Abstract:
A two pole contactor, particularly for a domestic electricity meter, comprising a solenoid with a plunger actuator and a movable contact for each pole mounted on a pivotal blade in a symmetrical opposed configuration. The plunger is connected to the blades by a leaf spring whose ends engage sliders connected to the blades to impart a similar and even movement to each blade.

Description:
FIELD OF INVENTION 
     The present invention relates to a two pole contactor, particularly for use in domestic electricity meters in which it is desired to have a total isolation between the utility or electricity supply metering side and the domestic circuits. 
     BACKGROUND TO THE INVENTION 
     The distribution system in North America is such that domestic premises are fed with a 2-phase (180° phase relationship) utility supply, the local transformer centre tap giving an artificial Neutral for normal low-current loads at 115 V, while the voltage across phases is 230 V for power loads such as air-conditioning, motor drives and heaters. The local transformer primary is usually fed from an overhead fused 25 KV supply, so that the contactor switch contacts must safely withstand any reasonable short-circuit fault on the load side of the meter. 
     Known contactor designs exist for performing such switching functions in association with domestic electricity meters used in North America. 
     In U.S. Pat. No. 4,388,535 the feed connections are provided with sets of fixed pairs of contacts, and related sets of sprung, contacted shorting bars are positioned in proximity to the fixed contact sets, such that when they are actuated the two switch sets make contact, connecting the feed or utility side to the domestic load side. 
     Actuation is achieved by a moving plunger within a power solenoid coil, and a set of pivoted bellcrank levers operate to push open the sprung shorting bars or to retract to close them, the spring forces providing the necessary contact closure. A microswitch is used to interrupt the solenoid coil drive during the OPEN and CLOSE actuation functions, ensuring that the energisation is only momentary, thus preventing the coil from over-heating and possible burn-out. 
     In U.S. Pat. No. 4,430,579 the construction is similar to U.S. Pat. No. 4,338,535, using sprung contacted shorting bar switch sets to create the 2-pole contactor function. But the actuation method adopted is different in that the solenoid is double-acting, the plunger being naturally attracted centrally into a power drive coil when energised, this being the point of greatest flux concentration. In being attracted centrally, the plunger is dynamically over-driven past its centre to mechanically latch at each end of its stroke. The coil power is typically 2,000 W for a reliable double-action mechanical latching function. 
     This solenoid double-action is used to translate the switching function via suitably guided roller-aided push rods, either to CLOSE or OPEN the two sprung switch sets, the contact closure force being provided by the compression springs behind each shorting bar. In order to ensure that the contacts do not separate under short-circuit fault conditions, a relatively high force must be applied by each compression spring. 
     The solenoid plunger is profiled in such a way as to perform both the translation and mechanical latching functions simultaneously. A variant of the profiled plunger uses a similarly profiled, hardened steel plate suitably pinned to the plunger, to perform the same mechanical translation and latching functions, respectively. A microswitch is again used to interrupt the solenoid coil drive to prevent the coil from over-heating. 
     It is an object of the present invention to provide an improved two-pole contactor. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention there is provided a two-pole contactor comprising a solenoid having a plunger actuator, a fixed contact and a moveable contact for each pole, the moveable contacts being each symmetrically mounted on a pivotal blade, in which the plunger is connected to the centre of a leaf spring, whereby in use the ends thereof impart a similar and even movement to each blade. 
     According to another aspect of the present invention there is provided a contactor having at least a single pole pair of contacts and a solenoid operated plunger to actuate the contacts, in which the part of the plunger external to the solenoid is made of non-magnetic material to reduce the influence of the interfering magnetic fields during the excess current or short-circuit fault conditions. 
     According to a further aspect of the present invention there is provided a contactor comprising a solenoid with a plunger actuator mounted within a metal frame and biased by a spring to the open condition of the contactor, the plunger contacting a stop on the frame in the closed condition, in which the status of the contactor is determined by passing a voltage between the frame and the spring, so that a circuit is made when the plunger contacts the stop in said closed condition. 
     Other features of the invention are defined in the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A contactor in accordance with the invention will now be described, by way of example only, with reference to the accompanying drawings in which: 
     FIG. 1 is a plan view of the contactor with the top removed to show the blade assemblies; 
     FIGS. 2A to  2 D are views of a U-frame for the shrouded solenoid, showing respectively a view from above, a plan view taken on the partial section line II—II of FIG. 2A, a side view, and a view from beneath the frame; 
     FIGS. 3A and 3B are views from one side and beneath respectively of a bus-bar assembly incorporating a moving blade; and 
     FIG. 4 is a plan view showing a status switch in the closed position. 
