CURRENT SENSING DEVICE

Abstract: A current device comprises a constant d.c. voltage power supply circuit (500) which is fed with a current flowing in one direction and outputs a positive potantial, a medium potential and a negative potantial at a positive output terminal (5a), a medium output terminal (5c) and a negative output terminal (5d), respectively, a current sensing resistive element (40) connected in series to the power circuit (500), a differential amplifier circuit (60) which is fed with power from the output of the power supply circuit (500), and converts the voltage drop across the current sensing resistive element (40) proportional to the one-direction current into an output signal based on the medium potential between the positive potential and the negative potential of the output of the power supply circuit (500),and a switching circuit (121); a control circuit (70) which is fed with power from the power supply circuit (500) and receives the output signal of the differential amplifier circuit (60) proportional to the one-direction current to control the closing operation of a switching circuit signalling device (81) which is activated by the closure of the switching circuit (121) to emit light or produce a warning sound; and a switching operation prohibitive circuit (50) which is connected to prevent the switching circuit (121) from being closed improperly when the output voltage of the power supply circuit (500) is not enough to operate the differential amplifier (60) or the control circuit (70) properly.

Description: The present invention relates to a current sensing device for detecting an electric current which flows through an a.c. line or a d.c. line, and particularly a current sensing device for use in a circuit breaker with an overcurrent trip device, according to the preamble of claim 1 as well as a circuit breaker including a trip contact inserted into an a.c. line and a current sensing device for opening the trip contact, according to the preamble of claim 5. (US-A-4 442 472) There have been known such types of current sensing devices, wherein an a.c. current flowing through a line causes an output current to be induced in the secondary winding of a current transformer, with a predetermined current transformer ratio and the output current is rectified by a full-wave rectification circuit to be fed to a shunt circuit. The waveform of a signal voltage in the shunt circuit changes only in one direction as known and is converted into a signal corresponding to its effective value or mean value, by a signal convertion circuit in the next step. The output signal from the signal convertion circuit is fed to a fault judgement circuit. When the fault judgement circuit determines that the sensed current value is above a predetermined level, it activates signalling means based on the level detection signal to light a lamp or sound an alarm. A power supply circuit for the fault judgement circuit is connected in parallel with the shunt circuit. Since the conventional current sensing devices are constructed as described above, only one part of the secondary current in the current transformer as the current sensing means flows through the power supply circuit. As a result, the current flowing through the shunt circuit does not always correspond to the current flowing through the a.c. line to be detected, which can cause an error in sensing the current level. The current flowing into the power supply circuit is not constant, so it is difficult to correct the error. US-A-4 442 472 describes a circuit breaker including a trip mechanism and a current sensing device for actuating the trip mechanism which senses line current more accurately. In US-A-4442472 a power supply circuit and a current sensing resistive element are connected in series between the terminals of a rectification circuit so that all the current from the rectification circuit flows through the current sensing resistive element. However, this document does not consider that when the load current flowing through the line to be detected is as small as 10% of rated current of the current sensing device, there is disadvantage that the current sensing device sometimes malfunctions because the output voltage of the power supply circuit is not sufficient to drive the fault judgement circuit. This is applicable not only to the case that a current flowing through an a.c. line is detected, but also to the case that a current flowing through a d.c. line is detected. It is an object of the present invention to eliminate the disadvatage of the conventional current sensing devices and to provide a new and improved current sensing device capable of providing greater accuracy in the level detection of the current, of avoiding the malfunction in sensing the current level when the output