Abstract:
A low arc AC contactor comprises an on/off mechanism electromagnetic-controlled contact, a pulse power source generating circuit, a low arc trigger generating circuit, and a multifunctional fault-detecting circuit. The contactor converts an instantaneous strong current into strong pulse magnetic field energy to actuate contacts of mechanical portion of the contactor, so that the power source load is on or off. Each of the actuating process will be controlled by the electric circuit to perform at the lowest potential of AC voltage for reducing arc&#39;s destructive power. Moreover, the contactor uses photoelectric coupling circuits, in which it can be controlled by external low voltage signal commands, as well as providing protection from overload, phase loss, short circuit, and leakage current.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to an AC (Alternating Current) contactor and, more particularly, to an AC contactor in which, during the lowest potential of an AC source, its mechanical contacts are operated upon a large magnetic force converted from an instantaneous strong current in order to control the on or off of AC source load circuit and which may be controlled by external low voltage signal commands, thus providing safety protection from overload, phrase loss, short circuit, leakage current, etc.  
       DESCRIPTION OF RELATED ART  
       [0002]     It is well known that commercially available conventional contactors are used for controlling the on or off of the power source load circuit, in which the mechanical contacts are operated upon a magnet field generated by the coil of contactor through that AC or DC flows. According to the requirements of the on-off procedure of the load circuit, as the mechanical contacts need to be in an engaging condition for a long time, the power for generating magnetic field must be continuously supplied to the coil for that period to retain the magnetic field, thus resulting in a higher consumption of power and bulky coil. Moreover, during the on-off procedure, the contacts of conventional contactor are engaged/disengaged in a random manner without a fixed schedule; hence during changing of engaging/disengaging condition, the contacts can be actuated at any point of the power cycle. However, as the load current is high or a short circuit occurs, a powerful arc will be generated so that the contacts are burn out or the life span thereof is shorten. Therefore, conventional contactor is generally equipped with metal or ceramic components as a heat sink and arc-suppression device, with the result that such the contactor are complex and bulky in structure. Furthermore, the commercially available conventional contactor merely has single purpose of engaging/disengaging the load circuit, and it must be cooperated with various electrical safety devices for protections from leakage current, overload or phase loss etc., so it would be inconvenient to handle. Finally, commercially available conventional contactor cannot be controlled directly by external signals with low voltages and small currents and it must be used with peripheral auxiliary devices in certain situation.  
       SUMMARY OF THE INVENTION  
       [0003]     To sum up, the present invention is directed at overcoming the technical problems of the conventional contactor, for example, higher power consumption, strongly arcing at changing of engaging/disengaging condition, single purpose and unavailable direct control by an external low voltage and small current signal.  
         [0004]     It is therefore an object of the present invention to provide a low arc AC contactor that can be controlled directly by an external low voltage and small current signal command.  
         [0005]     It is another object of the present invention to provide a contactor, which utilizes a pulse magnetic field to control engaging/disengaging condition of contacts of the contactor.  
         [0006]     It is a further object of the present invention to provide a multifunctional contactor, by which various circuit faults may be detected.  
         [0007]     The technical scheme of the present invention is as follows:  
         [0008]     With accordance to the present invention, a low arc AC contactor comprises an on/off mechanism of electromagnetic-controlled contacts having coils L 1  and L 2  for converting electric energy into magnetic energy, whereby allowing the contacts to be in engaging or disengaging condition, characterized in that said contactor further comprises:  
         [0009]     a pulse power source generating circuit in which its input is connected to R and T terminals for providing two pulse power sources, and both are controllably connected to coil L 1  and coil L 2  respectively; and  
         [0010]     a low arc trigger generating circuit in which a pulse DC (direct current) voltage signal generated by said pulse power source generating circuit is received in order to establish time coordinate signal of lowest potential in an AC power source cycle, and an external triggered on/off controlling command is received to provide with on-command trigger pulse to said pulse power source generating circuit so that a strong pulse magnetic field is generated in coil L 1 , thus resulting in the contacts being in engaging condition, as well as to provide with off-command trigger pulse or an power outage auto-cutout trigger pulse to said pulse power source generating circuit so that a strong pulse magnetic field is generated in coil L 2 , thus resulting in the contacts being in disengaging condition.  
