Patent Publication Number: US-6218921-B1

Title: Adjustable flux transfer shunt trip actuator and electric power switch incorporating same

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     This invention relates to actuators for electrically operating switches in electric power circuits, and to electric power switches incorporating such actuators. The actuator is a flux transfer shunt trip actuator in which a coil is pulsed to generate a field which bucks the holding force generated by permanent magnets so that the spring drives the actuator to an actuated position to trip the operating mechanism of the electric power switch. More particularly, it relates to an arrangement for adjusting the response of the flux transfer shunt trip actuator. 
     Background Information 
     Electric power switches, such as for example, circuit breakers, transfers switches, network protectors, disconnects, and the like, typically have a stored energy powered operating mechanism that is tripped to rapidly open the power contacts of the switch. It is common to have electrically actuated devices actuating or tripping the operating mechanism. Even in the case of a circuit breaker with a thermal-magnetic trip unit, an electrically actuated device is often also provided to alternatively actuate the operating mechanism from a remote location or for other conditions, such as under voltage, loss of phase, or off frequency. In many cases, an electronic trip circuit is used instead of a thermal-magnetic mechanism for overcurrent protection. 
     It is becoming wide spread for the electronic circuits now used to actuate these trip devices to generate logic levels signals which have insufficient energy to actuate solenoids previously used for these purposes. A low energy device which has been developed to trip the operating mechanism of an electric power switch in response to logic level signals, is the flux transfer shunt trip actuator. These devices utilize a spring to bias a plunger to an actuated position. Permanent magnets generate sufficient magnetic force to override the actuating force generated by this spring and hold the plunger in an unactuated, typically a retracted, position. The actuating signal is applied to a coil to generate an electromagnetic force which bucks the force generated by the permanent magnets. With the permanent magnet field at least partially cancelled, the spring actuates the plunger. A short duration pulse applied to the coil is sufficient to actuate the plunger and trip the operating mechanism of the switch. 
     While these flux transfer shunt trip devices are very effective, slight variations in the strength of the magnets, springs, or even plating thicknesses can affect the current required to release the plunger. U.S. Pat. No. 5,886,605 discloses a flux transfer shunt trip actuator which incorporates an arrangement for calibrating the device. In this patented actuator, the spring biasing the plunger to the tripped position is mounted inside the cylindrical plunger. A screw threaded into the free end of the plunger bears against the spring to adjust the spring preload. While this is effective for calibrating the device, it results in changing the effective length of the plunger, and therefore, varies the spacing between the plunger and a trip button on the operating mechanism of the electric power switch. 
     Accordingly, there is room for improvement in flux transfer shunt trip actuators and electric power switch incorporating them. 
     In fact, there is a need for a flux transfer shunt trip actuator and an electric power switch incorporating such an actuator, which provides an improved arrangement for calibrating the actuator. 
     More particularly, there is a need for such an actuator and electric power switch incorporating the actuator, in which the actuator can be calibrated without affecting the spacing between the actuator plunger and the operating mechanism of the electric power switch. 
     There is a more specific need, for an improved flux transfer shunt trip device and an electric power switch incorporating the actuator, in which the preload on a spring biasing the actuator plunger can be adjusted without affecting the positioning or length of the plunger. 
     SUMMARY OF THE INVENTION 
     These needs and others are satisfied by the invention which is directed to a flux transfer shunt trip actuator in which the actuator can be calibrated by adjustment of the preload biasing spring without changing the position or length of the actuator plunger biased by this spring. More particularly, the invention relates to a flux transfer shunt trip actuator which includes a housing having a magnetically permeable seat, a magnetically permeable plunger mounted by the housing for movement between a retracted position with a first end seated against the seat and an actuated position with the magnetically permeable plunger spaced from the seat. A biasing spring biases the plunger toward the actuated position. One or more permanent magnets generate a permanent magnet force sufficient to overcome the biasing spring to hold the magnetically permeable plunger in a retracted position seated against the magnetically permeable seat. A coil generates an electromagnetic field when energized which bucks the permanent magnet field so that the biasing spring moves the plunger to the actuated position. An adjustment device adjusts the preloading on the biasing spring without adjusting the position or length of the plunger. 
