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Abstract—The basic concept of direct torque control of induc-tion machines is investigated in order to emphasize the effects pro-duced by a given voltage vector on stator flux and torque vari-ations. The low number of voltage vectors which can be appliedto the machine using the basic DTC scheme may cause undesiredtorque and current ripple. An improvement of the drive perfor-mance can be obtained using a new DTC algorithm based on theapplication of the space vectormodulation (SVM) for prefixed timeintervals. In this way a sort of discrete space vector modulation(DSVM) is introduced. Numerical simulations and experimentaltests have been carried out to validate the proposed method.Index Terms—AC drives, direct torque control, sensorless con-trol.I. INTRODUCTIONIN RECENT years many studies have been developed to findout different solutions for the inductionmotor control havingthe features of precise and quick torque response, and reductionof the complexity of field oriented algorithms. 58588
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TheDirect torquecontrol (DTC) technique has been recognized as viable solutionto achieve these requirements [1]–[7].In principle the DTC selects one of the six voltage vectorsand two zero voltage vectors generated by a VSI in order tokeep stator flux and torque within the limits of two hysteresisbands. The right application of this principle allows a decou-pled control of flux and torque without the need of coordinatetransformation, PWM pulse generation and current regulators.However, the presence of hysteresis controllers leads to a vari-able switching frequency operation. In addition, the time dis-cretization due to the digital implementation besides the limitednumber of available voltage vectors determine the presence ofcurrent and torque ripple.In [8]–[11], different methods have been presented whichallow constant switching frequency operation. In general, theyrequire control schemes which are more complex with respectto the basic DTC scheme.With reference to current and torque ripple it has beenverified that a large influence is exerted by the amplitudeof flux and torque hysteresis bands, and the voltage vectorselection criteria [12], [13]. It can be noted also that a givenManuscript received November 4, 1998; revised December 20, 1999. Recom-mended by Associate Editor,
M. E. Elbuluk.The authors are with the Department of Electrical Engineering, Univer-sity of Bologna, Bologna I-40136, Italy (e-mail: domenico.casadei@mail.ing.unibo.it).Publisher Item Identifier S 0885-8993(00)05566-6.voltage vector has a different effect on the drive behavior athigh and low speed. Taking these considerations into account, agood compromise has been obtained using different switchingtables at high and low speed [13]. In general, the determinationof the switching tables is carried out on the basis of physicalconsiderations concerning the effects determined by radial andtangential variations of the stator flux vector on torque and fluxvalues. Although simple, this approach leads to unexpectedtorque variations in some particular operating conditions.The understanding of these phenomena requires a rigorousanalytical approach taking the electromagnetic behavior of themachine into account [14], [15].A substantial reduction of current and torque ripple could beobtained using, at each cycle period, a preview technique in thecalculation of the stator flux vector variation required to exactlycompensate the flux and torque errors [9], [10]. In order to applythis principle, the control system should be able to generate, ateach sampling period, any voltage vector (e.g., using the SpaceVector Modulation technique). This ideal behavior can be ap-proximated using a control system able to generate a numberof voltage vectors higher than that used in basic DTC scheme.An example of this concept is given in [16] and [17] where thenumber of available voltage vectors is increased using a doublethree-phase inverter or a double three-level inverter. These so-lutions are good for high power applications, but are not ac-ceptable for medium or low power applications owing to theincreased complexity of the power circuit.In this paper a new control technique is introduced whichallows the performance of DTC scheme in terms of flux andtorque ripple and current distortion to be improved. These re-sults can be achieved without increasing the complexity of thepower circuit and the inverter switching frequency. The newcontrol algorithmis based on a discrete space vectormodulation(DSVM) technique which uses prefixed time intervals within acycle period. In this way a higher number of voltage space vec-tors can be synthesized with respect to those used in basic DTCtechnique. The increased number of voltage vectors allows thedefinition of more accurate switching tables in which the se-lection of the voltage vectors is made according to the rotorspeed, the flux error and the torque error. The switching tablesare derived fromthe analysis of the equations linking the appliedvoltage vector to the corresponding torque and flux variations.These equations are obtained using a discrete model of the ma-chine valid for high sampling frequency.Several numerical simulations are presented to emphasize thedrive performance improvement obtained by using the DSVM technique.