    
    
     DETAILED DESCRIPTION 
     Referring first to FIG. 1, the contactor shown is designed to be fitted within a domestic electricity meter casing, or into a meter base moulding at the interface of a house, for isolating the mains utility power feed to domestic loads within the house. It may also be integrated into a proposed automatic meter reading (AMR) pre-payment and communication system, with the option of remote disconnection and reconnection of the customer&#39;s supply. The contactor comprises a stout moulded casing  8  made of an electrically non-conductive material and which forms a base into which are mounted two separate balanced and symmetrical mirror-image switching systems. 
     In order to avoid unnecessary repetition of references in the drawings, only the left-hand parts of the switch will generally be referred to, it being understood that the right-hand parts are essentially similar except where specifically stated. 
     Power is fed to the contactor from an inlet bus-bar  10  which is connected by a thin spring portion  12  to a bi-furcated moving blade  14  having a pair of inlet contacts  16  formed at the ends (see also FIGS.  3 A and  3 B). Power is delivered out of the contactor from an outlet bus-bar  18  which has fixed double contacts  20  for mating with the inlet contacts  16 . 
     Mounted centrally between the ends of the outlet bus-bars  18  is a solenoid actuator  24  comprising a ferrous plunger  26  slidable within a solenoid drive coil  22 . 
     A spigot  28  connected to a yoke  32  engages loosely within an aperture  30  in the plunger  26 , to which it is connected by a pivot pin  29 . At each end of the yoke  32  the lower face engages with a compression spring  34 , while a pair of projections  36  on the upper face engage with a pair of shaped leaf-springs  38 , held at their centre by a pin  39 A of a holder  39  made of aluminium casting. The end of each spring  38  engages in a slot of a moulded sliding lifter  40  (only one shown) made of an electrically non-conductive material and of which the upper end engages with the top and bottom sides of the moving blade  14 . 
     It should be pointed out here that the upper spring  38  and the upper lifters  40  are not shown in FIG. 1, and that the layout of the blades  12  is not only mirrored, but is symmetrical and balanced about the axis of the solenoid actuator  24 , thus presenting a consistent deflecting and actuating force via the two pairs of lifters  40  to each set of contacts in turn. 
     The moving blade  14  is thinned at one end for flexibility and suitably attached to the bus-bar  10  by soldering, brazing or ultrasonic welding. During manufacture of this assembly it is important not to generate excess heat, which could seriously distort the shape of, or affect the spring quality of the moving blade. Each assembly is tightly located and contained in slots and barriers within the moulded casing  8 . Suitable barriers within the casing provide the required safety isolation between the two individual switches which are at mains supply voltage, and the drive coil  22  which is at low voltage. 
     The feed bus-bar  10  and moving blade  14  are formed in such a way that they lie parallel to each other for a certain distance, with a small defined gap between, along their length. A larger gap exists at the flexible attachment of the spring portion  12  where the blade is relatively weak, to prevent damage when loaded under fault conditions. This blade arrangement is the basis of the so-called “blow-on” layout (as described and claimed in UK Patent Application Serial No. 2295726) [ref. 480.00/B] which is designed to give increased contact force and hence superior switching performance, especially under excessive or short-circuit current fault conditions. 
     Under such excessive/short-circuit fault conditions the current in the feed bus-bar  10  is in the opposite direction to that flowing in the respective adjacent moving blade  14 , so that electrodynamic forces are generated between them, trying to force them apart. The force is approximately proportional to the square of the current. Since the feed bus-bar  10  is comparatively rigid, these forces act directly upon the moving blade, thus increasing the forces between the contacts  16 ,  20  over and above the optimal overtravel force which is set when the solenoid adjustment takes place. 
     Opposing this increasing blow-on force, and attempting to open the contacts, is the so-called contact repulsion force, which is related to the geometry of the current flow through the contacts themselves. 
     The magnitude of this field-induced repulsion force is also approximately proportional to the square of the current, and is a function of the ratio of the contacting diameter to the actual contact diameter. In general the more “bedded” or “conditioned” the contacting surfaces are, the lower the repulsion forces between them. The effect of these two opposing forces is a net increase of the nominal contact force with increasing current, thus providing greatly improved and more efficient switching. 
