Patent Application
20150235794
Embodiments presented herein relate generally to electrical circuit interruption with circuit breakers, and more particularly, to circuit breaker mechanisms that are constructed as tri-stable planar flexure mechanisms with resilient components for use in Molded Case Circuit Breakers (MCCB).
MCCBs are employed to interrupt DC or AC, single-phase or multi-phase, electrical circuits for protection of the electrical infrastructure when an electrical fault condition occurs. The electrical fault conditions can include an instantaneous current in the circuit that exceeds a predefined instantaneous current limit (i.e., an electrical short exists) or a long-term current that exceeds a predefined long-term current limit (i.e., an overload condition exists). A single-break MCCB typically comprises one pair of electrical contacts for each phase, with each pair consisting of a stationary contact mounted to a stationary current loop and a movable contact mounted to a contact arm. A dual-break MCCB comprises two pairs of electrical contacts per phase. Per phase there are two stationary contacts and a contact arm with two movable contacts. A circuit breaker mechanism interrupts the flow of electrical current by separating the movable contacts from the stationary contacts, thereby transitioning from a closed to a tripped state.
In conventional approaches, the circuit breaker mechanism moves from the closed to the tripped state by releasing stored elastic strain energy from a helical tension spring and converting it to kinetic energy of the mechanism links. The release of elastic strain energy begins by disengaging a cradle from a latch assembly that helps hold the spring under tension in normal operation when the circuit breaker mechanism is closed. The latch assembly can be disengaged by an automatic trip unit that senses and responds to an electrical fault or manually by an operator pressing a “push-to-trip” button.
The energy to separate the movable contact and contact arm from the stationary contact may also come from an electromagnetic field that develops around the stationary current loop and the contact arm due to the short circuit currents that flow through these components. The interactions between the electromagnetic field and the short circuit current result in a repulsive force between the stationary current loop and the contact arm which causes them to move apart (i.e., “blow-open”). The magnitude of the repulsive force diminishes as the current and electromagnetic field intensity drop when the contacts start to separate. The circuit breaker mechanism has to be capable of preventing the contact arm from reclosing with the stationary contact.
Once tripped, the circuit breaker mechanism remains tripped and indicates this state to an operator. The circuit breaker mechanism is typically resettable after being tripped by moving a handle from the tripped position to the open (i.e., off) position. This increases the elastic strain energy of the spring and engages the cradle with the latch assembly. Once reset the electrical circuit can be closed by moving the handle from the open position to the closed (i.e., on) position. MCCBs may also be used to interrupt and close electrical circuits in absence of electrical faults by moving a handle between the closed and open positions.
The movement of the contact arm from the closed position to the tripped position should be fast to minimize the formation of arcs that may degrade the contacts and thereby increase the overall electrical resistance of the circuit breaker. Similarly, when a handle is used to transition the circuit breaker mechanism between the closed state and the open state, the contacts have to move quickly even if the handle motion is slow. This characteristic is called “quick make-break”. The circuit breaker mechanism has to be of suitable size to fit in a predefined circuit breaker casing or electrical panel. The circuit breaker mechanism should be insensitive to wear, contamination, long-term fatigue, vibrations, temperature and humidity to prevent unintended nuisance trips or a no-trip situation.
Due to these design criteria, conventional circuit breaker mechanisms have a variety of moving components. Correspondingly, they are difficult to assemble and have a high number of failure modes, sources for friction and other uncertainties affecting their performance characteristics. In addition, conventional circuit breaker mechanisms may be larger than desired and still not meet all of the design criteria with regard to the opening speed or repeatability of the displacement input or force input at which they can be tripped.
Therefore, there is a need for circuit breaker mechanisms that are less complex, have more repeatable performance characteristics, are easier to assemble, are smaller in size, have faster opening times and more repeatable displacement or force inputs at which they can be tripped.
In a first embodiment, an actuation system is disclosed. In accordance with the embodiment, the actuation system includes a latch assembly configured to releasably engage a cradle. The actuation system also includes a movable arm as well as a resilient component that includes a first end having a revolute joint on the cradle and a second end connected to the movable arm. The resilient component is elastically deformed and stores elastic strain energy in one of a first stable state or a second stable state when the latch assembly is engaged with the cradle. The resilient component is in a third stable state that is not substantially elastically deformed and stores substantially no elastic strain energy when the latch assembly is not engaged with the cradle. The actuation system also includes an input link engaged with the resilient component between the first end and the second end. Movement of the input link between a first position and a second position when the latch assembly is engaged with the cradle transitions the resilient component between the first stable state and the second stable state.
