Patent Publication Number: US-6222139-B1

Title: Rotary electric switch and contact therefore

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to an electric switch and more particularly to a four-break electric switch having two pairs of rotatably movable contacts and four fixed contacts and a contact suitable for use in such a switch. 
     Manually operated electric switches having ratings of up to several hundred amps are well known. Some such switches include an integral fuse element. Typically, the load contact, i.e. fixed contact, in such a switch is selectively engaged and disengaged by a movable blade, i.e. movable contact, that is cantilever supported in spaced relation to the fixed contact. Of course, to withstand high currents and effectively break the high currents, the contacts and the space in which the movable contacts move must be relatively large. To reduce the size of electric switches, it has been proposed to mount movable contacts to rotate about a central portion thereof to accomplish two breaks with two fixed contacts (one break at each end of the movable contact) in about the same space as one break is accomplished in the cantilever type of contact mounting. The word “break” as used herein refers to a location at which movable contacts can be selectively placed in contact with a fixed contact or other movable contacts to “make” or “break” current. Of course, the use of two breaks increases the effective contact area, i.e. the area in which the fixed contact touches the moving contact and thus increases switching capacity. U.S. Pat. No. 3,632,935 is representative of patents disclosing a two-break contact rotatable about a center thereof. 
     Further the use of movable contacts in pairs is well known. As disclosed in U.S. Pat. No. 3,632,935, the use of parallel movable contacts in combination with a single fixed blade or contact generates electro-motive force (EMF) that tends to pull the movable contacts towards one another (a force of attraction) and overcome the tendency of a movable contact to repel a fixed contact (a force of repulsion). In particular, the current flowing through the point of contact between fixed and movable contacts generates a force of repulsion at that point (technically known as “Crowding Effect”). Also, the entire current carrying area of a pair of movable contacts generates a force of attraction therebetween that can be used to overcome the force of repulsion to thereby avoid “popping”, i.e. separation of the contacts due to EMF which results in damage to the contacts and failure of the switch. However, it is difficult to implement pairs of contacts that are rotatable about a central portion in rotary switches having more than two breaks. In particular, in order to mount plural contact pairs (which are required for more than two breaks) on the same rotating member, it is necessary to offset the contact pairs to be in different planes to avoid mechanical interference between the contact pairs and to avoid electrical communication between the contact pairs. This increases the required dimensions of the switch. 
     The “S” Type Fused Combination Switches” sold by MEM SANTON SWITCHGEAR implements a four break rotary switch in a relatively small package by using four single knife contacts on a rotating member and fixed U shaped contacts having two extending portions on respective sides of the knife contacts disposed around the rotating member. However, the relatively small dimensions of the extending portions are not adequate to generate the high attractive forces necessary to avoid popping at high currents. Further, the size and configuration of the fixed contacts does not permit the extending portions to be easily flexed towards each other and thus even if a high force of attraction was generated, the contacts would not “grip” the movable contact and thus are not as stable as true contact pairs. Extending the size of the fixed contacts would increase the size of the device. 
     The complex interaction between mechanical and electrical forces in a rotary switch have rendered it difficult to implement four breaks in a compact design. Also, known four break rotary switches have relatively unstable contacts or are unduly large. 
     SUMMARY OF THE INVENTION 
     A first aspect of the invention is an electric switch comprising housing, four contacts mounted to the housing, a rotary member rotatably mounted with respect to the housing, and two pairs of movable contacts. Each movable contact has a central portion coupled to the rotary member and end portions adapted to contact a corresponding one of the contacts. The pairs of movable contacts are mounted in substantially the same plane. 
