Fuzzy based direct torque and flux control of induction motor drives Ning Chuang * Abstract— This paper investigates direct torque and flux control of an induction motor drive based on the fuzzy logic (FL) control technique. Direct torque and flux control has become a widely acceptable alternative to field oriented control The hysteresis-band controller for the stator flux and the electro- magnetic torque was designed using a fuzzy logic system. (FLS) in MATLAB. Simulation results show that the direct torque and flux control using an FL approach performs very fast dynamic response and has a simple structure which makes it to be more popularly used in the industry. I. I NTRODUCTION Induction motor (IM) drives may be classified into two main control strategies. Scalar control, of the IM voltage magnitude and frequency, is one of ac drives which produces good steady-state performance but poor dynamic response. There is an inherent coupling effect using the scalar control because both torque and flux are the functions of voltage or current and frequency. This results in sluggish response and is prone to instability because of 5 th order harmonics. However, vector control (VC) decouples these effects. A second IM drive method can be either the field oriented control (FOC) or the direct torque and flux control (DTFC or DTC). The principle used in these drive methods is to asymptotically decouple the rotor speed from the rotor flux in vector controlled ac motor drives. The two most commonly used methods of the vector control are direct vector control and indirect vector control. Control with field orientation may either refer to the rotor field, or to the stator field, where each method has own merits [1]. Direct torque and flux control is also denoted as direct self-control (DSC) which is introduced for voltage-fed pulse- width-modulation (PWM) inverter drives. This technique was claimed to have nearly comparable performance with vector-controlled drives [2]. Another significance mentioned in [3] is that DTFC does not rely on machine parameters, such as the inductances and permanent-magnet flux linkage. Consequently, a number of research approaches have been proposed for a wide range of industrial applications where [3] is proposed for direct-drive PMSG wind turbines. If an IM is being operated under its steady state, the three-phase drive can be easily presented as just one-phase because all the variables on the IM are sinusoidal, balanced and symmetrical. However, if the operation requires dealing with dynamics of motor speeds or varying torque demands in a sudden change, the motor voltages and currents are no longer in a sinusoidal waveform. Hence, the IM drive scheme using vector control or direct torque and field control is able to provide faster transient responses due to these dynamics. ————————————- * Ning Chuang, ning1@tpg.com.au. This paper is arranged as follows. Section II lists the notations used in this paper. Section III explains the induction motor dynamics. Section IV describes the direct torque and flux control. Section V presents the design of the fuzzy logic controller. Section VI illustrates the performance of the controller in the simulation. Specific conclusions are drawn in Section VII. II. NOMENCLATURE d e -q e synchronous reference frame direct, quadrature axes d s -q s stationary reference frame direct, quadrature axes u sd ,u sq stator voltages i sd ,i sq stator currents u rd ,u rq rotor voltages i rd ,i rq rotor currents ψ s ,ψ r stator, rotor flux vector R s ,R r stator, rotor resistance L s ,L r stator, rotor self-inductance L m magnetizing inductance σ resultant leakage constant ω e ,ω r , synchronous, rotor speed P number of motor pole pairs J Total inertia T e ,T L electromagnetic, load torque III. I NDUCTION MOTOR DYNAMICS A. Machine model in arbitrary reference frames In a three-phase ac machine, there are three main ref- erence frames of motion, which could be used to model its three main regions of operation. These are the station- ary reference frame for startup, the synchronous reference frame for equilibrium motion, and the rotor reference frame for changing speeds by acceleration or deceleration. The two commonly employed coordinate transformations with induction machines are the stationary and the synchronous reference frames as shown in Fig. 1. Fig. 1. Reference frame of motion in an ac machine In special reference frames, the expression for the elec- tromagnetic torque of the smooth-air-gap machine is similar to the expression for the torque of a separately excited dc machine.