In another embodiment, a method for actuating a movable arm is disclosed. The method includes: maintaining a resilient component under a state of elastic strain energy in a first stable state; holding a movable arm in a first stable position while the resilient component is in the first stable state; in response to a displacement input or a force input, releasing the elastic strain energy from the resilient component so that the resilient component is in a third stable state corresponding to a relaxed state; in response to the released elastic strain energy, rotating the movable arm so that it is no longer in the first stable position; and in response to the released elastic strain energy, moving an input link to a tripped position.
In a further embodiment, a circuit breaker assembly is disclosed. In accordance with the embodiment, the mechanism includes a latch assembly configured to releasably engage a cradle. The mechanism also includes a movable arm configured to move between a closed circuit position and an open circuit position. The mechanism also includes a leaf flexure comprising a first end having a revolute joint on the cradle and a second end connected to the movable arm.
These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying figures in which like characters represent like parts throughout the figures, wherein:
Embodiments presented herein relate generally to electrical circuit interruption, and more particularly, to the use of tri-stable planar flexure mechanisms that use resilient components. The mechanisms presented herein are, in one embodiment, for use in MCCBs. In certain such embodiments, the mechanisms presented herein are designed to move an electrical contact arm assembly in a single phase breaker or multiple contact arm assemblies in a multi-phase breaker in order to achieve the electrical circuit interruption. The MCCB may be configured as single-break with one stationary contact and one movable contact per phase or as dual-break with two stationary contacts and two movable contacts per phase.
As discussed herein, separation of the electrical contacts may be accomplished by releasing elastic strain energy stored in a resilient component and converting it to kinetic energy of the mechanism links. The resilient component may be provided as a leaf flexure element or other suitable component having a first end having a revolute joint on a cradle and a second end connected to the contact arm assembly. The release of elastic strain energy may be initiated by disengaging a cradle from a latch assembly that helps to constrain the resilient component in normal operation when there is no electrical fault. The latch assembly can be disengaged by a displacement or force input from a trip unit that senses and responds to an electrical fault or by an operator pressing a “push-to-trip” button. In certain embodiments discussed herein, the trip unit may use a bi-metallic strip that heats and deforms in an over-current situation and a magnetic flapper that deflects in the presence of electromagnetic fields that are generated in the presence of short circuit currents.
The energy to separate the movable contacts from the stationary contacts may also come from electromagnetic fields that develop around the stationary current loop and the contact arm due to the short circuit currents that flow through these components. The result is a repulsive force between the stationary current loop and the contact arm which causes them to move apart (i.e., “blow-open”). The magnitude of the repulsive force will diminish rapidly as the current levels drop when the electrical contacts start to separate. As discussed herein, the mechanisms disclosed are capable of preventing the movable contact arm from reclosing with the stationary contact after the repulsive force diminishes by being tripped by a trip unit and by releasing elastic strain energy to rotate the contact arm assembly so that it can no longer contact the stationary contact.
With the preceding in mind, the planar flexure mechanisms, as discussed herein, may be suitable for use in either residential, commercial, or industrial use MCCBs with operating voltages up to 1 kV DC or 1 kV AC at 50 Hz or 60 Hz and operating currents between 5 A and 2 kA. The rated short-circuit interrupt current can be more than tenfold higher than the rated operating current. The MCCB mechanisms, as discussed herein, may be suitable for opening times of the contact arm assembly between 1 ms and 100 ms and contact forces of 0.1 N to 100 N between the stationary and movable contacts. As discussed herein, various embodiments of the mechanism are described where resilient components are utilized that can function as both, devices that can store and release elastic strain energy and as structural components (i.e., links) in the kinematic chain (i.e., linkage) of the mechanism. As discussed herein, such resilient components can be leaf flexure elements, collapsible compression helical springs, volute springs, or curled flexure elements which are employed to maintain a contact arm assembly and a handle yoke in either a first stable closed position or a second stable open position while they have high elastic strain energy. In a third state, where the cradle is disengaged from the latch assembly and the resilient components are in a relaxed state with lower elastic strain energy, the circuit breaker mechanism may be in a tripped state distinct from the closed and open states. The resilient components as discussed herein may allow a reduction in the number of components in the circuit breaker assembly and, therefore, reduce the complexity of the circuit breaker, improve reliability and scalability of the design, and provide a lower mass and mass moment of inertia than a mechanism with conventional components, enabling higher speeds of operation.