     A second aspect of the invention is an electric switch comprising a housing, four contacts mounted to the housing, a rotary member rotatably mounted with respect to the housing, and two pairs of movable contacts. Each movable contact has a central portion coupled to the rotary member and end portions adapted to contact a corresponding one of the contacts. A longitudinal axis of the end portions extend at an angle with respect to a longitudinal axis of the central portion and the end portions are in substantially the same plane as the central portion. 
     A third aspect of the invention is a movable contact for an electric switch of the type having fixed contacts, a rotary member, and at least one pair of the movable contacts mounted on the rotary member. The movable contacts each comprises a central portion adapted to be coupled to the rotary member and end portions adapted to define breaks with a corresponding one of the fixed contacts. A longitudinal axis of the end portions extend at an angle with respect to a longitudinal axis of the central portion and the end portions are in substantially the same plane as the central portion. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The invention is described through a preferred embodiment and the attached drawing in which: 
     FIG. 1 is a top view of a switch in accordance with a preferred embodiment of the invention; 
     FIG. 2 is a sectional view taken along line  2 — 2  in FIG. 1 with the movable contacts beginning to contact the fixed contacts; 
     FIG. 3 is a perspective view of the contact carrier and movable contacts of the preferred embodiment; 
     FIG. 4 is a sectional view taken along line  4 — 4  in FIG. 3; 
     FIG. 5 is a top view of a movable contact of the preferred embodiment; 
     FIG. 6 is a sectional view taken along line  6 — 6  in FIG. 5; 
     FIG. 7 is a sectional view taken along line  7 — 7  in FIG. 5; 
     FIG. 8 is a side view of one type of fixed contact of the preferred embodiment; 
     FIG. 9 is an end view of the fixed contact of FIG. 8; 
     FIG. 10 is a side view of another type of fixed contact of the preferred embodiment; 
     FIG. 11 is a top view of the fixed contact of FIG. 10; 
     FIG. 12 is a sectional view taken along line  2 — 2  in FIG. 1 with the movable contacts fully removed from the fixed contacts, i.e., in the “off” position; and 
     FIG. 13 is a sectional view taken along line  2 — 2  in FIG. 1 with the movable contacts fully mated with the fixed contacts, i.e. in the “on” position. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     As illustrated in FIG. 1, switch  10  includes housing  20 , preferably made of a synthetic resin material or other electrically insulated material, removable cover  40 , and rotatable handle  46 . In the preferred embodiment, housing  20  substantially surrounds the other components disclosed below. However, housing  20  can take any form in which the other components are mounted thereto. For example, housing  20  can be in the form of a base plate. As illustrated in FIG. 2, two fixed contacts  22  and two fixed contacts  23  are fixedly disposed in housing  20 . Fixed contacts  22  and  23  of the preferred embodiment are essentially plate-like members and lie in substantially the same plane as described in greater detail below. A rotary member comprises shaft  28  coupled to handle  46  and contact holder  26  made of a material that has electric and thermal insulation properties as described in greater detail below. Two pairs of movable contacts  100  are mounted in contact holder  26  to rotate in the directions indicated by double-headed arrow A about an axis of shaft  28  when shaft  28  is rotated by rotating handle  46 . 
     It can be seen that rotation of handle  46 , and thus the rotary member comprised of shaft  28  and contact carrier  26 , in the clockwise direction in FIG. 2 causes end portions  102  (see FIG. 5) of movable contacts  100  to move towards corresponding fixed contacts  22  and  23 . Similarly, rotation of handle  46 , and thus the rotary member comprised of shaft  28  and contact carrier  26 , in the counter-clockwise direction in FIG. 2 causes end portions  102  of movable contacts  100  to move away from corresponding fixed contacts  22  and  23 . The position of movable contacts  100  illustrated in FIG. 2 corresponds to a position in which movable contacts  100  just begin to contact corresponding fixed contacts  22  and  23  as shaft  28  is rotated in the clockwise direction. It can be seen that each end portion  102  begins to contact fixed contacts  22  and  23  at the same time during rotation of handle  46 . Also, it can be seen that switch  100  defines four breaks corresponding to each of four pairs of end portions  102 . One fixed contact  22  and one fixed contact  23  are electrically coupled to terminals  32  to which fuse clips  42  are electrically coupled. Accordingly, fuse element  44 , such as a standard fast acting or slow blow thermal fuse, can be connected between two fixed contacts  22  and  23 . 