         [0011]     Moreover, the low arc AC contactor further comprises a multifunctional fault-detecting circuit, which is connected to the on/off mechanism of electromagnetic-controlled contacts-and by which a short circuit current pulse detecting signal or an overload current pulse detecting signal is transmitted to said low arc trigger generating circuit in order to output off-command trigger pulse, thus generating a strong pulse magnetic field in the coil L 2  and allowing the contacts to be in disengaging condition.  
         [0012]     Furthermore, said pulse power source generating circuit comprises a pulse DC voltage generating unit, a charging switch unit, a pulse power storage unit for onstate, and a pulse power storage unit for offstate to form a charging loop successively as well as an onstate discharging switch unit and an offstate discharging switch unit connected to the pulse power storage unit for onstate and the pulse power storage unit for offstate to form a discharging loop by electrically connects to coil L 1  or coil L 2  respectively.  
         [0013]     The aforesaid low arc trigger generating circuit comprises:  
         [0014]     a zero potential coordinate signal generating unit, by which a pulse DC voltage signal from said pulse power source generating circuit is received and to which a power outage auto-cutout triggering unit is connected;  
         [0015]     a photoelectric coupling on-command input unit, a on-command detecting unit and a on-command trigger unit to form a circuit successively and in which the on-command detecting unit is also controlled by a time coordinate signal of the lowest potential in AC power source cycle from zero potential coordinate generating unit, and  
         [0016]     a photoelectric coupling off-command input unit, a off-command detecting unit and a off-command trigger unit to form a circuit successively and in which the turn off command detecting unit is also controlled by a time coordinate signal of the lowest potential in AC power source cycle transmitted from zero potential coordinate generating unit.  
         [0017]     The said photoelectric coupling on-command input unit comprises a photoelectric coupling circuit PT 1  by which the ground terminal of the circuits of on-command trigger units is isolated from the ground terminal of said contactor.  
         [0018]     The input of the photoelectric coupling circuit PT 1  is connected to on-command input unit comprising a command key S 1 ;  
         [0019]     The photoelectric coupling off-command input unit comprises a photoelectric coupling circuit PT 2  by which the ground terminal of the circuits of off-command units is isolated from the ground terminal of said contactor.  
         [0020]     The input of the photoelectric coupling circuit PT 2  is connected to off-command input unit comprising a command key S 2 ;  
         [0021]     The multifunctional fault-detecting circuit includes a multifunctional fault-detecting unit, a short circuit current pulse detecting unit, and a short circuit signal coordinate signal generating unit, as well as an overload current pulse detecting unit connected to the multifunctional fault-detecting unit, wherein said short circuit signal coordinate signal generating unit and overload current pulse detecting unit input off-command signal to the low arc trigger generating circuit respectively;  
         [0022]     The multifunctional fault-detecting circuit is a fault-detecting device comprising plurality of mutual inductors capable of detecting short circuit, overload, leakage current, phase loss and imbalanced power, and  
         [0023]     The on/off mechanism electromagnetic-controlled contact is a mechanical self-locking one or a polarized magnetic field self-locking one.  
         [0024]     The present invention has advantages as follows:  
         [0025]     1. The multifunctional low arc contactor of the present invention is able to lay previously current in a capacitor in store for generating a pulse current by electronic circuit, so that when the pulse current passes through the coil of the contactor a strong pulse magnetic field is induced and by means of which its mechanical contacts are engaged together. At the same time, such a engaging condition is kept by using the mechanical self-locking method (see  FIG. 1, 10A ) or polarized magnetic field self-locking method. When the contacts need to be disengaged, the self-locking is released in the same manner as the above, i.e., by means of current stored in the capacitor. Therefore the contactor has low power consumption, compact size, and high in safe factor. Since the pulse current is applied to the coils, the contactor coils generally are at rest without consuming current and producing heat; therefore, it can be extremely small. The power consumption of the present contactor is only one-tenth of conventional contactor.  
