Research On Key Problems Of Direct Torque Control In Electric Traction System

Along with the large-scale construction of high speed and heavy haul railway network project, the locomotive and EMU that drive with asynchronous motor has a wide application prospect, and the AC drive control technique sealed the locomotive and EMU’s operational performance and it is the hard core of the locomotive and EMU. The AC drive control system adopted the asynchronous motor direct torque control technique has a torque ripple in low speed which cause wheel slide. The stator current distortion resulting from the limitation of Lower switching frequency provokes a pollution of the traction power network. The damage of speed sensor caused by bad operation environment lead to locomotive and EMU’s traction power loss.Therefore, studying the torque ripple characteristics of asynchronous traction motor when it start under low speed range and its ripple suppression method, realizing characteristics of the stator current distortion while the asynchronous traction motor was operated in high speed range and improving the dependability of the speed close loop control power unit in harsh conditions have important academic value and project significance.The torque ripple of asynchronous motor controlled by direct torque control method under low speed range was the research object. The components of torque ripple in the stator reference frame were studied. The influences of system’s sampling cycle, switching cycle and reference voltage vector on torque ripple were considered. The quantitative relationship between them was established. From the point of reference voltage vector decreasing the torque ripple, characteristics of ISC and Deadbeat-SVM-DTC were discussed and a new DTC method combined with select table DTC and SVM-DTC was put forward. The results show that under a certain sampling cycle and switching cycle, the components of torque ripple are constructing of back emf component which related to the speed, the resistance component which related to the torque and the component which caused by reference voltage. The torque ripple can be greatly depressed by compensate for the last two component decrement with optimizing reference voltage vector.Both advantages and disadvantages between synchronization PWM and DSC were discussed. The distoration characteristics of hexagon flux trace and single fold corner18-sides flux trace and its PWM method were studied. The double fold corner30-sides flux trace and its two approaches were put forward. A method for an even transferring different flux trace was put forward. The results show that DSC can depress the rush of stator current and has a less harmonic torque ripple and harmonic loss. The18-sides flux trace can suppress only one of the main low order harmonics but would lead to the raise of another low order harmonics, due to the limitation of the single fold corner. Most of the main low order harmonics can be eliminated to a much lower level and also the fold corner modulation can be simplified by the new double fold corner modulation method. The torque shock is also be restrained by the transferring measures.The influences of stator resistance error and DC offset on U-I model were considered, an observer combined with resistance adaptive and perpendicular feedback compensation was put forward. The relationship between U-I model, I-co model, U-co model and state equation and their susceptibility to motor parameter were studied. Based on the state equation of asynchronous motor, the resistance adaptive sliding mode stator flux observer was put forward to reduce the parametric sensitivity. The results show that perpendicular flux observer with compensation feedback is effective in suppressing the DC offset of stator flux but ineffective to the stator flux estimation error caused by the incorrect setting of stator resistance. All of the three models are special cases of state equation. The full order observer based on state equation is sensitive to the choice of parameter, while the sliding mode theory can decrease the sensitive. The stator flux calculated by perpendicular flux observer with compensation feedback and the sliding mode stator flux observer, both of which are stator flux adaptive in this paper has been proven to be high accuracy.In accordance with the characteristics of traction motor power unit operating in the severe environment and is easy to broke down. The speed sensor-less method for traction motor was studied to improve the reliability. In order to avoid the overcurrent and torque shock when the pulse was connected after the locmotive coasted and inverter pulses blocked, the strategy of restarting at unknown speed with speed-sensorless control were discussed. The results show that the precision of the open loop speed calculating is low. The high-frequency signal injection is invalid becase of the traction system’s low switching frequency. Based on the former stator flux observers, the model adaptive reference system theory was used to identify the stator resistance and rotor speed on the same time. The overcurrent and torque shock were also refrained.