     Cover  40  serves to isolate and protect fuse element  44  and is optional. For example fuse element  44  can be exposed or fuse element  44  can be omitted and terminals  32  can be electrically coupled in a direct manner to provide an unfused switch. The other fixed contacts  22  and  23  are electrically coupled to terminals  30  to which incoming (line) and outgoing (load) can be connected in a known manner. The line and load terminals are interchangeable in the preferred embodiment. One set of movable contacts  100  and fixed contacts  22  and  23 , i.e. two pairs of movable contacts  100 , two fixed contacts  22 , and two fixed contacts  23 , are illustrated in FIG.  2 . However, the invention can include plural sets of fixed contacts  22  and  23  and plural sets of movable contacts  100  mounted on a single shaft  28 . The preferred embodiment has six such sets as indicated in FIG. 1 by six sets of load terminals  30 . 
     As illustrated in FIGS. 3 and 4, movable contacts  100  are supported in contact carrier  26  as two opposing pairs of parallel contacts  100  extending through respective openings in contact carrier  26 . Springs  24 , in the form of a leaf spring in the preferred embodiment, are disposed in contact carrier  26 , as illustrated, to press movable contacts  100  into a seat defined in carrier  26  and towards an opposing movable contact  100 . FIGS. 5,  6 , and  7  illustrate one of moving contacts  100  in detail. Each moving contact  100  includes two end portions  102 , having longitudinal axis a, and a central portion  104 , having longitudinal axis b. The angle α between axis a and axis b is preferably in the range of 30°-50° inclusive, more preferably in the range of 35°-45°, and is 37° in the preferred embodiment. This angle defined between end portions  102  and central portion  104  permits contacts  100  to be placed in contact carrier  26  with the pairs of movable contacts  100  in substantially the same plane P and with end portions  102  in different quadrants of housing  20  while avoiding crossing of movable contacts  100  as illustrated in FIGS. 2 and 3. The separation of end portions  102  of a pair of movable contacts  100  pair with respect to end portions  102  of the other of movable contacts  100  pair afforded by the angled end portions  102  permits an adequate stroke of movement by end portions  102  with respect to fixed contacts  22  and  23 . Also, it can be seen that this configuration reduces the radial dimension of the space required by movable contacts  100  while still providing a relatively long movable contact to create the requisite attractive force as described in detail below. 
     As illustrated in FIGS. 5 and 7, each movable contact  100  has contact projection  106  formed on respective end portions  102 . Also, end portions  102  have tapered edges  108  defined thereon (see FIG. 3 also) defining sacrificial material which serves the purpose of providing material to burn off during arcing caused by current breaking. The geometry of the current path achieved by groove  60  in fixed contacts  22 / 23  described below pushes the seat of the arc to tapered edges  108 . This way the main contact area does not burn up. This configuration facilitates making and breaking electrical contact with fixed contacts  22 / 23  in the manner described below. Note that tapered edges  108  do not touch fixed contacts  22 / 23 . 
     FIGS. 8 and 9 illustrate one of fixed contacts  22  in detail and FIGS. 10 and 11 illustrate one of fixed contacts  23  in detail. The primary difference between fixed contacts  22  and fixed contacts  23  is the shape and orientations of groove  60  formed therein. The different groove shape in fixed contacts  22 / 23  ensures that the geometry of the current path at all 4 breaks is similar. Technically, the groove  60  is generated to get a “Bend-Back” effect, because the arc generated while breaking is pushed away from the point of break thereby minimizing the roughness caused by arcing at the point of the break (which is also the point of engagement at the time of making). This allows easy movement of movable contacts  100  during further engagement. Other aspects of fixed contacts  22  and  23  are similar and thus these elements will be discussed together with reference to FIGS. 8-11 in which similar elements are labeled with like reference numerals. 
