Optimised Direct Torque Control of Induction Motor For Electric Vehicle Propulsion

This paper presents Electric Vehicle (EV) propulsion using a three phase squirrel cage induction motor. The motor control at different operating conditions is carried out by a direct torque control technique with an energy optimization strategy. The operating flux of the motor is chosen optimally using model based control strategy. Simulation tests have been carried out using MATLAB SIMULINK to check the consistency and performance of the proposed scheme. Keywords—induction motor; EV propulsion; DTC; loss minimization;optimisation. INTRODUCTION Electric Vehicles (EVs) [1] are a solution for the environmental problems caused by vehicles with internal combustion engines. The advantages of EVs include energy efficiency, virtually lack of pollution, and the availability of electric energy through electric distribution systems. Among disadvantages, they have low energy density and long charging time for the present batteries. Hence, optimal energy management is very important in EVs. The other major factors include optimum design of the motor, selection of a proper drive, and optimal control strategy. The electric propulsion system of EV consists of the motor drive, transmission device, and wheels. The motor drive, comprising of the electric motor, power converter, and electronic controller, is the core of the EV propulsion system. Desired features of the propulsion system (motor) for an EV are high ratio of “torque/inertia” and “power/weight,” high maximum torque capability, high speed, low level of audible noise, low maintenance, small size, low weight, reasonable cost, high efficiency over low-and high-speed ranges, energy recovery on braking, and non sensitivity to acceleration forces. Squirrel-cage induction motors have most of the abovementioned features. Among the various control techniques available Direct Torque Control (DTC) appears to be very convenient for EV propulsion. The input currents are measured; flux, torque are estimated. The reference speed which is directly applied by the pedal of the vehicle is the input of the motor controller. [2][3] Furthermore, DTC typical advantages are not sufficient. EVs induction motor drive has also to possess a high efficiency in order to extend the running distance per battery charge. Therefore, DTC should be associated to a lossminimization strategy so as to maximize the drive efficiency. In this paper DTC control scheme along with a loss minimization strategy is implemented.[7] DIRECT TORQUE CONTROL In DTC the electromagnetic torque and stator flux linkage of the machine are controlled directly by the selection of optimum inverter switching modes. The use of a switching table for voltage vector selection provides quick response and it has a simple control structure. The flux and torque errors are restricted within respective flux and torque hysteresis bands with the optimum selection being made. The DTC control scheme utilizes hysteresis controllers for torque and flux to select the switching voltage vector. These hysteresis controllers maintain the flux and torque within an allowed upper and lower limit. [4] The generic Direct Torque Control scheme for an inverter fed induction motor drive is as shown in Fig. 1. The DTC scheme comprises of flux and torque estimator, hysteresis controllers for torque and flux and a switching table. Fig.1.Basic DTC Scheme A. Stator Flux and Torque Control The induction motor stator flux Ψs can be estimated as follows     SO S S S S dt I R V    where Vs is the stator voltage ,Is the stator current, Rs the stator resistance, Ψso the initial flux vector . Selection of appropriate voltage vector in the inverter is based on stator equation by (2) Pedal command ωr S(A,B,C) Stator currents and voltages