Abstract:
The present invention relates to a switching device ( 14 ) which is arranged to switch a load ( 12 ) to a direct voltage circuit ( 13 ). The switching device ( 14 ) comprises a switching member ( 14   a ) consisting of at least three contacts ( 7,8,9 ), a control member ( 10 ) which controls the contacts ( 7,8,9 ) of the switching member ( 14   a ) and has the positions open [ 0 ] and closed [ 1 ], and one or more spring members ( 16   a,    16   b ) which are tensioned by the control member ( 10 ) and control the contacts. The switching device is characterized in that the switching member ( 14   a ) of the switching device ( 14 ) comprises a limiter arrangement which is arranged to limit closing of at least one contact ( 7 ) when the control member ( 10 ) is turned from the open [ 0 ] position into the closed [ 1 ] position, the limiter arrangement comprising a limiter ( 1 ) and a toggle joint ( 17 ) connected thereto and a solenoid ( 4 ) arranged in the toggle joint, the toggle joint ( 17 ) being arranged to be controlled by the solenoid ( 4 ), as a result of which the toggle joint ( 17 ) turns the limiter ( 1 ) so that it releases the contact ( 7 ) of the switching member ( 14   a ) whose closing has been prevented, allowing the contact to close.

Description:
FIELD OF THE INVENTION 
     The invention relates to a switching device which is arranged to switch a load to direct voltage. The switching device of the invention is used in applications where a high capacitive load is switched to direct voltage, in which case the charging current of the capacitive load needs to be limited. 
     BACKGROUND OF THE INVENTION 
     A system which has a high capacitive load and is connected to a direct current circuit has to be protected during voltage switch-on. If the capacitors of the system are not charged, sudden switch-on of direct voltage generates a surge current, which may damage system components switched behind the switch. Usually the system is protected by using a three-position and three-pole switching device which supplies the charging current of the capacitive load through a charging resistor to the load during charging. The charging resistor limits the charging current and prevents problems that sudden charging may cause in the system. After charging the charging resistor is bypassed by switching the load directly to direct voltage. An example of a device with a high capacitive load is a frequency converter/converters which is/are switched to a direct voltage supply circuit. 
     One prior art solution is shown in FIG.  7 . This solution employs a manual three-position switching device for switching a high capacitive load to direct voltage. The switching device comprises a three-position control member which is controlled by turning the handle attached to the control shaft of the control device. During charging the current is supplied to charging resistors via an array of auxiliary contacts. The positions of both the switching device and the control member are Open [ 0 ], Charging [C] and Closed [ 1 ]. Turning of the control member and the switch from the switching position into the closed position is prevented by a limiter, such as a magnetic switch. The control member and the switch cannot be turned into the closed position until the limiter allows this. The operation of the limiter is controlled by a separate control signal, which is dependent on the charging state of the capacitive load. 
     A problem related to the prior art switching device is that the control device of the switching device has three positions and an array of auxiliary contacts. This means that controlling of the switching device includes a phase which is unnecessary for switching, i.e. charging phase, which requires that the person performing switching manually keeps the switching device in the charging [C] position until the limiter allows turning of the switch into the closed [ 1 ] position. This makes switching more difficult and involves the risk of misuse of the switching device if the person performing switching is unfamiliar with the special functions of the switching device. Secondly, the array of auxiliary contacts comprises a large number of moving mechanical parts which are easily damaged. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The object of the present invention is to eliminate disadvantages of the prior art by providing an improved switching device where the operation of the switching member corresponds to that of a three-position switching device but whose structure provides a two-position control device with only the positions open [ 0 ] and closed [ 1 ]. In that case the use of the switching device in switching is considerably more natural to switch of this kind as well as more reliable and safer. 
     This is achieved by a switching device which is characterized by what is disclosed in the claims. 
     The switching device is arranged to switch a load to a direct voltage circuit. It comprises a switching member consisting of at least three contacts, a control member which controls the contacts of the switching member and has the positions open [ 0 ] and closed [ 1 ], one or more spring members which control the contacts and are tensioned by the control member, and the switching device is characterized in that the switching member of the switching device comprises a limiter arrangement which is arranged to limit the closing of at least one contract when the control member is turned from the open [ 0 ] position into the closed [ 1 ] position, the limiter arrangement comprising a limiter, a toggle joint connected thereto and a solenoid arranged in the toggle joint, which is arranged to be controlled by the solenoid, as a result of which the toggle joint turns the limiter so that it releases the contact of the switching member whose closing has been prevented, allowing the contact to close. 