     Each fixed contact  22 / 23  is a substantially rectangular plate-like member having a corner removed to define a width w along which contact projection  106  moves as contact carrier  26  is rotated. Angle β preferably is about 40°-50° inclusive and is about 47° in the preferred embodiment. An edge of fixed contact  22 / 23  is chamfered to define sloped receiving surfaces  62  on either side of the edge of fixed contact  22 / 23  to facilitate making and breaking with movable contacts  100  as described below. The chamfered edge defines angle γ (see FIG. 9) that is preferably less than 10°, 4° in the preferred embodiment. 
     As best illustrated in FIGS. 2,  12 , and  13 , rotation of shaft  28  causes contact carrier  26  to rotate. Accordingly, end portions  102  of movable contacts  100  move toward and away from corresponding fixed contacts  22 / 23 . In particular, when movable contacts  100  are in the fully “off” condition illustrated in FIG. 12, no current flows between fixed contacts  22 / 23 . However, when shaft  28  is rotated clockwise in the drawing, end portions  102  of movable contacts  100  move towards fixed contacts  22 / 23 . When movable contacts  100  reach the position of FIG. 2, contact projection  106  of movable contact  100  slidingly engages the receiving surfaces  62  defined on fixed contacts  22 / 23 . Continued motion of end portions  102  causes the center of contact projection  106  to come in contact and slide along receiving surfaces  62  thus pressing each movable contact  100  away from the opposing movable contact  100  of the pair against the force of spring  24  and the attractive force of movable contacts  100 . Keep in mind that as soon as a pair of movable contacts  100  comes into contact with fixed contact  22 / 23 , current is conducted through movable contacts  100  and fixed contacts  22 / 23  (assuming that a load is applied to terminals  30 ). Continued rotation in the clockwise direction causes contact projections  106  to pass along width w of fixed contacts  22 / 23 . Similarly, rotation of shaft  28  in the clockwise direction from the position illustrated in FIG. 12, will cause contact projection  106  to move off of and away from fixed contacts  22 / 23  to break current of the load. It can be seen that end portions  102  travel through about 45° when moving from “off” to “on” or vice versa, within the quadrants defined by partitions  34  of housing  20 . A dwell mechanism can be incorporated in switch  10  to quickly move movable contacts  100  over and off of fixed contacts  22  and  23 . In other words, once movable contacts  100  pass a dwell point, a large spring force or the like can be used to accelerate movable contacts  100 . 
     As noted above, the angle of end portions  102  with respect to central portions  104  of movable contacts  100  overcomes many of the obstacles to the design of a four-break rotary contact switch. However, this angled configuration also raises other design considerations that must be overcome. In particular, the above-noted attractive and repulsive EMF act in a direction that is perpendicular to the direction of current flow through the contacts. The attractive force due to central portions  104  can be represented as vector Fa1 at the center of central portions  104  directed into the page in FIG.  5 . The attractive force due to end portions  102  can be represented as vectors Fa 2  at the center of end portions  102  directed into the page in FIG.  5 . It can be seen that Fa 2  is offset from the longitudinal axis of central portion  104  and thus will cause a moment, i.e. a torque, tending to twist or rotate movable contacts  100  in contact carrier  26  as illustrated by arrows b in FIG.  4 . 
     The repulsive force Fr acts at the point of contact between movable contacts  100  and fixed contacts  22 / 23  and thus can be represented as vectors Fr at the center of contact projections  106  directed out of the page in FIG.  5 . It can be seen that Fr is offset from the longitudinal axis of central portion  104  and thus will cause a moment, i.e. a torque, tending to twist or rotate movable contacts  100  in contact carrier  26  as illustrated by arrows c in FIG.  4 . Accordingly, the forces acting on movable contacts  100  are complex and the relative sizes and angles of movable contacts  100  must be designed to account for these forces to avoid popping of the contacts and damage to the switch. 
     Table 2 below illustrates Examples  1 - 19  of the various variables and forces which must be balanced to avoid failure of switch  10 . Table 1 lists the symbol, units, description and value of the various dimensions of an example of movable contact  100  with symbols correlated to FIGS. 5 and 7. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Symbol 
                 Unit 
                 Description 
                 Value 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 N 
                 No 
                 No. of moving contacts 
                 2 
               
