Patent Publication Number: US-4370635-A

Title: Undervoltage release device for a circuit breaker

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to an undervoltage release device for a circuit breaker. In particular, this invention relates to an undervoltage release device for a circuit breaker which is actuated by a trip coil. Still more particularly, this invention relates to an undervoltage release device for a circuit breaker which incorporates an electromagnetic device for sensing the undervoltage condition of a control voltage, and a chargeable trip spring for tripping the circuit breaker by means of the energy stored in the trip spring. 
     2. Description of the Prior Art 
     An undervoltage release device for a circuit breaker has the task of tripping the circuit breaker when the voltage under control or control voltage is reduced to a critical level. For this purpose, a circuit breaker is conventionally equipped with a normal trip coil. 
     In some presently known circuit breakers, electromagnetic sensing devices such as solenoids are used for sensing of the undervoltage condition of the control voltage. It is also known to trip a circuit breaker by means of energy stored in a charged spring. 
     In another presently known version of a circuit breaker, the undervoltage condition is sensed through a capacitor-relay system which energizes the trip coil of the circuit breaker. In such an approach, the aging of the capacitor has to be taken into consideration. The capacitor has only a definite life time, and therefore, the undervoltage release device of the circuit breaker has only a limited reliability. It is, however, an important objective in this technical field to design long-lasting tripping mechanisms for circuit breakers. 
     In some presently known circuit breakers, there may be a hazard that the circuit breaker is not tripped when the control voltage suddenly reduces to or below a critical value at which the normal trip coil of the circuit breaker does no longer activate. This means loss of control of the circuit breaker which is a great disadvantage especially as short circuit conditions. 
     It has to be understood, therefore, that the undervoltage release device should also perform a switching operation when the control voltage reduces to and below a critical voltage level at which the normal trip coil of the circuit breaker does not activate. 
     SUMMARY OF THE INVENTION 
     1. Objects 
     It is an object of this invention to provide an undervoltage release device for a circuit breaker which is adapted to trip the circuit breaker when the control voltage reduces to a critical value at and below which the normal trip coil of the circuit breaker does not activate. 
     It is another object of this invention to provide an undervoltage release device for a circuit breaker that is activated even at short circuit conditions. 
     It is still another object of this invention to provide an undervoltage release device for a circuit breaker which includes a spring charging mechanism and a latching mechanism for latching the spring charging mechanism, whereby these mechanisms directly act upon each other and a latch bar of the circuit breaker in a tripping operation. 
     It is still another object of this invention to provide an undervoltage release device for a circuit breaker, the control box of which may be conveniently located and isolated from the circuit breaker mechanism. 
     It is still another object of this invention to provide an undervoltage release device for a circuit breaker which includes a spring charging mechanism and which automatically resets the charging mechanism independently of the circuit breaker mechanism after a tripping operation. 
     It is still another object of this invention to provide an undervoltage release device for a circuit breaker which may be used on a DC or AC system. 
     2. Summary 
     According to this invention, an undervoltage release device for a trip coil actuated circuit breaker incorporates a solenoid for sensing an undervoltage, and a chargeable trip spring for storing energy and for discharging the stored energy and thereby tripping the circuit breaker. The trip spring is preferably a helical spring. The solenoid contains a plunger which is movable between an attracted position and a released position. The solenoid exercises a holding force to the plunger when it is energized by a control voltage. 
     The undervoltage release device further incorporates an electric charging motor and a driving device. The driving device which is operatively connected to the charging motor and to the trip spring, is used for precharging the trip spring. In operation, the driving device moves from an initial or tripped position to a stand-by position. 
     A blocking device is provided for stopping and holding the driving device in the stand-by position. 
     The undervoltage release device further incorporates a coil spring. One end of the coil spring is operatively connected to the plunger of the solenoid, and the other end of the coil spring is operatively connected to the driving device. The driving device applies a charging force to the coil spring in counteraction to the holding force of the plunger, when the driving device is in its stand-by position. The plunger is in its attracted position when the control voltage to be supervised has a normal voltage value and, therefore, the holding force of the solenoid is greater than the charging force of the coil spring. The plunger is in its released position when the holding force is smaller than the charging force. This will occur when the control voltage is an undervoltage, that is, drops below a predetermined threshold value. 
     The undervoltage release device finally incorporates a releasing device which is actuated by the plunger. This releasing device will act upon the locking device and thereby release the driving device out of its stand-by position, when the holding force of the plunger decreases in case of an undervoltage and the plunger therefore moves from its attracted position into its released position. 
     In the undervoltage release device according to the invention, the energy stored in the charged trip spring is used for tripping the circuit breaker. The trip spring may actuate a trip pin which in turn releases the main latch bar of the circuit breaker. 
