Abstract:
An electromagnetic actuator for a circuit breaker having a pair of relatively moveable contacts is disclosed. The actuator includes primary actuator coupled to at least one of the contacts by a link mechanism operable to provide closing and holding forces to the contacts of the circuit breaker and a secondary, faster acting actuator which, on tripping thereof, provides sufficient force to at least initiate opening of the contacts by the configuration of the link mechanism. The secondary actuator includes a stored energy latch which has a permanent magnet flux circuit for providing a holding force and a coil connected to receive a trigger signal to overcome the permanent magnet flux to trip the latch.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to electromagnetic actuator devices suitable for use in operating electrical switchgear, such as vacuum circuit breakers. The invention has particular, though not exclusive, relevance to direct current circuit breakers and vacuum circuit breakers in general. 
     2. Related Art 
     High power circuit breakers require large opening and closing forces to overcome various contact forces encountered. This requires the use of large and heavy actuators which are consequently much slower to operate than their smaller equivalents. This is disadvantageous, particularly in DC circuits where a fast circuit breaking action is required. 
     In addition, because the contacts of such circuit breakers tend to wear with use, it is desirable to include, in the circuit breaker mechanism, means to accommodate an increasing relative distance between the contact surfaces when open, ie. means to provide an increasing actuation distance during the lifespan of the contacts. This is typically achieved by providing an electromagnetic actuator which drives a moving contact through a closing spring coupling, which absorbs any difference between actuator stroke length and actual contact travel distance. This feature, however, results in the creation of a snatch gap which means that the actuator does not even start to open the contacts until part way through its opening stroke, thereby slowing still further the circuit breaking operation. 
     It is an object of the present invention to provide an improved circuit breaker providing high speed current interruption. 
     SUMMARY OF THE INVENTION 
     According to one aspect, the present invention provides a circuit breaker which comprises a heavy duly first, or primary, actuator coupled to provide the necessary power to provide closing and holding forces to relatively moveable contacts of the circuit breaker, and a secondary, faster acting, actuator coupled to provide only sufficient power to open, or initiate opening, of the contacts. 
     Preferably, the primary actuator is adapted to reset the secondary actuator during completion of the opening stroke, and may be further adapted to provide the closing stroke without assistance from the secondary actuator. 
     According to another aspect, the present invention provides an actuator for a circuit breaker that includes a drive shaft for coupling to a moveable contact of a circuit breaker, a primary actuator mechanism operable to propel the drive shaft between a first position and a second position and a secondary actuator mechanism which, upon receiving a trigger signal, shortens the effective length of the drive shaft. 
     Preferably, the drive shaft includes an actuator rod coupled to an armature of the primary actuator mechanism which actuator mechanism is configured to drive the actuator rod in a direction substantially parallel to its longitudinal axis, and a link or mechanism means, coupled at a first end to the actuator rod and configured for coupling at a second end to the moveable contact of the circuit breaker, the link or mechanism means having first and second link members substantially axially aligned with the actuator rod in a first condition and non-aligned in a second condition. 
     According to another aspect, the present invention provides an actuator for a circuit breaker having a drive link for coupling to a moveable contact of a circuit breaker, a primary actuator mechanism adapted to drive the drive link from a first position to a second position during a closing stroke, a secondary actuator mechanism, operable in concert with said primary actuator to drive the drive link from the second position to the first position during an opening stroke wherein the secondary actuator mechanism includes a latch which is tripped during the first part of the opening stroke, and which is reset by the primary actuator mechanism during a subsequent part of the opening stroke. 
     Preferably, the actuator drive link comprises a rotating arm which pivots about an axis, the position of the pivot axis being determined by the operation of the secondary actuator mechanism. 
     Preferably, the secondary actuator is coupled to the rotating arm by a spring link adapted to provide a snatch gap Alternatively, the spring link, to apply pressure to the moveable contact, could be coupled to the primary actuator. 
