Abstract:
A method and device for immunizing a contactor circuit from the inductive effects of a holding coil derived power supply, including: 
     sensing a voltage signal from a power supply, the voltage signal having a switching range comprising an upper voltage and a lower voltage; 
     providing a drive output responsive to the upper voltage; holding the drive output at a constant level as the voltage signal remains above the lower voltage; and, disabling the drive output in response to the voltage signal droppings below the lower voltage.

Description:
TECHNICAL FIELD 
     The present invention generally relates to contactor circuits. More specifically, to sensing a power supply derived from the contactor&#39;s holding coil and immunizing the contactor&#39;s circuitry from the adverse effects of transient induction inherent within the contactor circuit. 
     BACKGROUND OF THE INVENTION 
     There is a constant desire to increase speed, decrease energy consumption and decrease the amount of physical space required in electrical circuits. Circuit designers are continually redesigning circuits to be faster, smaller and more energy efficient. 
     One goal circuit designers frequently focus upon is reducing the amount of physical space required for a circuit. Various factors influencing the circuit&#39;s design include: power requirements, electric noise, component temperature, timing parameters, etc. Designers frequently utilize a variety of techniques to achieve their design goals. One such circuit design technique is to derive a power supply from the holding coil of a contactor. However, due to the physical structure of the contactor, transient inductive forces generated through normal use of the contactor may adversely affect the performance of the contactor&#39;s circuitry coupled to the derived power supply. For example, contactors or relays that utilize dc coils may have electronic circuits which are used to control the device. The contactor&#39;s electromechanical coil has resistance and this resistance can be used as a dropping resistor to derive a power supply for the electronics. When the coil resistance is used for the power supply, the power supply is exposed to transient forces, dL/dt, generated by the movement of the contactor&#39;s armature. 
     When an armature moves from an open to a closed position, the movement causes the inductance of the magnetic circuit to increase as the air gap between the magnet and the armature decreases. The dL/dt generates a back electromotive force (EMF) that causes the current in the coil to decrease. Once the armature seals with the magnet, the inductance becomes fixed and the dL/dt decreases to 0. The current in the coil recovers to its initial level. If the contactor control circuit&#39;s power supply is derived from the contactor holding coil, the derived power supplied to the contactor control circuit will decrease, or dip, with the contactor&#39;s holding coil current. The effect of the transient inductance generated by the derived power supply may turn off the controller circuit when the holding coil current of the contactor decreases, thus causing nuisance tripping. 
     It is now apparent that the effects of dL/dt cannot be avoided on this type of power supply. If the electronics of the contactor circuit are designed to be tolerant of a power supply decrease due to dL/dt, a power supply derived from the coil resistance can be utilized. To overcome the decreased coil current caused by the inductance created during the closing of the armature, larger filtering capacitors were utilized. However, these larger capacitors occupy valuable physical space. Another filtering technique implemented to combat the. transient inductive effects is to use electrolytic capacitors. However, electrolytic capacitors typically have a shorter life expectancy than the contactor circuit. 
     Prior to the present invention, a need existed to provide a power supply status circuit connected to a contactor circuit that monitors the power supply derived from the holding coil of the contactor. Also, a need existed for maintaining the output of the power supply status circuit while tolerating variations of the derived power supply caused by the inherent transient inductive effect of the contactor&#39;s physical and electrical structure. 
     This invention is designed to resolve these and other problems. 
     SUMMARY OF THE INVENTION 
     A power supply status (PSS) circuit is capable of monitoring a derived source of power from a contactor circuit while maintaining the PSS circuit&#39;s output. The PSS circuit is capable of tolerating variations in the derived power supply caused by the inherent, transient inductive effects of the contactor&#39;s physical and electrical structure. 
     According to the present invention, a robust PSS circuit has been developed with specific useful features for utilizing power derived from the holding coil of a contactor. As a result, use of the derived power eliminates the need for a separate and additional power supply. Also, the PSS circuit reduces the physical space previously required for filtering components necessary to utilize the derived power supply. In addition, the PSS circuit allows the use of longer lasting filtering components. 