voltage of the power supply circuit is not sufficient, and of signalling the results of the level detection without failure. It is another object of the present invention to provide a circuit breaker including a current sensing device, which can provide greater accuracy in fault current level detection precision, and which can prevent the malfunction of a control circuit when the output of the power supply circuit is not sufficient. The foregoing objects of the present invention have been attained by providing a current sensing device with the features of claim 1 and a circuit breaker with the features of claim 5. The control circuit can be constituted by a comparator which is fed with power from the power supply circuit and compares the output voltage of the differential amplifier circuit proportional to the one-direction current, with a reference voltage. The control circuit can be also constituted by a timer circuit which is fed with power supply from the power supply circuit and is connected to the differential amplifier circuit so as to receive its output signal proportional to the one direction current and provide predetermined time delays depending on the magnitude of the one direction current. Further object and advantages of the present invention will be apparent from the following description, reference being had to the accompanying drawings wherein preferred embodiments of the present invention are clearly shown. In the drawings: Figure 1 is a circuit diagram showing an embodiment of the current sensing device according to the present invention, Figure 2 is a circuit diagram showing an embodiment of the power supply and the switching operation prohibitive circuit according to the present invention, Figure 3 is an operational illustration of the power supply circuit and the switching operation prohibitive circuit, Figure 4 shows equivalent circuits of the power supply circuit and the differential amplifier circuit according to the present invention, Figure 5 shows a characteristic curve of the differential amplifier, Figure 6 is an operational illustration of the power supply circuit and the differential amplifier circuit, Figure 7 is a circuit diagram showing another embodiment of the current sensing device, Figure 8 is a circuit diagram of another embodiment of the current sensing device for a circuit breaker, according to the present invention, Figure 9 is a circuit diagram showing an embodiment of the power supply circuit and switching operation prohibitive circuit in the current sensing device as shown in Figure 8, Figure 10 shows a characteristic curve of the differential amplifier in the current sensing device, Figure 11 shows an example of the tripping characteristic curve of the circuit breaker using the current sensing device, and Figure 12 is a circuit diagram showing another embodiment of the current sensing device for circuit breakers. Referring now to the drawings, wherein like reference numerals designate like or corresponding parts throughout the several views, Figure 1 is a circuit diagram showing an embodiment of the current sensing device of the present invention. In Figure 1, a current transformer 21 for sensing a current to be detected is interposed in an a.c. line 11 between a line terminal 101 and a load terminal 301. Connected to the secondary winding of the current transformer 21 is a rectifier circuit 30 which rectifies the secondary output current. The rectifier circuit 30 is constituted by a parallel combination of a series circuit of diodes 31, 32 and another series circuit of diodes 33, 34. Corrected to the positive output terminal of the rectifier circuit 30 is a constant d.c. voltage power supply circuit 500 which is provided with a positive terminal 5a, a medium terminal 5c and a negative terminal 5d. Figure 2 is an embodiment of the power supply circuit 500. A voltage limiting element 502 such as a zener diode is connected in parallel with a smoothing capacitor 504 through a diode 503. A reference voltage generating circuit 505 is connected to the positive potential terminal 5a. A voltage limiting element 506 is connected in series with the reference voltage generating circuit 505. The junction between the voltage limiting element 506 and the reference voltage generating circuit 505 is connected to the medium terminal 5c of the power supply circuit 500. The output terminal of the reference voltage generating circuit 505 is connected to a reference voltage terminal 5b of the power supply circuit 500. As shown in Figure 1, a current sensing resistive element 40 has one end connected to the negative terminal 5d of the power supply circuit 500 and the other end connected to the negative output terminal of the rectifier circuit 30. This structure allows full wave rectification current to flow in the sensing