Abstract:
A motor drive employs a combination of gas discharge tubes and metal oxide varistors to create precisely tailored surge protection that has a low leakage current minimizing the triggering of ground fault detection circuitry when multiple motor drives are attached to a single line source.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0001]    -- 
       CROSS REFERENCE TO RELATED APPLICATION 
       [0002]    -- 
       BACKGROUND OF THE INVENTION 
       [0003]    The present invention relates to motor drives for electric motors and in particular to a surge protection circuit reducing erroneous false ground fault indications and providing improved component life. 
         [0004]    Motor drives control the frequency and amplitude of the electrical power applied to an electrical motor to improve motor operation, for example, by improving motor starting and stopping, motor speed and torque control, motor synchronization, load management, and energy efficiency. For this purpose, the motor drive will typically receive three-phase line power and rectify it to produce a DC bus voltage. The DC bus voltage is then received by a set of switching semiconductor devices, typically operating in switching mode, to synthesize multiphase AC electrical power from the DC bus voltage. The frequency and amplitude of the synthesized power is controlled by controlling the switching of the semiconductor devices. 
         [0005]    In order to comply with the standards of regulatory and certification agencies, motor drives must have over-voltage or “surge” protection on their connections to line power to protect the motor drive and its associated equipment from high voltage transients induced, for example, by lightning or the switching of inductive loads elsewhere on the line. The surge protection device of choice is a metal-oxide-varistor (MOV). As is understood in the art, an MOV provides a nonlinear resistor having a high resistance up to a threshold voltage at which point the resistance drops to provide for a substantially constant voltage regulation up to the power limits of the MOV device. MOV devices are typically constructed of sintered zinc-oxide with other metal oxide additives. 
         [0006]    A set of MOVs may be used to provide a network protecting the motor drive from excessive voltages between any of the three phase line inputs (“line-to-line over voltage”) and between any of the line inputs and ground (“line-to-ground over voltage”). 
         [0007]    One problem with MOV is that a small amount of current (“leakage current”) flows through these devices even when they are operating below their threshold voltage at normal line voltages. This leakage current, if flowing to ground, can trigger ground fault sensors used for fault detection particularly when several motor drives are connected to the same line inputs. 
         [0008]    Further, the leakage current to the MOV degrades the MOV over time thus reducing its reliability. 
       SUMMARY OF THE INVENTION 
       [0009]    The present invention provides a surge protection device utilizing a set of MOVs arranged in a network and combined with at least one gas discharge tube (GDT) in series with a network interconnection to ground. The gas discharge tube provides an essentially infinite resistance at voltages under its breakdown voltage thus ensuring very low current leakage to ground during normal operating conditions. At least one MOV is placed in series with the GDT preventing damaging current flows through the GDT when breakdown voltages are reached. 
         [0010]    Specifically then the present invention provides a motor drive with integrated surge protection, the motor drive having three-phase power line inputs receiving AC power and a rectifier unit communicating with the power line inputs and converting the AC power to a DC bus voltage. An inverter receives the DC bus voltage to synthesize a motor drive current. A surge protection network provides an interconnection between each line input and ground and an interconnection between each line input and each other line input, the surge protection network controlling a maximum surge voltage across each interconnection. The surge protection network provides a metal oxide varistor on each interconnection and at least one gas discharge tube between each line input and ground. 
         [0011]    It is thus one object of one embodiment of the invention to provide a surge protection device for motor drives that provides extremely low current leakage and thus that does not trigger ground fault detection devices when multiple motor drives are combined on a monitored power line. 
         [0012]    The invention may provide one gas discharge tube between each line input and each other line input. 
         [0013]    It is thus another object of one embodiment of the invention to provide low current leakage between line inputs such as may decrease the degradation of the MOV devices. 
         [0014]    The interconnection between input lines may be a Wye interconnection. 
         [0015]    It is thus another object of one embodiment of the invention to provide a surge protection network providing a neutral point. 
