ELECTRIC SWITCHING DEVICE

Abstract: A switching device comprising at least two operating coils (21, 22, 41, 42) for controlling at least two magnetic circuits (51, 52), a control unit (35) which controls the voltage supplied to the operating coils, and individual demagnetization circuits (31, 32) connected to each operating coil for achieving a simultaneous opening of the at least two magnetic circuits from all the operating coils.

Description: Electric switching device TECHNICAL FIELD The present invention relates to an electric switching device comprising at least one contact. More particularly, the invention relates to a switching device which comprises at least two operating coils driving at least two magnetic circuits . BACKGROUND ART To operate a switching device, such as a contactor, a magnetic circuit is often used. The magnetic circuit compri- ses an armature and a core, where the armature is the movable part of the magnetic circuit and the core is the fixed part of the magnetic circuit. Further, the switching device comprises an operating coil which influences the movement of the armature and a number of electric switches which are influenced by the armature. The operating coil which controls the closing and opening operations of the switching device is connected to a supply voltage source. The armature is drawn towards the core when a current flows through the operating coil, and when that occurs the circuit comprising the electric contacts, influenced by the armature, is closed. The current through the operating coil may be alternating current or direct current, and the armature is held in position as long as a current flows through the operating coil. When the current through the operating coil disappears, the armature separates from the core by means of, for example, springs which are tensioned upon closing of the switching device and the electric contact-containing circuit is broken. Electromagnetic switching devices are used, for example, as switching means between a voltage source and a load, for example an electric motor or a furnace . The current through a coil cannot abruptly change its value. Since the coil together with the magnet corresponds to an inductance, it takes a certain time for the current through the coil to reach its full value after connection of a voltage source. In a similar manner, when disconnecting the voltage source, the current through the coil decays within a certain time before the energy stored in the coil and the magnetic circuit is consumed by the circuit. Non-constant currents and voltages arise during a transition period after a change, for example when connecting and disconnecting voltage sources. Such transition periods are called transients. Transient currents flow until the circuit attains a state of equilibrium. A transient current through a coil induces an electromotive force (EMF) which opposes the change. If attempts are made to change the current through a coil too rapidly, harmfully high voltages may arise. A common example are the visible sparks which arise in the ignition coil of a car. To avoid high voltage transients, therefore, different forms of transient limitations are used in operating coils in switching devices, for example varistors, RC circuits, or freewheeling diodes. These components even out changes in the circuit comprising the operating coil. When breaking the supply voltage to the operating coil, for example, the components carry the current resulting from the energy stored in the coil until all energy has been been dissipated, that is to say, until the coil is demagnetized. However, in switching devices it is still important to rapidly divert the energy stored in the operating coil so that the opening movement is not damped. The armature and the core may be held closed for a longer time because of the energy stored in the magnetic circuit and the coil. The armature and the core start separating only when said energy has been consumed. In addition to this opening operation, per se, being delayed, the opening speed is damped by the fact that the movement of the armature gives rise to currents in the operating coil which rotate freely in the transient-limiting circuit, and hence the opening movement is slowed down by the oppositely directed EMF which is generated in the coil by the movement itself. For the breaking capacity of a switching device it is impor- tant to have a fast opening operation and no damping of the opening speed. A known technique for increasing the opening speed and reducing the delay of the separation of the armature from the core is to introduce a load or an impedance in the transient-limiting circuit during the opening operation, in order to create a so-called demagnetizing circuit which consumes the energy in a suitable manner without giving rise to too high transients. This functions well when there is only one single magnetic circuit. In cases where two or more magnetic circuits with series-connected coils are used, for example for driving several magnetic circuits located in one or more switching devices, it is, however, not sufficient to have one single load in the demagnetizing circuit since, when the first magnetic circuit starts separating, an energy is fed back to the other operating coils and the other magnetic circuits are retained harder, thus delaying the opening of these circuits. A very high impedance is required to obtain the desired simultaneous opening operation, and the voltage transients will then be high. SUMMARY OF THE INVENTION It is an object of the present invention to provide a switching device which comprises at least two magnetic circuits which