Abstract:
A circuit breaker includes a coil arrangement for actuating an armature to open the breaker. To prevent false tripping during high transient currents drawn by a load, the coil arrangement has first and second coils, electrically connected in series. The second coil is interposed between the first coil and the armature and is wound in an opposite direction from the first coil to produce an opposing field.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to electrical-power-interrupter devices and, in particular, to a circuit breaker with means for preventing false tripping. 
     2. Description of Related Art 
     In some applications circuit breakers are utilized to protect electrical loads having high initial inrush currents. To avoid false tripping of a breaker during such initial inrush currents, it is known to include in the breaker mechanical and/or electronic means for delaying tripping during transient overcurrent conditions. FIG. 1 illustrates one type of known circuit breaker which is particularly effective in such applications. The illustrated breaker comprises a plastic housing in which are secured a switch 30, a switch control arrangement including a toggle lever 32 and an armature A, both mechanically connected to the switch by linkage 33, and an armature actuation arrangement including a pole piece P, a core assembly M, and a coil L. 
     The switch 30 includes a contact 30A, which is affixed to one end of a conductive line terminal T A , and a contact 30B, which is affixed to one end of a conductive arm 34 that is pivotable around a pin 36. Arm 34 is electrically connected to one end of the coil L through a conductive lead 37. The opposite end of the coil is electrically connected to a conductive load terminal T B  through another conductive lead 35. The linkage 33 is pivotably attached to the switch arm 34 (by a pin 38) and to the toggle lever 32 (by a pin 40) such that, when the toggle lever is pivoted clockwise around a pin 42, the linkage latches the switch 30 in a closed state with the contacts 30A and 30B forced together. 
     The armature A is a magnetically-permeable metal member which, together with an integral arm 44, is pivotable around a pin 46 secured in an L-shaped magnetically-permeable metal frame 48. This frame has an opening 50 in which the core assembly M is secured. The core assembly includes a cup-shaped tubular member 52 formed of non-magnetic material, such as copper, which is closed at its top end by the pole piece P to form a sealed container. This container is filled with a damping fluid, such as silicone oil (not visible in the figure), in which a solid tubular delay core 54 of magnetically-permeable material, such as non-leaded 1010 steel, is immersed. The delay core 54 has a reduced-diameter portion 54A for supporting a cylindrical spring 56 which extends from a ridge 54B to the pole piece P. The delay core 54 is shorter in length than the tubular member 52, but is of sufficient length to extend from the pole piece P to the opening 50 when positioned against the pole piece P. 
     During steady-state operation of the known circuit breaker shown in FIG. 1, current supplied by a source (not shown) that is connected to line terminal T A  passes through the closed switch 30, through coil L, and to a load connected to load terminal T B . As the current passes through the coil L, it produces a torroidal magnetic field having a concentration of flux lines in the magnetically-permeable delay core 54. The force developed by this field urges core 54 toward pole piece P to eliminate the gap between them. However, spring 56 produces a counteracting force which is sufficient to prevent the core from moving toward the pole piece at all currents below the rated tripping current of the breaker. 
     During the occurrence of transient moderate-overload currents, such as 125-200% of the rated breaker tripping current, the force developed by the coil field overcomes the spring force and the core 54 begins to move through the obstructing damping fluid toward the pole piece P. The viscosity of the damping fluid determines how quickly the core moves, and thus determines how long the transient must last before the core reaches the pole piece. If the core reaches the pole piece, it cooperates with the L-shaped magnetically-permeable frame 48 to form a U-shaped magnetic circuit that is separated from the armature A by air gaps G1 and G2, causing flux lines to concentrate in these two gaps. These flux lines develop sufficient force to instantly close the air gaps G1 and G2, by causing the armature A to pivot around pin 46 against the force of a return spring (not shown). As this happens, integral arm 44 pushes against a lever 58 of the linkage 33 causing unlatching and opening of the switch 30, thereby interrupting current flow to the load. 
     During the occurrence of high overload currents, such as 500% or greater, the flux in gaps G1 and G2 is sufficient to cause the armature to close these air gaps, thereby instantaneously opening the switch 30. This is undesirable if the load is of a type which typically draws such currents during startup. Examples of such loads are electronic circuits with highly capacitive input impedances and low-resistance motor windings. The known armature actuation arrangement of the type illustrated in FIG. 1 cannot be easily modified to solve this problem. If the delay is increased by, for example, reducing the number of turns in the coil L and/or increasing the force of the armature return spring and/or increasing the gaps G1/G2, the circuit breaker could lose its ability to interrupt a low-level continuous overcurrent (e.g. 125%). 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide an electromagnetically-operated circuit breaker including simple means for maintaining the breaker in the closed state during predetermined transient high-overcurrent conditions. 
     It is another object to provide such a circuit breaker which trips substantially instantaneously upon the occurrence of a destructive overcurrent condition. 