As used herein, resilient components are integrated into the mechanism such that these components have a mode of higher elastic strain energy and such that the mechanism has a first stable state and a second stable state while the cradle stays engaged with a latch assembly. In the embodiments presented herein, when the mechanism is in the first stable state, reaction forces and reaction moments of the resilient components create a moment about a revolute joint of the contact arm assembly so that the movable contacts contact the stationary contacts and so that the handle yoke is held in the closed position. When the mechanism is in its second stable state, reaction forces and reaction moments of the resilient components create a moment about a revolute joint of the contact arm assembly to disconnect the movable contacts from the stationary contacts and to hold the handle yoke in the open position. A transition between the first stable state and second stable state of the mechanism can be made by means of the handle yoke that is mechanically coupled to the resilient component. The opening and closing of the stationary contacts and the movable contacts happen rapidly and are nearly independent of the handle yoke speed once the resilient component transitions through an unstable bifurcation point. This behavior is referred to as “mechanism snap-through” and provides a “quick make-break” characteristic. Furthermore, at a third stable state, where the resilient components have a mode of lower elastic strain energy, the cradle is disengaged from the latch assembly, and the mechanism is in a tripped state. The mechanism can transition from the first stable state to the third stable state by means of releasing the cradle from the latch assembly and from the third stable state to the second stable state by moving the handle yoke from tripped to closed.
By way of example, and as discussed in greater detail below, in a first embodiment, the resilient component is a leaf flexure element that is mounted between two ends, one of which is rigidly fixed to a crank which is part of a contact arm assembly and that forms a revolute joint with the mechanism frame. The rigid connection between the leaf flexure element and the crank is located at the revolute joint of the crank. The other end of the leaf flexure element has a revolute joint on a cradle. A handle yoke having a revolute joint on the mechanism frame connects to the leaf flexure element through a slot and a protrusion on the flexure forming a revolute joint and a prismatic pair between the handle yoke and the leaf flexure element. In this example, in normal operation, the cradle is held stationary by a latch assembly with which it engages. When the handle yoke is moved from the closed position to the open position the leaf flexure element eventually reaches a bifurcation point and transitions from the first stable position to the second stable position. The “mechanism snap-through” happens quickly and nearly independent of the speed of the handle yoke. Similarly, there is “mechanism snap-through” behavior when the handle yoke is moved from the open position to the closed position. Further, when disengaged from the latch assembly, the cradle rotates about a revolute joint on the mechanism frame as a result of the reaction forces exerted upon it by the leaf flexure element. The rotation of the cradle causes the revolute joint between the cradle and leaf flexure element to relocate with respect to the mechanism frame, and the leaf flexure element reaches its unstressed shape with low elastic strain energy content. As a result of the leaf flexure element transitioning from a mode of high elastic strain energy to a mode of low elastic strain energy, the crank and contact arm assembly as well as the handle yoke accelerate from the closed position to the tripped position.
With the preceding in mind, and turning to
In the depicted example, the leaf flexure element 150 is in a first stable closed position, held in a stressed and deformed state of high elastic strain energy by the cradle 116 which is secured to the latch assembly 186. One end of the leaf flexure element 150 has a flexure revolute joint 152 on the cradle 116 (i.e., there still can be relative rotational motion between the cradle and the leaf flexure element). The other end of the leaf flexure element 150 forms a rigid flexure connection 151 with the crank 118 (i.e., there is no relative motion possible between the crank and the leaf flexure element). The crank revolute joint 117 is in close proximity to the rigid flexure connection 151. In one embodiment the leaf flexure element 150 consists of a 65 mm×15 mm×1 mm unidirectional fiber glass composite that has a rigid flexure connection 151 to the crank 118. The rigid flexure connection 151 is located less than 10 mm away from the crank revolute joint 117.
In the depicted embodiment, mechanism 100 includes a link in the form of a handle yoke 114 that is connected to a handle switch 112 and that has a handle yoke revolute joint 113 on the mechanism frame 102. The handle yoke 114 has a handle yoke slot 122 that is engaged with a flexure protrusion 153, thus forming a prismatic pair and a revolute joint, respectively, between the handle yoke 114 and the leaf flexure element 150 (i.e., a pivot is formed that can move linearly with respect to the handle yoke). In this manner, a change in state of the leaf flexure element 150, either by operation of the latch assembly 186, the contact arm assembly 154, the cradle 116, or the handle yoke 114, may be communicated to the other interconnected parts to change the positions of those interconnected parts.