         [0026]     2. The multifunctional low arc contactor of the present invention utilizes electronic circuit for calculation, so that engaging/disengaging process is controlled to proceed at the lowest potential of AC voltage. As a result, the harmful effects of arcing on contacts are minimized; therefore, the contactor is compact in size and high efficiency without using expensive arc-suppression device.  
         [0027]     3. The multifunctional low arc contactor of the present invention utilizes photoelectric coupling circuit, which can be directly controlled by external signal command; hence it is simple, and convenient.  
         [0028]     4. The multifunctional low arc contactor of the present invention is equipped with a power safety protection device, by which the contacts of the contactor can be instantaneously disengaged when faulty occurs in the circuit load, including overload, leakage current, phase loss, short circuit, imbalance power or electric shock, etc. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0029]     These and other objects as well as advantages of the present invention will become clear by the following description with reference to the accompanying drawings, in which:  
         [0030]      FIG. 1  is a block diagram of the entire mechanism of the present invention.  
         [0031]      FIG. 2  is a block diagram of a pulse power source generating circuit of the present invention.  
         [0032]      FIG. 3  is a block diagram of a low arc trigger generating circuit of the present invention.  
         [0033]      FIG. 4  is a block diagram of a multifunctional fault-detecting circuit of the present invention.  
         [0034]      FIG. 5  is an embodiment of a pulse power source generating circuit and a mechanism for engaging/disengaging electromagnetic-controlled contacts of the present invention;  
         [0035]      FIG. 6  is an embodiment of a low arc trigger generating circuit of the present invention.  
         [0036]      FIG. 7  is an embodiment of the multifunctional fault-detecting circuit of the present invention.  
         [0037]      FIG. 8  is another embodiment of the mechanism for engaging/disengaging the electromagnetic-controlled contacts of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0038]     An embodiment of a multifunctional low arc AC contactor of the present invention is illustrated in  FIGS. 1-7 .  
         [0039]     Referring now to  FIG. 1 ,  FIG. 2  and  FIG. 3 , the low arc AC contactor of the present invention includes a mechanism  2  for engaging/disengaging electromagnetic-controlled contacts, wherein the contacts are connected to AC power supply and the load; a pulse power source generating circuit  1 , which is two-way electrically connected with coil L 1  and a coil L 2  in the on/off mechanism  2  of the electromagnetic-controlled contacts; as well as low arc trigger generating circuit  3 , by which a pulse DC signal from the pulse power source generating circuit  1  is received the lowest potential of the AC voltage is detected, and an on-command low arc triggering signal is transmitted to the pulse power source generating circuit  1  in order to generate strong current to induce strong pulse magnetic field in coil L 1 , thus engaging the contacts, and an off-command low arc triggering signal or outage auto-cutout triggering signal is transmitted in order to generate strong current to induce strong pulse magnetic field in coil L 2 , thus disengaging the contacts. Furthermore, the present invention also includes a multifunctional fault-detecting circuit  4  connected to the mechanism  2  for engaging/disengaging electromagnetic-controlled contacts, as shown in  FIGS. 1 and 4 , by which a short circuit pulse current signal when short circuit is detected or overload pulse current signal when other faults such as overload, leakage current, phase loss or imbalanced power is provided to the low arc trigger generating circuit  3  as an off-command or signal in order to generate strong pulse magnetic field in coil L 2 , thus disengaging the contacts.  
