Effect of power factor on torque capability of FOC induction machine in field weakening region for propulsion systems

Propulsion system used in applications like electric traction, rolling stock, etc. requires high torque at low speed for starting and climbing and nearly constant power at high speed. Induction machines, which are rugged, reliable, low cost and has less maintenance, satisfy the characteristics of the propulsion and reinstating the traditionally used DC series motor for this purpose. Field oriented control (FOC) of induction machines can decouple its torque control from field control and extended speed range operation with constant power beyond rated speed by field weakening (FW). It is desirable to retain maximum torque capability in the FW region for propulsion applications at high speed. To produce maximum torque that machine could possibly develop at FW region, the maximum inverter voltage and current must be appropriately utilized. This paper proposes an innovative approach to improve the torque capability of induction machine in FW region by modifying the design of induction machine to change the bahaviour of the machine to suit propulsion requirements. Extensive simulation of the design options generated from the design modification has been carried out using MATLAB/Simulink and torque-speed characteristics are compared.