         [0016]    In one embodiment the motor drive may have a first series connected MOV and GDT interconnecting a first line input and a common point; a second series connected MOV and GDT interconnecting a second line input and the common point; a third series connected MOV and GDT interconnecting a third line input and the common point; and an MOV interconnecting the common point to ground. 
         [0017]    It is thus an object of one embodiment of the invention to provide for the leakage on line-to-line and line-to-ground pads while minimizing the number of GDTs. 
         [0018]    The interconnection between input lines may be a Delta interconnection. 
         [0019]    It is thus one object of one embodiment of the invention to provide a network where a common neutral point is not required. 
         [0020]    The motor drive may include a first series connected MOV and GDT interconnecting a first line input and a second line input; a second series connected MOV and GDT interconnecting a second line input and a third line input; a third series connected MOV and GDT interconnecting a third line input and the first line input; a fourth series connected MOV and GDT interconnecting the first line input and ground; a fifth series connected MOV and GDT interconnecting the second line input and ground; and a sixth series connected MOV and GDT interconnecting the third line input and ground. 
         [0021]    It is thus another object of one embodiment of the invention to provide for both line-to-line and line-to-ground low leakage in a Delta configuration 
         [0022]    Alternatively the motor drive may provide a first MOV interconnecting a first line input and a second line input; a second MOV interconnecting a second line input and a third line input; a third MOV interconnecting a third line input and the first line input; a fourth series connected MOV and GDT interconnecting the first line input and ground; a fifth series connected MOV and GDT interconnecting the second line input and ground; and a sixth series connected MOV and GDT interconnecting the third line input and ground. 
         [0023]    It is thus an object of one embodiment of the invention to provide a Delta connection system with reduced GDTs. 
         [0024]    The motor drive may provide only one gas discharge tube between ground and a common connection leading to each of the line inputs. 
         [0025]    It is thus an object of one embodiment of the invention to provide an embodiment requiring only a single GDT. 
         [0026]    The motor drive may provide a first MOV interconnecting a first line input and a common point; a second MOV interconnecting a second line input and the common point; a third MOV interconnecting a third line input and the common point; and a series connected MOV and GDT connecting the common point to ground. 
         [0027]    It is thus another object of one embodiment of the invention to produce a cost minimized protection network. 
         [0028]    The MOVs and GDT are rated for breakdown voltage greater than the peak value of the AC input line voltage. 
         [0029]    It is thus an object of one embodiment of the invention to substantially increase the life of MOV in high voltage applications. 
         [0030]    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 
         [0031]      FIG. 1  is a simplified schematic representation of a standard motor drive using the present invention and showing ground and the multiple line inputs; 
           [0032]      FIG. 2  is a schematic representation of a first embodiment of the invention employing a Wye connection and providing low line-to-line leakage and low leakage to ground; 
           [0033]      FIG. 3  is a schematic representation of a second embodiment of the invention minimizing the use of gas discharge tubes; 
           [0034]      FIG. 4  is a schematic representation of a third embodiment of the invention employing a Delta connection; and 
           [0035]      FIG. 5  is figure of a fourth embodiment of the invention employing a Delta connection and providing reduced line-to-line leakage. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0036]    Referring now to  FIG. 1 , a solid-state motor drive  10  may receive three-phase power at line inputs  12  having phases labeled: R, S, and T, the line inputs providing three-phase power with respect to a ground  14  (labeled G). 
         [0037]    The three-phase power at the line inputs  12  may be received by a rectifier circuit  16  producing a DC voltage on DC bus  18  as is understood in the art. The DC voltage may be received by inverter  20  which synthesizes new three-phase power  22  provided to a motor  24 . By converting the three-phase power to DC and then back into three-phase power  22 , the frequency, amplitude, and phase of the motor voltages and currents may be adjusted for precise motor control. 
         [0038]    Generally, the three-phase power at line inputs  12  may have voltage surges caused by disturbances on the powerline including, for example, lightning. Accordingly a surge protector network  26  may be attached to the line inputs to suppress these transient voltages. 