         [0040]     Referring now to  FIG. 2 , the pulse power source generating circuit  1  includes a pulse DC voltage generating unit  10 , charging switch unit  11 , a pulse power storage unit  12  for onstate, and a pulse power storage unit  13  for offstate, as well as an onstate discharging switch unit  14  and an offstate discharging switch unit  15  connected respectively with said pulse power storage unit  12  for onstate and pulse power storage unit  13  for offstate.  
         [0041]     Referring now to  FIG. 5 , the pulse DC voltage generating unit  10  consists of a full bridge rectifier formed by four rectifier diodes D 1 -D 4 , in which it is connected with the terminals R. T of the AC power source, thus generating a pulse DC voltage, and the current flows through resistors R 6 , R 7 , a bias current keeping diode D 6  and a resistor R 8  to trigger switching diode CR 1  on. The current flows through the switching diode CR 1  for charging up capacitors C 2 , C 3  until potential across the capacitors C 2 , C 3  is higher than that of the limitation Zener diode ZD 1 , where by make the diode D 6  reverse-biased and the switching diode CR 1  turnoff. When a trigger switching diode CR 2  receives a trigger pulse from an on-command trigger unit  34  in the low arc trigger generating circuit  3 , an instantaneous strong current from the capacitor C 2  flows through coil L 1  and diode D 9  to discharge to trigger switching diode CR 2 , thus instantaneously generating a strong pulse magnetic field in coil L 1 . By means of such strong magnetic field, the contacts of the electromagnetic-controlled on/off mechanism  2  of the contactor of the present invention are engaged together. Since trigger switching diode CR 2  is on, trigger voltage of the trigger switching diode CR 1 , through the diode D 8 , makes trigger switching diode CR 2  short-circuited. As a result, the switching diode CR 1  stops working until the capacitor C 2  finishes discharging and trigger switching diode CR 2  has no current to maintain its operation. Similarly, when trigger switching diode CR 3  is triggered by off-command signal voltage from the operational amplifiers OA 2  and OA 9  in the low arc trigger generating circuit  3  through OR gates D 7 , D 14  respectively, an instantaneous strong current from the capacitor C 3  flows through coil L 2  and diode D 12  for discharging to trigger switching diode CR 3 , thus generating strong pulse magnetic field in coil L 2  and then releasing the self-locking mechanism from the electromagnetic-controlled on/off mechanism  2  of the contactor. In the embodiment, the electromagnetic-controlled contacts utilizes a mechanical self-locking or releasing. While a pulse current flows through coil L 1  and an instantaneous strong magnetic field is induced, the contacts of mechanical portion are engaged and such an engaging condition is kept in a mechanism self-locking manner. When a pulse current flows through coil L 2  and an instantaneous strong magnetic field is induced, the self-locking mechanism is attracted and contacts are disengaged. This configuration is characterized as follows:  
         [0042]     1. Coils L 1 , L 2  have no current consumption except that the pulse current there through during startup.  
         [0043]     2. The pulse current in coils L 1 , L 2  during startup is ten times higher than the operating current of the conventional contactor.  
         [0044]     To sum up, the pulse DC voltage generating unit  10  is a full bridge rectifier consisting of D 1 -D 4 .  
         [0045]     The charging switch unit  11  includes a trigger switching diode CR 1  and a current limiting resistor R 6  being in series therewith as well as a resistor R 7 , a bias current keeping diode D 6 , a resistor R 8 , a limiting Zener diode ZD 1  and diodes D 8 , D 10 .  
         [0046]     The pulse power storage unit  12  for onstate is a capacitor C 2  and the pulse power storage unit  13  for offstate is a capacitor C 3 , between both a separating diode D 11  is provided.  
         [0047]     The onstate discharging switch unit  14  includes a diode D 9  and a trigger switching diode CR 2  being in series connection, and voltage divider resistors R 9  and R 13  connected to trigger pole of switching diode CR 2 . The offstate discharging switch unit  15  includes a diode D 12  and a trigger switching diode CR 3  being in series connection, and voltage divider resistors R 10  and R 11  connected to trigger pole of switching diode CR 3  as well as diodes D 7 , D 14  consisting of OR gates.  
