1358 IEEE TRANSACTIONS ONINDUSTRY APPLICATIONS, VOL. 42, NO. 6, NOVEMBER/DECEMBER 2006 A New Torque and Flux Controller for Direct Torque Control of Induction Machines Nik Rumzi Nik Idris, Senior Member, IEEE, Chuen Ling Toh, and Malik E. Elbuluk, Senior Member, IEEE Abstract—This paper presents the implementation of a high- performance direct torque control (DTC) of induction machines drive. DTC has two major problems, namely, high torque ripple and variable switching frequency. In order to solve these problems, this paper proposed a pair of torque and flux controllers to replace the hysteresis-based controllers. The design of these controllers is fully discussed and a set of numerical values of the parameters for the proposed controllers is given. The simulation of the proposed controllers applied to the DTC drive is presented. The simulation results are then verified by experimental results. The hardware implementation is mainly constructed by using DSP TMS320C31 and Altera field-programmable gate array devices. The results prove that a significant torque and stator flux ripples reduction is achieved. Likewise, the switching frequency is fixed at 10.4 kHz and a more sinusoidal phase current is obtained. Index Terms—Constant switching frequency, direct torque con- trol (DTC), DSP control implementation, field-programmable gate arrays (FPGAs), flux controller, induction machines, torque con- troller, torque ripple. I. I NTRODUCTION N OT UNTIL the mid 1980s, when direct torque control (DTC) was first introduced, was field-oriented control (FOC) of induction machines considered the only scheme capable of delivering high-performance torque control in ac machines [1]. The first DTC drive was marketed by ABB in 1996 and since then has gained popularity in applications, which previously utilized FOC drives. In DTC, the torque and flux are directly controlled using optimum voltage vectors; whereas, the FOC, which uses q- and d-axis components of the stator current to control the torque and flux, respectively, re- quire current regulated pulsewidth modulation or space-vector modulation (SVM) for implementation. Unlike the FOC drives, where frame transformation is a must, no frame transformation Paper MSDAD-06-07 presented at the 2005 Industry Applications Society Annual Meeting, Hong Kong, October 2–6, and approved for publication in the IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS by the Industrial Automation and Control Committee of the IEEE Industry Applications Society. Manuscript submitted for review October 15, 2005 and released for publication July 21, 2006. N. R. N. Idris is with the Electrical Energy Conversion Department, Univer- siti Teknologi Malaysia, 81310 UTM, Skudai, Malaysia (e-mail: nikrumzi@ ieee.org). C. L. Toh was with the Electrical Energy Conversion Department, Universiti Teknologi Malaysia, 81310 UTM, Skudai, Malaysia. She is now with Intel Microelectronics (M) Sdn. Bhd. (Company No. 302251-K), Penang, Design Center (PG12), Halaman Kampung Jawa, 11900 Penang, Malaysia (e-mail: chuen.ling.toh@intel.com). M. E. Elbuluk is with the Department of Electrical and Computer Engineer- ing, University of Akron, Akron, OH 44325-3904 USA (e-mail: melbuluk@ uakron.edu). Color versions of Figs. 13–15 are available at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TIA.2006.882685 is required in DTC, and in its basic configuration, only the stator resistance is required to estimate the stator flux and torque. It is also shown that DTC of induction machine gives faster torque response compared to the FOC of induction machines. The basic configuration of the conventional DTC drive consists of a pair of hysteresis comparators, torque and flux estimators, a voltage vector selector, and a voltage source inverter. Although gaining popularity, DTC has some drawbacks, which need to be rectified. Variable switching frequency and high torque and flux ripples are the major problems. The root of variable switching frequency in DTC is the use of torque and flux hysteresis controllers, as originally pro- posed in [1]. Hysteresis controllers not only produce a variable switching frequency but also produce large torque and flux rip- ples. Therefore, various methods have been proposed to over- come these problems including the use of variable hysteresis bands [2], predictive control schemes [3], SVM [4], and in- telligent control techniques [5]. The use of these techniques, however, has somehow increased the complexity of the control technique in DTC and diminished the main feature of DTC, which is simple control structure. A torque controller, which produces constant torque switch- ing frequency with low ripple, has been presented in [6]–[8]. The method replaces the conventional hysteresis torque com- parator with a fixed switching frequency controller. Subse- quently, an almost fixed switching frequency is obtained by comparing the triangular waveforms with the compensated torque error signal. There are two major limitations associated with the method presented in [6] and [7]. First, since the triangular waveform is generated by the digital signal processor, which is also responsible for performing other tasks, the fre- quency of the triangular wave, hence the switching frequency, is limited by the sampling period of the processor. Second, the switching frequency still varies with operating conditions. This is mostly due to the fact that the flux controller still uses the hysteresis-based comparator. In [8], we have removed the first limitation by introducing the field-programmable gate array (FPGA) to perform some of the important tasks, hence reducing the burden of the processor. The tasks include the generation of the high-frequency triangular waveform, performing the comparisons, as well as the implementation of the lookup table. By doing so, we have managed to increase the torque switching frequency from 2.7 kHz (in [6] and [7]) to about 10 kHz (in [8]). This paper expands on the work presented in [6]–[8]. Instead of just the hystersis-based torque controller as in [6]–[8], here, we also replace the flux hysteresis-based controller with a constant frequency controller. The consequences of introducing this new flux controller are twofold. First, it would remove 0093-9994/$20.00 © 2006 IEEE