     In accordance with the invention, a circuit breaker of the above-described type comprises an armature actuation arrangement including first and second coils arranged around a tubular core-holding member. The first coil is wound for conducting current in a first direction around a first length of the tubular member which is remote from an attached pole piece which defines the air gap with the armature. The second coil is wound for conducting current in the opposite direction around a second length of the tubular member which is proximate the pole piece. In response to current flowing through the breaker switch, the first coil produces a first magnetic field acting on the core and urging it toward the pole piece to effect actuation of the armature. In response to at least a portion of the current flowing through the breaker switch, the second coil produces a second magnetic field which opposes and has a smaller strength than the first magnetic field. This opposition field will increase the delay for high overload currents, but will not prevent tripping of the breaker for low-level overload currents. At currents above a predetermined level, the effect of the first coil will dominate and the breaker will trip instantaneously. The specific currents at which delayed or instantaneous tripping occurs can be adjusted by adjusting the relative strengths of the fields produced by the first and second coils. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 a cross-sectional view of a known circuit breaker. 
     FIG. 2 is a cross-sectional view of an embodiment of a circuit breaker in accordance with the invention. 
     FIG. 3 is a schematic circuit diagram illustrating a simple power distribution network incorporating a circuit breaker in accordance with the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 2 illustrates a circuit breaker which is similar to that of FIG. 1, but which has been modified in accordance with a preferred embodiment of the invention. Identical parts in both figures are identified with the same reference numbers. More specifically, the first and second coils in this embodiment are identified as L A  and L B , respectively. The directions of the windings are indicated by the cross-sectional symbol &#34;&#34; for current flowing out of the page and the symbol &#34;x&#34; for current flowing into the page. One end of the coil L A  is electrically connected to the switch arm 34 through conductor 37, one end of coil L B  serves as a terminal T C , and the opposite ends of these coils are electrically connected at a junction point to a lead 35, which is connected to terminal T B . Thus the switch 30 and the coils are electrically connected in series between the terminals T A  and T C , while terminal T B  provides a connection to the junction where the coils are connected to each other. 
     The increased delay provided by the circuit breaker of FIG. 2 is believed to occur because of a localized negative influence of the magnetic field produced by coil L B  on the field produced by coil L A . That is, the field produced by coil L B  weakens the field produced by coil L A  in the vicinity of the armature A and pole piece P to a greater extent than it weakens it in the vicinity of the core 54. Thus, when an overcurrent condition begins, the core moves more slowly toward the pole piece P. Once it reaches the vicinity of the pole piece, however, the breaker will trip if the resultant flux in the air gap is sufficient to move the armature A. 
     The third terminal T C  enables an alternative usage for the circuit breaker of FIG. 2. It enables the breaker to be utilized in a power distribution network where a circuit breaker must protect an upstream circuit, while also providing current to a second circuit breaker, or other type of current interrupter device, which protects a downstream circuit. A problem which occurs in such a network is that, when a short circuit occurs in the downstream circuit, it will cause opening of not only the locally-affected current interrupter device, but also of the upstream circuit breaker. 
     FIG. 3 illustrates utilization of a circuit breaker 1, of the type illustrated in FIG. 2, in such a network. The circuit breaker 1 provides power directly to a circuit 10, including parallel-connected loads 10-1, 10-2, 10-3, 10-4, and indirectly (via a downstream, serially-connected, conventional circuit breaker 2 having a lower current rating) to a circuit 20, including parallel-connected loads 20-1, 20-2, 20-3. For the sake of brevity, only this simple two-breaker, two-circuit network will be described to explain operation of the invention. However, one or more circuit breakers in accordance with the invention can be utilized advantageously in networks of much greater complexity or in networks which substitute a different type of power interrupter device, such as a fuse, for the conventional downstream circuit breaker 2. 
     The circuit breaker 1 includes a line terminal T1 A  for connection to a source of electric current, a first load terminal T1 B  for supplying current directly to the circuit 10, and a second load terminal T1 C  for supplying current indirectly to the circuit 20, through the circuit breaker 2. The terminals T1 A , T1 B  and T1 C  correspond to terminals T A , T B  and T C  in FIG. 2. Similarly, breaker 1 includes switch SW1, armature A1, and coils L1 A  and L1 B  corresponding to switch 30, armature A, and coils L A  and L B  in the breaker of FIG. 2. The circuit breaker 2 includes a line terminal T2 A , which is electrically connected to load terminal T1 C , for receiving current from circuit breaker 1, and a load terminal T2 B  connected to the circuit 20. 
     The conventional circuit breaker 2 includes an electromagnetically-actuated switch SW2 through which the current supplied to circuit 20 must pass. It also includes a coil L2 which is electrically connected in series with the switch SW2 for sensing this current. The coil L2 is wound around a magnetically-permeable core M2, which is disposed proximate an armature A2 for actuating switch SW2, thereby interrupting current flow to circuit 20, if a rated current for breaker 2 is exceeded. 
     In operation, the tripping characteristics of breaker 1 are dependent on the currents drawn by both circuits. In a first situation, where circuit 20 draws no current, the reverse winding L1 B  will not produce any field and breaker 1 will respond to current flowing in circuit 10 similarly to the breaker of FIG. 1. In a second situation, where circuit 10 draws no current, breaker 1 will respond to the load current drawn from terminal T1 C  as described in connection with the description of FIG. 2. In the expected situation, where current is drawn by both circuits, operation of breaker 1 will fall somewhere between the first and second situations. In any situation, provided that circuit breaker 2 is functioning correctly, a destructive overload at terminal T2 B  will cause it to trip without causing tripping of circuit breaker 1.