         [0048]     Referring now to  FIG. 3  and  FIG. 6 , as shown in  FIG. 3 , the low arc trigger generating circuit  3  of the present invention includes the above mentioned pulse DC voltage generating circuit  10 , and a zero potential coordinate signal generating unit  30  and an outage auto-cutout trigger unit  31  to form a circuit in turn; and  
         [0049]     A photoelectric coupling on-command input unit  32 , a on-command detecting unit  33  and a on-command trigger unit  34  to form a circuit in turn, wherein said on-command detecting unit  33  receives a zero potential detecting signal from the zero potential coordinate signal generating unit  30 ; as well as  
         [0050]     A photoelectric coupling off-command input unit  35 , a off-command detecting unit  36  and a off-command trigger unit  37  to form a circuit in turn, wherein said photoelectric coupling off-command detecting unit  35  receives a zero potential detecting signal from the zero potential coordinate signal generating unit  30 .  
         [0051]     Referring now to  FIG. 6 , wherein voltage comparator OA 1  and OA 2  formed a zero potential coordinates generating unit  30  and a outage auto-cutout trigger unit  31  consist of voltage comparators OA 1  and OA 2  respectively, two objectives of which are as follows: the first objective is to control the discharging/charging of D 13 , C 301 , D 201 , and D 202  constantly at zero potential of the input source and the second one is to turn trigger switching diode CR 3  on by allowing OA 2  to output a high potential signal, when the input AC source is outage, thus disengaging the contacts of mechanic portion of the contactor.  
         [0052]     The voltage comparator OA 1  compares the power source voltages value of the terminals of the divider resistors R 1 , R 3  at the entrance of pulse DC voltage generating unit  10  with a predetermined voltage value of resistor R 4 . When the terminal voltage value of R 3  is lower than that of R 4 , OA 1  outputs a high potential, so that C 202  in the photoelectric coupling off-command input unit  35  and C 301  in the on-command detecting unit  33  are ready to be charged until the terminal voltage value of R 3  is higher than that of R 4 , and then the output of OA 1  changes to a low potential, so that the diodes D 5 , D 13 , D 201  are connected to the ground. As a result, the voltage and current stored in C 202 , C 301  and C 1  are quickly discharged to the said diodes to produce a time coordinate signal for the on-command detecting unit  33 . Secondly, since the C 1  is slowly charged and quickly discharged, the usual voltage value of the C 1  terminals is greatly lower than that of the R 4  terminals. Output of OA 2  is in a status of low potential, when the input power is suddenly interrupted, the voltage value of R 4  terminals is higher than that of R 3  terminals for extended period, and OA 1  output a high potential to charge the C 1  until the voltage value of C 1  terminals is higher that of the R 4  terminals. Then, the outputs of OA 2  changes to a high potential, thus making the trigger switching diode CR 3  through the OR gate diode D 7  turned on, and disengaging the contacts of the mechanical portion.  
         [0053]     The objective of installing a photoelectric coupling on-command input unit  32  and a photoelectric coupling off-command input unit  35  is to isolate the ground terminal of the command input units ( 32 , 35 ) from the ground terminal of contactor circuit, and to be directly controlled by an external signal voltage command.  
         [0054]     When a commend key S 1  is pressed, PT 1  is turned on. OA 3  outputs a high potential through R 12  and charges up C 301  once OA 1  outputs a high potential, until the potential output of OA 1  becomes low, C 301  discharges to OA 1  through D 13 , thus triggering on-command detecting unit  33  to operate.  
         [0055]     When an off-command key S 2  is pressed, PT 2  is turned on and outputs a high potential through R 206  and charges up C 202  once OA 1  outputs a high potential, until the potential output of OA 1  becomes low, C 202  discharges to OA 1  through D 201 , thus changing the output status of OA 6 , OA 7 , and making off-command contactor unit  37  operated.  