     Referring to FIGS. 3A and 3B, the pair of moving blades  14  are shown in a condition in which the bifurcated contacts  16  are open. 
     Adjacent its contact end the moving blade  14  is formed with a slightly U-shaped portion  15  so as to freely engage with the sliding lifter  40 , one half below and the other half above, for free actuation of the blade. The bottom end of the lifter  40  is engaged with the lower one of the two leaf-springs  38  within the holder  39  (only the bottom one being shown). Both split lifter sets are contained by and run smoothly in grooves (not shown) within the base and lid mouldings of the contactor. 
     As the leaf-spring holder  39  is freely pinned to the solenoid actuator plunger  26 , and lies symmetrically between the two lifter/moving blade systems, this ensures that actuation forces translated from the solenoid plunger to the blades via the two leaf springs  38  are evenly distributed on both sides, thus giving similar, distributed contact forces and reliable switching. Furthermore, as each leaf spring  38  is entrapped by the central pin  39 A, giving three fixing points within the holder  39 , one limb on each side being pre-tensioned to exert a slightly greater pick-up force than the other, the result is that during actuation, one half blade contact is slightly advanced with respect to the other, creating an early closure with its mating fixed contact, followed rapidly with closure of its counterpart. 
     The pre-tensioning is designed in such a way that at the end of the stroke or overtravel, all four contacts  16 ,  20  receive approximately the same, consistent nominal contact force. Also, by virtue of the blow-on electrodynamic forces, a considerably lower nominal contact force is required for operation at normal current levels, in this case 200 A rms. Typically, each contact force is in the region of 300 to 400 g (3 to 4 Newtons). 
     This is the basis of a “sacrificial” contact pair on each set; one contact taking the brunt of the early closure and late opening, with the other contact carrying the load current. In practice, however, both contacts should share the load current equally. 
     The advantages of bifurcated contacts with such a sacrificial contact pair are as follows: 
     a) Since the total load current is equally shared between the bifurcated contact sets, it can be shown that the total heating effect is approximately halved. 
     b) Halving of the load current through each pair of “sharing” contacts more than halves the total resultant contact repulsion force which is attempting to open the contacts. 
     c) The combined effect of a) and b) above allows a lower leaf spring force to be utilised. 
     This also makes the blow-on layout less critical, while still giving an improved reliable switching life to the contactor. 
     The solenoid actuation  24  is latched by rare earth magnets  37  and only requires a short DC pulse for its operating and release functions, the latched hold force being considerably greater than the total contact force exerted via the double leaf-springs  38 . This surplus hold ensures that the contactor function is not susceptible to shock and vibration, or excess current forces. 
     The actuator thus being magnet latching, and only requiring a short momentary DC pulse to perform the operating and release functions, no quiescent power is necessary. This virtually eradicates any self-heating, as is the case in a non-magnet latching solenoid. Typical coil actuation power is only of the order of 20 to 30 W (compared with 2000 W for the known contactors cited earlier), with actuation times of typically 20 ms. 
     As shown, the solenoid actuator  24  is wound for a single coil, requiring e.g. a positive DC pulse to operate (CLOSE) and a negative DC pulse to release (OPEN) the contactor switches, and requiring a simple reversing-bridge type of drive circuit. Alternatively, however, the solenoid may be wound with two coils with a common center tap, requiring DC pulses of the same polarity (say negative going with respect to a positive center-tap common, from separate conducting transistors), so as to achieve the operating (CLOSE) and release (OPEN) contactor functions. 
     Alternatively in a preferred single coil option, drive is taken directly from the AC supply e.g. via opto-isolated triacs, where it is only necessary for a positive half-cycle to operate (CLOSE) and for a negative half-cycle to release (OPEN) the contact function. 
     In this case, it is advantageous for the triac drive to be triggered from the so-called zero-crossing of the supply, ensuring that the contacts open and close on a rapidly declining load current (or preferably at the next zero-crossing), resulting in minimal arcing, enhanced switching and longer contact life. 
     To assist the release function, the two push-off springs  34  are located between the leaf spring holder  39  and the contactor casing  8 . The solenoid axial position is adjustable so that a minimum contact force is achieved, which is then fixed with a pair of screws  54  (see FIG. 4) in holes in the casing, and glued for added retention during the contactor life. A moulded top cover provided with suitable catches, tightly contains and integrates the entire assembly within the casing. 