     The preferred embodiments of the invention are disclosed in the dependent claims. 
     For charging a capacitive load, the switching device is provided with a limiter arrangement, i.e. a mechanism which limits the movement of the third contact of the switching device when the control device is turned from the open [ 0 ] position into the closed [ 1 ] position and prevents the third contact of the switching member from closing when the control device is turned. The mechanism leaves the contact open to wait for release in an excited state generated by spring force. The contact is released after a certain charging state of the capacitive load has been reached. 
     The mechanism consists of a limiter, which is preferably a lever arm, and of a toggle joint attached thereto. The first end of the lever arm is connected to the toggle joint and the second end is connected to the control member of the third contact in the switching device. The toggle joint comprises a fold where an electrically-controlled solenoid is arranged. The solenoid generates a pulse in the fold of the toggle joint so that the toggle joint can be folded and the spring tension released, which allows the third contact to close. 
     In an embodiment the joint at the first end of the limiter, where the limiter is connected to the shaft of the control device, comprises a clearance. The clearance allows the shaft to rotate by a certain angle without moving the limiter. This way the spring member of the switching device is tensioned for release but turning of the shaft does not influence the position of the limiter. The clearance is preferably implemented as follows: the first end of the limiter, where the limiter is connected to the shaft of the control device, is provided with an opening through which the shaft is guided. The opening is shaped to allow rotation of the shaft by a certain angle without moving the limiter, as a result of which the spring member of the switching device is tensioned for release without the turning of the shaft affecting the position of the limiter. 
     In another embodiment all contacts of the switching device are controlled by a common shaft. In that case the necessary clearance can be implemented by controlling the first and the second contact with a common shaft. At the third contact the shaft comprises a connecting sleeve for implementing the clearance. 
     A sensor, such as a voltage sensor, is arranged to sense the charging state of the capacitive load and to control the solenoid. 
     The switching device of the invention thus has three operating positions, i.e. open [ 0 ], charging [C] and closed [ 1 ], whereas the control member of the switching device preferably has only two positions, i.e. open [ 0 ] and closed [ 1 ]. This makes the switching device simple and safe to use since the control member of the switching device needs to be turned only between the open [ 0 ] position and the closed [ 1 ] position thanks to the fact that the mechanism of the invention provides the switching device with automatic operation for controlling the operation of the switching device between the positions charging [C] and closed [ 1 ]. 
     A further advantage of the solution is that no auxiliary contacts are needed in the switching device implemented according to the invention, and consequently the structure of the device is simpler and more reliable. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     In the following, the invention will be described in greater detail by means of the accompanying drawings, in which 
     FIG. 1 is a wiring diagram of the frequency converter output, showing a switching device according to the invention, 
     FIG. 2 schematically illustrates the limiter mechanism of the third contact in the switching device  1  according to FIG. 1 when the switch is in the open position, 
     FIG. 3 schematically illustrates the limiter mechanism of the third contact in the switching device  1  according to FIG. 1 when the switch is in the charging position, and 
     FIG. 4 schematically illustrates the limiter mechanism of the third contact in the switching device  1  according to FIG. 1 when the switch is in the closed position, 
     FIG. 5 illustrates the structure of the sleeve for providing the shaft controlling the contacts of the switching device with a necessary clearance for limiting closing of the third contact, 
     FIG. 6 schematically illustrates an embodiment for providing the clearance between the limiter of the limiter mechanism and the control member in the third contact of the switching device according to FIG. 2, and 
     FIG. 7 illustrates a prior art switching device which comprises a three-phase control device and a charging circuit. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An example of the structure and function of a preferred embodiment of the typical switching device according to the invention will be described with reference to FIGS. 1 to  6 . 
     According to FIG. 1, the switching device  14  comprises a switching member  14   a,  which consists of contacts  7 ,  8 ,  9 , which are closed and opened by turning the handle  6  of the control member  10  of the switching device  14 . The switching device  14  is arranged between a direct current circuit  13  and a load  12 . The load  12  is e.g. a frequency converter which drives an electric motor. The switching device  14  is thus arranged to switch the load  12  to the direct voltage  13 . 