         [0056]     The operating of the low arc trigger generating circuit  3  is independent from input time of the “ON” signal so that the discharging/charging of C 301  and C 302  in the circuit is always at zero potential which delaying the signal appropriately that associates with the engaging action time (T 1 ) of the contacts for turning the trigger switching diode CR 2  on and engaging the contacts at the position of lowest potential of input voltage, such as to minimize the harmful effects of arcing.  
         [0057]     Before “ON” signal is input, C 301  will only be charged by means of voltage through the R 12  at the time that OA 1 &#39;s output potential changes from low to high, until OA 1  output potential changes from high to low again. Once C 301  discharges, OA 4  output potential changes from low to high to charge up C 303  until the C 301  finishes discharging in which the discharging time depends on values of C 301  and VR 301 . When OA 4  backs to the low potential, C 303  discharges into OA 4 , thus allowing OA 5  to output a high potential and making trigger switching diode CR 2  turned on.  
         [0058]     The working principle of OA 8  and OA 9  of the off-command trigger unit  37  is the same as that of OA 4  and OA 5  of the on-command trigger unit  34 .  
         [0059]     To sum up, in  FIG. 6 , the zero potential coordinate signal generating unit  30  is a voltage comparator connected to a rectifier output terminals of the pulse DC voltage unit  10 , which includes a operational amplifier OA 1  and divider resistors R 1 , R 3  connected to the input end thereof, and R 2 , R 4  and an integrating circuit R 5 C 1  and a limiter diode D 5 . The outage auto-cutout trigger unit  31  is a voltage comparator OA 2  connected to the zero potential coordinate signal generating unit  30 .  
         [0060]     The photoelectric coupling on-command input unit  32  includes a photoelectric coupling on-command circuit PT 1 , an on-command key S 1 , a resistor R 201 , and independent resistors R 204 , R 205 , and an operational amplifier OA 3  connected thereto through resistor R 203  and capacitor C 203 .  
         [0061]     The on-command detecting unit  33  includes an operational amplifier OA 4 , a resistor R 12  connected to the output end of OA 3  and the input “−” end of OA 4 , a capacitor  301 , an Zener diode ZD 2  connected to “−” end of OA and a current limiting potentiometer VR 301 , as well as a diode D 13  connecting to a common node of R 12 , C 301  and output end of OA 1 .  
         [0062]     The on-command trigger unit  34  includes an operational amplifier OA 5 , wherein “+” end of which is grounded, and the input “−” end of which is connected to the divider resistors R 301 , R 302 , as well as at the point of voltage division, a capacitor C 303  is connected to the output end of OA 4 .  
         [0063]     The photoelectric coupling off-command input unit  35  includes a photoelectric coupling on-command circuit PT 2 , a off-command key S 2  and a resistor R 202  connected to the input terminal of PT 2 , as well as an output high-frequency filter capacitor C 201  and an isolating resistor R 206 .  
         [0064]     The off-command detecting unit  36  includes an operational amplifier OA 6 , divider resistors R 207 , R 208  and input coupling capacitor C 201  connected to “−” input end of OA 6 , and a diode D 201  connected to C 201  and output terminal of OA 1 .  
         [0065]     The off-command trigger unit  37  includes operational amplifiers OA 7 , OA 8 , OA 9 , divider resistors R 210 , R 211  connected to “+” input end of OA 7 , a resistor R 209  and two input OR gate diodes D 202 , D 203  connected to “−” input end of OA 7 ; voltage regulation units VR 302 , ZD 3  and coupling capacitor C 302  connected to “−” input end of OA 8 ; divider resistors R 303 , R 304  and coupling capacitor C 304  connected to the “−” input end of OA 9 , and the “+” input end of OA 8  and OA 9  are grounded.  
