Abstract:
A device is provided for riding through brownouts, or transitory losses of control power applied to electromagnetic devices such as contactors and relays. An energy storage circuit stores charge that can be discharged into a contactor operating coil during partial or full losses of control power and for short durations. Momentary reductions in control power can thus be experienced without opening or otherwise changing the logical state of the contactor. Emergency stop override circuitry may be provided for preventing the ride-through performance, thus enabling the device to provide for both ride-through of short losses of control power, while permitting emergency stopping of loads due to the override.

Description:
BACKGROUND  
       [0001]     The invention relates generally to the field of electrical switch gear used to supply energizing power to a load. More particularly, the invention relates to a technique for maintaining power applied to such loads during transient losses of power from a grid or other power supply.  
         [0002]     A wide range of applications exist for powered loads, particularly in industry, but also in mining applications, material handling applications, marine, commercial, and other fields. In most of these applications power from a source is switched by various switch gear, typically contactors, to apply power to the driven load. The contactors, which may be sometimes referred to as relays of various types, will generally include an electromagnetic operator and a set of moveable contacts which can be moved by energizing the operator. When a control voltage is applied to the operator coil, which essentially forms an electromagnet, an armature is moved by the magnetic flux produced by the coil to open or close the contacts in the device. Opening and closing contacts thus establishes or interrupts a current carrying path through the device to control application of power to the load.  
         [0003]     In many applications, the provision of control power is subject to fluctuations in power from the source. That is, relatively small or intermittent loss of power is possible due to fluctuations in the grid, other loads that are switched on and that share the same power source circuitry, and so forth. Certain areas and power networks tend to suffer from such short losses in power on a fairly regular basis. Depending upon the nature of the load and process to which the load is applied, such transient losses of power, which would may be termed “brownouts” can be extremely problematic. For example, driven loads can be caused to stop or start, or circuitry may need to be reset following such common brownouts. The questionable reliability of certain grids and power sources in many locations makes the consistent application of power to loads via contactors and relays particularly problematic, to the point of jeopardizing the ability to reliably drive the process at all.  
         [0004]     There is a need, therefore, for techniques that will allow for switch gear to survive short losses in power and maintain power either applied to a load or not applied to a load, depending upon the circuit configuration. There is a particular need for arrangements which will provide for such brownout ride-through while allowing the systems to work with other common circuit configurations, particularly emergency stops that override the ride-through function.  
       BRIEF DESCRIPTION  
       [0005]     The invention provides a novel approach to brownout ride-through devices designed to respond to such needs. The invention may be applied in a wide range of settings, and generally allows for energization of an electromagnetic operator to continue despite short losses in control power. The circuitry may be designed to provide for various degrees of brownout ride-through, such as for fractions of a second to several seconds, and from partial to complete loss of control power. The circuitry may also be designed to permit rapid operation (e.g., energization) of an operator coil despite the presence of energy storage circuitry that is used to power the coil during a transient loss of power. Moreover, the circuitry is specifically designed to accommodate emergency stop equipment and the like typically found in certain industrial settings, while still providing the brownout ride-through function when no emergency stop signal is received or where power has not been interrupted to the entire circuit by an emergency stop. 
     
    
     DRAWINGS  
       [0006]     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:  
         [0007]      FIG. 1  is a diagrammatical overview of a brownout ride-through circuit including an emergency stop override circuit, and other features in accordance with the present invention;  
         [0008]      FIG. 2  is a graphical representation of a voltage versus time performance of the circuitry of  FIG. 1  in the event of a brownout; and  
         [0009]      FIG. 3  is a graphical representation of a current versus time performance of the circuitry of  FIG. 1  during a temporary loss of power as diagrammed in  FIG. 2 . 
     
    
     DETAILED DESCRIPTION  
       [0010]     Turning now to the drawings, and referring first to  FIG. 1 , a brownout ride-through circuit, designated generally by reference numeral  10 , is shown for controlling power applied to a load via a contactor  12 . Although reference is made throughout the present discussion to a “contactor” the present invention is not intended to be limited to any particular design of such switch gear. Accordingly, as will be appreciated by those skilled in the art, a contactor is generally a powered electromechanical device that opens and closes a current carrying path in response to a powered control signal. Such devices may be variously referred to as relays, circuit interrupters, and so forth. In general, the invention is directed to controlling or maintaining a desired state of such devices throughout a period of partial or complete loss of control signal power.  
