A Simple and Low-Cost Method for Three-phase Induction Motor Control in High-Speed Applications Y. Sangsefidi, S. Ziaeinejad, A. Shoulaie Electrical Engineering Department Iran University of Science and Technology, Tehran, Iran y�sangsefidi@elec.iust.ac.ir, s�ziaeinejad@elec.iust.ac.ir, shoulaie@iust.ac.ir Abstract� This paper presents a simple method for speed control of induction motors on the basis of direct control of the stator flux modulus. The proposed method needs neither mechanical sensor nor current transducers, so it can be compared with "Variable Voltage Variable Frequency (vI" method. High-speed operation of induction motor controlled by the proposed method is compared with that of vlf method that leads to the fact that in addition to control of motor dynamic behavior, the proposed method uses DC-link voltage in a more efficient way. In the stator flux modulus control, the amplitude of electromagnetic torque ripple and average switching frequency are computable and analytical formulas for calculation of them are derived. The performance of proposed method and accuracy of presented formulas are validated in MATLAB/Simulink. Keywords- Scalar control, Vector control, Stator flux modulus control, Stationary reference frame I. INTRODUCTION Due to their significant advantages such as simple and robust structure and high reliability, induction motors are widely used in many household and industrial applications. vlf method is widely used for speed control of induction motors that provides control of them without mechanical sensors and current transducers. This low-cost method can be implemented using either scalar or space vector approaches. In the scalar approach, the phase voltage waveform is obtained by comparing modulation waveform with triangular carrier. Intersections of these two waveforms deteine switching instants. This approach is based on volt-second balance of output voltage waveform and modulation waveform. In space vector approach, time intervals of applied active and zero voltage vectors are adjusted in order to rotate voltage vector with desired amplitude and equency. Implementation of vlf method with scalar approach is easier. With proper modulation waveforms, performances of these two approaches are identical [1]. One of disadvantages of vlf method is improper use of DC-link voltage [2]. Another defect which limits the use of this method in high performance industrial applications is poor dynamic behavior of the motor. The method is based on equations authentic in steady-state conditions [3]. This method controls the amplitude and equency of the stator flux only in steady-state. In vlf control method, the electromagnetic torque and flux cannot be directly or indirectly controlled in transient state. In fact, the motor response to applied voltage and equency determines transient electromagnetic torque behavior and the stator flux trajectory. These behaviors are not controlled by drive system [4]. Accordingly, represented torque response is weak. Moreover, lack of flux control in transient conditions may cause the motor to operate in saturated regions. Vector control methods are in contrast with vlf method. These methods e based on dynamic model of induction motor which are capable of controlling the motor in both transient and steady-state conditions. Vector control is a general philosophy which can be implemented in different ways [3]. In Field-Oriented Control (FOC) [5],[6], voltage, current and flux vectors are conolled in two-phase synchronous reference ame. This method is based on independent control of electromagnetic torque and flux which uses position sensor as well as current transducers. Direct Se Control (DSC) [7] and Direct Torque Control (DTC) [8] provide efficient motor control without position sensor. In these two methods, motor variables are transfeed to two-phase stationary reference ame that obviates the need of using mechanical sensor. In DSC method, stator flux vector traverses a hexagonal path. The most significant characteristic of this method is its low switching equency that makes it justifiable in high-power applications. On the contrary, DTC method, based on direct control of stator flux in a circular path and dect control of electromagnetic torque is preferred in low- and medium-equency applications [9]. In comparison with FOC, DTC represents better dynamic response, but suffers om problems like high torque ripple, vulnerability to DC-offset of cuent sensors and improper performance in very low speed applications [9],[10]. Since the representation of different vector control methods, these methods have been significantly developed [11]-[14]. Also, some methods have been proposed that are combination of FOC and DTC methods [9]. Despite evolution of vector control methods, there are still two problems. One problem is that in vector control methods, the value of motor parameters is requed and control system is sensitive to variations of them. The other problem is that vector control methods need current transducers which impose considerable cost to drive system. In contrast, being simple and inexpensive cause vlf method -mainly scalar approach- to be still justifiable in low-cost applications [15]. In this paper, stator flux modulus as a vector control method is proposed. In this method, dynamic equations of IEEE Catalog Number: CFP1211J-ART 212 ISBN: 978-1-4673-0113-8/12/$31.00 ©2012 IEEE