Abstract:
An undervoltage circuit breaker has an electromagnet for triggering a switching device and a driver circuit that supplies power to the electromagnet. The driver circuit contains a pulse generator for generating a holding current for the electromagnet and a capacitance dimensioned so that delayed triggering of the electromagnet is possible. Due to the variable pulse duty factor of the pulse generator, the energy content of the capacitance is utilized favorably. An elevated operating voltage can also be selected and therefore a smaller capacitor may be selected to advantage.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an undervoltage circuit breaker for monitoring the voltage of a single-phase or multi-phase system including an electromagnet for triggering a switching device, and a driver circuit that supplies power to the electromagnet. The driver circuit includes a rectifier circuit for obtaining a direct current from the system, a capacitance and a threshold circuit for interrupting a holding current that keeps the electromagnet in an energized condition when the voltage of the system falls below a predetermined level. The driver circuit is designed as a pulse generator to generate a holding current that is essentially independent of the line voltage when the latter is sufficiently high. The driver circuit also contains an operational amplifier that is controlled by a timer and an electronic switch that is controlled by the operational amplifier and is connected in series with the electromagnet and with a resistor, wherein the voltage dropping at the resistor is applied to the timer. The capacitance is designed as an energy storage device to supply power to the electromagnet if triggering is to be delayed. 
     BACKGROUND INFORMATION 
     A conventional undervoltage circuit breaker with these features is described in U.S. Pat. No. 4,890,184. 
     Although undervoltage circuit breakers are used in electrical installations to prevent damage to loads whose operating voltage must not fall below a predetermined level, it may nevertheless be desirable not to have each voltage reduction lead to shutdown of the load. In particular, it may be desirable to disregard brief interruptions or brief dropping of the voltage below a minimum. This is accomplished by the above-mentioned delay in triggering. The small space provided in compact low-voltage circuit breakers, for example, for installation of an undervoltage circuit breaker does not, however, usually make it possible to accommodate capacitors with a suitably large capacitance in addition to the electromagnet and the components of the driver circuit. Likewise, it is not usually possible to accommodate suitable capacitors outside the switching device because the installation space provided for the power switch in switchgear or control cabinets is dimensioned only for the power switch. Therefore, the delay that can be achieved is limited to relatively small values. 
     One of the objects of the present invention is to create a predetermined required triggering delay when there is an undervoltage, using a smallest possible capacitance. 
     SUMMARY OF THE INVENTION 
     An undervoltage circuit breaker according to the present includes: 
     a series circuit of another electronic switch and a resistor is connected in parallel to another resistor which is in series with an electromagnet, 
     the resistor of the series circuit has a lower resistance than the resistor that is connected in series with the electromagnet, and 
     an additional electronic switch can be controlled by a threshold circuit. 
     Thus, a current flowing through the electromagnet can be reduced to a minimum required holding current when the voltage to be monitored drops below a limit value and triggering is to occur after a time delay. Due to a reduction in the holding current, energy content of the capacitance is better utilized to generate a longer possible time delay. 
     As explained above, an essential property of the undervoltage circuit breaker according to the present invention provides that a relatively long time delay can be achieved while using a comparatively small capacitance. Therefore, an electromagnet, a respective driver circuit and a capacitor which is provided as the energy storage device for the delay time can be combined to form a single unit. A further embodiment of the undervoltage circuit breaker according to present invention provides that the driver circuit (which includes the capacitor) is arranged on a printed circuit board that matches the width of the electromagnet and extends beyond the electromagnet M in the longitudinal direction, with the total capacitance required provided by at least one capacitor that is mounted on the part of the circuit board projecting beyond the electromagnet in such a way that substantially the only space required is the space that extending in the longitudinal direction of the electromagnet. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a block diagram of the driver circuit of an undervoltage circuit breaker according to the present invention. 
     FIG. 2 shows a perspective view of an undervoltage circuit breaker with a driver circuit illustrated in FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The voltage to be monitored is applied at terminals  1  and  2 . Then an operating voltage for the driver circuit is obtained from rectifier diodes V 1 , V 2 , V 3  and V 4  via a bridge circuit. A resistor R 1  limits the starting current, while overvoltages are limited by a non-linear series resistor R 2 . A capacitance C 1  is charged via another diode V 5 . With a suitable rating of capacitance C 1 , the half-waves supplied from the bridge circuit are formed and an energy storage device is formed to supply power to an electromagnet M shown in the lower right portion of FIG.  1 . Electromagnet M is provided to trigger a power switch LS. 
