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

A novel, hybrid solution employing a combination of a load-commutated inverter (LCI) and a voltage-source inverter (VSI) is proposed for 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 LCI-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 LCI, controlled by the VSI in the entire speed region of the induction motor and 2) a dc-link current control loop to ensure minimum VSI rating. Advantages of the proposed solution over the conventional LCI-based induction motor drives include the following: 1) sinusoidal motor phase current and voltage based on the instantaneous motor speed control; 2) fast dynamic response by the VSI operation; and 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 to support the use of the proposed system are also included for a 1-hp induction motor laboratory setup.