mode choke helps to damp common mode voltage and current reflections. 
         [0018]    The resistors may be connected in parallel across each inductor of the common mode choke. 
         [0019]    It is thus an object of one embodiment of the invention to damp common mode reflections with an independent set of resistors. 
         [0020]    The resistors may be substantially equal to the common mode characteristic impedance of the power cable. 
         [0021]    It is thus an object of one embodiment of the invention to provide a simple method of determining resistor value. 
         [0022]    The resistor is substantially 50 ohms. 
         [0023]    It is thus another object of one embodiment of the invention to provide a resistor value suitable for common shielded cables. 
         [0024]    The invention may further include a resistor connected in parallel across each inductor of the differential mode choke. 
         [0025]    It is thus an object of one embodiment of the invention to damp differential mode reflections with a different, independent set of resistors. 
         [0026]    In an alternative embodiment, the resistors may be connected in parallel across the series connected inductors of the differential mode choke and the common mode choke. 
         [0027]    It is thus an object of one embodiment of the invention to provide a circuit topology that, by permitting the use of similar values of the shunting resistors for common mode and differential mode reflections, allows one set of resistors to attenuate both common mode and differential mode reflections. 
         [0028]    The resistors may be substantially equal to the average of the differential mode characteristic impedance and common mode characteristic impedance of the power cable. 
         [0029]    It is thus an object of one embodiment of the invention to provide for a single resistor value for cables where the desired resistance for the common mode and differential mode impedances are similar but not the same. 
         [0030]    The resistors may be substantially 50 ohms. 
         [0031]    It is thus an object of one embodiment of the invention to provide a simplified circuit topology and resistor value that may be used with common cable types. 
         [0032]    The common mode choke and differential mode choke may be wound on magnetically separate cores. 
         [0033]    It is thus an object of one embodiment of the invention to permit the invention to be fabricated from commonly available components without requiring specialized choke construction. 
         [0034]    In an alternative embodiment, the resistors may be connected in parallel across an integrated choke that provides both common mode and differential mode impedance. 
         [0035]    It is thus an object of the invention to provide an embodiment with reduced component count. 
         [0036]    The resistors may be substantially 50 ohms 
         [0037]    It is thus an object of one embodiment of the invention to permit the invention to be fabricated from commonly available components that allow for a compact design. 
         [0038]    These particular features and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0039]      FIG. 1  is a block diagram of a standard motor drive employing a filter device as described in the background of the invention; 
           [0040]      FIG. 2  is a schematic representation of the filter device of  FIG. 1  providing a common mode and differential mode choke; 
           [0041]      FIG. 3  is a diagram of a common mode choke of the prior art; 
           [0042]      FIG. 4  is a diagram of a differential mode choke of the prior art; 
           [0043]      FIG. 5  is a diagram of an integrated common mode and differential mode choke of the prior art; 
           [0044]      FIG. 6  is a discretized representation of the distributed inductance and capacitance of the typical power cable illustrating the divergence between common mode impedance and differential mode impedance underlying the present invention; 
           [0045]      FIG. 7  is a schematic representation similar to that to that of  FIG. 2  of a filter device in a first embodiment of the present invention; 
           [0046]      FIG. 8  is a schematic representation similar to that of  FIG. 7  of an alternative embodiment of the present invention providing dual use for the damping resistors; and 
           [0047]      FIG. 9  is a schematic representation similar to that of  FIG. 7  of an alternative embodiment of the present invention using an integrated common mode and differential mode magnetic device and providing dual use for the damping resistors. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0048]    Referring now to  FIG. 6 , the conductors  30  of power cable  24  may have an intrinsic inductance represented by discrete inductors  60  distributed in series along the length of each power conductor  30 . 
         [0049]    A differential mode impedance will be determined by these inductors  60  together with capacitors  62  passing between each conductor  30  and its neighbor representing the distributed capacitance among the conductors  30 , and capacitors  64  passing from each conductor  30  to ground typically presented by a shield around the conductors  30 . 
         [0050]    The common mode characteristic impedance of the power cable  24 , in contrast, will be determined by inductors  60  together with capacitors  64  passing from each conductor  30  to ground. 