     More specifically, the housing comprises a magnetically permeable U-shaped frame having a bight and a pair of legs spaced apart and extending from the bight. The permanent magnet or magnets are mounted between the legs and define a gap. A magnetically permeable seat member projecting from the bight toward the gap terminates in a free end which forms the seat. The first end of the magnetically permeable plunger extends into the gap when seated in the retracted position against the seat. The biasing spring has a first end bearing against the magnetically permeable plunger and the adjusting device comprises an adjusting member and a mount mounting the adjusting member for movement relative to the bight toward and away from the magnetically permeable plunger. The adjusting member bears against the second end of the biasing spring to adjust the preload. Thus, the actuator is calibrated through adjustment of the preload on the biasing spring, yet the plunger length and position do not change. 
     Even more specifically, the mount for the adjusting member is a threaded shaft which is coaxial with the seat member. The adjusting member is moved along the threaded shaft by a threaded member which engages the threaded shaft and bears against the adjustment member to set the position of the adjusting member on the threaded shaft and, therefore, the preload on the spring. In a preferred embodiment of the invention, the threaded shaft projects from the bight of the frame in a direction opposite the orientation of the legs of the frame. The adjusting member has at least one projection extending toward and engaging the first end of the biasing spring. In a particularly preferred embodiment, these projections extend through apertures in the bight of the frame and are a pair of opposed cylindrical sections each bearing against the biasing spring which is a helical compression spring coaxially mounted on the seat member together with the coil. 
     The invention also embraces the electric power switch incorporating such an adjustable flux transfer shunt trip actuator. 
     It is therefore, an object of the invention to provide an improved flux transfer shunt trip actuator and an electric power switch incorporating such an actuator. 
     It is a further object of the invention to provide such an improved actuator and electric power switch incorporating the actuator in which the actuator can be calibrated without affecting the length of the actuator plunger. 
     It is a more specific object of the invention to provide such actuator and electric power switch which can be calibrated by adjusting the preload on the biasing spring without changing the position or length of the plunger. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A full understanding of the invention can be gained from the following description of the preferred embodiments when read in conjunction with the accompanying drawings in which: 
     FIG. 1 is a schematic illustration of an electric power switch in the form of a circuit breaker employing an adjustable flux transfer shunt trip actuator in accordance with the invention; 
     FIG. 2 is an exploded isometric view of the adjustable flux transfer shunt trip actuator; 
     FIG. 3 is a longitudinal sectional view of the actuator assembled and in the retracted position; and 
     FIG. 4 is a longitudinal sectional view similar to that of FIG. 3 showing the actuator in the actuated position. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The invention is directed toward an adjustable flux transfer shunt trip actuator for an electric power switch, such as, for example, a circuit breaker, a transfer switch, a disconnect switch, and the like. For purposes of illustration, the invention will be described in relation to a circuit breaker. 
     Referring to FIG. 1, an electric power circuit  1  having conductors  3  is protected by a circuit breaker  5 . The circuit breaker includes a pair of separable power contacts  7  connected in the conductors  3 . While a single line representation of the conductors  3  is used for clarity, the electrical power system can be multiphase, typically  3  phase, in which case, separable contacts  7  would be connected in series with each phase conductor. 
     The circuit breaker  5  includes an operating mechanism  9  which is typically a spring powered mechanism which opens and closes the separable contacts  7 . As is conventional, the operating mechanism can be manually operated to open and close the separable contacts  7 . 
     The operating mechanism can also be automatically operated to open the separable contact  7  in response to overcurrent conditions in the conductors  3  by a trip unit  11 . The trip unit can be either of the well-known thermal-magnetic type or electronic type. In either case, the trip unit  11  monitors the current in the conductors  3  and trips the operating mechanism  9  under certain overcurrent conditions. 
     The circuit breaker  5  also includes a flux transfer shunt trip actuator  13  having a plunger  15  which is normally held in a retracted position shown in FIG.  1 . In the circuit breaker shown, the flux transfer shunt trip actuator  13  provides an auxiliary means for opening the separable contacts  7 . The actuator  13  is actuated by an electrical signal provided on the leads  17 . This signal can be generated in several ways. For instance, the signal can be generated at a remote location as by an operator in a control room or at a panel board. Also, the signal on the lead  17  can be generated by an under voltage relay (not shown), a loss of phase relay (not shown) or an off-frequency relay (also not shown). When the flux transfer shunt trip actuator  13  is actuated, the plunger is extended to an actuated position in which it engages the trip unit, such as through a trip button  19 . In accordance with the invention, the flux transfer shunt trip actuator  13  has an adjustment device  21  which allows the actuator to be calibrated for response to the actuating signal delivered on the leads  17 . 
