Direct Torque Control for Sensorless Induction Motor Drives Using an Improved H-Bridge Multilevel Inverter Javier Pereda, Student Member IEEE Juan Dixon, Senior Member IEEE Pontificia Universidad Católica de Chile, Santiago, Chile jepereda@ing.puc.cl, jdixon@ing.puc.cl Mauricio Rotella ABB Chile Local Division Manager, Automation Product mauricio.rotella@cl.abb.com Abstract — This paper presents the application of a Sensorless Direct Torque Control (DTC) for an induction motor, using an improved H-Bridge multilevel inverter with 27-Levels. The inverter topology reduces the power sources from nine to only four active sources and three ultracapacitors. The power sources are unidirectional and non-redundant; scaled in power of three to optimize the number of voltage levels with a minimum of semiconductors and power sources. For the improved inverter topology, two additional control strategies are introduced; 1) an Inhibit Negative Currents (INC) controller, which solves the regeneration problem when unidirectional sources are used; and a 2) Proportional-Integral (PI) controller, which keeps the ultracapacitor voltages at the reference value. Both controls work on a Pulse Width Modulation (PWM) signal, where the INC control decides the levels among the PWM operates, and the PI controller changes the duty-cycle. A closed-loop estimator called Model Reference Adaptive System (MRAS) was used for the speed estimation, with the advantage of using the stator voltages and currents already obtained for the DTC. The application of the system was simulated and implemented in Matlab®/Simulink® software using the industrial controller AC800PEC from ABB, obtaining satisfactory results. The multilevel inverter was specially designed and built for this application. Index Terms—Direct Torque Control (DTC), Multilevel Inverter, Pulse Width Modulation (PWM), AC Machine Drives, Sensorless. I. INTRODUCTION HE squirrel cage induction motor has been preferred by the Industry for years due to its low cost, high performance, reliability, ruggedness, and applicability. Nevertheless, the use of an induction motor in high performance applications requires complex electronic converters and controllers to obtain an adequate control of the motor speed, torque, current, and magnetic flux. Moreover, problems of conventional inverters include noise, harmonic contamination, torque jerk, motor deterioration, and losses in the inverter. The H-Bridge multilevel inverter is a considerable improvement in the voltage waveform of the motor in comparison with conventional inverters [1]. Even more, the number of voltage levels can be maximized if the H-Bridges use asymmetrical power sources scaled in power of three [2]. The increase in the number of voltage levels reduces: i) the Total Harmonic Distortion (THD); ii) abrupt changes in voltage (dv/dt); iii) voltage and currents of common mode; iv) output filters; and v) switching losses. Switching losses are reduced because the main bridges, which carry 80% of total power, works at very low frequency when power sources are asymmetrical [2-4]. The drawbacks of asymmetric H-Bridge inverters are the large number of bidirectional floating power sources and the lack of modularity. To reduce these drawbacks, a special topology and control strategy was used [5]. This solution simplifies and reduces the number of power sources from nine bidirectional to only four; one source for the three phases of the main H-Bridges, which can be unidirectional or bidirectional, and three unidirectional sources for the intermediate H-Bridges (Aux-1). The smallest H-Bridges (Aux-2) use floating ultracapacitors. The Direct Torque Control (DTC) [6, 7] is considered as one of the strategies of control with the highest performance in AC Drives and is commercially available for conventional inverters. The DTC already has been used in multilevel inverters [8, 9] showing advantages over its application in inverters with fewer levels regarding the switching frequency and torque jerk [10]. The sensorless drives have the advantages to being economic and reliable. They have had an important development and commercial acceptation, and can estimate the rotor speed using the stator voltages and currents, so a tachometer is not necessary. These systems already have been tested in DTC obtaining satisfactory results [11-13]. The purpose of this work is to develop a sensorless DTC drive for induction motors, using an improved multilevel inverter with a reduced number of unidirectional power sources though new topologies and control strategies [5]. II. MULTILEVEL INVERTER TOPOLOGY The improved 27-Level inverter used is shown in Fig. 1. Each phase has three sources scaled in power of three (V, 3·V and 9·V) to maximize the number of levels to 27 (3 3 ), where V is the voltage of each level. Furthermore, this topology reduced from nine bidirectional power sources to only four unidirectional power sources plus three ultracapacitors. T 978-1-4244-4649-0/09/$25.00 ｩ2009 IEEE 1110 Authorized licensed use limited to: Pontificia University Catolica de Chile. Downloaded on February 22,2010 at 23:28:10 EST from IEEE Xplore. Restrictions apply.