         [0051]    In practice, the differential mode impedance is measured from one end of any conductors  30  to the other two conductors connected to each other at that end and with all three conductors connected to each other at the other end. This differential impedance measurement normally involves the application of a voltage step across the conductors  30  and measurement of the amplitude of the resulting current pulse. 
         [0052]    In contrast, the common mode impedance measurement connects the ends of all three of the conductors  30  together and applies a similar step voltage between ground and the commonly connected conductors  30 . 
         [0053]    For a typical shielded power cable  24 , the differential mode impedance may be approximately 50 ohms whereas the common mode impedance may be approximately 15 ohms. 
         [0054]    Referring now to  FIG. 7 , in a first embodiment, the present invention provides a filter device  32  having a common mode choke  34  with three inductors  36  and a differential mode choke  40  with three inductors  38 . Each inductor  36  is connected in series with one corresponding inductor  38 . As described above with respect to  FIG. 2 , each of the inductors  38  may be connected in parallel with a resistor  42 . In contrast to the prior art, each of the inductors  36  of the common mode choke  34  is also shunted by resistors, in this case by the parallel connection of a resistor  70  across each of the inductors  36 . 
         [0055]    For common mode transients  28   a  represented by a voltage source applied to each of the conductors  30 , the differential mode choke  40  will provide a low impedance passing these transients  28   a  to the common mode choke  34 . The common mode choke  34 , in contrast, presents a relatively high impedance to the transients  28   a  so that the impedance experienced by common mode transients  28   a  will be determined by the resistors  70 . 
         [0056]    For a typical power cable  24  having a common mode impedance of approximately 15 ohms, impedance matching will occur when each of the resistors  70  has a resistance of approximately 45 ohms. The common mode transient  28   a  will thus experience an impedance within the filter device  32  of three 45 ohm resistors in parallel, equaling 15 ohms. 
         [0057]    For differential mode transients  28   b,  a high impedance will be presented by the inductors  38  of the differential mode choke  40  presenting an impedance to the transients  28   b  characterized by the resistors  42 . The common mode choke  34 , in contrast, presents a relatively low impedance to the transients  28   b  effectively bypassing the effect of resistors  70 . 
         [0058]    For a typical power cable  24  having a differential mode impedance of approximately 50 ohms, each resistor  42  will also have a value equal to 50 ohms. The differential mode transient  28   b  will experience an impedance within the filter device  32  equaling the value of each resistor  42  of 50 ohms. 
         [0059]    As will be understood from this description, for common power cables  24 , resistors  70  and  42  may have similar values (e.g. 45 ohms and 50 ohms). In this case, a compromise may be made approximating the value of resistor  70  as 50 ohms (or resistor  42  as 45 ohms). Through this compromise, by making resistors  70  and  42  equal to a compromise value somewhere between (or including) the two actual values, it will be understood that the function of the resistors  70  and  42  may be combined into single resistor  80  (as shown in  FIG. 8 ). 
         [0060]    Referring to  FIG. 8 , resistors  80  shunt the series connected combination of the inductors  36  of the common mode choke  34  and the inductors  38  of the differential mode choke  40  so that one resistor  80  is in parallel with the series connected inductors  36  and  38 . This reduction in the number of resistors is possible because the common mode choke  34  and differential mode choke  40  serve to steer transients  28   a  and  28   b  separately to resistors  80 . Thus, common mode choke  34  provides a high impedance steering common mode transients  28   a  to resistors  80 , and differential mode choke  40  provides a high impedance steering differential mode transients  28   b  to resistors  80 . This dual function of resistors  80  will require resistors  80  to have a higher wattage value than resistors  70  and  42 . 
         [0061]    Referring to  FIG. 9 , resistors  80  shunt an integrated magnetic structure  41  that provides both common mode and differential mode impedance. The resistors  80  function to damp both differential mode and common mode transients, and hence will be required to have a higher wattage value than resistors  70  and  42 . 
         [0062]    Because the common mode characteristic impedance and differential mode characteristic impedance of power cable  24  are essentially independent of cable length, a filter device  32  may be constructed and used freely with cables of a variety of lengths long enough to present significant reflection problems. 
         [0063]    It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein and the claims should be understood to include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims. For example, while the present invention describes three-phase motors and drives it will be understood that the present invention will apply to higher order phases and that the term “three-phase” should be considered to embrace any system having at least three phases.