     Turning to FIGS. 2-4, the flux transfer shunt trip actuator  13  includes a magnetically permeable frame  23  in the form of a U-shaped member  25  having a bight  27  and a pair of spaced apart legs  29  extending from the bight. A seat member in the form of a stationary pin  31  is secured to the bight by threaded shaft  33  having a first threaded end  35  which extends through an aperture  37  and threads into a tapped bore  39  in the seat member  31 . The seat member  31  extends between the pair of legs  29  and terminates in a seat  41 . 
     The magnetically permeable plunger  15  is supported for axial movement toward and away from the seat  39  by a nonmagnetically permeable guide  43  mounted in an end plate  45 . The end plate  45  has tabs  47  which snap into openings  49  in the free ends of the pair of legs  29 . A helical compression spring  51  is concentrically mounted on the seat member  31  and has a first end  53  which bears against a circular flange  55  forming a first end of the magnetically permeable plunger  15 . The other or second end  56  of the spring  51  bears against the bight  27  of the U-shaped member  25 . 
     A coil  57  is wound on one end of a molded bobbin  59  and is encapsulated by an insulating cover  61 . The bobbin is supported between the pair of legs  29  on the U-shaped member  25  with the coil coaxial with and extending over the seat member  31  and the helical compression spring  51 . The forward end  63  of the bobbin  59  forms a support for a pair of permanent magnets  65  which are spaced apart to form gap  66 . These permanent magnets  65  generate a permanent magnet field which circulates through the magnets  65 , the gap  66 , the seat member  31 , the bight  27  and the pair of legs  29 . 
     The helical compression spring  51  biases the plunger  15  away from the seat  41  toward an extended or actuated position shown in FIG.  4 . However, when the plunger is pushed inward so that the flange forming a first end  55  seats against the seat  41 , the permanent magnet field generated by the magnets  65  is sufficient to overcome the bias generated by the spring and thereby magnetically latch the plunger in the retracted position shown in FIG.  3 . 
     The coil  57  is wound such that when energized it generates an electromagnetic field which bucks the permanent magnet field generated by the permanent magnets  65 . The bias force generated by the spring then exceeds the magnetic latching force and the plunger is driven by the spring to the actuated position shown in FIG.  4 . With the coil deenergized, the actuator can be reset by physically pushing the plunger back to the retracted position. 
     As previously mentioned, variations in the strength of the permanent magnets  65 , the spring  51  and even plating thicknesses on the U-shaped member  25 , the seat member  31 , or the plunger  15  can affect the current that must be applied to the coil in order to release the plunger. In order to compensate for these variations and provide for calibration of the actuator, the flux transfer shunt trip actuator  13  is provided with an adjusting device  67  for adjusting the preload on the bias and spring  51 . The adjusting device  67  includes an adjusting member  69  which is moveable along a second threaded end  71  of the threaded shaft  33  which projects from the bight  27  in a direction opposite the direction of the pair of legs  29  and forms a mount for the adjusting member. This adjusting member has at least one projection, preferably a pair of opposed cylindrical sections  73  which extend through complementary apertures  75  in the bight  27  and bear against the biasing spring  51 . The position of the adjusting member  69  on the threaded shaft  33  is set by a threaded member in the form of nut  77 . By turning the nut  77  to move the adjusting member  69  toward the bight  27 , the preload on this biasing spring  51  is increased. The threaded shaft  33  has a hexagonal center section  79  by which it may be gripped for threading the first threaded end  35  into the seat member  31  during assembly. 
     Thus, it can be appreciated that movement of the adjusting member  69  toward and away from the bight  27  increases or decreases the preload on the spring  51 , respectively, but does not affect the position, or the length of the plunger  15 . Also, since the adjustment is made at the rear end of the actuator, the adjustments can easily be made with the actuator in place. 
     While specific embodiments of the invention have been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular arrangements disclosed are meant to be illustrative only and not limiting as to the scope of invention which is to be given the full breadth of the claims appended and any and all equivalents thereof.