control one or more switches located in either the same or in different circuits. It is another object to provide a switching device which allows simultaneous opening of at least two magnetic circuits. It is a further object to attain a switching device which comprises at least one or more series-connected operating coils where the voltage transients are minimized. To prevent the operating coils from influencing each other during the opening operation and to achieve simultaneous opening, an individual circuit for demagnetization is connected to each operating coil. Each demagnetizing circuit comprises an impedance for increasing the speed during the opening operation and the impedance consumes the energy stored by its magnetizing circuit and its operating coil. A control unit, for example a switching transistor, controls the voltage supplied to the operating coils . Experiments have shown that this results in a simultaneous opening operation from a number of magnetic circuits with operating coils connected in series. BRIEF DESCRIPTION OF THE DRAWINGS The invention will be explained in greater detail by description of a few embodiments with reference to the accompanying drawing, wherein Figure 1 shows an example of part of a switching device comprising two operating coils with one single demagnetizing circuit according to the prior art, Figure 2 shows an example of part of a switching device comprising two Operating coils with individual demagnetizing circuits for each operating coil according to one embodiment of the present invention, Figure 3 schematically shows an example of part of a switching device in which the operating coils, a control unit and a control circuit are shown, Figure 4 schematically shows an example of a switching device comprising two operating coils with individual demagnetizing circuits according to the present invention, which control the switches supplying voltage to two three-phase ac motors. DESCRIPTION OF THE PREFERRED EMBODIMENTS Figure 1 shows two operating coils 11, 12 connected in series and one demagnetizing circuit according to the prior art. A switch 13 controls the supply of voltage to the coils. The demagnetizing circuit of the operating coils comprises an impedance 14 and a freewheeling diode 15 connected in anti-parallel with the coil. The switch 16 is opened during breaking of the supply voltage of the opera- ting coils, which occurs via the switch 13. This circuit would require a very high impedance 14 to obtain a simultaneous opening, and the voltage transients will then be high. Figure 2 shows two operating coils 21, 22 connected in series. A switch 23 controls the supply of voltage to the coils. The first operating coil 21 is connected to its own demagnetizing circuit which comprises an impedance 24 and a freewheeling diode 25 connected in anti-parallel with the coil. The second operating coil 22 is connected to its own demagnetizing circuit which comprises an impedance 27 and a freewheeling diode 28 connected in anti-parallel with the coil. The switches 26 and 29 are opened upon breaking of the supply voltage of the operating coils, which occurs via the switch 23. With this ' ' circuit, a " simultaneous opening is obtained without high voltage transients occurring. Figure 3 shows a circuit comprising two operating coils with their respective demagnetizing circuits 31 and 32. The operating coils together operate a contactor (not shown) . The contactor is closed and is kept in the closed position by supplying a supply voltage U. to the connection terminals 33. The contactor is opened on disconnection of the supply voltage. The contactor is intended to be connected to either an ac voltage or a dc voltage. A supply voltage U. is supplied to the contactor via a full-wave rectifier 34, the output voltage U s of which is thus a constant dc voltage in the case of dc-voltage supply or a full-wave-rectified ac voltage in the case of ac-voltage supply. U s is supplied to the operating coils 31 and 32 of the contactor. The coils are connected in series with a switching transistor 35, for example a MOSFET or an IGBT and a series resistor Rl arranged for the voltage measurement. A control circuit 36 is adapted to control the voltage across the operating coils by pulse-width modulation, by means of the transistor 35. The control circuit 36 supplies a control signal U c which controls the transistor with a constant pulse frequency and with a variable pulse width. The control circuit 36 supplies a control signal U c to the switching transistor 35. The control circuit 36 is supplied with the voltage U m arising across the measuring resistors Rl, which voltage U m constitutes a measure of the current through the operating coil. A voltage divider, formed by resistors R2 and R3 , supplies to the control circuit a measurement signal U εm which is proportional to the voltage U s . The control circuit 36 receives a controlled supply voltage U f from a voltage controller 37. The control circuit 36 comprises components adapted, before a closing operation, to sense the voltage U. of the supply voltage source and, during the closing operation, to control the voltage supplied to the operating coils in dependence on the sensed supply voltage. The advantage of the type of control circuit described here is 'that, during the closing operation, the ' operating coils will be supplied with a voltage that is a constant and independent of the supply voltage. The closing operation thus always follows a certain desired sequence with regard to acceleration and speed of the armature of the contactor. Further, by supplying to the operating coils, during the closing