     Embodiments of the present invention will now be described in detail by way of example and with reference to the accompanying drawings in which: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIGS. 1 and 2 show schematic cross-sectional diagrams of a magnetic actuator useful in explaining the principles of a circuit breaker according to the present invention; 
     FIG. 3 shows a side view of a circuit breaker according to the present invention; 
     FIG. 4 shows a perspective view of the circuit breaker of FIG. 3; 
     FIGS. 5,  6  and  7  show a detailed schematic side view of a circuit breaker according to the present invention in three stages of operation, respectively closed, tripped and open; and 
     FIGS. 8,  9  and  10  show schematic diagrams of a circuit breaker in various stages of operation, namely closed (FIG.  8 ), partially opened (FIG. 9) and fullly opened (FIG.  10 ). 
    
    
     DETAILED DESCRIPTION 
     Throughout the present specification, references to relative orientation of parts of the described mechanisms (eg. upward, downward, leftward and rightward) are used for clarity referring only to the orientations shown in the drawings. It will be understood that the mechanisms described can be provided in any orientation. 
     With reference to FIGS. 1 and 2, an exemplary bistable magnetic actuator  1  suitable for use as a primary actuator mechanism of the present invention will now be described. The actuator  1  comprises a moving armature  2  coupled to, and co-axial with, a non-magnetic drive rod  3 , a solenoid or coil  4  surrounding and co-axial with the armature and drive rod, a cylindrical permanent magnet  5  radially polarized and also co-axial with the armature and drive rod. The armature  2  and drive rod  3  are axially displaceable with respect to the coil  4  and permanent magnet  5 . The actuator  1  is housed within a mild steel casing  6  which provides an external magnetic circuit. An opening spring  7  may be provided to assist in providing bias to the armature and drive rod in one direction. 
     The actuator  1  is shown in FIG. 1 in the open contacts position, in which the armature is in the lower of two stable positions. It is held in that position by magnetic flux from the permanent magnet  5  forming a magnetic circuit as indicated by the flux path  10  (bearing double arrows) and by the opening spring  7 . There is also another secondary permanent magnet flux path  11  (bearing single arrows). However, there will be very little flux in this magnetic circuit due to the presence of an air gap  15  between the armature  2  and the upper pole piece  16  of the external magnetic circuit of casing  6 . The armature  2  is therefore very firmly held in the open position. 
     In order to close the circuit breaker, the actuator coil  4  is energized by a pulse of direct current setting up a magnetic flux as indicated by flux path  12  (bearing triple arrows). This flux is in opposition to the permanent magnet flux  10  holding the circuit breaker open and is in the same direction as the weak permanent magnet flux  11  across the air gap  15 . As the current increases in the coil  4 , the point is reached where the increasing flux across the air gap  15  creates a greater attractive force than the decreasing holding force at the bottom of the actuator and the armature  2  begins to move upward. Once the armature  2  has moved, the holding force at the bottom becomes very low as an air gap  17  (FIG. 2) has been introduced and the air gap  15  begins to close at the top, further increasing the closing force. 
     The armature  2  moves to the upper position, closing the circuit breaker and compressing the opening spring  7  during the closing stroke. The actuator is now in the position shown in FIG.  2  and is held in this position by the strong permanent magnet flux of flux path  21  (bearing double arrows). The permanent magnet flux through path  20  (bearing single arrows) is very low. The holding force is designed to be sufficiently greater than the forces of the contact pressure and opening spring  7  and the blow-open forces of short-circuit current such that under all conditions of temperature, component variation, shock etc, the circuit breaker will remain closed. 
     To trip the circuit breaker, the actuator coil is pulsed with direct current in the opposite direction to that required to close the circuit breaker, setting up the flux shown in path  22  (bearing single arrows). This flux opposes the holding flux thereby reducing the holding force to such an extent that the opening spring and contact pressure forces can cause the armature  2  to move in a downward direction. The trip current is generally much less than the closing current. 
     With reference to FIGS. 3 and 4, there is shown one embodiment of a circuit breaker  40  which effectively accelerates the opening stroke beyond that which would be provided solely by a primary actuator  30 . The circuit breaker generally includes a heavy duty primary actuator  30  in conjunction with a faster acting secondary actuator  70 , coupled to a contact arm of the circuit breaker by a link mechanism  50 . 