resistive element 40 so as to correspond to the load current to be detected. A differential amplifier circuit 60 is provided to convert the voltage drop across the sensing resistive element 40 into a signal based on the medium potential V₀ of the power supply circuit 500. The differential amplifier circuit 60 can comprise an operational amplifier 63 and four resistive elements 64, 65, 66 and 67. The operating power of the differential amplifier circuit 60 is supplied by the power supply circut 500. The input terminals of the differential amplifier circuit 60 are connected across the current sensing resistive element 40. The control circuit is constituted by a comparator 70 as known. It is provided to compare the output voltage of the differential amplifier circuit 60 with the reference voltage. When the output voltage of the differential amplifier circuit exceeds the reference voltage, the comparator outputs an output signal at its output terminal 70a. A light emitting element 81 is connected to the positive output potential terminal 5a of the power supply circuit 500. A switching circuit 121 has one end connected in series to the light emitting element 81 through a resistive element 62 and the other end connected to the negative output terminal 5d of the power supply circuit 500. The light emitting element 81 is constructed so that it emits light when the switching circuit 121 is closed. A switching operation prohibitive circuit 50 can be constituted by a comparator 53 and a voltage divider circuit 51 as shown in Figure 2. The voltage divider circuit 51 can comprise resistive elements R₇ and R₈. The divider circuit is connected between the positive output terminal 5a and the medium output terminal 5c. The junction between the resistive elements R₇ and R₈ is connected to one of the input terminals of the comparator 53. The other input terminal is connected to the reference output potential terminal 5b. The switching operation prohibitive circuit 50 has an output switch 55. The output switch 55 is constructed so that when it is closed, the switching circuit 121 is allowed to close, as clearly seen from Figure 1. In operation, when a current to be detected flows through the a.c. line 11, a secondary current having a intensity determined by a predetermined current transformer ratio is induced to flow in the secondary winding of the current transformer 21. The secondary current is rectified by the rectifier circuit 30 to become a one-direction current. The output current of the rectifier circuit 30 passes through the power supply circuit 500 and the sensing resistive element 40 and returns to the rectifier circuit 30. At this time, a full wave rectification current corresponding to the load current in the a.c. line 11 flows through the power supply circuit 500 and the sensing resistive element 40. When the rectified current flows through the power supply circuit 500, the power circuit generates voltages +V, V ref , V₀ and -V at its output terminals 5a, 5b, 5c and 5d. The voltages +V, V ref and -V are based on the voltage V₀ at the medium output terminal 5c as grounded, as shown in Figure 3. The output voltage +V at the output terminal 5a is allowed to include a ripple component. The relation between the output voltage +V and the output voltage V ref is as follows: (+V) > (V ref ) The output of the power supply circuit 500 is applied to the prohibitive circuit 50 which includes the comparator 53 having predetermined hysteresis. When the output voltage +V becomes higher than the ON level as indicated in Figure 3, the output switch 55 is closed. On the other hand, when the output voltage +V becomes lower than the OFF levels, the output switch 55 is opened. The difference between the ON level and the OFF level is set to be greater than the ripple component in the output voltage +V, so that the output switch 55 is prevented from being repeatedly closed and opened due to the presence of the ripple component. In other words, only when the output voltage +V is higher than a prescribed value, the output V ref is well stable and the output voltage -V is at a prescribed value, the prohibitive circuit 50 closes the output switch 55. Power is supplied to the differential amplifier circuit 60 by the power supply circuit 500. The input voltages of the amplifier circuit 60 are fed by the current sensing resistive element 40. The voltage gain A of the amplifier circuit 60 is obtained as follows, as can be seen from Figure 4: The output voltage characteristic B as shown in Figure 5 is obtained. In order to properly operate the differential amplifier circuit 60, the following conditions must be satisfied: The conditions will be explained by use of equivalent circuits of the power supply circuit 500 and the differential amplifier circuit 60 as shown in Figure 4. In order