         [0066]     Referring now to  FIGS. 1, 4  and  7 , since the electromagnetic-controlled contacts mechanism  2  of the low arc contactor of the present invention is connected with the multifunctional fault-detecting circuit  4 , it can detect various faults in AC power source load circuit, such as faulty signals of short circuit, overload, leakage current, phase loss and imbalanced power etc., namely the short circuit current pulse signal or the overload current pulse signal; these signals are fed into the low arc trigger generating circuit  3  as an off-command or signal whereby generating an offstate trigger signal for actuating the coil L 2  to generate a pulse magnetic field and disengage the contacts. Thus, the present invention provides with a multifunctional low arc AC contactor.  
         [0067]     Referring now to  FIG. 4 , the multifunctional fault-detecting circuit  4  of the present invention includes a multifunctional fault-detecting unit  41 , a short circuit current pulse detecting unit  42 , and a short circuit coordinate signal generating unit  43  to form a circuit successively, whereby transmitting short circuit pulse to the off-command detecting unit  36  through the OR gate diode D 203 . Furthermore, an overload current pulse detecting unit  44  is connected to the multifunctional fault-detecting unit  41  for sending the overload current pulse signal to the photoelectric coupling off-command input unit  35  to provide a offstate trigger pulse. Thus, a pulse magnetic field is generated in coil L 2  to disengage the contacts in order to cut out the AC power source load circuit. In an embodiment as shown in  FIG. 7 , the multifunctional fault-detecting unit  41  consists of plurality of mutual inductors, while the short circuit current pulse detecting unit  42  and the overload current pulse detecting unit  44  consist of voltage comparators made of operational amplifiers OA 11 , OA 12  respectively, wherein the patented circuit disclosed in Chinese Patent No. 95115982.8, and U.S. Pat. No. 6,163,444 has been permitted using in the present invention.  
         [0068]     The design principle of short circuit coordinate signal generating unit is as follows:  
         [0069]     In accordance with practical situation, the first short circuit signal is not taken into account since it can exist at any position of the AC source. When the second short signal appears, it can be sure that the signal voltage is generated in the process of the AC voltage rising from low to high.  
         [0070]     When the AC power source has a short circuit fault, the OA 11 , in the short circuit current pulse detecting unit  41  will output a high potential pulse signal, at one end of which a capacitor C 402  is charged, and at another end of which it is transmitted to OA 7  through the resistor R 14  and OR gate diode D 203 . Since OA 10  remains unchanged without being affected by the charging of C 402 , the output of OA 10  is still in low potential; therefore, the signal voltage is short circuit to ground through R 14  to diode D 401 . When the OA 11  outputs pulse signal changing from high to low, C 402  discharges to change the output status of OA 10  and OA 11  outputs a high potential to charge C 401  maintaining the output status of OA 10  for a certain period. When the second short circuit signal pulse is generated, it instantly triggers OA 7  through R 14  and D 203  and, then sends through the operational amplifiers OA 8 , OA 9  in the off-command trigger unit  37 , and D 14  to the trigger switching diode CR 3 .  
         [0071]     Referring now to  FIG. 8 , as an alternative of above mentioned embodiment, the electromagnetic-controlled contacts on/off mechanism  2  of the contactor utilizes a polarized magnetic field self-locking structure. When a pulse current flows through the coil L 1 , an instantaneous strong magnetic field is induced to polarize a magnetic core, thus attracting the mechanical portion of the contacts to be engaged. Such a situation will be maintained until a pulse current flow through the coil L 2 , thus inducing an instantaneous strong counter magnetic field and counteracting the magnetic force. Thus, the contacts are disengaged. Such mechanism has the following advantages:  
         [0072]     1. The coils L 1 , L 2  have no current consumption except that the pulse current flows there through during startup, and  
         [0073]     2. The pulse current at start-up of the coils L 1 , L 2  is ten times higher than the operating current of the conventional contactor.