Reduced Torque Ripple Operation with a PWM Scheme in SCR-based CSI fed Medium Voltage Induction Motor Drive

This paper presents a Pulse Width Modulation (PWM) technique to reduce the torque pulsation in Current Source Inverter (CSI) fed Medium-Voltage High-Power (MV-HP) induction motor drives. The drive uses Silicon Controlled Rectifier (SCR) based CSI topologies for their reliability, cost-effectiveness, and high-power handling capabilities. However, the quasi-square current provided by SCR-based CSI introduces significant lower-order current harmonics. These current harmonics generate lower-order torque ripple, which degrades motor performance, especially during startup and low-speed operation of the drive. Therefore, reducing lower-order harmonic torque ripples in drives, especially during starting and low-speed operation, is important. The paper proposes a harmonic minimization PWM technique for a 12-pulse CSI-fed multiphase induction machine drive to minimize such lower-order harmonic torque ripples. The motor has an additional low-voltage winding that is connected to a small power-rated Voltage Source Inverter (VSI). This VSI assists in commutating the CSI-SCRs from the motor side. As a result, SCR commutation and PWM operation are achieved without requiring external commutation circuitry of high voltage and power ratings. This paper provides detailed information on the design considerations, operating principles, commutation technique and control architecture of the PWM technique in enhancing motor drive performance.