that the circuit 60 can work as a differential amplifier, the input voltages V₁, V₂ of the operational amplifier circuit 60 should be at values between the output voltages +V and -V of the power supply circuit 500 (see Figure 6). The value of the input resistive element R in and that of the output resistive element R out for the differential emplifier circuit 60 are set to satisfy the conditions as specified above. When the output of the differential amplifier circuit 60 exceeds a predetermined current region, the comparison circuit 70 produces an output signal. The output of the comparison circuit 70 is connected to the input of the switching circuit 121 to be able to trigger it through the output switch 55 of the switching operation prohibitive circuit 50 to close the switching circuit 121 so as to make the light emitting element 81 emit light. There is a case where the current flowing through the a.c. line 11 is as small as 10%-20% of rated current and the output voltage of the power supply circuit 500 is not enough to operate the comparison circuit 70 properly. In order to prevent the comparison circuit 70 from outputting improperly in that case, the output switch 55 of the switching operation prohibitive circuit 50 is opened to prevent the switching circuit 121 from being closed. Another embodiment of the current sensing device according to the present invention will be explained referring to Figure 7. In the embodiment, a buzzer 83 is used instead of the light emitting element 81. It is driven by another control power source 84. The buzzer is operated through a solid state relay 82 for warning a user. It is desired that the buzzer sounds when the current flowing to the a.c. line 11 is beyond a predetermined level which is approximate to the rated current, for example. With regard to the embodiments, the explanation on the cases wherein the present invention is applied to the current sensing device for the a.c. line 11 has been made for the sake of clarity. The present invention can be applied to a current sensing device for a d.c. line. As explained, in accordance with the present invention, the power supply circuit is connected in series with the current sensing resistive element. All the current that flows through the power supply circuit flows through the sensing resistive element to prevent an error in detecting the current from being caused. As a result, greater accuracy in the current level detection can be obtained. In addition, there is provided the switching operation prohibitive circuit. As a result, when the current flowing through the power supply circuit is too small, the current sensing device can be prevented from malfunctioning. Now, further embodiments wherein the current sensing device according to the present invention is used in a circuit breaker will be explained. Figure 8 is the circuit diagram of an embodiment wherein the current sensing device according to the present invention is used in the circuit breaker. In Figure 8, the line terminal 101 is connected to a three phase power supply. The line terminal 101 is connected to the load terminal 301 through a tripping contactor 201. The control circuit is constituted by a timer circuit 90. The timer circuit 90 includes an instantaneous tripping circuit, a short time delay tripping circuit and a long time delay tripping circuit as known but not shown. The output terminals of the tripping circuits are connected in parallel with one another to constitute the output terminal 90a of the timer circuit 90. An electromagnetic trip coil 80 has one end connected to the positive output terminal of the rectifier circuit 30. The electromagnetic trip coil 80 has the other end connected in series to one end of a first switching circuit 120. The other end of the switching circuit 120 is connected to the negative terminal 5d of the power supply circuit 500. The electromagnetic trip coil 80 is mechanically interlocked with the tripping contact 201 through tripping means 100 so that the tripping contact 201 is opened when the first switching circuit 120 is closed. A second switching circuit 121 has one end connected in series to one end of the light emitting element 81 which has the other end connected to the positive output terminal 5a of the power supply circuit 500. The second switching circuit 121 has the other end connected to the negative terminal 5d of the power supply circuit 500. The light emitting element 81 is constituted so that it emits light when the switching circuit 121 is closed. The switching operation prohibitive circuit 50 has output switches 54 and 55 which are positioned in lines connecting the output terminals 90a, 90b of the timer circuit 70 to the inputs of the first and second switching circuits 120 and 121, respectively. When the output switches 54 and 55 are