It is a matter of common knowledge to use a magnetic actuator with high force density to operate moving contacts for a purpose of electrical power interruption in the medium-voltage field of technology. Known magnetic actuators have a design with a fixed core in the center of the device, and two moveable plungers, one above and one below the core, that are connected with a plunger rod. Such a device is supposed to generate a high static holding force in the closed position to latch opening and contact springs. The magnitude of this static holding force is the key parameter for the design of the entire circuit breakers and for space and weights reasons it is generally advantageous to generate this force with a small magnetic actuator. In the open position, a lower static holding force is needed to keep the circuit breaker in open position. For bringing the actuator from close to open position feeding the electrical coil of the actuator with electrical energy is needed.
The document EP 0 898 780 B1 describes a magnetic actuator with a ferromagnetic armature which is displaceable linearly between two limit positions and which is mechanically connected to a circuit breaker and which in the limit positions is under the influence of magnetically generated forces. The armature and the ferromagnetic shunt body are arranged in succession in a space between first and second abutment. The abutments are pole surfaces of magnetic circuits which include at least one permanent magnet for generating a holding force for the armature. This known device is as well supposed to operate a vacuum circuit breaker. In the closed position, the ferromagnetic shunt body is apart from the armature. The shunt can now be moved towards the armature to initiate the opening operation of the circuit breaker. The known solution is based on a design that does not use the full potential of the static holding force as the effective area between the moveable armature and the fixed yoke is limited to the area that is inside the coil. As a result, the actuator is almost twice as big as needed.
WO 03/030188 A1 discloses a further magnetic actuator, such as for a vacuum circuit breaker design being large in size. Two electrical coils are needed in order to operate the magnetic actuator or bringing a connected circuit breaker from an open to a closed switching position. A first magnetic flux is generated by the armature and the yoke in such a way that the armature is held in one limit position and the electrical coil generates a second magnetic flux that actuates the armature. The permanent magnet is located between the yoke and a fixed magnetic return element, in such a way that the magnetic flux runs via the magnetic return element. In addition, the armature outside the yoke covers a front face of the yoke, said face running perpendiculary to the direction of displacement of the armature. Since the permanent magnet is provided to hold the magnetic armature in one of the two limit positions, neither mechanically latching nor a constant electrical current supply is specified.
Also this known solution uses the armature for generating the static holding force in both limit positions. This solution implies a second magnetic path from the magnets to the armature that is only effective in the open limit position. This second magnetic path increases sizes again and weights of the magnetic actuator. The known solution also specifies a closed room around both the armature. The ferromagnetic shunt body forms the two abutments that need to fulfill magnetic functions. This increases the size and weight of the actuator further. The known solution entails the driving of the ferromagnetic shunt body back to the lower abutment during the opening operation. This driving should include additional energy that is not available for the opening operation, which is the most critical operation of a circuit breaker in case of short circuit switching.