In the depicted example, a latch assembly 186 is also shown which, when engaged holds the mechanism in either the closed or open states. The depicted latch assembly includes a primary latch 182 that may be engaged with cradle 116 at primary latch interface 172 and with a secondary latch 178 that may be engaged with primary latch 182 at secondary latch interface 174. In the depicted example, a latch bias spring 180 is provided between the primary latch 182 and the secondary latch 178 such that, when disengaged from one another, the latch bias spring 180 can bias the primary latch 182 and the secondary latch 178 towards one another such that the cradle 116 can be reengaged by moving the handle yoke 114 after the mechanism has been tripped. As will be appreciated, the depicted latch assembly 186 is a dual latch assembly (i.e., there are two engagements, the first between the primary latch 182 and the secondary latch 178 and the second between the primary latch 182 and the cradle 116). In other embodiments, the latch assembly may differ, such as being a single latch assembly between a primary latch and cradle, or a triple or greater latch assembly, having more than two latches.
Contact arm assembly 154 comprises a contact arm 110 that has a contact arm revolute joint 155 located on crank 118. A contact arm torsion spring 156 creates a moment between crank 118 and movable contact arm 110. In the depicted closed state of the MCCB mechanism 100, the crank 118 is oriented such that the contact arm 110 is rotated away from contact arm hard stop 162 which results in a contact force between the movable contact 108 and the stationary contact 106. The deflection of the contact arm 110 with respect to the contact arm hard stop 162 is known as “contact arm depression”. In other embodiments discussed herein, the torsion spring 156, the contact arm revolute joint 155, and the contact arm hard stop 162 may be absent, and the resilient component may directly attach to the contact arm 110. In this case the resilient component directly generates the moment required for a contact force between the movable contact 108 and the stationary contact 106. Also in this case, the leaf flexure element 150 may directly provide the necessary compliance for a “blow-open” event to occur.
In
In general, it may be desirable to move the circuit breaker mechanism 100 between the closed position depicted in
Turning to
As shown in
To move the MCCB mechanism 100 in
In the tripped state of the MCCB mechanism 100, and as shown in
The mechanical motions of the components described above are determined by various kinematic pairs provided as part of the assembly. For example, certain kinematic pairs may be revolute joints (i.e., pivots), prismatic pairs (i.e., sliders), or cams-followers (i.e., surface-to-surface contacts). Referring to
Conversely, various kinematic pairs are moving with respect to the mechanism frame 102 and are defined by the structure depicted in
By way of example, in the embodiment of MCCB mechanism 100 shown in
By way of further example, in the embodiment of mechanism 100 shown in
Further, in the embodiment shown in
In an additional example, in the embodiment of the MCCB mechanism 100 shown in
By way of example, in the embodiment of the MCCB mechanism 100 shown in
Further, in this embodiment, the handle yoke 114 has two separate structures parallel to one another with the collapsible compression spring 140 positioned between these separated structures. The interface between the collapsible compression spring 140 and handle yoke 114 is shown in the form of a pair of crossbars 136 connecting the separated components of the handle yoke 114 through which the collapsible compression spring 140 passes.
Turning to
Turning to
The preceding examples have depicted planar, and other, flexure mechanisms for MCCBs that may be used in single-phase, single-break embodiments for simplicity and to facilitate explanation. However, it should be appreciated that any of the aforementioned approaches may be applied in other configurations, such as in multi-phase (e.g. three-phase) or dual break arrangements and in AC or DC circuits. For example, and tuning to
Technical effects of the invention include the construction and use of a MCCB mechanism incorporating resilient components that link a cradle to a contact arm assembly. In a first embodiment, the resilient component is a leaf flexure element that is rigidly connected to a contact arm assembly. In a second embodiment the leaf flexure element has a revolute joint on the contact arm assembly that is offset from the crank revolute joint. In a third embodiment, the resilient component is a volute spring. In a fourth embodiment, the resilient component is a curled flexure, and in a fifth embodiment the resilient component is a collapsible compression spring. A handle yoke may also interface with the resilient component to transition the mechanism from a first stable closed state to a second stable open state when the cradle is held stationary. The handle yoke is moved to a tripped position by the resilient component when the mechanism is tripped to transition to a third stable tripped state. The handle yoke can be used to reset the mechanism by reengaging the cradle with the latch assembly, adding elastic strain energy to the resilient component, and transition the mechanism to the open state. The components of the mechanism may provide a low mass and mass moment of inertia allowing high angular acceleration of the contact arm assembly.
This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.