         [0011]     As illustrated in  FIG. 1 , the contactor  12  is disposed in a circuit path between a line power source  14  and a load  16 . As will be appreciated by those skilled in the art, many other circuit components may typically be included in the arrangement, but have not been shown in  FIG. 1  for the sake of clarity. For example, various fused and manual disconnects may be provided upstream of the contactor  12 ; signal conditioning circuitry may be included; safety relays may be coupled to the contactor, and so forth. Moreover, the line power source  14  may simply be a power grid, but may also include various power converters, inverters, and so forth. Similarly, the load  16  may include any appropriate load, such as electric motors, valve actuators, or any other suitable load in the overall machine process or system.  
         [0012]     Within the contactor  12  a moveable conductive element which may be referred to as a spanner or bridge  20  is displaced to bring moveable contacts  22  into contact with stationary contacts  24 . The line power source  14  and the load are coupled to these stationary contacts, such that the bridge  20  and moveable contacts  22  complete a current carrying path through the device when these elements are brought into contact with one another. It should be noted that other contactor designs are known and may be used with the present invention, including devices that have single set of moveable contacts, as well as devices that have multiple sets, such as for three-phase operations.  
         [0013]     The moveable contacts are generally held or biased in a default position, in this case an open position, by a biasing device, such as a spring  26 . An armature  28  is coupled to the moveable contacts and contact bridge and is attracted by magnetic flux produced by an operator coil  18  when the operator coil  18  receives and energizing voltage. As will be appreciated by those skilled in the art, the operator coil  18 , with associated core or armature structures (not shown) forms an electromagnet that produces flux by virtue of a circulating current from the control power source  30 . The circulating current produces a Lorentz force that attracts the armature and displaces the moveable contacts toward the stationary contacts against the biasing force of the spring  26  when the control power is applied. As will be readily appreciated by those skilled in the art, the entire structure of the contactor may be wired to operate as a normally-open or normally-closed device, depending upon the exigencies of the application (e.g., whether the load should be normally energized or normally unpowered).  
         [0014]     The brownout ride-through circuit  10  is designed to retain and store a charge from the control power source  30  and to provide power to coil  18  in the event of a partial or a complete loss of power from the source for a short duration. As illustrated, the circuit  10  includes an energy storage circuit  32  which will typically include a capacitor or a bank of capacitors designed to store the necessary charge. A charge control circuit  34  is coupled upstream of the energy storage circuit  32  and regulates the charging of the capacitors. That is, to permit rapid energization of the coil  18 , it is preferred that the energy storage circuit  32  be charged more slowly over time so as to permit most or all of the current initially applied to the operator coil to produce flux for operation of the contactor. Once energized, and with the contactor in its operative state, the charge control circuit  34  then begins to charge the energy storage circuit  32  until the desired charge is reached.  
         [0015]     To permit emergency shutoff of the power to load  16 , and to any other desired loads in an installation, the brownout ride-through circuit  10  includes an E-stop override circuit  36 . The E-stop override circuit  36  is coupled to an E-stop input  38 . As will be appreciated by those skilled in the art, the E-stop input  38  may be any one of a range of typically manual override inputs, such as a conspicuous push button which can be depressed to stop loads in a manufacturing or other process. As will also be appreciated by those skilled in the art, the E-stop input will typically cause an actual interruption in power throughout an application, so as to prevent damage to equipment, work pieces, personnel and so forth. The E-stop input  38  is thus able to force interruption of brownout ride-through power from the energy storage circuit  32  to the coil  18 . Thus, when an E-stop signal is received, despite the ability of the energy storage circuit  32  to provide energizing power to the operator coil  18 , such power will be completely interrupted by the E-stop override circuit  36 .  
         [0016]     Finally, the brownout ride-through circuit  10  also includes a bleed off circuit  40  coupled to the energy storage circuit  32 . In practice, the bleed off circuit  40  may be part of a charge control circuit  34 , and may be powered, such as by the control power source  30 . The bleed off circuit  40  effectively drains the charge from the energy storage circuit  32  when control power is purposefully removed from the operator coil  18 . That is, the control power source  30  may provide a signal to the bleed off circuit  40  which causes the draining of the energy storage circuit  32  to avoid the discharge of the energy storage circuit  32  to any other device during down time of the contactor and load.  
