A hybrid converter system for high performance large induction motor drives

A novel, hybrid converter system employing combination of a load-commutated inverter (LCI), a DC-DC buck converter, and a voltage-source inverter is proposed for the large induction motor drives. By avoiding the use of output capacitors and a forced decommutation circuit, this system can eliminate all disadvantages related to these circuits in the conventional LCI based induction motor drives. Improved quality of output current waveforms and faster dynamic response can be achieved. In addition, the use of the buck converter enables reduction of the delink inductor size and eliminating requirement for the controlled rectifier. The proposed hybrid scheme features the following tasks: 1) safe commutation angle for the LCI controlled by the voltage source inverter over all speed regions, and 2) a delink current control loop to ensure minimum voltage source inverter rating. Advantages of the proposed solution over the conventional LCI-based induction motor include the followings: 1) sinusoidal motor phase current and voltage based en the instantaneous motor speed control; 2) fast dynamic response through the operation of voltage source inverter and buck converter; 3) elimination of motor circuit resonance and motor torque pulsation; 4) compact and simple system implementation due to small size of the delink inductor. 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.