Sliding Mode Controlled drives for wide speed operation of Threephase Induction Motor

Induction motor is widely used due to advantages in terms of performance, size, maintenance and efficiency compared to dc motor. Induction motor is either Scalar Controlled or Vector Controlled. Magnitude of voltage and frequency is controlled in scalar control and drive has better performance under steady state. The coupling effect of flux and torque makes the drive sluggish. In vector control of induction motor drive, the flux and torque producing currents are independent of each other, making the transient response of the system better. Direct and indirect vector control is classified depending upon how the magnitude and position of the flux vector are determined. The specifications or parameters of the induction motor have least effect on the performance of the induction motor drive. This is because the measured or estimated flux is processed in the feedback loop for speed control operation of the drive. However, use of flux sensor within the machine which makes the drive uneconomical. In indirect vector control, rotor position signals are used for instantaneous rotor flux magnitude and position estimation. Motor parameters, used for estimation of flux vector vary with frequency, magnetic saturation and temperature. Vector Control is achieved using any of the rotating flux vectors in the induction motor. The vector control must have independent control of flux and torque, rotor flux orientation provides the independent control or natural decoupling. This decoupling leads to improved stability and enhanced dynamic response. Sensitivity is the problem associated with terminal quantities based flux observers. Research has been carried out to overcome the aforementioned problems by implementing accurate estimation of rotor flux. The selection criterion for the control principle depends upon the cost, accuracy, reliability and stability requirements. The uncertainties in the system parameters give way to a more robust and dynamic controller. The sliding mode control is one of the popular control techniques used to handle the uncertain system parameters, model uncertainties and external load variations which exist in induction motor drives. The induction motors used in traction require low speeds only during starting and low speed operations. The power output of the motor can be maximized by limiting the current drawn by the motor to rated value and lowering the reference voltage. The sliding mode control offers a robust tracking of a given speed demand when subjected to disturbances from both input and output side. This gives better performance compared to other techniques. Control Algorithm: Sliding Mode Controlled three-phase induction motor is shown in Fig. 1. It consists of three-phase rectifier converting the ac grid voltage into dc voltage which forms dc bus for PWM Inverter. Sliding mode algorithm is used for controlling the inverter which powers the three-phase induction motor.The aim of the control algorithm is to control the inverter voltage so that the induction motor tracks the desired speed. Two-line voltages and phase currents are sensed into the controller. These line quantities are converted to α-β components by the conventional 3-phase to 2-phase transformation. Synchronous speed is determined using frequency estimator and then estimated mechanical speed is compared with speed reference to give speed error. This error forms the input for the Sliding Mode control. In addition to this, torque current reference and actual torque current component are compared to give the voltage reference Vds. The voltages are converted to Va, Vb and Vc using the reverse transformation. By conventional Sine-triangle comparison, switching pulses are generated. SIMULATION RESULTS: Motor response with Indirect Field Oriented Control is as shown in Fig. 2. It can be observed that the motor accelerates at the starting with maximum torque. As the motor reaches the desired speed, the generated torque becomes minimum. At t = 0.5 sec, load torque is applied on the motor resulting in the transient shown.Fig. 2 - Electromagnetic torque and Speed of the three phase induction motor with IFOC algorithm. For the same transient conditions, motor is controlled using Sliding Mode control algorithm. The results are shwon in Fig. 3. The motor torque and speed response shows lower ripple which which validates the robustness of the algorithm.Fig. 3 - Electromagnetic torque and speed of the three phase induction motor with Sliding Mode Control.