Implementation of a reduced order stator-flux observer for three level NPC inverter-fed induction motor drive

This paper presents a three-level neutral point clamped (NPC) inverter fed induction motor for high power traction applications in Indian Railway Technology. This drive is operated under direct torque control (DTC) method, and it is well known for faster torque and flux control system. This method is simple to implement but DTC do not show good performance in low speed region due to two reasons. First is due to stator flux estimation that relates to variation in stator resistance and secondly due to the fluctuation in the mid-point or neutral point of dc bus voltage. The current flow in and out of this neutral point of dc-bus causes voltage imbalance between the upper and lower capacitors. During low speed operation, due to imbalance in the capacitor voltage, motor speed fluctuation becomes another demerit. A robust reduced order stator flux estimator is designed and the effect of stator resistance variation due to temperature is also taken care. In the proposed torque control scheme, the low speed operation has been successfully achieved keeping two capacitor voltages equal and balanced. Simulation and laboratory prototype test results demonstrate the validity of the proposed torque control method.