Synchronous induction motor speed control

Claim 1

MEANS FOR CONTROLLING THE SPEED OF A SYNCHRONOUS INDUCTION MOTOR OF WHICH THE SPEED, AT SYNCHRONISM WITH A THREE-PHASE SUPPLY, IS SMALL AS COMPARED WITH ITS MAXIMUM SPEED, COMPRISING A RECTIFIER, A CHOKE, AN INVERTER, MEANS FOR CONTROLLING THE FIRING ANGLE OF THE INVERTER, AND A SWITCH MEANS, SAID SWITCH MEANS OPERABLE TO CONNECT THE MOTOR ROTOR WINDINGS TO A THREE-PHASE SUPPLY AND TO APPLY THE RESULTANT SLIP ENERGY APPEARING AT THE MOTOR STATOR TO SAID RECTIFIER AND THROUGH SAID COKE TO SAID INVERTER SUCH THAT SAID INVERTER FEEDS ITS OUTPUT TO THE THREE-PHASE SUPPLY AT THE VOLTAGE AND FREQUENCY OF THE LATTER, WHEREBY THE SPEED OF THE MOTOR UP TO SYNCHRONOUS SPEED IS CONTROLLED BY THE FIRING ANGLE OF SAID INVERTER, AND SAID SWITCH MEANS OPERABLE WHEN SAID MOTOR HAS ACHIEVED SYNCHRONOUS SPEED TO APPLY TO D.C. EXCITATION TO THE MOTOR ROTOR WINDINGS SO AS TO PRODUCE A SUFFICIENT AC. MOTOR STATOR VOLTAGE FOR THE OBTAINING OF NATURAL COMMUNTATION OF SAID INVERTER, AND TO CONNECT THE THREE-PHASE SUPPLY FOR THE MOTOR THROUGH SAID RECTIFIER, CHOKE AND INVERTER TO DRIVE THE MOTOR FROM THE OUTPUT OF SAID INVERTER OVER A WIDE SPEED RANGE DETERMINED BY CONTROLLING THE FIRING ANGLE OF THE INVERTER.