Abstract:
A contactor may operate to interrupt current in a circuit while the circuit is operating under load. A shunt is provided to by-pass surge power current around contacts to reduce arcing. The shunt includes a solid-state switch that may be operated in a series of pulses during movement of the contacts. The pulse control unit may detect a potential for arcing and then provide for periodic pulsing operation of the shunt. Because the solid-state switch may operate discontinuously, the contactor may be constructed with a switch that is selected on a basis of its pulse rating.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention is in the field of electrical switches, and more particularly, contactors for high-power direct current (DC) circuits. 
         [0002]    In certain circumstances there is a need to interrupt current in a DC circuit while the circuit is carrying a high current (e.g. 50 to 200 amps). These circumstances may arise, for example, when an electrical load on the circuit becomes excessive or when a short-circuit fault develops. In order to accommodate such eventualities, high-current DC circuits may incorporate heavy-duty contactors. 
         [0003]    Rapid interruption of current may produce an induced surge of energy. This energy may produce arcing in a contactor. Some heavy-duty contactors may be constructed so that this arcing may be tolerated. Other prior-art contactors may be constructed so that such arcing is reduced. 
         [0004]    In some prior-art contactors, a gas-tight or liquid-tight enclosure may be provided for the contactor or its contact elements. A gas or liquid may surround the contact elements and prevent oxidation of the elements when arcing occurs. In other prior-art contactors, selected arc-tolerant metallic alloys may be used for contact elements. 
         [0005]    Some prior-art contactors may be provided with an electrical shunt that may by-pass an energy surge around the contact elements. Such a shunt may comprise a high-power field-effect transistor (FET) or similar device. The FET must be able to tolerate a high-current surge without damage. For example, a shunt or by-pass rated at about 1500 amps may be needed for a contactor rated at 150 amps that may be required to open with a “short circuit” condition. 
         [0006]    Prior-art high-power contactors with protected contact elements or with by-pass shunts are expensive, heavy and complex. These characteristics of prior-art contactors are of particular concern to aircraft designers. Aircraft designs are evolving in a direction that is often referred to as “more electric architecture” (MEA) design. In new MEA designs various operational functions which were formerly performed with hydraulic and pneumatic systems are now performed electrically. These electrical operations are often performed with high amperage DC motors and controls. In this context, MEA designs may incorporate an increasing number of contactors which may interrupt high-amperage DC. MEA designs could be improved if high-power contactors could be made lighter, less expensive and more reliable than prior-art contactors. 
         [0007]    As can be seen, there is a need to provide improved contactors which are capable of interrupting high amperage DC. Additionally, there is a need to provide such contactor with low weight so that they may be effectively employed in aircraft. 
       SUMMARY OF THE INVENTION 
       [0008]    In one aspect of the present invention, an apparatus for interrupting current in a circuit comprises contacts through which the current passes. The contacts move away from one another during current interruption. A shunt is provided to by-pass surge power around the contacts when current is interrupted. The shunt is operative during a portion of time period that the contacts move and the shunt is inoperative during a portion of said time period. 
         [0009]    In another aspect of the present invention, an electrical power circuit comprises a contactor with movable contacts, an electrical shunt to by-pass current around the contacts, and a pulse control unit to periodically operate the shunt during movement of the contacts. 
         [0010]    In still another aspect of the present invention, a method for interrupting current in a circuit under load conditions comprises the steps of moving conducting contacts away from one another for a predetermined time period, detecting electrical power at the contacts during the step of moving the contacts, determining if the detected power is sufficient to initiate arcing at the contacts, operating an electrical shunt around the contacts for a portion of the predetermined time period if the detected power is sufficient for arcing initiation, and disabling the electrical shunt for a portion of the time period. 
         [0011]    These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is graphical representation of an arc initiation relationship in accordance with the invention; 
           [0013]      FIG. 2  is a schematic diagram of a contactor in accordance with the invention; 
           [0014]      FIG. 3  is a symbolic graphical representation of operational aspects of a contactor in accordance with the invention; 
           [0015]      FIG. 4  is a block diagram of a current interruption system in accordance with the invention; and 
           [0016]      FIG. 5  is a flow chart of a method of performing current interruption in accordance with the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0017]    The following detailed description is of the best currently contemplated modes of carrying out the invention. The description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating the general principles of the invention, since the scope of the invention is best defined by the appended claims. 
         [0018]    Broadly, the present invention may be useful for interrupting high-amperage current in a circuit. More particularly, the present invention may provide light-weight shunted contactors to perform such interruption. The present invention may be particularly useful in vehicles such as aircraft. 