closed, the first and second switching circuits 120 and 121 are ready for being closed, respectively. Other constituent elements are indicated by the same reference numerals as used in Figure 1. Explanation on those elements will be omitted for the sake of the clarity. Figure 9 is an embodiment of the power supply circuit 500. In Figure 9, the power supply circuit is shown to include an impedance circuit 501 which facilitates the operation of the electromagnetic trip coil 80. In the impedance circuit 501, the series combination of a resistive element R₄ and a resistive element R₅ is connected to the collector of a transistor T r1 , and the junction between the resistive elements R₄ and R₅ is connected to the base of the transistor T r1 through a resistive element R₆. The junction between the resistive elements R₄ and R₅ is also connected to the emitter of the transistor T r1 through a zenerdiode ZD₁. Connected in series to the impedance circuit 501 is the voltage limiting element 502, such as a zenerdiode. The diode 503 has the anode connected to the junction between the impedance circuit 501 and the voltage limiting element 502. The smoothing capacitor 504 is connected between the cathode of the diode 503 and the negative terminal 5d of the power supply circuit 500. The cathode of the diode 503 is also connected to the positive terminal 5a of the power supply circuit 500. Other constituent elements in the power supply circuit are indicated by the same reference numerals as used in Figure 2. Explanation on those elements will be omitted for the sake of clarity. The operation of the circuit breaker with the current sensing device according to the present invention will be explained. When a current flows through the a.c. line 11 like the first embodiment, the differential amplifier 60 produces outputs as shown in Figure 10. The switching operation prohibitive circuit 50 is different from the one of the first embodiment in that it can prohibit the closing operation of both first and second output switches 54 and 55. When the output of the differential amplifier 60 exceeds the instantaneous tripping current region, the short time delay tripping current region or the long time delay tripping current region as shown in Figure 11, the output terminal 90a of the timer circuit 90 produces the output signals accordingly. In addition, the timer circuit 90 includes a comparator (not shown) which provides an output the output terminal 90b when the detected current is beyond a predetermined level. When the output terminals 90a, 90b of the timer circuit 90 triggers the inputs of the switching circuits 120 and 121 through the output switches 54 and 55 of the switching operation prohibitive circuit 50, respectively, the switching circuits are closed to energize the electromagnetic trip device 80. The fact that the load current is beyond the predetermined level is visible by light emission of the light emitting element 81, which is given by the closing of the switching circuit 121. When the electromagnetic trip device 80 is activated, the tripping contact 201 is opened to cut off a fault current. The tripping characteristic curve is shown in Figure 11. When the current flowing in the tripping contact 201 is as small as 10%-20% of rated current, the output voltage of the supply circuit 500 is not enough to operate the timer circuit 90 properly. In order to prevent the timer circuit 90 from malfunctioning in that case, the output switches 54 and 55 of the switching operation prohibitive circuit 50 remain opened to avoid the closing of the switching circuits 120 and 121. Another embodiment wherein the current sensing device according to the present invention is used in the circuit breaker will be explained, referring to Figure 12. In this embodiment, the buzzer 83 is used instead of the light emitting element 81 like the second embodiment of the current sensing device according to the present invention. The buzzer 83 is operated by the control power source 84 through the solid state relay 82. It is desired that solid state relay is set to sound the buzzer when the current flowing through the tripping contactor 201 is beyond a predetermined value which is approximate to the rated current. With regard to the third and fourth embodiments, the case wherein the current sensing device according to the present invention is applied to a circuit breaker for cutting off a single-phase a.c. line 11 has been explained for the sake of clarity. The current sensing device according to the present invention is also applicable to circuit breakers for cutting off a multi-phase a.c. line. As explained, the current sensing device according to the present invention can provide greater accuracy in the tripping characteristics and enhance the safety of the tripping characteristics economically.

