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The rotor angular speed has beenconsidered constant during the torque step response.The simulation results given in Figs. 16 and 17 show that,owing to the use of a five-level torque hysteresis comparator, theDSVMtechnique does not deteriorate the good dynamic perfor-mance of basic DTC schemes.VIII. EXPERIMENTAL RESULTSIn order to make the experimental validation of the proposedDSVM technique a DSP based system has been used. The ex-perimental set-up includes a fully digital controlled IGBT in-verter and a 4 kW, 220V, 50 Hz, 4-poles standard inductionmotor. The control scheme has been implemented on a 20 MHz TMS320E15 DSP, adopting a cycle period of 160 s. It can benoted that this value is much higher than that strictly required toimplement the new DSVM algorithm. In this way a large com-putational time is available for on-line parameter adaptation, fil-tering routine, protection and diagnostic facility.Fig. 18 shows the experimental results obtained with a rotorspeed of 100 rpm and a torque command of 25 Nm.Fig. 18(a) represents the estimated value of the torque, readout through a D/A converter. In Fig. 18(b), the squared valueof the estimated stator flux magnitude is represented. Fig. 18(c)shows the measured motor current.As it could be expected from the numerical simulation theperformance of the drive system based on the DSVM techniqueis satisfactory, showing an acceptable level of torque and currentripple. In addition, owing to the new modulation technique, theacoustic noise is sensibly reduced with respect to that of basicDTC scheme, particularly in the low speed range.IX. CONCLUSIONSThe analysis carried out in this paper was aimed to modifythe basic DTC control scheme in order to improve the perfor-mance of an inductionmotor drive in terms of torque and currentripple. For this purpose a SVMtechnique that uses prefixed timeintervals within a cycle period has been proposed. In this wayit is possible to synthesize a higher number of voltage vectorswith respect to basic DTC scheme, without the need of timers orPWM signal generator. It is then possible to define more accu-rate switching tables which allow a sensible reduction of torqueand current ripple in the whole speed range.The implementation of the DSVM technique requires only asmall increase (25–30%) of the computational time required bybasic DTC scheme. Taking into account that the same switchingfrequency is obtained doubling the cycle period in the new DTCscheme, a large amount of time is available for parameter adap-tation, protection and diagnostic facility. It can be noted also thatusing a cycle period equal to the required computational time afurther improvement of the drive system performance can beachieved, even if with a higher switching frequency.Numerical simulations at low and high speed have been car-ried out showing the advantages of the new DTC scheme withrespect to the basic one.The feasibility and the validity of the proposed controlsystem, based on the DSVM technique, has been proved byexperimental tests.REFERENCES[1] I. Takahashi and T. Noguchi, “A newquick-response and high-efficiencycontrol strategy of an induction motor,” IEEE Trans. Ind. Applicat., vol.IA-22, pp. 820–827, Sept./Oct. 1986.[2] M. Depenbrok, “Direct self-control (DSC) of inverter-fed induction ma-chine,” IEEE Trans.
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