A Novel PWM Scheme for Thyristor Based Current Source Inverter fed Induction Machine Drive with Improved Torque Profile

Load-commutated inverter-based topologies are popularly used in high-power drive applications mainly due to the advantages offered by the thyristor such as high reliability, lesser losses, lower cost, and availability at high power levels. The LCI-fed drives suffer from the well-known disadvantage of low-frequency toque harmonics originating due to the quasi-square wave nature of machine current. These low-frequency oscillations are more critical at starting and low-speed operation of the drive, as they can excite mechanical resonance and cause permanent damage to the system. This paper proposes a selective harmonic minimization-based PWM method for a CSI fed multiphase induction machine drive which offers an improved torque profile during starting and low-speed operation. In the proposed topology, the MV-rated CSI operates in PWM mode and supplies all the power requirements of the machine. The CSI commutation is achieved by a low-voltage VSI without any additional complex commutation circuitry. The proposed topology is experimentally validated on a 37.5 kW multi-phase induction machine with 1.6 kV rated HV winding and 400 V rated auxiliary LV winding.