Implementation of Low-Cost Direct Torque Control Algorithm for Induction Motor without AC Phase Current Sensors

Abstract

ABSTRACT: This paper presents a novel low -cost and simple phase-current reconstruction algorithm for three-phase induction motor (IM) under direct torque Control (DTC) using the information obtained from only one shunt resistor (in series with low side switches in a conventional three -phase inverter). The main objective is to develop a low -cost high-performance IM drive. The proposed algorithm is robust and very simple. It uses the dc current to reconstruct the stator currents needed to estimate the motor flux and the electromagnetic torque. A theoretical concept is developed, the modified look -up table is presented and current -access tables are designed and used in the phase-current reconstruction. The limitations are also studied and presented. Simulation results are given to prove the ability of the proposed scheme of reproducing the performances of a traditional DTC IM drive. KEYWORDS: Direct torque control (DTC), Induction motor (IM), sensor count reduction, single current sensor. I.INTRODUCTION Direct torque control (DTC) of induction motors has gained popularity in industrial applications mainly due to its simple control structure from its first introduction in 1986. An electric motor drive controlled with the DTC technique exhibits performance similar to a field oriented drive despite a simpler structure. In fact, a DTC scheme achieves the closed-loop control of the motor stator flux and the Electromagnetic torque without using any current loop or shaft sensor. Many researchers are interested in this control technique because of its wide area applications used with various ac machine types as induction motor, PMSM, PM Brushless, and reluctance motor. The DTC scheme requires information about the stator currents and the dc -link voltage, which is used with the inverter, switches states, to estimate the values of stator flux and electromagnetic torque. The current feedback for the closed -loop control is usually obtained by sensing instantaneous phase currents by current sensors. In general, galvanically isolated current sensors such as Hall Effect sensors and current transducers are widely used in many applications. They are typically used on, at least, two outputs of the power inverter to provide current feedback signals. Such a kind of sensors performs well, but brings disadvantages to the overall drive system in terms of cost, Encumbrance and somehow nonlinearity. Recently, single current sensor operation has been proposed to reconstruct phase currents from the DC link current sensor. In this way, various approaches have been proposed in the literature. Some methods adjust the pulse -width modulation (PWM) signals to ensure that two-phase currents can be sampled in each control period. Other strategies introduce modifications of the modulation algorithm in order to guarantee the reliability of the measurements from the dc -link current sensors under all the operating conditions. Other interesting approaches are based on the estimation of the motor phase currents using prediction-correction algorithms, thus introducing additional computational burden to the drive system. Only a few papers deal with the DTC technique for induction motor and PMSM. The algorithm used in these works operates in two stages. First, it predicts the stator currents from a model of the motor and then adjusts the prediction on the basis of the sensed dc -link current. This algorithm requires an additional computation burden and the knowledge of the stator transient inductance. In this paper