     The switching device  14  includes a switching member  14   a  consisting of three closable contacts  7 ,  8 ,  9 . Naturally the number of the contacts can be larger than three if the application requires more contacts. The first contact  8  is arranged to connect the negative pole  13   c  of the direct voltage circuit  13  to the load. The second contact  9  is arranged to connect the positive pole  13   b  of the direct voltage circuit  13  to the load  12  via a charging resistor  11  connected in parallel with the contact  9 . The third contact  7  is arranged to connect the positive pole  13   a  of the direct voltage circuit  13  to the load  12  after a delay. 
     The contacts  7 , 8 , 9  of the switching member  14   a  are controlled by a control member  10 , which comprises a control shaft  10   a  whose one end is provided with a handle  6 . Depending on the embodiment, the control shaft  10   a  may comprise one or more shafts and mechanical components for providing the shaft with clearances needed in this type of embodiments. Closing and opening of the contacts  7 , 8 , 9  of the switching member  14   a  is controlled by spring members  16   a  and  16   b,  which are tensioned when the control member  10  is turned either into the open position or into the closed position, and the spring members  16   a,    16   b  are arranged to be released as the handle  6  of the control member  10  reaches either the closed or the open position of the switch, thus causing sudden closing or opening of the contacts  7 , 8 , 9  of the switching member  14   a.  In that case the control shaft  10   a  is connected to the spring members  16   a,    16   b  e.g. via a cam  20  and turning of the shaft  10   a  presses the spring of the spring member  16   a,    16   b  together if it is a coil spring and thus the spring  16   a,    16   b  is tensioned. When the cam  20  revolves past its dead centre, the spring  16   a,    16   b  is released and it closes or opens the contact. 
     The switching device  14  has three operating phases, including the following positions: open [ 0 ], charging [C] and close [ 1 ]. The control member  10  of the switching device  14 , on the other hand, has two operating phases, including the positions open [ 0 ] and closed [ 1 ]. Thus a two-phase control member  10  is used for controlling a three-phase switching device  14 . For this purpose the switching device  14  is, according to FIGS. 2 to  4 , provided with a mechanism which limits closing of the third contact  7  of the switching member  14   a  when the control member  10  is turned from the open [ 0 ] position into the closed [ 1 ] position and prevents closing of the contact  7  due to turning of the control member  10 . The mechanism leaves the contact  7  open to wait for external release in an excited state. 
     According to an embodiment, the mechanism consists of a limiter  1 , which is preferably a lever arm  1 , and of a toggle joint  17  connected thereto. The first end of the limiter  1  is connected to the shaft  10   a  of the control member  10  and the second end to the first end of the first lever arm  2  of the toggle joint  17 . The second end of the first lever arm  2  of the toggle joint  17  is connected to the first end of the second lever arm  3 , in which case a fold  15  is formed in the toggle joint  17  at this point. The second end of the second lever arm  3  of the toggle joint  17  is attached to a fixed supporting point  18  by means of bearings. An electrically controlled solenoid  4  is arranged in the fold  15  of the toggle joint  17 , the solenoid being arranged to generate an impulse, such as a strike, in the fold  15  of the toggle joint  17 . 
     According to FIG. 6, the operation of the limiter  1  is based on the fact that the joint at the first end of the limiter  1  for connecting the limiter  1  to the shaft  10   a  of the control member  10  comprises a clearance. This clearance is implemented e.g. by providing the first end of the limiter  1 , where the limiter  1  is connected to the shaft  10   a  of the control member  10 , with an opening  10   b  through which the shaft  10   a  is guided. The opening  10   b  is shaped to allow the shaft  10   a  to rotate by a certain angle to tension the spring member  16   b  without the turning of the shaft  10   a  moving the limiter  1 . One way of implementing clearance of this kind is to use a shaft  10   a  with a quadrangular cross-section and a cross-shaped opening  10   b  with rounded edges in the limiter  1  as shown in FIG.  4 . In that case the shaft  10   a  is connected to the spring member  16   b  by a cam  20 , for example, and turning of the shaft  10   a  presses the spring member  16   b  together if the spring member is a coil spring, thus tensioning the spring  16   b.  At the third contact  7  the limiter  1 , however, limits the movement of the cam  20  to a point where the cam has revolved slightly past its upper dead centre, in which case the spring member  16   b  cannot release the contact  7  into the closed position. It is not until the solenoid  4  arranged in the fold  15  of the toggle joint  17  generates a release impulse in the fold  15  of the toggle joint  17  that the toggle joint  17  folds and the spring  16   b  is released, thus closing the contact  7 . 