     The first embodiment of the present invention is directed to a method of immunizing a contactor circuit from the inductive effects of a derived power supply, including: sensing a voltage signal from a power supply, the voltage signal having a switching range comprising an upper voltage and a lower voltage; providing a drive output responsive to the upper voltage; holding the drive output at a constant level as the voltage signal remains above the lower voltage; and, disabling the drive output in response to the voltage signal dropping below the lower voltage. 
     According to a second embodiment, the invention is directed to a device for immunizing a contactor circuit from the inductive effects of a derived power supply. The device comprising the contactor circuit having an input that receives a voltage signal. The voltage signal having a switching range comprising an upper voltage and a lower voltage. The contactor circuit providing a drive output in response to the voltage signal. The drive output being held at a constant level as the voltage signal remains above the lower voltage; and the drive output being disabled in response to the voltage signal dropping below the lower voltage. 
    
    
     Other advantages and aspects of the present invention will become apparent upon reading the following description of the drawings and detailed description of the invention. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a contactor circuit; 
     FIG. 2 is a schematic diagram of a contactor circuit; 
     FIG. 3 is a side view of a contactor magnet and armature, the arrows show directional movement of the armature; 
     FIG. 4 is a signal trace showing the pickup current when the contactor is operated; 
     FIG. 5 is a signal trace showing the pickup current when diode D 2  is blocking; 
     FIG. 6 is a schematic diagram showing the coupling between the pickup coil and the holding coil; 
     FIG. 7 is a schematic diagram of the preferred embodiment of the invention; 
     FIG. 8 is a schematic diagram of the preferred embodiment of the power supply status circuit for a contactor; 
     FIG. 9 is a schematic diagram of a timer circuit for a contactor; and, 
     FIG. 10 is a schematic diagram of a gating circuit for a contactor. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     While this invention is susceptible of embodiments in many different forms, there is shown in the drawings and will herein be described in detail preferred embodiments of the invention. The present disclosure is to be considered as an exemplification of the principles of the invention and is not intended to limit the broad aspect of the invention to the embodiments illustrated. 
     An electronic coil design utilizes a dual wound DC coil that pulls an armature  32  into the picked up or closed position, and holds the armature  32  in the closed position. The first coil is called the pickup  28 , or inrush coil. It is a low impedance, high current coil that generates the large amount of flux, NI, necessary to pull the armature  32  in from the dropped out or open position. The other coil is the holding coil  30 . The holding coil  30  is a high impedance, low current coil that generates the smaller amount of flux, NI, necessary to hold the armature  32  in the closed position. The pickup coil  28  should not be allowed to remain continuously on after the armature  32  has been pulled in. The large amount of wattage that the pickup coil  28  generates could thermally damage the coil assembly if it were to remain on. It is the primary purpose of the electronic circuits to turn the pickup coil  28  on until the armature  32  is fully pulled in, and then turn the pickup coil  28  off. The pickup  28  and holding  30  coils have distinct and separate responsibilities; however, the two coils are magnetically coupled by an iron core (armature and magnet) that they share. In some aspects this causes the two coils to act like a transformer. 
     Contactors or relays that utilize dc coils typically have electronic circuits that are used to control the contactor. Typically, a contactor circuit  10  may contain many components and contactor control sub-circuits, such as: a power supply  38 , a timing circuit  14 , a gating circuit  16 , a power supply status (PSS) circuit  20 , an insulated gate bipolar transistor (IGBT)  22  and metal oxide varistors (MOV)  24 ,  26 . 
     The contactor circuit  10 , specifically the pickup coil  28 , is responsive to the output of the operably connected sub-circuits. As shown in FIG. 1, the sub-circuits and components are operably connected. The gating circuit  16  is responsive to the output of the PSS circuit  20  and the timer circuit  14 . FIG.  10 . If both outputs of the PSS circuit  20  and the timer circuit  14  are high, the output of the gating circuit&#39;s op-amp  48  will be high and the IGBT  22  will be turned on. If either of the outputs from the PSS circuit  20  or the timer circuit  14  is low, the output of the gating circuit&#39;s op-amp  48  will be low and the IGBT  22  will be turned off. 
