Frequency changer for frequency conversion of three phase alternating currents

Claim 1

IN A FREQUENCY CHANGER FOR FREQUENCY CONVERSION OF THREE-PHASE ALTERNATING CURRENTS, PARTICULARLY FOR THE PRODUCTION OF LOWER FREQUENCY THREE-PHASE ALTERNATING CURRENTS FROM THREE-PHASE ALTERNATING CURRENTS OF MAINS FREQUENCY, THE COMBINATION COMPRISING THREE GROUPS OF SIX RECTIFIERS EACH CORRESPONDING TO THE PHASES, SAID RECTIFERS BEING OF THE CONTROLLABLE TYPE, MEANS ARRANGING THE SIX RECTIFIERS OF EACH GROUP IN THREE PAIRS WITH THE RECTIFIERS OF EACH PAIR ARRANGED ANTI-PARALLEL, SAID RECTIFIER PAIRS BEING CONNECTED RESPECTIVELY TO THE CORRESPONDING WINDINGS OF A THREE-PHASE SUPPLY TRANSFORMER OPERATING AT THE MAINS FREQUENCY, MEANS CONNECTING SAID THREE GROUPS OF RECTIFIERS IN DELTA FORMATION, THE SUPPLY OF THE LOWER FREQUENCY THREE-PHASE ALTERNATING CURRENTS TO THE CONSUMER BEING TAPPED AT THE ANGLE POINTS OF SAID DELTA FORMATION, AND MEANS CONTROLLING OPERATION OF SAID RECTIFIERS OF EACH GROUP SUCH THAT AT ANY GIVEN MOMENT ONLY TWO RECTIFIERS OF EACH SIX-RECTIFIER GROUP ARE FREE TO CONDUCT CURRENT WHILE THE OTHER FOUR RECTIFIERS OF THAT GROUP ARE BLOCKED AND PREVENTED FROM CONDUCTING CURRENT.