         [0019]    In contrast to prior-art contactors, among other things, the present invention may provide a pulse-rated shunt for a contactor. The present invention, instead of employing a prior-art steady-state rated shunt for a contactor, may, utilize a lower-rated shunt. The lower-rated shunts may be operated in a series of conducting pulses to reduce or preclude arcing in a contactor. By avoiding continuous conduction of current through the shunt, a smaller, lower-rated shunt (e.g. an FET) may be used to protect a contactor from arcing damage. 
         [0020]    Referring now to  FIGS. 1 and 2 , a series of graph lines show various combinations of surge voltage Vs and surge current Is that may initiate arcing between contacts  10  and  12  of a contactor  14  during interruption of current being provided to an electrical load. High surge voltages and currents may arise in conductors  16  and  18  during such an interruption. A graph line  100  may represent an arc-initiation relationship between surge voltage Vs and surge current Is when the contacts  10  and  12  are separated by a first distance (e.g. 0.5 millimeters [mm]). A graph line  102  may represent an arc-initiation relationship between Vs and Is when the contacts  10  and  12  are separated by a second distance (e.g. 1.0 mm). In other words, to use graph line  100  as an example, at a contact spacing of 0.5 mm, an arc may not develop if the surge voltage is less than a Vs (min) or if the surge current is less than Is (min). Furthermore an arc may not develop at any combination of Vs and Is that is below the graph line  100 . The graph line  100  may be considered to represent a surge-power limit curve, i.e., a plot of a Vs*Is. It may represent the concept that if surge power remains below the graph line  100 , then an arc may not initiate at 0.5 mm spacing between the contacts  10  and  12 . 
         [0021]    It may be seen that as spacing between the contacts  10  and  12  increases, a combination of Vs and Is must become larger in order for an arc to initiate. Graph lines  102 ,  104 ,  106  and  108  may illustrate this concept. Graph line  102  represents a surge-power limit curve for contact spacing of 1.0 mm. Graph lines  104 ,  106  and  108  may represent surge-power limit curves for contact spacings of 1.5 mm, 1.8 mm and 3.0 mm respectively. 
         [0022]    Referring now to  FIG. 3 , a graph  300  may symbolically illustrate how arc initiation may be delayed or entirely precluded in accordance with the invention. The graph  300  may represent surge power on a vertical axis  302 . Spacing between the contacts  10  and  12  of  FIG. 2  may be represented on a horizontal axis  304 . When the contactor  14  of  FIG. 2  interrupts current, the contacts  10  and  12  may move away from one another during a brief but finite time period (e.g., about 1 to 2 milliseconds [msec]). Thus, the axis  304  may also represent time. 
         [0023]    A sloped line  306  may represent a compilation of the surge-power curves of  FIG. 1  plotted against time. In other words, the line  306  may represent a surge power boundary below which arcing may not initiate between the contacts  10  and  12 . As the contacts  10  and  12  move further and further apart, increasing amounts of power may pass between the contacts  10  and  12  without initiation of arcing. 
         [0024]    Referring now to  FIGS. 2 and 3  a novel application of a shunt in accordance with the present invention may be understood. The contactor  14  may be provided with a shunt  22  interconnected so that current may by-pass the contacts  10  and  12 . The shunt  22  may comprise a solid-state switch  24  such as a field effect transistor (FET) or any of a number of conventional solid-state switching devices. The shunt  22  may operate responsively to surge power that may develop during current interruption. When surge power exceeds a predetermined limit, the switch  24  may close and allow current to by-pass the contacts  10  and  12 . An apparatus and method for producing selective operation of the shunt  22  is described hereinbelow with reference to  FIGS. 4 and 5 . 
         [0025]    In  FIG. 3 , a graph line  308  may represent surge power as a function of time. It may illustrate dynamic conditions that could arise when the contacts  10  and  12  are moved away from one another while current is being supplied to the load  15 . Surge power may begin developing and increasing as soon as the contacts  10  and  12  no longer touch one another (time T 0 ). At a time T 1  the surge power may have increased to a level at which the surge power may exceed the surge power boundary  306 . Under this condition an arc could initiate between the contacts  10  and  12 . But, if the switch  24  is closed at or before time T 1 , then surge power may be shunted away from the contacts  10  and  12  and the surge power at the contacts may be diminished. In the event of such shunting, the surge power at the contacts  10  and  12  may be represented by a graph line  310 . 
         [0026]    If shunting were not to occur at or before time T 1 , surge power at the contacts  10  and  12  could continue to increase in accordance with the graph line  308 . In such a case, arcing could initiate and continue until surge power is dissipated, i.e., until a time T 2  on the graph  300 . 
