2116 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 22, NO. 6, NOVEMBER 2007 High-Performance Torque and Flux Control for Multilevel Inverter Fed Induction Motors Samir Kouro, Student Member, IEEE, Rafael Bernal, Hernán Miranda, Student Member, IEEE, César A. Silva, Member, IEEE, and José Rodríguez, Senior Member, IEEE Abstract—This paper presents a high-performance torque and flux control strategy for high-power induction motor drives. The control method uses the torque error to control the load angle, obtaining the appropriate flux vector trajectory from which the voltage vector is directly derived based on direct torque control principles. The voltage vector is then generated by an asymmetric cascaded multilevel inverter without need of modulation and filter. Due to the high output quality of the inverter, the torque response presents nearly no ripple. In addition, switching losses are greatly reduced since 80% of the power is delivered by the high-power cell of the asymmetric inverter, which commutates at fundamental fre- quency. Simulation and experimental results for 81-level inverter are presented. Index Terms—Direct torque control, induction motor, multilevel inverters, switching losses. I. INTRODUCTION I NDUCTION motors are today the most widely used alterna- tive in adjustable speed drives. Field-oriented control (FOC) [1]–[3] and direct torque control (DTC) [4]–[7] have emerged as the standard industrial solutions for high dynamic performance operation of these machines. On the other hand, multilevel inverters have become a very attractive solution for high-power applications, due to higher voltage operation capability, reduced common-mode voltages, near-sinusoidal outputs, low ’s, and smaller or even no output filter [8]. FOC can be naturally extended for multilevel inverter-fed drives, only the modulation method has to be upgraded to multilevel pulsewidth modulation (PWM) (with multiple carrier arrangements) or multilevel space vector modulation (SVM). On the contrary, traditional DTC cannot be extended easily for multilevel inverters, due to the high amount of possible voltage vectors available for selection. Therefore, some contributions have adapted DTC to multilevel topologies Manuscript received December 1, 2006; revised May 10, 2007. This work was supported by the Chilean National Fund of Scientific and Technological Development (FONDECYT), under Grants 1060423 and 1060436 and by the Industrial Electronics and Mechatronics Millenium Science Nucleus of the Uni- versidad Técnica Federico Santa María. Recommended for publication by As- sociate Editor A. Consoli. S. Kouro, H. Miranda, C. A. Silva, and J. Rodrígurez are with the Electronics Engineering Department, Universidad Técnica Federico Santa María, Val- paraíso, Chile (e-mail: samir.kouro@ieee.org; hernan.miranda@elo.utfsm.cl; cesar.silva@elo.utfsm.cl; jrp@elo.utfsm.cl). R. Bernal is with the Komatsu Chile S.A., Quilicura, Santiago, Chile (e-mail: rafael.bernal@komatsu.cl). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TPEL.2007.909189 [9], [10]. However, they are conceived for particular inverter topologies and for a certain amount of levels. A more gener- alized view is presented in [11] and [12] with good dynamic performance, nevertheless at expense of complicated imple- mentation issues including torque derivatives and prediction. Other recent contributions, for two-level inverter-fed drives, combine DTC principles together with PWM and SVM to reduce torque ripple and fix the switching frequency of the inverter [13]–[20]. Although DTC is known by the absence of modulators and torque linear controllers, this approach has shown significant improvements together with high dynamic performance. This paper presents a load angle control-based DTC to en- able the natural incorporation of multilevel inverters. The ap- proach is intended for inverters with high number of levels (over nine levels), more commonly observed in modular structures like the cascaded H-bridge inverter. In particular in this paper, an asymmetric-fed cascaded inverter is used [21]–[24]. The high number of levels and consequently of voltage vectors provided by these inverters makes the modulation stage unnecessary. In addition, the semiconductors of the high-power cells of the in- verter only perform few commutations per cycle, reducing the switching losses of the inverter, which is specially attractive for high-power applications. Experimental results obtained for a 81-level inverter-fed induction motor confirm the accuracy and high dynamic performance of the proposed method. II. THE INVERTER A. Power Circuit The power circuit of the Asymmetric Cascaded H-Bridge In- verter is illustrated in Fig. 1. The inverter is composed by the series connection of two or more H-bridge inverters fed by in- dependent dc-sources provided by individual secondaries of a transformer or batteries (if used in electric or hybrid vehicles for example). These sources are not equal, i.e., , for each phase . The use of asymmetric input voltages can reduce, or when properly chosen, eliminate redundant output levels, maximizing the number of different levels generated by the inverter. There- fore this topology can achieve the same output voltage quality with less number of semiconductors. This also reduces volume, costs, losses and improves reliability. When cascading three level inverters like H-bridges (output levels: , 0 and , the optimal asymmetry is obtained by using voltage sources scaled proportional to the power of three. Applying this criteria 0885-8993/$25.00 © 2007 IEEE