     The output  31  of the primary actuator  30  is coupled to the link mechanism  50  which connects the actuator  30  with a moveable contact arm  60 . The moveable contact arm  60  is mounted on a pivot  63  and is shown in its closed condition in FIGS. 3 and 4, biased against a non-moving contact  61  by the action of the primary actuator  30 . An opening spring  62  provides an opening bias to the moveable contact arm  60 . 
     The link mechanism  50  comprises a first link arm  51  and a second link arm  52  which are pivotally attached to one another at au intermediate pivot  53  and, respectively, to the output  31  of the actuator  30  (at pivot  54 ) and to the moveable contact arm  60  (at pivot  55 ). In the contacts closed position shown, the first link arm  51  and the second link arm  52  are approximately in axial alignment with the output  31  of the actuator  30 . 
     The secondary actuator  70  has an actuator rod  71  which is connected to the link mechanism  50  at the intermediate pivot  53  and is displaceable by the secondary actuator stroke in a direction which is non-parallel, and preferably approximately orthogonal to, the first and second link arms. It will be understood that the actuator rod  71  need not be coupled to the link mechanism at the intermediate pivot  53 , but could be coupled at any suitable position along the lengths of either the first or second link arms  51 ,  52  in order to vary the ratio of secondary actuator stroke length to intermediate pivot  53  displacement. The secondary actuator  70  is pivotally coupled to the same chassis or sub-frame (not shown) as the primary actuator  30  and contact assembly, by an anchorage  73 . 
     The function of the circuit breaker  40  Will now be described with reference to the FIGS. 5,  6  and  7 , which provide a detailed schematic view of preferred embodiments of primary and secondary actuator mechanisms  30 ,  70  and a drive shaft connecting the primary and secondary actuators to the moveable contact  60 . 
     FIG. 5 shows the circuit breaker in closed condition; FIG. 6 shows the circuit breaker in tripped condition; and FIG. 7 shows the circuit breaker in open condition. Where components have the same or similar functions to the components described in connection with FIGS. 1 and 2, the same reference numerals have been used. 
     The primary actuator  30  uses the same principles of bistable operation as described in connection with actuator  1  of FIGS. 1 and 2, but uses an internal closing and contact pressure spring, to accommodate variations in maximum contact separation, by provision of a snatch gap. It will be understood, however, that tie particular type of actuator mechanisms used for the primary and secondary actuators may be varied. 
     Referring to FIG. 5, the primary actuator  30  includes a short moving armature  2  which is in axial sliding engagement with the non-magnetic drive rod  3  which passes axially therethrough. The primary actuator  30  includes a coil  4 , cylindrical permanent magnet  5  and a steel casing  6  which provides the external magnetic circuit. The actuator also includes an internal closing spring  37  which resides within a flux conducting cylinder  9 . The armature is magnetically bistable in both left and right positions of FIGS. 5 and 7 using similar principles as explained in connection with FIGS. 1 and 2. 
     The armature  2  transmits its leftward motion (corresponding to opening the circuit breaker) to the drive rod  3  by way of a first collar  32  attached to the drive rod  3 , and transmits its rightward motion (corresponding to closing the circuit breaker) to the drive rod  3  by way of closing spring  37  and a second collar  33  attached to the drive rod  3 . In the closed position shown in FIG. 5, the closing spring  37  is in compression, leaving a small gap  34  between the first collar  32  and the left hand face  38  of the armature  2 , and a corresponding gap  35  between the second collar  33  and the internal radial face  39  of the flux conducting cylinder  9 . These gaps  34 ,  35  correspond to a degree of overtravel of the armature  2  to effect contact closure which thereby allows for contact wear and provides sufficient degree of closing spring  37  compression to give the necessary holding force to resist the blow-open forces and opening spring forces. 