operation, a voltage which is independent of the coil current, the current-reducing effect caused by the armature movement will exert full influence and reduce the final speed of the armature. This causes a lower final speed of the armature of the contactor, and in this way a considerably smoother closing operation is obtained, whereby the disadvantages such as wear, mechanical stresses and contact bouncing are reduced, which is of special importance in large contactors . This type of equipment is described in more detail in US 5914850. Figure 4 shows a switching device that comprises two opera- ting coils 41 and 42. Each operating coil has its own demagnetizing circuit that comprises impedances 43 and 44 and freewheeling diodes 45 and 46 connected in anti-parallel with the operating coils. The demagnetizing circuits comprise switches 47 and 48 which are opened when interrupting the voltage supply to the operating coils, which occurs through the switch 49. A control unit 50 controls the voltage supplied to the operating coils . The operating coils 41 and 42 operate two armatures 51 and 52 which, for example, are U-shaped or E-shaped magnets. Each armature influences a number of electric switches 53- 58. When the armature and the core close the magnetic circuits, the electric switches 53-58 close power-supply lines 59-64 which feed two three-phase ac motors 65 and 66. When the voltage supply to the operating coils 41 and 42 is interrupted, the armatures 51 and 52 separate from the cores with the aid of, for example, springs (not shown) which are tensioned upon closing of the switching devices, and at the same time the power supply to the motors 65 and 66 is interrupted . In one embodiment according to the present invention, the demagnetizing circuits comprise varistors or RC circuits. In an alternative embodiment, the switches are enclosed in, for example, a hermetically sealed capsule, where the components of the switching device are surrounded by a pure and dry gaseous volume which is enclosed in a diffusion-tight casing. In an additional embodiment, the circuit containing the operating coils is enclosed in one casing and the switches are enclosed in another casing with a connection between the casings. The switching devices described above are only examples of how a switching device may be designed according to the invention. A large number of other embodiments are feasible within the scope of the invention as defined in the appended claims .

Claims: CLAIMS 1. A switching device comprising at least two operating coils (21, 22, 41, 42) connected in series for operating at least two magnetic circuits (51, 52) , characterized in that each operating coil has its own demagnetizing circuit (31, 32) . 2. A switching device according to claim 1, characterized in that each demagnetizing circuit comprises an impedance (24, 27, 43, 44) . 3. A switching device according to claim 1 or 2 , characterized in that each demagnetizing circuit comprises a freewheeling diode (25, 28, 45, 46) connected in anti- parallel with the coil. 4. A switching device according to claim 1 or 2 , characterized in that each demagnetizing circuit comprises a varistor. 5. A switching device according to claim 1 or 2 , characterized in that each demagnetizing circuit comprises an RC circuit. 6. A switching device according to any of the preceding claims, characterized in that the components of the switching device are enclosed in a capsule. 7. A switching device according to claim 6, characterized in that said components are surrounded by a pure and dry gaseous volume which is enclosed in a diffusion-tight casing. 8. A switching device according to any of the preceding claims, characterized in that a control unit (35) and a control circuit control the voltage supplied to the operating coils. 9. A switching device according to claim 8, characterized in that the control circuit is adapted to supply to the operating coils a pulse-width-modulated voltage by means of the control unit (35) , the pulse width being maintained at a fixed value during the closing operation of the switching device . 10. A switching device according to any of the preceding claims, characterized in that the control circuit comprises components adapted to sense the voltage U. of the supply- voltage source prior to a closing operation, and to control the voltage supplied to the operating coils during the closing operation in dependence on the sensed supply voltage . 11. A method for operating at least two magnetic circuits (51, 52) of a switch in a switching device having at least two operating coils (21, 22, 41, 42), characterized in that a control unit (35) receives a signal from a control circuit and that operating coils are set in motion at the same time and that a simultaneous opening of all the contactors is achieved. 12. Use of a switching device according to any of claims 1 to 10 or the method according to claim 11 in a system which controls several electromagnetically controlled switches in one and the same piece of equipment. 13. Use of a switching device according to any of claims 1 to 10 or the method according to claim 11 in a system which controls several switches in two or more pieces of equipment by means of a centralized switching device which comprises at least two operating coils . 14. Use of a switching device according to any of claims 1 to 10 or the method according to claim 11 as a switching member between a voltage source and at least one load.

Inventors: JOHANSSON GUNNAR, JOHANSSON, GUNNAR
Applicants: ABB AB, JOHANSSON GUNNAR, JOHANSSON, GUNNAR
Application year: 2001
Main classification: H01H47
IPC codes: H02H9