Direct Torque Control Of Induction Motor Used In Electric Vehicles

In order to deal with the problem of energy and environment, Governments payhighly attention to the development of electric vehicles. Induciton motors have theadvantages of simple structure, low noise, low cost, no maintenance, so it willbecome the mainstream product of electric vehicle driving system.Direct torque control (DTC) has a simple structure, just need stator resistance,fast dynamic response, so it’s suitable for electric vehicles. In this thesis, it startsfrom the mathematical model of induction motor and circular flux DTC scheme, andanalyze the relationship between voltage space vector and the stator flux, torque.flux and torque adjustment using hysteresis control, and then deduce the voltagevector selection table.Accurate observation of stator flux is the key to realize DTC. Voltage model iscommonly used on flux observer. In order to improve low-speed and dynamicperformance of the voltage model, a modified low-pass filter (LPF) algorithm isproposed. Firstly, the tracking differentiator is brought in to modulate the measuredstator current, which suppresses the measurement noise, and then amplitude andphase compensation is made towards the stator EMF, after which the stator flux isobtained through a LPF. The cutoff frequency of LPF varies with the stator angularfrequency, set as20%to30%. This method can eliminate the dynamic error of fluxfiltered by LPF and improve low-speed performance.DTC model of induction motor is Established in Simulink, the various modulesare analyzed detailedly, using S-function to achieve LPF with variable cutofffrequency. Through Simulation analysis, it verifies the feasibility of DTC system,and get the related parameters of tracking differentiator and speed PI regulator.Experiment platform of DTC for induction motor is built based onTMS320F28335. Tests uses a185W three-phase squirrel cage induction motor tocarry on the contrast experiment, and write control program respectively about thetraditional flux observer method and the flux observer method proposed in this paper.Test results show that porposed scheme improves the dynamic performance and lowspeed performance of induction motor DTC system, the flux observer accuracy isnot affected by the stator frequency mutations, reduce the current harmonics andrestrain the speed fluctuations.