     The secondary actuator  70  is, in principle, a stored energy latch device which includes an actuator rod  71  coupled telescopically to the anchorage  73  which is pivotally attached to the chassis (not shown). The telescopic coupling includes a trip spring  72  which provides an extending bias to the telescopic connection. The trip spring  72  is compressed in the closed position of FIG.  5 . The drive rod  71  supports a magnetic disc  75  which is normally retained by a permanent magnet flux circuit holding force provided by an electromagnetic mechanism  74  of the secondary actuator. The mechanism  74  also includes a coil which, upon receiving a trip signal, overcomes the permanent magnet holding flux such that the trip spring  72  can displace the rod  71  and disc  75  rapidly in an upward direction. 
     The upper end of the actuator rod  71  is connected to the link mechanism  50  which connects the output  31  of the primary actuator  30  to the movable contact arm  60 . As previously discussed, the link mechanism  50  is preferably formed from first and second link arms  51 ,  52  angularly displaceable in relation to one another in the form of a knee joint about pivot  53 . The two link arms  51 ,  52  together, in effect, form a variable length extension of the drive rod  3 . In the closed condition of FIG. 5, the two link arms are substantially in alignment with one another and with the drive rod  3 , provide a fill length extension to maintain the moving contact  60  in engagement with the non-moving contact  61 . 
     Referring now to FIG. 6, an overcurrent condition is detected and is is conveyed to both the primary and the secondary actuator. The secondary actuator, being of a faster acting type, energises its coil to overcome the permanent magnet holding force on disc  75  and thereby releases actuator rod  71  under the power of the trip spring  72 . This causes the knee joint formed by link arms  51 ,  52  to pivot with a consequent effective shortening of the link mechanism. This occurs prior to the slower acting primary actuator commencing its opening movement, as shown in FIG. 6 as the intermediate “stripped” condition. The trip signal is generated either by a control circuit, and/or the direct current itself may be used to energise the coil in the secondary actuator  70 . The primary current may itself flow through the secondary actuator and cause it to unlatch. 
     In preferred embodiments, the action of the secondary actuator  70  can be designed to have a number of effects. As shown in FIG. 6, the secondary actuator  70  may have sufficient energy and stroke length to completely open the contacts  60 ,  61  of the circuit breaker ahead of the opening stroke of the primary actuator  30 . The force available to open the contacts can be varied according to a number of design parameters, including: the strength of the trip spring  72 ; the mechanical advantage offered to the secondary actuator by the position of its connection to the link arms  51  or  52  (ie. The geometric configuration); and the strength of the closing spring  37  of the primary actuator  30  in combination with the inertial mass of the spring  37 /drive rod  3  combination and the size of gaps  34 ,  35 . 
     In another embodiment, the secondary actuator  70  may be designed simply to close the snatch gap  34 ,  35  such that the primary actuator  30  is able to immediately commence movement of the drive rod  3  during its opening stroke. 
     In either of the above cases, once the moving contact  60  is fully opened (as limited by a mechanical stop, not shown), either before or during movement of the primary actuator  30  in its opening stroke, the completion of the opening stroke of the primary actuator  30  can be used to recharge or assist in recharging the trip spring  72  of the secondary actuator  70 . Once the moving contact reaches its maximal opening position as shown in FIG. 6, the continued leftward movement of drive rod  3  acts to return the link mechanism  50  to its extended condition with or without assistance from the electromagnetic mechanism  74 . Once in the fully open position (FIG.  7 ), the disc  75  is retained by the permanent magnet flux from the mechanism  74  to retain the secondary actuator  70  in its charged condition. Thus, subsequent closure of the circuit breaker  40  by the closing stroke of the primary actuator  30  can be effected without any action by the secondary actuator  70 . The pivotable connection of the secondary actuator to the chassis (not shown) ensures that the primary actuator can close the contacts independent of the secondary actuator. 
     It will be understood that the link mechanism  50  can be effected in a number of different ways. The embodiment shown uses a knee-type joint coupled to an electromagnetic secondary actuator  70  to achieve a shortening of the effective length of the link mechanism and thus of the primary actuator overall drive shaft. 
     The link mechanism  50  could, for example, alternatively be provided by a sprung telescopic link biased to a contracted condition, with a mechanical release latch which is triggered by a suitable electromechanical or electromagnetic actuator. 