     The gating circuit  16  controls the operability of the contactor via the electromechanical coils and movement of the armature  32 . As the timer circuit  14  output goes low, the gating circuit=s op-amp  48  is switched off. The contactor&#39;s timer circuit  14  is derived from a RC timing network (two resistors R 13 , R 14  and capacitor C 2 ), and a single op-amp  46  being used as a comparator. FIG.  9 . The capacitor C 2  begins charging through resistor R 13  as soon as the PSS circuit&#39;s output goes high. Resistor R 14  affects the time it takes the capacitor C 2  to charge as well as providing anti-telegraphing. As power is removed from the circuit  14 , approximately 500 milliseconds elapse before sufficient charge drains off capacitor C 2  to reset the timer circuit  14  for another operation. This protects the devices from conditions such as fluttering switches. The output of the timer circuit  14  remains high until the voltage across capacitor C 2  exceeds the voltage at the non-inverting input of the op-amp  46 . At this time, the output of op-amp  46  pulls down low. 
     The armature  32 , magnet  40  and air gap form a magnetic circuit. An electromechanical coil (not shown), supplies NI, or magnetomotive force (MMF), needed to drive the flux in the circuit. The equations below illustrate how the inductance of the circuit is determined by the number of turns of the coil and the reluctance.          L   =       N   2     R       ;                          
     where N=number of turns and R=       R   =         I   magnet         μ   magnet     ×     A   magnet         +       I   armature         μ   armature     ×     A   armature         +       I   gap         μ   gap     ×     A   gap                                  
     where A=cross sectional area, and μ=permeability        L   =       N   2     ×     1         I   magnet         μ   magnet     ×     A   magnet         +       I   armature         μ   armature     ×     A   armature         +     (       I   gap         μ   0     ×     A   gap         )                                  
     The terms I magnet  and I armature  are the lengths of the magnetic paths within the armature  32  and magnet  40 . These values are fixed and do not change as the air gap changes. The last term (shown in parenthesis) is the reluctance due to the air gap. This last equation illustrates how the inductance of the circuit is controlled by the size of the air gap (position of the armature). As the size of the air gap, I gap , reduces, the inductance, L, will increase. 
     The contactor&#39;s armature  32  moves during the pickup interval. When the armature  32  is in motion, the air gap between the magnet&#39;s face and the armature  32  is no longer fixed. See FIG.  3 . As the armature  32  moves closer to the magnet  40 , the air gap gets smaller, simultaneously driving the reluctance of the circuit down. The permeability of the flux path (metal and air gap) rises greatly, causing the inductance of the two coils to increase very rapidly. The large change in inductance (dL/dt) that occurs affects the pickup coil current  28 . This dL/dt generates a back electromotive force (EMF) that causes the current in the coil to dip and collapse to nearly 0. Once the armature  32  seals, the inductance becomes fixed and the dL/dt goes to 0. This change in inductance is a function of the armature&#39;s  32  motion. 
     If the circuit&#39;s power supply  38  is derived through the holding coil  30 , the power supply  38  will dip along with the holding coil current. The power supply  38  may turn the circuit off when the dip occurs; this will cause nuisance tripping. FIG. 4 depicts the pickup current when a device is operated. The current increases when the device is first turned on, decreases to almost 0 when the armature  32  seals closed with the magnet  40 , and increases again to steady state until the device is switched off. 
     The holding coil derived power supply  38  provides the power to drive the contactor control circuits that turn the pickup coil  28  ON and OFF. See FIG. 2. A 16V Zener diode D 6  is used to provide regulation for the 16V bus that will power the contactor control circuits. There is an additional external power resistor called the power supply resistor  36 . This is an eight (8) watt power resistor that is located in a different location from the printed wiring board (PWB). The power supply resistor  36  is placed in parallel with the 16V power supply. This is done to reduce the amount of current that the 16V Zener diode D 6  will sink and prevent excessive heating of the component or the potting compound. 
