THE STRUCTURAL ROBUSTNESS OF THE INDUCTION MOTOR STATOR CURRENTS SUBSYSTEM Luis Amezquita-Brooks, Eduardo Liceaga-Castro and Jesús Liceaga-Castro ABSTRACT Stator-currents control is essential for several high-performance induction motor control schemes such as field oriented control. There are numerous reports dealing with sophisticated control schemes for this subsystem. However, classical linear controllers remain widely used due to their experimental success and simplicity. Considering that the induction motor stator currents subsystem is normally represented by a fifth order non-linear multivariable model, it is remarkable that simple fixed linear controllers, such as typical proportional integral schemes, are able to provide adequate robustness and performance in practice. In fact, it is normally assumed that this subsystem is “easy” to control, and the difficulties are mostly technical. Moreover, it is common practice to consider a stable first order linear single input single output (SISO) system as a design model. On the other hand, it is widely known that stable and minimum phase uncertain SISO systems are also “easy” to control. In this article it is formally demonstrated that the stator currents subsystem of the induction motor is the multivariable equivalent of such SISO systems. That is, it is formally demonstrated that this process is “easy” to control. This result may assist with better induction motor control and may serve as an example of the evaluation of similar multivariable systems. Real time experimental results are included. Key Words: Multivariable control, decentralized control, induction motor control, frequency domain analysis and control. I. INTRODUCTION Induction motors are widely used as actuators in many industrial and research applications. In recent decades the evolution of digital processing systems and power electronics have made possible the extended use of high-performance induction motor control systems such as field oriented control (FOC) and direct torque control (DTC). Among these control strategies FOC has been traditionally shown to be a viable every-day solution for most induction motor control applica- tions [1,2]. There are many kinds of FOC schemes, however, the most successful FOC strategies are based on rotor flux and torque decoupling [3]. Several schemes recently pro- posed in the literature are designed under this strategy [1,4,5]. These strategies aim at modifying the behavior of the induc- tion motor into that of a classical direct current (DC) motor. With DC motors it is easy to manipulate the flux and the torque separately by driving different physical currents, namely field and armature currents. However, induction motors do not share this characteristic. Therefore, non-linear control elements are introduced in order to have virtual flux-torque producing currents. This strategy is commonly implemented following a two-step procedure [1–5]. The first step consists of controlling the stator currents by using a voltage source inverter (VSI) as an actuator. By controlling the stator currents, the fifth degree non-linear model of the induction motor may be simplified into a third order system. The second step is to design a non-linear torque-flux control law for this system. It is in this second step that most FOC schemes are introduced. Control strategies other than FOC may be used in the second step while preserving the stator currents control loop [6–10]. In this context, the appropriate control of the stator currents subsystem is vital to reduce the burden imposed on the robustness and perturbation rejection requirements of the torque-flux control law. While there are many reports dealing with the torque-flux control loop, there are only a few formal analyses of the stator currents subsystem when using linear controllers [11–16]. For instance, in [11] a linear control for the flux-speed subsystem of the induction motor is consid- ered, nonetheless the stator currents operate in an open-loop manner. This is a common practice which reduces the pertur- bation rejection capabilities of this subsystem. In references [12,13] other practical aspects of induction motor control, such as parameter identification and anti-windup control, are treated without attention to the stator currents subsystem. In [14] a solution for induction motor control using cerebellar model based control is presented. However, the presented control scheme is rather complex considering that the system Manuscript received February 25, 2013; revised November 8, 2013; accepted January 19, 2014. Luis Amezquita-Brooks (correspondng author, e-mail: luis.amezquitab@uanl.mx) and Eduardo Liceaga-Castro (e-mail: e.liceaga.c@gmail.com) are with CIIIA-FIME, Universidad Autónoma de Nuevo León, Monterrey, México. Jesús Liceaga-Castro is with the Depto de Electrónica UAM-Azcapotzalco, DF, México (e-mail: ucastro21@hotmail.com). Asian Journal of Control, Vol. 16, No. 6, pp. 1632–1645, November 2014 Published online 14 April 2014 in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/asjc.875 © 2014 Chinese Automatic Control Society and Wiley Publishing Asia Pty Ltd