               
                 l1 
                 m 
                 Length of central portion 
                 14.8 
               
               
                 l2 
                 mm 
                 Length of end portion (derived) 
                 5.78 
               
               
                 x 
                 mm 
                 Distance between Center points of contact 
                 24 
               
               
                   
                   
                 projections 
               
               
                 y 
                 mm 
                 Center of bent portion from edge (derived) 
                 4.25 
               
               
                 z 
                 mm 
                 Distance of center points from edge 
                 6 
               
               
                 k 
                 mm 
                 Width of the moving contact 
                 5 
               
               
                 Ar 
                 cm2 
                 Cross sectional area at contact zone 
                 0.11 
               
               
                 a 
                 mm 
                 Separation between moving contacts in a pair 
                 3 
               
               
                 t 
                 mm 
                 Thickness of moving contact 
                 1.6 
               
               
                 w 
                 mm 
                 Width of fixed contact 
                 11 
               
               
                 Fa 
                 N 
                 Electrodynamic Force of Attraction 
               
               
                 Fa1 
                 N 
                 Attraction force due to length x of movable 
               
               
                   
                   
                 contact 
               
               
                 Fa2 
                 N 
                 Attraction force due to length y of movable 
               
               
                   
                   
                 contact 
               
               
                 Fr 
                 N 
                 Repulsions force at contact points 
               
               
                 Fs 
                 N 
                 Spring force on each movable contact 
                 10.5 
               
               
                 Mr 
                 N-m 
                 Moment of Fr about edge of movable contact 
               
               
                   
                 m 
               
               
                 Ma 
                 N-m 
                 Moment of Fa about edge of movable contact 
               
               
                   
                 m 
               
               
                 Ms 
                 N-m 
                 Moment of Fs about edge of movable contact 
               
               
                   
                 m 
               
               
                   
               
            
           
         
       
     
     Table 2 shows the results of the calculation of the various forces on the contacts for various levels of instantaneous current. 
     
       
         
           
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                 Symbol 
                 I* 
                 Fa1 
                 Fa2 
                 Ma 
                 Ms 
                 Fr 
                 Mr 
                 Ma + Ms 
               
               
                 Units 
                 kA 
                 N 
                 N 
                 N-m 
                 N-m 
                 N 
                 N-m 
                 N-mm 
               
               
                   
               
             
            
               
                 Ex. 1 
                 4  
                 0.54 
                 0.21 
                  3.15 
                 89.25 
                  2.23 
                  26.75 
                  92.40 
               
               
                 Ex. 2 
                  4.5 
                 0.68 
                 0.27 
                  3.98 
                 89.25 
                  2.76 
                  33.14 
                  93.23 
               
               
                 Ex. 3 
                 5  
                 0.85 
                 0.33 
                  4.92 
                 89.25 
                  3.34 
                  40.12 
                  94.17 
               
               
                 Ex. 4 
                  5.5 
                 1.02 
                 0.40 
                  5.95 
                 89.25 
                  3.97 
                  47.68 
                  95.20 
               
               
                 Ex. 5 
                 6  
                 1.22 
                 0.48 
                  7.08 
                 89.25 
                  4.65 
                  55.81 
                  96.33 
               
               
                 Ex. 6 
                  6.5 
                 1.43 
                 0.56 
                  8.31 
                 89.25 
                  5.37 
                  64.48 
                  97.56 
               
               
                 Ex. 7 
                 7  
                 1.66 
                 0.65 
                  9.64 
                 89.25 
                  6.14 
                  73.69 
                  98.89 
               
               
                 Ex. 8 
                  7.5 
                 1.90 
                 0.74 
                 11.07 
                 89.25 
                  6.95 
                  83.43 
                 100.32 
               
               
                 Ex. 9 
                 8  
                 2.16 
                 0.84 
                 12.59 
                 89.25 
                  7.81 
                  93.69 
                 101.84 
               
               
                 Ex. 10 
                  8.5 
                 2.44 
                 0.95 
                 14.21 
                 89.25 
                  8.70 
                 104.45 
                 103.46 
               
               
                 Ex. 11 
                 9  
                 2.74 
                 1.07 
                 15.93 
                 89.25 
                  9.64 
                 115.71 
                 105.18 
               
