Means for smoothly varying the speed of a star-connected polyphase induction motor

Claim 1

A VARIABLE FREQUENCY D.C. LINK INVERTER MEANS FOR SUPPLYING A STAR-CONNECTED POLYPHASE INDUCTION MOTOR SUCH THAT ITS SPEED CAN BE VARIED SMOOTHLY FROM ZERO TO VERY HIGH SPEEDS AND SUCH THAT IT DEVELOPS SUBSTANTIALLY SMOOTH TORQUE AT ALL SPEEDS WITHIN ITS RANGE, COMPRISING AN INVERTER HAVING A NUMBER OF OUTPUT PHASES EQUAL TO THE NUMBER OF PHASES OF THE MOTOR, SAID INVERTER INCLUDING SWITCHING ELEMENTS FOR SEQUENTIALLY CONNECTING ITS SAID OUTPUT PHASES TO SAID MOTOR PHASES RESPECTIVELY, TWO D.C. VOLTAGE SUPPLIES FOR SAID INVERTER, A COMMON TERMINAL TO WHICH SAID D.C. VOLTAGE SUPPLIES ARE CONNECTED TOGETHER, ONE OF SAID D.C. VOLTAGE SUPPLIES HAVING A POSITIVE VOLTAGE WITH RESPECT TO SAID COMMON TERMINAL AND THE OTHER OF SAID D.C. VOLTAGE SUPPLIES HAVING A NEGATIVE VOLTAGE WITH RESPECT TO SAID COMMON TERMINAL, A CONNECTION BETWEEN SAID COMMON TERMINAL AND THE STAR POINT OF SAID MOTOR, A CIRCUIT FOR SMOOTHLY AND REPEATEDLY VARYING THE VOLTAGES OF SAID SUPPLIES FROM ZERO TO MAXIMUM INVERSELY AND IN SYNCHRONISM, A CIRCUIT FOR VARYING THE FREQUENCY AT WHICH THE VOLTAGE VARIATIONS OCCUR, SAID SWITCHING ELEMENTS ADAPTED SEQUENTIALLY TO CONNECT ONE MOTOR PHASE DURING A PERIOD IN EACH HALF CYCLE TO BE SUPPLIED FROM SAID INVERTER WITH AN INCREASING CURRENT DERIVED FROM THE ONE OF SAID VARIABLE VOLTAGE D.C. SUPPLIES WHOSE VOLTAGE IS INCREASING, AND IN THE SAME PERIOD TO CONNECT ANOTHER MOTOR PHASE TO BE SUPPLIED FROM SAID INVERTER WITH A DECREASING CURRENT DERIVED FROM THE OTHER OF SAID VARIABLE VOLTAGE D.C. SUPPLIES WHOSE VOLTAGE IS DECREASING, AND MEANS FOR SYNCHRONIZING THE VARIATION IN VOLTAGE OF SAID D.C. SUPPLIES WITH THE SWITCHING ACTION OF SAID INVERTER.