     An electronic switch V 11  and a resistor R 15  are connected in series with electromagnet M. Electronic switch V 11  can be controlled by an operational amplifier N 2  having a first input for receiving a reference voltage over R 10 , R 12  and second input for receiving voltage from a timing circuit that includes a resistor R 13  and a capacitor C 5 . The voltage drop at resistor R 15  is applied to timing circuit R 13 , C 5 . Because of the periodic operation of the circuit described above, a current with a variable pulse duty factor flows through electromagnet M. Therefore, the average current remains essentially unchanged when there is a fluctuating operating voltage as long as the threshold circuit (to be described below) does not respond. To form a switching hysteresis, resistor R 11  acts (on the reference potential) on voltage divider R 10 , R 12 . A diode V 9  serves for non-delayed charging of timer R 13 , C 5 . 
     The operating voltage available at the input of the circuit is divided by a voltage divider R 3 , R 4  and sent via a Zener diode V 6  to a flip-flop K. Flip-flop K controls another electronic switch V 12  as well as an integrated circuit N 1  through its input  6 . Blocking of electronic switch V 12  causes a resistor R 14  to become ineffective; this resistor is connected in parallel to resistor R 15  and its relatively low resistance determines the operating current of electromagnet. Terminal points  1 ,  2  and  3  of integrated circuit N 1  are wired with a combination of fixed and adjustable resistors R 5 , R 6  and a capacitance C 2 , so a delayed control signal is obtained at output  8 . Integrated circuit N 1  may be, for example, the component available commercially via a code designation 4060. 
     To ensure that the operation of integrated circuit N 1  and operational amplifier N 2  will be substantially independent from the capacitance of capacitor C 1 , a resistor combination R 9  is provided in combination with a Zener diode V 7  and a capacitor C 4 . Another diode V 10  is connected in parallel to electromagnet M and allows the current to continue flowing during the periodic shutdown of electronic switch V 11 . 
     The processes providing a condition of an undervoltage is explained below with respect to FIG.  1 . In normal operation, the voltage at terminal points  1  and  2  may assume any values above a predetermined limit up to the overvoltage range. Operational amplifier N 2  in combination with electronic switch V 11  and the respective components described above ensures that a largely uniform current through electromagnet M is maintained by an adapted variation in the pulse duty factor. If the voltage at points  1  and  2  drops below the above-mentioned limit, flip-flop K applies voltage to integrated circuit N 1  and the other electronic switch V 12  over terminal point  6 . Electronic switch V 12  is directly blocked, so that current flowing through the electromagnet M is reduced to a low holding current. At the same time, the time delay set by an adjustable resistor R 5  begins to run; when this time elapses, operational amplifier N 2  receives a voltage over output  8  of integrated circuit N 1  and a diode V 8 , and electronic switch V 11  is blocked. Electromagnet M then drops and triggers power switch LS. 
     When the voltage is restored at points  1  and  2  or this voltage increases above the predetermined limit, electronic switch V 12  is released again, so resistor R 14  becomes active again. Therefore, a higher current needed for electromagnet M can flow again as soon as operational amplifier N 2  also begins operating and releases electronic switch V 11 . 
     FIG. 2 shows an example of the mechanical design of an undervoltage circuit breaker UA that is installed in a low-voltage power switch. Electromagnet M, which was previously described with respect to FIG. 1, is designed as a solenoid plunger magnet and has a solenoid plunger TA for suitably connecting to a tripping shaft in the breaker mechanism of the power switch. A spring energy storage device is a component of electromagnet M, but is not shown in FIG. 2. A base plate MG connected to electromagnet M is provided to mount undervoltage circuit breaker UA in the power switch. 
     A printed circuit board LP is applied to the top side of electromagnet M (whose border is indicated with a dash-dot line in the area of electromagnet M in FIG.  1 ). All the elements of the circuitry described above with respect to FIG. 1 are arranged on circuit board LP. Capacitance C 1  is distributed between two capacitors KO, as is also indicated as a circuit variant in FIG.  1 . 
     Circuit board LP is designed as shown in FIG. 2 so that it corresponds approximately to the width of electromagnet M but is longer than the electromagnet M. Therefore, circuit board LP has a projecting part in the longitudinal direction of the electromagnet M, which corresponds to the longitudinal axis of solenoid plunger TA. The two capacitors KO are mounted on this projecting part of circuit board LP so they extend down. Therefore, the space required is especially small because essentially only space extending in the longitudinal direction of electromagnet M is needed. Only a small height is needed because the other components are all mounted so they lie flat on circuit board LP. A protective cap SK covers circuit board LP and protects the circuit board LP from external influences.