SLIDING MODE DIRECT TORQUE CONTROL OF THREE PHASE INDUCTION MACHINE APPLICABLE IN ELECTRIC VEHICLES
The electric vehicle relies on electric motors for propulsion. In this report, the case was made for the induction motor as a suitable candidate for this tractive application. Being able to control the electric motor invariably implies efficient control of the vehicle. This work reports the six-step operation of the constant volts/hertz (V/f) control technique for voltage fed induction motor drives. Mathematical equations supporting the principle is discussed and the drive is modelled and implemented in the open loop with results. The line and phase voltage outputs of the laboratory-implemented v/f drive is presented. The modelled results show the behaviour of the v/f drive under varying load conditions. The reference speed is set at 1800rpm while the actual speed only follows the reference speed until torque is applied at 0, 2, 3 and 4 seconds with torque values 0, –11, 11 and 0 Nm respectively. At 2 seconds, the actual speed changes (increases) from the reference speed slightly and reduces slightly when the positive torque is applied, but the speed does not totally match the reference speed until the torque is completely withdrawn. The Direct Torque Control (DTC) scheme is analysed mathematically and the principle of Variable Structure Systems (VSS) theory is applied to the DTC for robustness and tolerance to disturbances. DTC simulation results are presented, the system is run at steady state conditions, at time t = 0.4s, a load-torque disturbance causes it to reduce to one-half of its initial value. The objective of this drive scheme is to keep the load speed constant at its initial value. This causes a sharp increase in speed but it returns to the set reference speed in about 0.4s. This shows the control efficiency and robustness of the DTC scheme as compared with the v/f control method.