         [0027]    If shunting occurs at or before time T 1 , overall surge power may continue to increase as a function of time but there may be a reduced amount of the surge power at the contacts  10  and  12 . The graph line  310  may represent a portion of the surge power at the contacts  10  and  12 , i.e., a “contact portion”. A graph line  312  may represent a “shunt portion” of surge power as a function of time. 
         [0028]    The shunt portion line  312  may have a pulsed configuration. This configuration may be associated with a novel operation of the shunt switch  24  in accordance with the invention. The switch  24  may be closed at or before the time T 1 . At that time the surge power may pass through the switch  24 . At a later time, T 12 , the switch  24  may open and surge power may once again be applied to the contacts  10  and  12 . An exemplary time period between T 1  and T 12  may be about 5 to 10 microseconds (μsec). The contact portion of surge power at time T 12  may be greater than the contact portion at time T 1 , but the contacts  10  and  12  may be further apart at the later time T 12 . If the contact portion of surge power remains below the surge power boundary (line  306 ) after time T 12 , then arcing may not initiate. 
         [0029]    Surge power may continue rising after time T 12 . If such rising were left to proceed, the contact portion of surge power may exceed the surge power boundary  306  at a later time T 16 . But, at or before the time T 16  (e.g., at a time T 14 ), the switch  24  may again close. Surge power may once again by-pass the contacts  10  and  12 . Consequently the surge power boundary  106  may not be crossed by the contact portion of surge power and arcing may not initiate. 
         [0030]    A similar sequence of events may occur at a time T 18  when the switch  24  may again open. At the time T 18 , contact surge power may begin to rise at a rate that may result in the contact portion of surge power crossing the surge power boundary at a later time T 22 . Such a crossing may be precluded if the switch  24  were to close at or before the time T 22  (e.g., at a time T 20 ). 
         [0031]    The time period between T 1  and T 12  may be considered a pulse period  314  for the switch  24 . Similarly a time period between T 14  and T 16  may be considered a pulse period  314  for the switch  24 . A series of similar pulse periods  314  may develop during a surge period  316 , i.e., a period of time between T 0  and T 2  required for dissipation of the surge power. For purposes of simplicity, only a few of the switch pulse periods  314  are shown symbolically in  FIG. 4 . It may be noted that if the surge period  316  extends for an exemplary 1 msec to 2 msec., then up to about twenty of the 5 μsec to 10 μsec switch pulses  314  may be produced in that time period. 
         [0032]    In a pulsed mode of operation, the switch  24  may conduct current during a fractional part of the surge period  316 . Pulsed operation of the switch  24  may allow for use of a solid-state switch (e.g. a FET) with a lower current rating lower than a FET that may be required to continuously conduct current throughout the surge period  316 . For example, in the prior-art, a FET with a nominal rating of 1500 amps may be required to continuously shunt all of the surge power for an exemplary 150 amp circuit. But, in the case of the present invention, an exemplary FET may be used with a “pulse-rating” of 1500 amps. Pulse rating for an FET may be about 2.5 times as great as its nominal rating. Thus, a FET with a nominal rating of 600 amps (1500 amps/2.5) may be used to provide arc suppression for a contactor in the exemplary 150 amp circuit. In other words, the switch  24  of the present invention may have a nominal rating that is at least 50% lower than a nominal rating of a prior-art shunt switch. 
         [0033]    An FET with a nominal rating of 600 amps may be smaller, lighter and less expensive than a FET with a nominal rating of 1500 amps. It may be seen therefore that when contactors are constructed and operated in accordance with the present invention, the contactors may be smaller, lighter and less costly than their prior-art counterparts. 
         [0034]    Referring now to  FIG. 4  a block diagram may illustrate how the contactor  14  may be constructed and operated in accordance with the invention. A pulse control unit  400  may provide switching signals  402  to the solid-state switch  24 . The pulse control unit  400  may produce the switching signals  402  responsively to voltage and current information from the contactor  14 . In particular a voltage signal V 1 , indicative of voltage in the conductor  16  may be provided to the pulse control unit  400 . A second voltage signal V 2  indicative of current in the conductor  18  may also be provided to the pulsing circuit  400 . 