     In another embodiment, the secondary actuator mechanism could be housed in the same casing as the primary actuator mechanism. 
     In another embodiment, now described in connection with FIGS. 8 to  10 , the secondary actuator may be operative to displace a pivot point of a drive link. 
     Referring to a schematic FIG. 8, a primary actuator  100  has au armature which is operable between a first position indicated at A, and a second position indicated at B. Preferably, the actuator includes a spring bias toward position B indicated by spring  111 . The primary actuator  100  is coupled, via first, second and third drive links  101 ,  102  and  103  to a moving contact assembly  104  of a circuit breaker, which circuit breaker also has a fixed contact assembly  105  and an opening stop  106  to limit travel of the moving Contact, which fixed contact and opening stop are fixed relative to a supporting structure, not shown. 
     The first and second drive links  101 ,  102  are pivotable relative to one another by a pivot  106 ; the second and third drive links  102 ,  103  are pivotable relative to one another by a pivot  107 ; and the third drive link  103  is pivotable relative to the moving contact  105  by a pivot  108 . The second drive link  102  is also rotatable about an intermediate point along its length at pivot  109 . The moving contact  104  is preferably pivoted about a fixed reference point relative to the supporting structure at pivot  110 . 
     The pivot  109  is not, however, fixed relative to the supporting structure, but moves according to a secondary actuator  120  represented in FIG. 8, the operation of which is described hereinafter. The secondary actuator  120  is operable to move between a latched position (indicated by C) as shown in FIG.  8  and an unlatched position (indicated by D) as shown in FIG.  9 . The actuator  120  also includes a spring bias to position D, as represented by  121 . The secondary actuator  120  and the spring  121  are operative to drive a fourth drive link  122 , about a pivot  123  fixed relative to the support structure, between positions indicated by E and F (see FIGS. 8 and 9, respectively). 
     A first end of a contact spring link  125  is coupled to the drive link  122  by a pivot  124 . At the other end of the contact spring link  125  is the moving pivot  109 . The contact spring link  125  does not, however, provide a fixed distance between the pivot  124  and the pivot  109 : the distance between pivot  124  and pivot  109  is extendable within predetermined limits, and is biased by a contact spring represented at  126  to an extended state. This provides for the necessary snatch gap which allows for contact wear and maintenance of contact pressure as discussed earlier. This extendable nature of the link can be provided in a number of ways well understood by the person skilled in the art. 
     The operation of the circuit breaker will now be described, starting from the closed condition indicated by FIG.  8 . To trip the circuit breaker open, a release signal is provided to the secondary actuator  120  in similar manner to that described in connection with the secondary actuator  70  (FIG.  6 ), which causes rapid acceleration of the link  122  in an anticlockwise direction about pivot  123  under the bias of spring  121 . The first part of this motion closes the snatch gap in the contact spring link  125 ; the second part of the motion opens the moving contact  104 . 
     Now referring to FIG. 9, the moving contact  104  has fullly opened and hit the opening stop  106  preventing further movement of the moving contact. At the same time as, or some time later than, the secondary actuator  120  is operated, the primary actuator moves through its opening stroke from position A to position B, thereby propelling the drive link  101  so that drive link  102  rotates in a clockwise direction about moving pivot  109 . 
     Of course, depending upon the precise relative timing of operation of the secondary and primary actuators  100 ,  120 , the rotation of the drive link  102  will be accelerated or slowed. However, as soon as the position of FIG. 9 is reached, further movement of the pivot  109  toward the contact  104  is prohibited by the opening stop  106 , and the primary actuator continues with its opening stroke from position A to position B, which motion recharges the contact spring link  125 , and thereby latches and resets the secondary actuator. 
     Control of die primary actuator  100  movement may be effected in a number of ways, including electronic control. The opening stroke may be triggered by way of a microswitch or other device linked to the actuation of the secondary actuator. 
     Return of the moving contact to the closed position of FIG. 8 from the open position of FIG. 10 is effected by operation of the primary actuator  100  alone, to drive the armature from the position indicated at B to the position indicated at A. The secondary actuator remains latched during this closing stroke.