Claims: A current sensing device comprising a voltage power supply circuit (500) which is fed with a current flowing in one direction and outputs different potentials; a current sensing resistive element (40) connected in series to the power circuit (500); a differential amplifier circuit (60) which is fed with power from the output of the power supply circuit (500), and converts the voltage drop across the current sensing resistive element (40) proportional to the one-direction current into an output signal; a switching circuit (121); a control circuit (70) which is fed with power from the power supply circuit (500) and receives the output signal of the differential amplifier circuit (60) proportional to the one-direction current to control the closing operation of the switching circuit (121); characterized in that the voltage power supply (500) is a constant d.c. voltage power supply circuit and outputs a positive potential, a medium potential and a negative potential at a positive output terminal (5a), a medium output terminal (5c) and a negative output terminal (5d), respectively; the output signal of the differential amplifier circuit is based on the medium potential between the positive potential and the negative potential of the power supply circuit outputs; and that the current sensing device further comprises signalling means (81) which is activated by the closure of the switching circuit (121) to emit light or produce a warning sound; and a switching operation prohibitive circuit (50) which is connected to prevent the switching circuit (121) from being closed when the output voltage of the power supply circuit (500) is not sufficient to operate the differential amplifier (60) or the control circuit (70) properly. The current sensing device according to claim 1, characterized in that the control circuit (70) is a comparison circuit which is fed with power from the power supply circuit (500) and compares the output voltage of the differential amplifier circuit (60) proportional to the one-direction current with a reference voltage. The current sensing device according to claim 1 or 2, characterized by a current transformer (21) for detecting the current flowing through an a.c. line (11), and a rectifier circuit (30) which is connected to the secondary winding of the current transformer (21) to convert the secondary a.c. current of the current transformer (21) into said one-direction current, wherein the power supply circuit (500) and the current sensing resistive element (40) are connected between the output terminals of the rectifier circuit (30). The current sensing device according to claim 3, characterized in that the control circuit (70) is a timer circuit which is fed with power from the power circuit (500) and is connected to receive the output signal of the differential amplifier circuit (60) proportional to the one-direction current and provide predetermined time delays depending on the magnitude of the one-direction current. A circuit breaker including a trip contact (201) inserted into an a.c. line (11) and a current sensing device for opening the trip contact, the current sensing device comprising a current transformer (21) for detecting a current flowing through the trip contact (201); a rectifier circuit (30) which is connected to the secondary winding of the current transformer (21) to convert the secondary a.c. current of the current transformer (21) into a one-direction current; a voltage power supply circuit (500) which is connected between the output terminals of the rectifier circuit (30) and outputs different potentials; a current sensing resisitive element (40) connected in series to the power supply circuit (500) between the output terminals of the rectifier circuit (30); a differential amplifier circuit (60) which is fed with power from the output of the power supply circuit (500) and converts the voltage drop across the current sensing resistive element (40) proportional to the one-direction current into an output signal; a timer circuit (90) which is fed with power from the power supply circuit (500) and is connected to receive the output signal of the differential amplifier circuit (60) proportional to the one-direction current and provide predetermined time delays depending on the magnitude of the one-direction current; a first switching circuit (120) which is closed by an output of the timer circuit (90); an electromagnetic trip coil (80) which is connected in series to the first switching circuit (120), the series combination of the trip coil (80) and the first switching circuit (120) being connected between the junction of the rectifier circuit (30) and the power supply circuit (500) and the junction of the power supply circuit (500) and the current sensing resistive element (40); tripping means (100) which is driven by the electromagnetic trip coil (80) when energized due to the closure of the first switching circuit (120), so as to open the trip contact (201); characterized in that the voltage power supply circuit (500) is a constant d.c. voltage power supply circuit and outputs a positive potential, a medium potential and a negative potential at a positive output terminal (5a), a medium output terminal (5c) and a negative output terminal (5d), respectively; the output signal of the differential amplifier circuit (60) is based on the medium potential between the positive potential and the negative potential of the output of the power supply circuit (500); and that the current sensing device further comprises a second switching circuit (121) which is closed by an output of the timer circuit (90); a switching operation prohibitive circuit (50) which is connected to prevent the first and the second switching circuit (120, 121) from being closed when the output voltage of the d.c. voltage supply circuit (500) is not sufficient to operate the differential amplifier (60) and the timer circuit (90) properly; and signalling means (81) which is driven by the closure of the second switching circuit (121) to emit light or produce a warning sound.

Inventors: ISHII, KAZUHIRO MITSUBISHI DENKI K.K.
Applicants: MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1988
Main classification: H02H3
IPC codes: H02H1, H02H3