A Fast Torque Response Control Scheme for Medium Voltage CSI Fed Induction Motor Drives

The Current Source Inverter technology is well suited for medium voltage induction motor drives. Because the CSI drive generally features simple converter structure, motor friendly waveforms, inherent four-quadrant operation capability and reliable fuse less short circuit protection. The main draw back lies in its dynamic torque response. An improved Direct Torque Control method is proposed here to overcome the drawback. The DTC of a CSI fed induction motor drive involves direct control of rotor flux linkage and electromagnetic torque by applying the optimum switching vectors. In DTC fast response is possible to control directly the modulus of rotor flux linkage space vector through the rectifier voltage and electromagnetic torque by the supply frequency of the CSI. The input variables of the proposed control scheme are motor torque and rotor flux. Control variables are direct & quadrature axis current components in synchronous reference frame and phase angle between them. The motor current and voltage, needed for flux estimation, are reconstructed from the DC link measurements and inverter switching states, in conventional DTC they are measured directly. The simulation model is developed in MATLAB/Simulink using sim-power systems block library that allows a very real representation of the power section. The proposed scheme has the exceptional dynamic performance features, many of which are obtained without the need for an encoder or tachometer to monitor shaft position or speed.