     The model shown in FIG. 6 shows the coupling between the pickup coil  28  and the holding coil  30 . The holding coil  30  has approximately four times as many turns as the pickup coil  28 , so if the coupling were perfect, the holding coil  28  would have an induced voltage approximately four times greater than that applied to the pickup coil  28 . The resistance of the pickup coil  28  drops most of the rectified AC bus voltage during inrush; however, when the pickup current falls, the pickup coil inductance voltage increases. It is this voltage across the pickup coils&#39;s inductance that couples to the inductance in the holding coil  30 . 
     In FIG. 5, it can be seen how the voltage across the diode D 12  is largest when the pickup coil  28  current is smallest. During the time that the armature  32  seals, the pickup current falls to near 0. At this time, there is maximum voltage coupled to the holding coil  30  inductor. The holding coil&#39;s inductor now pushes current in reverse as shown in FIG.  6 . The current returns to the holding coil  30  by flowing in through the pickup coil  28 , the IGBT  22 , the power supply resistor  36 , and back to the holding coil  30 . It is this current that applies a reverse voltage across the power supply resistor  36  which diode D 12  blocks. During this time, the power supply capacitor C 3  cannot receive any more charging current; however, the sub-circuits still draw current from the power supply capacitor C 3 . The voltage across the power supply capacitor C 3  decays during this interval. 
     The contactor&#39;s control circuits are designed to tolerate voltage decreases and yet maintain gate voltage on the IGBT  22 . Specifically, the PSS circuit  20  must tolerate the dip in power supply voltage while deactivating the circuit if power is removed altogether. 
     Preferably, it is desired to use a source of power for an electrical circuit that is stable and free from the effects of transient forces. To conserve space, power for a control circuit may be derived from the holding coil  30  of the contactor circuit  10 . The contactor&#39;s electromechanical coil has resistance and this resistance can be used as a dropping resistor to derive a power supply for the electronics from the input lines. In addition, an external power resistor, referred to as the economizing resistor  34 , is used in conjunction with the holding coil  30  to dissipate some of the wattage that would otherwise need to be handled by the holding coil  30 . See FIG. 2. A diagram of the power supply topology is shown in FIG.  7 . 
     When the coil resistance is used for the power supply, the power supply is exposed to transients generated by the movement of the armature  32 . Due to the transient effects inherent within the contactor circuit  10 , the functionality of the contactor&#39;s control circuit will be adversely affected. 
     It is now clear that the dL/dt cannot be avoided on these type of power supplies, so a method must be devised that will keep it from being a problem. If the electronics of the contactor circuit are designed to be robust enough to ignore the power supply dip due to the dL/dt phenomenon, the power supply derived from the coil resistance can be utilized. 
     The circuit that allows the electronics to tolerate the dL/dt transient is the PSS circuit  20 . FIG.  8 . As power is first applied to the circuit  20 , the power supply, V BUS , charges up. Once V BUS  charges to 12.5 volts nominally, the PSS signal goes high. The PSS signal will remain high until V BUS  drops below 6.5 volts nominally. This allows the power supply to sag by over 6 volts and still not toggle the PSS output low. Allowing the power supply to sag by this amount, allows the power supply capacitor C 3  to be much smaller. Utilizing a smaller power supply capacitor C 3  allows the capacitor itself to be a tantalum capacitor which has no known wear out mechanisms. This is important because contactors generally have long life expectancies; twenty (20) or more years is not uncommon. If a large capacitor was placed on the power supply to completely eliminate the power supply dip, much more board space would be required, or possibly an electrolytic capacitor would be required. Electrolytic capacitors have wear out mechanisms and usually much shorter lives than tantalum capacitors. 
     The circuit  20  utilizes hysteresis to control the gating of the IGBT  22  that turns on the contactors&#39;s pickup coil  28 . The circuit  20  senses when there is sufficient voltage available on the power supply and then drives the IGBT  22  to turn on. This hysteresis is implemented within the PSS circuit  20  to control the gating of the IGBT  22  that actuates the contactor&#39;s pickup coil. The PSS circuit  20  delays until there is sufficient voltage available from the derived power supply before sending a drive output to turn the IGBT  22  on. Once the IGBT  22  is turned on, the PSS circuit  20  will remain high unless the power supply dips below approximately 6.5 volts. The only way the derived power supply will decrease below this value is if the user commands the circuit to turn off. When the user commands the circuit to turn off, V BUS  will decrease so that the PSS circuit will rapidly drop its output signal low, thus ensuring the circuit&#39;s turn off. The hysteresis enables the circuit to tolerate the effects of dL/dt associated with the derived power supply; and it also makes the circuit very resilient to electrical noise. 
