Improvement of low speed characteristics of railway vehicle by sensorless control using high frequency injection

This paper presents a way to improve the torque control characteristics of railway vehicle at low speed region using the high frequency injection method. In traction drives, the demanded robustness, which can only be reached with expensive, specially constructed speed sensors, limits the speed quantization (often only 50-100 pulses per revolution) so that the speed information is inaccurate and is not available instantaneously. Due to this reason precise torque control with FOC (field oriented control) is degraded at the standstill and low speed region. The sensorless control with high frequency signal injection method has been proposed and it is known that this high frequency injection method gives the feasibility of FOC at the standstill and the robustness against parameter variation. In this study, the rotor flux angle is estimated by the high frequency injection method at low speed region and when the speed exceeds a threshold, the control is transitioned to the indirect vector control mode. The proposed strategy makes the precise torque control so that the slip between wheel and rail on the starting can be eliminated and electrical braking can be extended to the standstill. The strategy has been validated by simulation study, experiments with 7.5 kW laboratory setup, and experiments with 1.2 MVA IGBT traction drive system with four 210 kW induction motors