     A flexible cable can be used between the trip pin and the trip spring on the one hand and the control box of the undervoltage release device on the other hand for triggering the charged trip spring. 
     The driving device may preferably include a crank drive which acts upon a pivot drive for charging the coil spring. The pivot drive in turn may actuate an electric switch which turns off the charging motor when the stand-by position is reached. The blocking device may simply be a latch or cam associated with the pivot drive. The releasing device may be a latch lever assembly which is actuated by the plunger. The pivot drive and the latch lever as well as the crank drive are preferably rotatable about axes which are parallel to each other. The pivot drive and latch lever may have the same axis of rotation. 
     The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIG. 1 is a top or planar view of an undervoltage release device for a circuit breaker in its tripped position with the trip coil discharged; 
     FIG. 2 is a top or planar view of the undervoltage release device of FIG. 1 in its charged position with the trip coil charged; 
     FIG. 3 is a side view of the undervoltage release device of FIG. 2 along line 3--3&#39;; 
     FIG. 4 is a side view of the undervoltage release device of FIG. 2 along line 4--4&#39;; 
     FIG. 5 is a schematic diagram of the lever assembly of the undervoltage release device of FIG. 2 in its charged position, illustrating various control forces; 
     FIG. 6 is a schematic diagram of a circuit showing the electrical connections for the undervoltage release device for direct current; and 
     FIG. 7 is a schematic diagram of a circuit showing the electrical connections for the undervoltage release device for alternating current. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to FIGS. 1 through 4, the undervoltage release device for a circuit breaker contains an armature 1 which is a plate essentially of V-shape. As will become apparent later, the armature 1 is the eccentric lever or crank arm of a crank drive. It is the essential part of a driving device. The upper or acuate portion of the armature 1 contains a recess, see FIG. 3, and a first pin 1a projects through the recess. The armature 1 is rotatable about an axis which is located in its lowr or broader portion. In a distance e aside from the connection line between the axis of rotation and the first pin 1a is located a second pin 1b. The distance e characterizes the eccentricity of the crank arm or armature 1. Both pins 1a and 1b are parallel to each other and to the axis of rotation. The axis of rotation is formed by the driving or motor shaft 2a of a motor 2. The motor 2 is connected to a ground or base plate P which is part of a box. The armature 1 is pivotly connected to the shaft 2a. 
     The second pin 1b is used as a connection point for a flexible push-pull cable 3. 
     Below the armature 1 is provided a drive bar 4. This drive bar 4 is firmly connected to the shaft 2a. The distance between the connecting point and the outer end of the drive bar 4 is greater than the distance between the shaft 2a and the second armature pin 1b. In operation, the rotating drive bar 4 will pick up the armature 1 and rotate it about the shaft axis 2a. 
     The undervoltage release device also contains a swinging lever or a lever 5 which is L-shaped. The &#34;L&#34; has a smaller bottom portion and a longer main portion. The end face on the bottom portion of this lever 5 is arranged obliquely with respect to the longitudinal axis of the main portion. An end plate 5e is firmly secured to the oblique end face. The lever 5 is part of a pivot drive for charging a coil spring, as will be apparent later. It is also part of a blocking device. 
     Of great importance to the operation of the undervoltage release device is a cam or latch 6 which extends into the direction of the motor shaft 2a from a latch lever 7. The latch 6 is part of a releasing device, as will be explained later. The latch lever 7 is an elongated rectangular bar which is rotatable about a pivot 8. The pivoting point is located at the lower end. The pivot 8 is parallel to the shaft 2a. The end portion of the main part of the lever 5 is also pivotally connected to the pivot 8. Secured to the upper end of the latch lever 7 is a smaller pivot 8a. Pivotally connected to this pivot 8a is another lever 9 which is also of L-shape. If the shorter bottom portion of the lever 9 is moved from left to right in FIG. 2, the lever 9 will pivot about the pivoting point 8a which in turn will pivot about the pivoting point 8. 
     An electromagnet or solenoid 10 is firmly secured to the base plate P. The solenoid 10 is supplied by the control voltage to be monitored. A plunger 10a extends from the solenoid 10 perpendicularly to the motor shaft 2a. The outer end of the plunger 10a is pivotally connected to the smaller bottom portion of the L-shaped lever 9. 
