Mathematical Model Associated to Three-Phase Induction Servomotors in the Case of Scalar Control SORIN MUSUROI, CIPRIAN SORANDARU, VALERIU-NICOLA OLARESCU, MARCUS SVOBODA Department of Electric Machines and Drives POLITEHNICA University of Timisoara, Faculty of Electrical Engineering Timisoara, Bd. Vasile Parvan nr.2, RO-300223 ROMANIA sorin.musuroi@et.upt.ro, ciprian.sorandaru@gmail.com, vali_olarescu@yahoo.com, marcus.svoboda@et.upt.ro, http://www.et.upt.ro Abstract: - Scalar control of induction servomotors was implemented on their steady-state model. If the machine is powered via a frequency and voltage converter, due to the presence in the motor input voltage wave of higher time harmonics, both its parameters and functional characteristic values will be more or less different comparing to the case of sinusoidal supply. The presence of these harmonics will result in the appearance of a distorting regime in the machine, with adverse effects in operation. In this work is realized a theoretical study of the behavior of the asynchronous servomotor in the presence of distorting (non-sinusoidal) regime and also a mathematical model for its scalar control is proposed. Key-Words: - Mathematical model, Asynchronous servomotor, Non-sinusoidal regime, Power converter, Scalar control. 1. Introduction Three-phase asynchronous servomotors are now spread to a growing extent. Theses eliminate the disadvantages of the d.c. servomotors linked to the collector-brush system; moreover they are robust, having a simple construction, a lower friction and a lower cost price. Asynchronous servomotors also present a number of drawbacks in that efficiency and power factor are lower, dimension and weight are greater and control is more complicated than the d.c. servomotors Compared to the usual three-phase asynchronous servomotors, which do not distinguish from the point of view of construction, at the servomotors of the same type on can remark: - A higher length / diameter ratio relative to the rotor, which has the drawback that the heat losses transfer from the rotor is most difficult; - A stronger strengthening of the stator insulation in order to resist to the often transient processes; - Taking into account of the rotor heating which becomes important. There is a high variety of schemes for automatic position control made with three-phase induction servomotors. Despite its simple and robust construction, the motion control for this type of servomotors should take into account the complexity of the dynamic model which is nonlinear and variable in time and that the physical parameters of the machine are not always known with great precision. Under these conditions the motion control means controlling the speed and/or the position, respectively torque control. As on obtain a faster torque response as the motion control is more efficient. Mainly there are two control strategies: scalar control and vector control. Scalar control or scalar regulation can be done in open or closed speed control loop and can be accomplished by connecting scalar values, for example, u s = f(f 1 ) or i s = f(f 2 ), where u s and i s are the stator voltage and current and f 1 and f 2 are the stator and rotor frequencies. Usually, it is necessary to impose the condition for keeping the stator flux constant and equal to the nominal one (ψ s = ψ sn = const.). This strategy is based on a steady-state simple induction servomotor model. The advantage of the scalar control resides in the simplicity of the control circuits but has the drawback of generally obtaining variable speeds with low accuracy; the dynamic performances of the system are also low. It follows a satisfactory adjustment only when the machine works with stationary speed for long periods. If the fluctuations occur in tension, disturbances in load or if the servo-system requires fast accelerations or decelerations, the open loop WSEAS TRANSACTIONS on SYSTEMS Sorin Musuroi, Ciprian Sorandaru, Valeriu-Nicola Olarescu, Marcus Svoboda ISSN: 1109-2777 1125 Issue 10, Volume 8, October 2009