Circuit breaker with moving magnetic core for low current magnetic trip

A circuit breaker for responding to abnormal currents in conductors associated with each phase in a multiphase electrical system comprising:

• a set of electrical contacts for each phase of the multiphase electrical system completing an electrical circuit through an associated conductor when closed and interrupting the circuit when open;
• a latchable spring powered operating mechanism operable to open all of said sets of electrical contacts when unlatched;
• a trip bar rotatable from a biased position to a trip position to unlatch said operating mechanism;
• a magnetic trip assembly for each phase of the multiphase electrical system each comprising:
• an elongated conducting bimetal through which current from said conductor flows to generate a magnetic flux;
• a pivotally mounted armature rotatable about a pivot axis and having a free end rotatable toward said bimetal, said free end spaced from said bimetal in a latch position in which said armature latches said operating mechanism, said armature is attracted towards said bimetal by the magnetic flux produced by an abnormal current in said bimetal to an unlatched position which unlatches said operating mechanism;
• a generally U-shaped fixed magnetic yoke having an elongated base and two outwardly extending legs partially surrounding one end of said bimetal to concentrate the magnetic flux in the direction of said armature to concentrate the magnetic flux in the direction of the armature and to form a primary air gap therewith;
• a generally U-shaped movable magnetic core having a base and two outwardly extending legs, the base of said movable core being adjacent the base of said fixed yoke and movable relative to said fixed yoke between an extended position and a retracted position with the legs of said movable core extending beyond the legs of said fixed yoke into said primary air gap to shorten said primary air gap between the armature and said fixed yoke in the extended position to further concentrate the magnetic flux and to generate the magnetic force required to attract the armature toward the bimetal at a lower current level, said core in the extended position is engaged by said armature as it pivotally rotates toward said fixed yoke, and said armature as it continues to rotate urges said movable core into said retracted position in which the legs of said movable core extend toward the armature about as far as the legs of said fixed yoke, said movable core having means mounting said movable core with said movable core being movable on said mounting means from said extended position to said retracted position;
• and
• biasing means biasing said armature away from said bimetal to said latching position.