         [0039]    Referring now to  FIG. 2  in a first embodiment the surge protector network  26  may provide for three interconnections  28   a ,  28   b , and  28   c , each interconnection  28  passing between a single one of the line inputs  12  (R, S, T) and a neutral point  30  in a so-called Wye connection. A fourth interconnection  32  passes from the neutral point  30  to ground  14 . 
         [0040]    Each of the interconnections  28   a - c  provides a series connected gas discharge tube (GDT)  34  and MOV  36  each having substantially equal breakdown voltage. The order of the GDT  34  and MOV  36  in the series connection is not important. The fourth interconnection  32  may provide series conduction through a single MOV  38 . 
         [0041]    As is understood in the art, a GDT  34  is a crowbar type surge protection device that operates on the principle of arc discharge. GDT s are hermetically sealed capsules containing a rare earth gas, such as argon or neon, at a predetermined optimum pressure to create a controlled arc between two opposed electrodes at a given breakdown voltage. The electrodes may include ignition aids on their surface to ensure quick response to rapidly rising surge voltages. At voltages less than the breakdown voltage, virtually no current flows through the GDT  34 . At voltages above the breakdown voltage, current flows and voltages across the GDT  34  are clamped at a low value of approximately 10 to 25 V. 
         [0042]    The GDTs  34  and MOV  36  each have a breakdown voltage defined by one half of the maximum desired voltage limit between any two line inputs  12 . Thus, for example, if it is desired to limit the maximum line-to-line surge voltages to 550 V, then each GDT  34  and MOV  36  will have a breakdown voltage of approximately 275 V. The maximum line-to-ground breakdown voltage may be the same or different by using an MOV  38  with a different breakdown voltage as desired. 
         [0043]    In this configuration, any path between any of the line inputs  12  to another line inputs  12 , or between any of the line inputs  12  and the ground  14  must pass through a GDT  34 . Therefore each of the interconnections  28   a - c  and  32  experiences extremely low current leakage, resulting in extremely low line-to-line current leakage and line-to-ground current leakage. 
         [0044]    Referring now to  FIG. 3 , in an alternative embodiment, the multiple GDTs  34  may be removed in favor of a single GDT  34  placed in series with MOV  38  on interconnection  32 . Interconnection  28   a ,  28   b  and  28   c , in this case, provide only the MOV  36  without accompanying GDTs. This configuration provides some leakage currents along interconnections  28   a - c , and thus some line-to-line leakage current; however, leakage current along interconnections  32  is blocked providing low line-to-ground leakage current. This embodiment further significantly reduces the number of GDTs and thus the cost of the surge protector network  26 . 
         [0045]    Referring now to  FIG. 4 , in an alternative embodiment, three interconnections  40   a ,  40   b , and  40   c  may join the ground  14  to one of the line inputs  12  (R, S, T) associated with each of the interconnections  40   a - c . In this case each interconnection  40   a - c  provides a series connected MOV  42  and GDT  44 . Again, the order of the MOV  42  and GDT  44  in the series connection is not important. 
         [0046]    In this embodiment, three more interconnections  46   a ,  46   b  and  46   c  may each join one of the line inputs  12  to one other line input  12  in a so-called Delta connection. In this case each interconnection  46  consists of a single MOV  48 . This topology provides essentially the same protection as that provided by the embodiment of  FIG. 3 , permitting some line-to-line leakage current but blocking line-to-ground leakage current by means of GDTs  44 . In this embodiment the maximum surge voltage between line inputs  12  (line-to-line surge voltage) may be selected independently of the maximum surge voltage between line inputs  12  and ground  14  (line-to-ground and surge voltage) with the former being either higher or lower than the latter. Further, failure one of the MOVs  48  does not compromise surge protection with respect to ground, as would be the case for the embodiment of  FIG. 3 . 
         [0047]    Referring now to  FIG. 5 , low line-to-line leakage current may be provided between the line inputs  12  by using the circuit of  FIG. 4  and replacing the single MOV  48  in each of the interconnections  46   a - c  with a series connected MOV  48  and GDT  50 . Again, the order of the GDT  50  and the MOV  48  in the series connection is arbitrary. 
         [0048]    It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein, but 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.