Field Weakening Combined Over-modulation and Six-step Operation of Induction Motor with Improved Torque Control

In this paper, field weakening combined six-step operation of the induction motor is presented for electrical vehicle application. At first, to operate the induction motor beyond the base speed, a voltage closed loop based field weakening system (VCFS) is constructed, which reduces the magnetizing current based on the maximum operating voltage of the inverter. To improve maximum torque capability for the given DC-link voltage, the over-modulation with six-step operation is implemented. Due to the saturation of stator voltage in the current controller, the adapted rotor field-oriented control (RFOC) cannot give decoupled control of torque and flux in the six-step mode. To overcome this, voltage angle-based torque control is implemented. The seamless transition between RFOC and voltage angle control is achieved based on minimum flux required in the induction machine. The implemented over-modulation along with the six-step mode of operation not only increases the DC-link utilization and maximum torque capability but also widen the constant torque and constant power operating regions. The entire control algorithm is implemented in Matlab / Simulink platform and the results are analysed up to three times the base speed.