     Referring now to FIGS. 2A to  2 D there is shown a secondary U-frame  42  for shrouding the solenoid. 
     The frame comprises a base  44 , a pair of sides  48 , from each of which extends a fixing lug  48 , a top side  50  and a lower end  52  having a small central hole  54 . The lugs  48  are secured to the moulded base  8  by fasteners, as shown in FIG.  1 . 
     The frame  42  thus consists of a four-sided box structure, which is also enclosed at the lower end, and by the aluminium holder  39  beyond its upper end, thereby excluding large magnetic fields produced by the blade assemblies during excess or short-circuit fault conditions. 
     Auxiliary status switch for actuator/contactor function 
     Some end applications require an auxiliary low-voltage switch, for signalling to the drive electronics, or indicating remotely, as part of a pre-payment or Automatic Meter Reading (AMR) system, the status of the contactor (or at the very least, the status of the solenoid actuator). A simple version of such a status switch is shown in FIG.  4 . 
     While the contacts  16  and  20  are open, the moving plunger  26  is isolated in a plastic bobbin from a metal end stop  56  and the solenoid frame  42  (at the bottom end) by the stroke distance, typically 2-3 mm. However, the plunger is in continuity with the aluminium leaf-spring holder assembly and both push-off springs  34 . 
     As already mentioned, the functionality of the present contactor relies upon the successful latching of the magnet solenoid, fundamentally involving a strong, intimate attraction of the metallic plunger  26 , the stop  54  and frame  42 , when the contacts are closed. This latching hold force is typically several kilogrammes, and forms an ideal low-voltage, low-current switch. 
     A wire connection  58  is made to one of the fixing screws  54  for the frame  42 , and a similar wire connection  60  is made to the adjacent push-off spring  34  by means of a tag (not shown) trapped under the spring. The wire connections  58  and  60  are fed to a flag circuit to show the status of the switch. 
     When the contactor is in the closed position shown, a continuity loop is formed as shown by the dotted line  62 . Thus an electric circuit is formed as follows: from the wire  60  through the spring, along one arm of the aluminium yoke  32 , through the pivot pin  29  and the plunger  26 , across the nickel plated interface with the stop  56 , along the side of the frame  42 , and out from the screw  54  to the wire  58 . The wires  58  and  60  are fed to a flag circuit to show the status of the contactor, e.g. by a indicator light (not shown). 
     Immunity to large generated magnetic fields 
     Some USA and IEC specifications require normal operation of the contactor following a 6,000 A rms 6 cycle, or a 10,000 A rms ½ cycle fault. During such excessive/short-circuit faults very large magnetic fields are generated by the bus-bars  10 , the moving blades  14  and load wiring connections. 
     The effect of these large magnetic fields is to interfere with or influence the standing hold conditions of the magnet latch solenoid which in some cases may actually force the solenoid to drop out, opening the contactor contacts, with catastrophic consequences. 
     The interfering magnetic fields may enter a magnet latching solenoid in three ways: 
     1) by inducing forces via the plunger end face at the leaf spring carrier  39  (which is in close proximity to one of the moving blades), thus directly affecting the nett hold of the solenoid to the point of dropping out, or 
     2) by inducing forces directly into the plunger  26  and/or end-stop parts within the coil area, again affecting the nett hold of the solenoid, or 
     3) by partially demagnetising conventional existing Ferrite magnets  37  momentarily during actuation. 
     In order to reduce the effect of the large interfering magnetic fields at fault conditions the present design provides the following features: 
     1) The ferrous plunger  26  is shortened so that only the magnetically-active portion is contained within the magnet latch solenoid, the external actuation portion linking it to the aluminium leaf-spring holder  39  being non-magnetic eg. insert-moulded plastic or an extension of the holder  39 . This considerably reduces the interfering influence of the large fault-condition magnetic fields. 
     2) The rest of the solenoid is shrouded and enclosed by the secondary U-frame  42 , such that further reduction is achieved in the interfering influence of the large magnetic fields. 
     3) The use of rare-earth magnets  37  which not only provide considerably higher hold forces, but also makes them inherently difficult to demagnetise because of their greater bulk B.H.max product, which is typically 30 to 35 Mega.Gauss.Oersteds (MGO) compared with 3 to 6 MGO for the best grades of Ferrite material that are currently used. 
     The combination of these three improvements is believed to virtually eradicate the problem of the magnetic field influence, giving a reliable, immune, solenoid performance under the most arduous excess/short-circuit fault conditions.