Abstract— Nowadays the control of stator voltage at a constant frequency is one of the traditional and low expense methods in order to control the speed of induction motors near its nominal speed. The torque of induction motor is a nonlinear function of the firing angle, phase angle and speed. In this paper the speed control of induction motor regarding various load torque and under different conditions will be investigated based on a fuzzy controller with inverse training. Keywords— Three phase induction motor, AC converter, speed control, fuzzy control. I. INTRODUCTION OLTAGE control at constant frequency is used increasingly in order to speed controlling of a three phase induction motor at low and medium powers especially when the load torque is proportional to the square of motor speed [1]. In this method the voltage of the stator is controlled between zero and its maximum value by controlling the firing angle of thyristors symmetrically. Although the speed control of induction motors by means of variable voltage and constant frequency seems to be simple but its needs to some complex analysis. Two main problems will be encountered in analyzing the behavior of induction motor by this method [2]: 1) The differential equation that describes the motor's behavior is completely nonlinear. 2) Determination of initial and bound conditions is very difficult. In this paper by applying a variable controlled voltage through a three phase chopper upon the circuit's model of induction motor, the instant and effective values of currents and voltages are computed. It will be depicted that the torque of such a motor in the steady state is a nonlinear function of firing angle, phase angle and rotation speed. Hence achieving the produced torque for a specific firing angle and speed depends on solving nonlinear equation and calculating complex integrals that are time consuming and complicated. This problem appears when it is required to control the speed of the motor due to variation of load torque, by adjusting the firing angle of thyristor. In the proposed method, in this paper, the induction motor plus to AC converter is considered as a nonlinear system that receives the firing angle and load torque as inputs and produces the speed as output. The proposed fuzzy controller indeed will be an inverse fuzzy model of the thyristorized induction motor with torque and speed as its inputs and the firing angle as its output. II. MODELING In most of studies, the system of thyristorized induction motor is considered as shown in Fig. 1 [1]. Fig. 1: The model of induction motor-AC converter. In this system, ignoring the effect of saturation and core dissipation, it is possible to consider the induction motor as a linear circuit. In such case the equivalent circuit is considered as R-L load [2]. Fig. 2 depicts the single phase equivalent circuit of such system. Fig. 2: The 1-phase equivalent circuit of 3-phase induction motor. The parameters of the depicted system in Fig. 2 are: Rs : The equivalent stator resistance Rr' : The equivalent rotor resistance referred to stator side Xs : The equivalent stator reactance Xr': The equivalent rotor reactance referred to stator side Xm : The magnetic reactance s : The slip The equivalent impedance of this circuit can be written as follows: ) ( ' ' 2 2 ' ' ' ' ' r m r m r m s r r m r m r m s X X X X s R X DX j D s R X X X X s R X DR Z (1) Where: D X X R s m r r ' ' 2 2 (2) The operational mode and consequently the analysis of the Abolfazl Jalilvand 1 , Mohammad Reza Feyzi 2 , Sohrab Khanmohammadi 2 , Mohammad Bagher Bana Sharifian 2 and Ali Sajjadi 2 Electrical Engineering group, Islamic Azad University of Abhar, Abhar, Iran 1 Faculty of Electrical and Computer Engineering, University of Tabriz, Tabriz, Iran 2 Fuzzy Control of a Three Phase Thyristorized Induction Motor V PROCEEDINGS OF WORLD ACADEMY OF SCIENCE, ENGINEERING AND TECHNOLOGY VOLUME 1 JANUARY 2005 ISSN 1307-6884 PWASET VOLUME 1 JANUARY 2005 ISSN 1307-6884 274 © 2005 WASET.ORG