2621 Disturbance Observer Based Current controller for Vector Controlled IM Drives Mehmet Dal * and Remus Teodorescu ** * Kocaeli University/VSK Electrical Department, Kocaeli, Turkey ** Aalborg University/Instituted of Energy Technology, Aalborg, Denmark Abstract - In order to increase the accuracy of the current control loop, usually, well known parameter compensation and/or cross decoupling techniques are employed for advanced ac drives. In this paper, instead of using these techniques an observer-based current controller is proposed for vector controlled induction motor (IM) drives. The control design, based on synchronously rotating d-q frame model of the machine, has a simple structure that combines the proportional portion of a conventional PI control and output of the observer. The observer is predicted to estimate the disturbances caused by parameters change in current control loop and, also to remove undesired cross coupling existing between components of the stator current. The observer uses the measured stator currents and estimated PWM voltages, and produces a disturbance signal with a low pass filter. The proposed control scheme reduces cross coupling effects and increase robustness against parameters change without requiring any other compensation strategies. The experimental implementation results are provided to demonstrate validity and performance of the proposed control scheme. I. INTRODUCTION An accurate current loop control is essential to obtain a fast torque response under the transient and the steady- state conditions for advanced ac drives. The main requirements concerning to increase accuracy of current control loop for the high performance ac drive system are rejection of disturbance existing due to change of the parameters, removing undesired cross coupling between the torque and flux producing currents of the stator and reducing harmonics created by the power converter switches[1]. To overcome these problems, many different current control strategies have been suggested in the literature, but the control method preferred in the most of industrial applications generally is PI type, and it is reported that the synchronously rotating frame PI current controller is a common choice for ac machine drives [2], [3]. A comprehensive survey pertaining to other commonly known current control strategies can be found in [4], [5]. The advanced current controllers usually use the compensation strategies based on estimate of the parameters to reduce the disturbances caused by parameters change and the decoupling strategies based on adding a feedforward term to the PI controller output to remove undesired cross coupling effects. The decoupling strategy works well if leakage inductance L is updated online by estimate, but it is generally weak at low speed dynamic decoupling scheme that presented in [6], and other improved strategies like one based on dynamic decoupling scheme that presented in [6] generally are complicated. Improvement of the synchronous frame PI current regulator based on complex state variable is another advanced strategy that removes the cross coupling effect and it is discussed widely in [1],[3]. In [6] a dynamic decoupling scheme to solve the cross coupling effect and in [7] a variable structure current controller (VSC) to improve performance of the motor drives with rejecting the disturbances are presented. The strategy uses a disturbance observer to increase accuracy of the control is well accepted in the motion control field [7], [8] but it is rarely used in the current control loop. In this paper, a disturbance observer based current control scheme aimed to reject the disturbances and remove voltage cross coupling in the current control loop is presented. The proposed control scheme that structured on d-q frame model of the machine is very simple and the resulted output signal of the controller combines the proportional portion of the conventional PI controller and output of the disturbance observer. The idea using disturbance observer comes from the intention to simplify the stator current dynamical equation (1),(2); at the right hand side of this equation, except the last term concerning to the stator voltage, the rest covers the resistive voltage drop, the cross coupling term and the back emf voltage. Sources of the disturbances existing in current control loop are these three terms; therefore extracting them totally through the disturbance observer employing the measured stator currents and voltages is objected. The stator voltage is estimated (as done usually) using the calculated duty cycles of the PWM pulses and the measured DC bus voltage of the power converter. Due to the need to have state derivative, a differentiator with a low pass filter is inserted in the observer structure. The proposed control scheme was simulated and also implemented; the results confirm that proposed control scheme reduces the cross coupling existing between current components and improves robustness against parameters change as required for high performance ac drives. The remaining of the paper is organized as follows: In section II machine dynamic model in the synchronous frame is defined. In section III observer structure and design of current controller with stability analysis and in section IV simulation and experimental results are presented and finally the summary concluded in section V. 978-1-4244-1668-4/08/$25.00 ©2008 IEEE Authorized licensed use limited to: Aalborg Universitetsbibliotek. Downloaded on March 16, 2009 at 07:41 from IEEE Xplore. Restrictions apply.