         [0035]    The V 1  and V 2  signals may be provided to the pulse control unit  400  through a conventional signal conditioning and protection block  404 . The pulse control unit  400  may comprise an analog to digital (A/D) converter  406 , a multiplier  407  and an arcing-condition determination block  408 . The block  408  may analyze a digital representation of the V 1  and V 2  signals against a clock signal (not shown) to determine if their combined power may initiate arcing between the contacts  10  and  12 . The block  408  may perform its analysis repetitively at an exemplary sampling rate of about 0.1 μsec. In the event that arcing potential is determined by the block  408 , a driver  410  may be activated to close the solid-state switch  24 . This may shunt surge power through the switch  24 . If current through the switch  24  increases beyond a predetermined level, an over-current block  412  may produce a signal  412 - 1  to an OR gate  414 . An over-on-time block  416  may determine a length of time that the switch  24  is closed or “on”. This on-time may be compared against a predetermined time (e.g., a switch pulse period of 5 to 10 μsec.). An over-on-time signal  416 - 1  may be provided to the OR gate  414  after the predetermined amount of on-time for the switch  24 . If either of the signals  412 - 1  or  416 - 1  are received by the OR gate  414 , a switch-opening signal  414 - 1  may be provided to the driver  410  and the switch  24  may be directed to open. A shunt of current of a desired magnitude and time duration may thus be produced based on the predetermined level of current that may be established in the block  412  and the predetermined time that may be established in the block  416 . 
         [0036]    Effectiveness of the present invention may be dependent on a proper selection of shunt pulse time. In an exemplary case of a surge period of about 1 msec. it has been found that a shunt pulse period of about 5 μsec may be effective in reducing or even eliminating arcing. One of the contactors  14  may experience some brief arcing (less than 5 μsec in duration) or none at all when the shunt  18  is operated with 5 μsec pulses. 
         [0037]    However, it has also been found that a shunt pulse period of about 1 μsec may not effective in reducing or precluding arcing. When, in the same exemplary case, the shunt  18  is operated with pulses of about 1 μsec, an arc may initiate and may continue for about 900 μsec. Thus there appears to be a lower limit for effective shunt pulse time and that lower limit is about 1 μsec. 
         [0038]    There may also be an upper limit for effective shunt pulse time in the context of the present invention. The present invention allows for shunting with a solid-state switch employed at its pulse rating. As described in an earlier example, a switch with a pulse rating of 1500 may be much smaller and lighter than a switch with a continuous conduction rating of 1500 amps. In order to safely use the smaller and lighter switch, it must be allowed to conduct for only brief periods, i.e., pulses. If the pulses are too long or are too closely spaced in time, the smaller and lighter switch may no longer perform safely. It has been found that a cumulative elapsed time of all shunt pulses in a single current interruption should not exceed 50% of the surge period. Furthermore, it has been found that no single one of the shunt pulses should exceed 20% of the surge period. In the exemplary case under consideration these principles suggest that a shunt pulse should not exceed 20 μsec. 
         [0039]    In one embodiment of the present invention, a method may be provided for interrupting current in a circuit under load conditions. Such a method  500  may be illustrated in flow-chart format in  FIG. 5 . 
         [0040]    In a step  502 , voltage may be continuously detected at current-interruption contacts (e.g., the voltage V 1  may be detected at the contact  10  of the contactor  14 ). In a step  504 , a voltage signal may be produced which is indicative of current at the contacts (e.g., a voltage drop V 2  across a resistor may be indicative of current in the conductor  18  as well as current at the contact  12  of the contactor  14 ). 
         [0041]    In step  506  the voltages of steps  502  and  504  may be periodically sampled (e.g., by the arcing-condition determination block  408 ). In a step  508  a combination of the voltages of steps  502  and  504  may be analyzed to determine if sufficient power is present at the contacts to initiate arcing (e.g. the block  408  may perform an analysis of V 1  and V 2  and make a time-related comparison to determine if surge power is high enough to initiate arcing). In the event that arcing potential is determined to exist, a step  510  may be initiated in which shunting of current around the contacts may be performed for a predetermined time (e.g., the solid-state switch  24  may be closed responsively to a signal  414 - 1  from the driver  414 ). In a step  512 , the shunt may be opened (e.g., the switch  24  may open in response to signal  414 - 1  from the driver  414 , which may act responsively to signals  412 - 1  or  416 - 1 ). 
         [0042]    After step  512  may be completed, the step  508  may be re-initiated to determine in arcing potential may exist. If arcing potential is determined to exist, step  510  and  512  may be re-initiated. When and if performance of step  508  may determine that arcing potential does not exist, step  510  may not be initiated. 
         [0043]    It should be understood, of course, that the foregoing relates to exemplary embodiments of the invention and that modifications may be made without departing from the spirit and scope of the invention as set forth in the following claims.