     In another embodiment all contacts  7 , 8 , 9  of the switching device are controlled by a common shaft  10   a.  In that case the necessary clearance can be implemented by controlling the first contact  8  and the second contact  9  by a common shaft and by providing the shaft  10   a  at the third contact  7  with a connecting sleeve  19  of FIG. 5 for providing the clearance. The shaft  10   a  from the first and the second contact to the first end of the sleeve  19  is e.g. quadrangular and the sleeve  19  is provided with a quadrangular receiving member for receiving the shaft end, the shape of the member corresponding to that of the shaft  10   a.  Correspondingly, the second end of the sleeve  19  is provided with a cross-shaped receiving member  10   b  with rounded edges for receiving the quadrangular shaft  10   a,  which controls the third contact. This clearance implemented by the sleeve  19  allows the shaft  10   a  to rotate by a certain angle to tension the spring member  16   b  without the turning of the shaft  10   a  moving the limiter  1 . Otherwise the embodiment functions in the same way as the one described above. 
     The switching device  14  functions as follows: In the initial situation the handle  6  of the control member  10  of the switching device  14  is in the open [ 0 ] position according to FIG.  2 . All contacts  7 , 8 , 9  of the switching member  14   a  are open. When the control member  10  of the switching device  14  is turned into the closed [ 1 ] position according to FIG. 3, the first contact  8  and the second contact  9  of the switching member  14   a  close simultaneously. The first contact  8  connects the load  12  to the negative pole  13   b  of the direct voltage circuit  13 . A resistor  11  is connected in parallel with the second contact  9  to restrict the current in the circuit. The second contact  9  connects the load  12  to the positive pole  13   c  of the direct voltage circuit  13  through the resistor  11 . At the third contact  7  turning of the shaft  10   a  into the closed [ 1 ] position tensions the spring member  16   b.  The limiter  1 , however, limits release of the third contact  7  into the closed position. The device that constitutes the load  12  comprises a sensor  5 , such as a voltage sensor, which senses the voltage of the load  12  capacitance and thus the charging state. When the charging state reaches a predetermined level, the sensor  5  gives a switching signal to the solenoid  4  e.g. by closing the contact  5 , which switches voltage to the solenoid  4  and thus the solenoid  4  generates an impulse in the fold  15  of the toggle joint  17  and releases the spring  16   b.  In that case the toggle joint  17  folds as shown in FIG. 4, releasing the limiter  1  that limits the closing of the third contact  7  of the switching member  14   a.  Consequently, the third contact  7  closes and switches the positive pole  13   a  of the direct voltage circuit  13  to the load  12  of the second current path in parallel with the current path that comprises the charging resistor  11 . In the final situation all three contacts  7 , 8 , 9  are thus closed. 
     The load  12  is switched off from the direct voltage circuit as follows: In the initial situation the handle  6  of the control member  10  in the switching device  14  is closed [ 1 ]. All contacts  7 , 8 , 9  of the switching member  14   a  are closed. The control member  10  of the switching device  14  is turned into the open [ 0 ] position. The contacts  7 , 8 , 9  of the switching member  14   a  open simultaneously and switch off the load  12  from the direct voltage circuit  13 . 
     If the handle  6  of the control member  10  in the switching device  14  is turned from the closed [ 1 ] position back into the open [ 0 ] position before the third contact  7  of the switching member  14   a  has closed, the spring member  16   b  at the third contact  7  of the switching member is released without releasing the contact  7  and the contacts  8 , 9  of the switching member  14   a  open simultaneously and disconnect the load  12  from the direct voltage circuit  13 . 
     It is to be understood that the above description and the related drawings are only intended to illustrate the invention. Thus the invention is not limited only to the above description or to the embodiment disclosed in the claims, but it is obvious to a person skilled in the art that the invention can be varied and modified in several ways within the inventive concept defined in the appended claims.