         [0017]     As noted above, the foregoing functional components permit the contactor to remain energized during transitory periods of partial or full interruption of control power to the contactor coil.  FIGS. 2 and 3  graphically illustrate the voltage and current levels that may be experienced by the operator coil during such operation. In particular,  FIG. 2  generally illustrates the voltage performance or curve  42  that will be seen by the coil. As noted in  FIG. 2 , the voltage prior to a brownout event will generally be a constant line voltage  44 . At the onset of a brownout event, indicated generally by time line  46 , the voltage will drop, or be completely or partially interrupted. The end of the brownout event is indicated generally by the time line  48 . Between these two times, the voltage of the control power source may droop or be lost completely. The duration  50  of the brownout event is the time required for charge current to be supplied to the coil by the energy storage circuit.  
         [0018]     In general, in the event of a loss of power, a drop in voltage will be expected, such as indicated at reference numeral  52  in the absence of the energy storage circuit  32 . That is, line  52  represents the actual line voltage during the brownout event. In accordance with one presently contemplated embodiment, a voltage drop  54  corresponding to the lowest level experienced for the line voltage  52  will be on the order of 40% of the rated line voltage  44 . In a further embodiment, a deeper droop  56  and line voltage can be experienced and ridden through, corresponding to a voltage drop of 50% as indicated by reference numeral  58  in  FIG. 2 . Moreover, in presently contemplated embodiments, the duration  50  of the brownout event that can be experienced and ridden through by the foregoing circuitry is on the order of 200 ms. However, other designs are presently contemplated that can experience and thrive through voltage drops of a duration of up to two seconds, and with full loss of line voltage.  
         [0019]     As will be appreciated by those skilled in the art, and as described in greater detail below, the energy storage circuit  32  discussed above will be required to store sufficient charge to provide energizing current to the coil to support the voltage in event of a brownout depending upon the size of the coil, the flux required to maintain the moveable armature in the energized position, the resistance of the conductors between the energy storage circuit and the operator coil, and so forth.  
         [0020]      FIG. 3  represents a typical current performance trace  60  that might be anticipated during a loss of control power through the use of the foregoing circuitry. The droop in current in the trace  60  from a full or steady state current level  62  is illustrated between the same onset and termination times for the brownout event discussed above with respect to  FIG. 2 . In the absence of the energy storage circuit  32 , then, a droop would be anticipated in the current applied to coil  18  throughout the brownout period. One would anticipate that some support for the current would be experienced by the breakdown of the field within coil  18 , but ultimately the current would drop precipitously throughout the brownout period, depending upon the severity of the loss of power. The anticipated current loss may be represented by trace  64  in  FIG. 3 , while the current droop experienced by the coil by virtue of the discharge of energy storage circuit  32  is represented by segment  66 . As can be seen from  FIGS. 2 and 3 , the discharge of current from the energy storage circuit  32  to the coil  18  effectively supports the voltage, as indicated by segment  68  in  FIG. 2  to levels above those that would be experienced if the present brownout ride-through circuitry were not coupled to the coil.  
         [0021]     As will be appreciated by those skilled in the art, the charge required to be made up by the energy storage circuit  32  may be represented as the area between trace segment  66  and trace  64  in  FIG. 3 . That is, the charge is equal to the product of the current provided by the energy storage circuit  32  to the coil times the duration of time for which the current is provided. The selection of the particular capacitors or other components of the energy storage circuit  32 , then, is made based upon the amount of charge required to be provided.  
         [0022]     As will also be appreciated by those skilled in the art, and as illustrated in  FIG. 3 , in a typical contactor or relay, the force required to maintain the moveable armature in the energized position is balanced by (and must exceed) the force of the biasing spring against the Lorenz force exerted by the energized coil. So long as the biasing force of the spring is overcome by the magnetic forces, the contactor will resist changing its logical state. This force generally will correspond to some minimum current level as indicated by reference line  72  in  FIG. 3 . With this in mind, the energy storage circuit  32  effectively provides a current sufficient to maintain this force balance throughout the desired period. As shown in  FIG. 3 , at some relatively short time following a loss of control power, it would be anticipated that the contactor would open by virtue of a drop in current, as indicated by trace  64  below the minimum current required to maintain the contactor energized (i.e., the intersection between line  72  and trace  64 ). The brownout ride-through circuitry described above, then, effectively extends the period through which the device may remain energized by maintaining the current above the threshold current  72 .  
         [0023]     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.