A novel hybrid solution for load commutated inverter-fed induction motor drives

A novel, hybrid solution employing a combination of a load-commutated inverter and a voltage-source inverter is proposed for the induction motor drives. By avoiding the use of output capacitors and a forced DC-commutation circuit, this solution can eliminate all disadvantages related with these circuits in the conventional load commutated inverter based induction motor drives. In addition, improved quality of output current waveforms and faster dynamic response can be achieved. The proposed hybrid scheme features the following tasks: (1) the safe commutation angle for the load commutated inverter, controlled by the voltage source inverter in all speed region of the induction motor and (2) a DC-link current control loop to ensure minimum voltage source inverter rating. Advantages of the proposed solution over the conventional load commutated inverter based induction motor include the followings: (1) sinusoidal motor phase current and voltage based on the instantaneous motor speed control; (2) fast dynamic response by the voltage source inverter operation; (3) elimination of motor circuit resonance and motor torque pulsation. The feasibility of the proposed hybrid circuit for the high power drive system is verified by computer simulation for a 500 hp induction motor. Experimental results are also under way in our laboratory to show the validity of the proposed technique.