Qualitative performance evaluation of torque control methods for industrial applications Anurag Tripathi 1 * ,y , Bharti Dwivedi 1 and Dinesh Chandra 2 1 Department of Electrical Engineering, Institute of Engineering and Technology, Lucknow, UP, India 2 Department of Electrical Engineering, MNNIT, Allahabad, UP, India SUMMARY Robust control of torque has been a motivation for research in the servo and electric drives industry. For industrial drives and servo applications AC machine drives are gaining prominence over the conventional DC-motor drives. Two methods, (a) field oriented control (FOC) and (b) the direct torque control (DTC) have been proposed to control an AC motor. FOC is a current control method and needs co-ordinated transformation. Torque dynamic depends upon the modulator, which also decides the steady state performance. DTC on the other hand is more robust and has physically the fastest torque dynamic. These methods have succeeded in achieving a fast and dynamically decoupled torque control but at what expense? Problems of FOC are saturation of current controllers, current control in the overmodulation region, and parameter sensitivity. Problems with DTC are variable switching frequency and sampling period dependence of hysteresis controllers, during digital implementation. This paper highlights the problems and explains a method of achieving direct control of torque using a method of stator flux vector control. The analysis carried out here is backed with appropriate simulation results. Copyright # 2009 John Wiley & Sons, Ltd. key words: direct stator flux control; overmodulations I & II; torque ripple 1. INTRODUCTION In field orientation, the motor input currents are adjusted to set a specific angle between fluxes produced in the rotor and stator windings, in an approach that follows from the operation of a separately excited fully compensated DC motor. The approach applies direct-quadrature (d-q) axis analysis methods directly to the torque control issue. When the dynamic equations for an AC machine are transformed by means of appropriate transformations into a reference frame that aligns with rotor flux, the performance becomes similar to the dynamic behavior of a DC machine. This allows the AC motor’s stator current to be separated into a flux-producing component and an orthogonal torque-producing component, analogous to a DC machine field current and armature current. In theory, field orientation provides decoupled control of torque and a flux, which is inherently possible in the DC machine. However, there are certain fundamental differences between the two. A significant difference is the need for precise information of motor parameters in case of an AC machine. In a DC machine, small parameter errors will alter the output torque but not the decoupling. In an AC machine, parameter errors alter the transformation and can cause torque ripple and other problems. To enhance the dynamic torque performance, it is necessary to apply the maximum voltage vector to the machine. This is possible only if the modulator is capable of achieving over modulation as in space vector modulation (SVM) [1]. In the range of operation from over modulation I to six-step, low frequency current harmonics are produced. This imposes a limit on the bandwidth of the linear controllers which otherwise are required to be of very high bandwidth for fast torque dynamics. This creates a dubious condition during over modulation and a compensating control [2] is needed. Saturation of linear controllers is another problem associated with field- oriented control (FOC) that limits the dynamic performance. Direct torque control (DTC) is a technique similar in intention as the field orientation, in which induction motor voltages or currents are set to values as close as possible to the ones needed to generate a EUROPEAN TRANSACTIONS ON ELECTRICAL POWER Euro. Trans. Electr. Power 2010; 20:461–469 Published online 9 February 2009 in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/etep.329 *Correspondence to: Anurag Tripathi, Department of Electrical Engineering, Institute of Engineering and Technology, Sitapur Road, Lucknow, UP, India. y E-mail: shuklakavita@rediffmail.com Copyright # 2009 John Wiley & Sons, Ltd.