     Also connected to the smaller bottom portion of the lever 9 is one end of a helical or coil spring 11. The coil spring 11 will exercise a certain spring force F1 to the plunger 10a in counter action to the holding force F4 of the plunger 10a. The plunger 10a is in an attracted or left position when the control voltage has a normal voltage value and when the holding force F4 of the solenoid 10 is therefore greater than the charging force F1 of the coil spring 11. The plunger 10a is in a released or right position when the holding force F4 is smaller than the charging force F1. The other end of the coil spring 11 is connected to an ear or loop 12 which is secured by a lead screw and a nut in the upper part of the lever end plate 5e. 
     A return spring 13 is provided close to the spring coil 11. One end of this return spring 13 is secured to a connecting point 14 in the base plate P, and the other end is hooked into an opening provided close to the upper end of the end plate 5e. The return spring 13, which is also illustrated as a coil spring, exercises a return spring force F2 to the end plate 5e and thereby to the levers 5, 7 and 9 and the solenoid plunger 10a. 
     Secured to the base plate P is a bracket 15. The upper portion of this bracket 15 contains an adjustable stop screw 16. In the charged position (see FIG. 2), the end plate 5e engages the tip of the screw 16. In this position, the lower part of the end plate 5e engages and depresses the switching arm of an electrical switch 17. This switch 17 is also secured on the ground plate P. The switch 17 is connected electrically to the motor 2 as is described below in connection with FIGS. 6 and 7. When the end plate 5e is in its right position, as shown in FIG. 2, the electric circuit for supplying the motor 2 is interrupted. 
     As illustrated in FIGS. 1 and 2, the driving and release mechanism (illustrated on the right side) is connected to the trip coil device (illustrated on the left side) by means of the flexible cable 3. This cable 3 is guided through a bushing or jacket 18a out of a side wall of the control box. The bushing 18a is secured to the side wall by means of a lead screw and two nuts. A similar bushing or jacket 18b is also provided on the side of the trip coil device. 
     The trip coil device contains two mounting plates 19 and 20 which are arranged and fixed parallel to each other. Contained in the space between the mounting plates 19, 20 is a trip pin 21 which is aligned with the bushing 18b. The trip pin 21 has secured thereon an annular retainer leaf spring 22 which engages an end ring 23. Between the end ring 23 and the mounting plate 19 is arranged a chargeable trip spring 24. The trip spring 24 is shown to be a coil spring. By means of the flexible cable 3, the trip spring 24 can be transferred from a discharged position (as shown in FIG. 1) to a charged position (as shown in FIG. 2), and vice-versa. The trip spring force is denoted as F3. The outer end of the trip pin 21 is positioned close to the normal trip or latch bar 26 of the circuit breaker, as can be seen in FIG. 2. The latch bar 26 is rotatable about an axis 27. The latch roller of a four-bar linkage of the circuit breaker is denoted as 28. 
     The latch 6 exerts a latch force F5 the armature pin 1a in the charged position, as illustrated in FIG. 2. 
     Thus, the illustrated direct acting undervoltage release device utilizes as its main parts an electromagnet or a solenoid 10 that senses an undervoltage, a trip spring 24 that stores energy in order to trip the circuit breaker at undervoltage conditions, a release mechanism 6-9 that actuates the trip spring 24, and a charging motor 2 and a lever assembly that automatically resets the undervoltage release device after a tripping operation has taken place, provided the control voltage recovers its normal value. 
     A direct acting feature is herein achieved by letting the trip pin 21 directly drive the latch or latch bar 26 of the circuit breaker in order to trip the circuit breaker. 
     As indicated above, the tripping force F3 is supplied by the charged trip spring 24. This charged trip spring 24 may be located in the circuit breaker together with the trip pin 21 directly and parallel with the regular trip solenoid 29 of the circuit breaker by means of the mount-into plates 19 and 20. 
     The operation of the undervoltage device will now be explained. In the trip position, as illustrated in FIG. 1, where the trip spring 24 is discharged, the motor 2 is energized through a suitable electric source (not shown) via the switch 27. This causes the drive bar 4 (which is locked to the motor shaft 2a to pick up the armature 1 in a clockwise rotation. As the armature 1 turns, its second pin 1b pulls the flexible cable 3. This compresses and thereby charges the trip spring 24. This retracts the trip pin 21 away from the latch bar 26 of the circuit breaker. 
     During the same rotation of the armature 1, the first armature pin 1a pushes the lever 5 to the right to charge the coil spring 11, thereby counteracting the solenoid holding force F4 on the plunger 10a. 
     When the cable 4 crosses over the center line of the motor shaft 2, the torque applied on the armature 1 by the charged trip spring 24 reverses in direction (toggle action). 
     This causes the armature 1 to rotate freely, that is independently of the motor 2, until its first pin 1a rests on the latch lever 7. The lever 5 activates the switch 17 just when toggle occurs in order to de-energize the motor 2. Thereby an overdrive against the latch lever 7 is prevented. 