     The equations for selecting the values of V BUS  that will toggle the state of the PSS circuit  20  are listed below. Using these equations allows the user to optimized the circuit  20  for each specific application.          V   BUS     =       V   D5     ×         R   8     +     R   11         R   11                   V   BUS     =     12.5                 V                            
     When PSS is low, 12.5 V is the value of V BUS  that causes PSS to switch from low to high.          V   BUS     =               5.6   ×     (       R   8     +     R   10       )       +     5.6   ×     (       R   10     +     R   11       )       +                 5.6   ×     (       R   8     +     R   11       )       +       V   D1     ×     (       R   8     +     R   11       )                   R   11     ×     (       R   8     +     R   10       )                   V   BUS     =     6.5                 V                            
     When PSS is high, 6.5 V is the value of V BUS  that causes PSS to switch from high to low. 
     A first op-amp  42  acts as a comparator. Two resistors R 8 , R 11  form a voltage divider that determines the voltage at the non-inverting input terminal of the op-amp  42 . The voltage at the inverting terminal of the op-amp  42  is determined by the Zener diode D 5  in series with a resistor R 2 . The diode D 5  does not allow any current to flow through the resistor R 2  until the voltage on VBUS exceeds the 5.6 V required to avalanche the diode D 5 . This maintains the voltage at the inverting input of the op-amp  42  at approximately 0 volts until VBUS exceeds 5.6 V. The diode D 5  and the resistor R 2  ensure that the output of the op-amp  42  is high when V BUS  initially begins to charge up. Once V BUS  exceeds 5.6 V, the voltage at the inverting and non-inverting pins of the op-amp  42  are as follows:          V   INV     =       V   BUS     -     5.6                 V                 V     NON        -        INV       =       V   BUS     ×       R   8         R   8     +     R   11                                  
     The equations for the inputs of the op-amp  42  can be set equal to each other to solve for the value of V BUS  where the output of the comparator  42  will switch from high to low. As mentioned earlier, this occurs when V BUS  equals 12.5 V. 
     Once the output of the op-amp  42  switches low, diode D 1  is forward biased and resistor R 10  is placed in the feedback path back to the non-inverting input. The feedback creates hysteresis and lowers the voltage at the non-inverting input of the comparator  42  so that V BUS  must now fall lower than 12.5 V to cause the output of the comparator  42  to toggle back to high. The voltage at the non-inverting input is defined by:          V     NON        -        INV       =       (         V   BUS     ×     R   10       +       V   OUT     ×     R   11       +       V   D1     ×     R   11         )     ×       R   8           R   8     ×     R   10       +       R   11     ×     R   10       +       R   11     ×     R   8                                    
     The diode D 1  is preferably a Schottky diode. Substituting 0.4 V for V D1 , 0 V for V OUT , and the proper values for R 8 , R 10 , and R 11 ; the voltage at non-inverting input of the comparator  42  is: 
     
       
         V NON-INV =0.84×V BUS +0.339 
       
     
     Since the voltage at the inverting input of the comparator  42  is known, the value of V BUS  that will toggle the output of the comparator  42  to high again can be determined. 
      V NON-INV =V INV   
     
       
         0.084×V BUS +0.339=V BUS −5.6 
       
     
     
       
         V BUS =6.5V 
       
     
     The second op-amp  44  acts as an inverter. This op-amp  44  has its non-inverting input established by a voltage divider. The voltage divider is equal to approximately 0.195AV BUS . When the output of the comparator op-amp  42  is low, the output of the inverter op-amp  44  is high. 
     While the specific embodiments have been illustrated and described, numerous modifications come to mind without significantly departing from the spirit of the invention and the scope of protection is only limited by the scope of the accompanying claims.