Estimation of Stator Resistance in Direct Torque Control Synchronous Motor Drives

Direct torque control (DTC) is a high-performance method that provides effective control of stator flux modulus and electromagnetic torque of electric motors. However, error in estimation of stator resistance significantly degrades the performance of a DTC drive system, especially for a synchronous motor. This paper discusses the problems associated with the error in stator resistance and proposes an analytical approach to investigate its effect on the actual and estimated variables of a synchronous motor. Based on how this error affects the estimated angle between the stator flux and current vectors, a method to track stator resistance variations is proposed. The presented analytical method and the proposed stator resistance estimation are validated using simulation and experimental case studies.

I. Introduction

As an alternative to conventional vector-controlled synchronous motor drives, direct torque control (DTC)–based drives were proposed in the 1990s [1], [2]. Desirable features such as fast dynamic response, no need for mechanical sensors, simple structure, and less dependence on motor parameters [3] –[5] have made DTC drive systems an attractive option for the industry. DTC provides effective control of stator flux modulus and electromagnetic torque in a synchronous motor. However, one of its main limitations is that a priori knowledge of the stator resistance is required to estimate the stator flux vector and electromagnetic torque. Due to variations in temperature and frequency, stator resistance can vary considerably [3], [6]. A basic DTC synchronous drive with no provisions for correction for stator resistance is unable to track these variations, and consequently, its performance may degrade.