     The surface of the latch 6 is concentric with the pivot 8 so that the line of action of the latch force F5 passes through the center of the pivot 8 avoiding stray torque on the lever 7. The effect of any stray torque on the latch 6 caused by an impact between the first pin 1a and the latch 6 and shock-force from breaker operation is minimized by means of a large mechanical advantage L2/L1, as can be seen in the schematic diagram of forces in FIG. 5. L1 is the radius of the surface of the latch 6 with respect to the pivot 8, and L2 is the effective distance of the spring force F1 with respect to the pivot 8. 
     When the control voltage has a normal value, the holding force F4 of the solenoid 10 is greater than the charging force F1. The holding force F4 of the solenoid 10 decreases with the control voltage. When the spring force F1 exceeds the holding force F4, the lever 9 moves to the left causing the lever 7 rotates about the pivot 8. The combined rotation is clockwise about the pivot 8. This will cause the latch 6 to disengage and release the first pin 1a of the armature 1. This, in turn, results in the rotation of the armature 1 due to a torque which is determined by the trip spring force F3 times the eccentricity e. The trip spring 24 will be discharged, thereby activating the latch bar 26 of the circuit breaker. 
     Any inertia of the solenoid plunger 10a, and the levers 9 and 7 towards the lever 5 that could damage the switch 17 is arrested by the adjustable stop screw 16 mounted on the bracket 15. 
     As soon as the armature 1 is disengaged from the lever 5, the light return spring 13 (return spring force F2) resets the solenoid plunger 10a and all the levers 5, 7, 9 to their normal positions. The lever 9 is coupled to and rotatable with the lever 7 about the pivot 8a to allow a free axial movement of the solenoid plunger 10a. 
     The eccentricity e of the cable 3 with the center line of the armature 1 can be selected to minimize the impact on the latch 6 and to provide adequate control of the motor cut-off. The flexible push-pull cable 3 allows mounting of the parts of the mechanism on the base plate P at a convenient location in the circuit breaker. 
     Some important details of the operation of the undervoltage release device will briefly be stressed again: It is assumed that in the tripped position of the device (see FIG. 1) the control voltage reaches its normal value after a short time. This causes the motor 2 to be energized. The drive bar 4 (which is locked on the motor shaft 2a) picks up the armature 1 to pull the cable 3 and charge the trip spring 24. As the armature 1 turns, its first pin 1a pushes the lever 5 to charge the coil spring 11, thereby counteracting the holding force F4 of the solenoid 10 on the plunger 10a. When the cable 3 crosses over the center of the motor shaft 2a, the cut-off of the switch 17 is activated by the lever 5 and its end plate 5e to de-energize the motor 2. The torque on the armature 1 caused by the trip spring 24 reverses in direction making the armature 1 rotate freely until it latches on the latch plate 6. The driving device of the undervoltage release device has arrived in its stand-by position. 
     When the control voltage drops down, the holding force F4 of the solenoid 10 on its plunger 10a drops proportionally. The plunger 10a is released when the spring force F1 exceeds the holding force F4. When this occurs, the first pin 1a of the armature 1 is released by the latch 6 causing it to rotate freely, and the energy stored in the spring 24 is dumped on the latch bar 26 to open the circuit breaker. 
     FIGS. 6 and 7 schematically show the circuits for DC and AC operation, respectively, of the undervoltage release device according to the invention. As noted, control voltages of 125 V DC, 250 V DC, 115 V AC and 230 V AC may be used. Since the circuit is basically designed for 125 V DC, a rectifier 29 is used when the device is applied on 115 V AC and 230 V AC and dropping resistors 30 are used on 250 V DC and 230 V AC. 
     It should be borne in mind that the undervoltage release device according to the invention is primarily intended for use as an ancillary device to trip a circuit breaker when the control voltage of the breaker drops below the operating range of the trip solenoid (solenoid 29 in FIG. 2). The device according to the invention operates in parallel with the trip solenoid both electrically and mechanically. After the tripping operation, the device resets automatically when the control voltage returns to normal. The normal drop-out range of the device may be 30 to 60% of the nominal control voltage; however, the actual drop-out may be in the 20 to 40% range due to the spurious effect of the shock and vibration of the breaker operation on the device. Similarly, the actual pick-up range may be 70 to 85% of the nominal control voltage with the normal pick-up range of 60 to 100%. 
     While the forms of the undervoltage release device herein described constitute preferred embodiments of the invention, it is to be understood that the invention is not limited to these precise forms of assembly, and that a variety of changes may be made therein without departing from the scope of the invention.