               
                 Ex. 12 
                  9.5 
                 3.05 
                 1.19 
                 17.75 
                 89.25 
                 10.62 
                 127.46 
                 107.00 
               
               
                 Ex. 13 
                 10   
                 3.38 
                 1.32 
                 19.67 
                 89.25 
                 11.64 
                 139.70 
                 108.92 
               
               
                 Ex. 14 
                 10.5 
                 3.73 
                 1.46 
                 21.69 
                 89.25 
                 12.70 
                 152.40 
                 110.94 
               
               
                 Ex. 15 
                 11   
                 4.09 
                 1.60 
                 23.80 
                 89.25 
                 13.80 
                 165.57 
                 113.05 
               
               
                 Ex. 16 
                 11.5 
                 4.47 
                 1.75 
                 26.20 
                 89.25 
                 14.93 
                 179.20 
                 115.27 
               
               
                 Ex. 17 
                 12   
                 4.87 
                 1.90 
                 28.33 
                 89.25 
                 16.11 
                 193.29 
                 117.58 
               
               
                 Ex. 18 
                 12.5 
                 5.28 
                 2.06 
                 30.74 
                 89.25 
                 17.32 
                 207.82 
                 119.99 
               
               
                 Ex. 19 
                 13   
                 5.71 
                 2.23 
                 33.25 
                 89.25 
                 18.57 
                 222.78 
                 122.50 
               
               
                   
               
               
                 *TotaI Instantaneous Current  
               
            
           
         
       
     
     When the following equation is satisfied, movable contacts  100  will not pop and switch  10  will operate properly: 
     
       
         Ma+Ms&gt;Mr  
       
     
     where: 
     Ma is the total moment due to the attractive forces; 
     Ms is the total moment due to the force of spring  24 ; and 
     Mr is the total moment due to the repulsive forces. 
     In the table, it can be seen that the equation above is satisfied until the instantaneous current through the switch approaches 8.5 kA (Ex.  10 ). Accordingly, the switch having contacts of the listed dimensions can withstand instantaneous currents of up to almost 8.5 kA which corresponds to accepted ratings for a 63 A switch. Of course the dimensions can be varied to achieve other desired current ratings without contact popping as long as the equation is satisfied. Of course, as the attractive forces overcome the repulsive forces, they will offer resistance to moving contacts  100  sliding over the fixed contacts  22 / 23 , if this passage of high current happens at the time the switch is in the process of being moved to the ON position. This resistance should therefore be overcome by the mechanism that drives the contacts. 
     Fixed contacts  22 / 23  and movable contacts  100  can be made of any appropriate electrically conductive material. For example, copper, silver plated copper, aluminum or the like can be used. The above noted forces on the contacts place a high degree of force on the seats of contact carrier  26 . Accordingly, contact carrier  26  must be made of a temperature and pressure resistant material. Applicant has found that the material sold under the number TW241F10 and trade name Stanyl by DSM Polymers International is suitable for contact carrier  26 . This material has a heat distortion temperature at 1.8 MPa (HDT A) of &gt;290° C. Also, this material has a peak temperature (1 minute) per UL 746 B of &gt;250° C. Applicant has found that use of this material yields a contact carrier that does not distort despite the moments applied to movable contacts  100 . 
     The shape and dimensions of the contacts can be varied based on the mechanical dimensional, and electrical requirements of the switch. Plural sets of movable contact pairs can be mounted on a single shaft to provide a switch of higher capacity or to provide a multi-pole device. Plural sets can also be configured by connections in series or parallel to offer different ratings of current or voltage. The contact pairs or sets can be in the form of modules that can be easily added or removed from the switch. Switches with higher current ratings and lower switching duties can be constructed by using two breaks in parallel instead of four breaks in series. The switch can be fused or non-fused. The movable contacts can be movably mounted in the switch through any appropriate mechanism. 
     The invention has been described through a preferred embodiment. However, various modifications can be made without departing from the scope of the invention as defined by the appended claims.