This paper presents novel hybrid SVPWM (HSVPWM) methods for direct torque controlled induction motor drives without angle estimation to reduce steady state ripple in current, torque and flux. The proposed PWM technique is designed based on the notion of stator flux ripple, which is a measure of line current ripple. Expressions for RMS ripple, over a sub-cycle are derived for each switching sequence in terms of reference voltage vector, imaginary switching times and subcycle duration. This analysis together with the THD performance is used to design new HSVPWM techniques, which result in reduction in THD. The proposed PWM methods simplify the control algorithm and also reduce the execution time. Also to improve the speed performance under uncertainties caused by load torque, a robust speed controller with an integral sliding switching surface is used. To validate the proposed method, the results are presented. Index Terms-Direct torque control, flux ripple, hybrid SVPWM, sliding mode controller. I. NOMENCLATURE Rs, Rr stator and rotor resistances Ls Lr, Lm self and mutual inductances P number of poles Vs stator voltage phasor is ir stator and rotor currents phasors V/s' 1/'r stator and rotor flux linkage phasors COr rotor electrical speed in radians Te electromagnetic torque B friction coefficient J inertia constant of the induction motor TL load torque. VARAIBALE speed induction motor drive, based on direct torque control (DTC), is receiving wide attention in the literature [1]-[5], [9], [11]. The conventional DTC (CDTC) [1] consists of adaptive motor model, two hysteresis comparators, a switching table and a voltage source inverter (VSI). In DTC the flux and torque are controlled independently by selecting one of the voltage space vectors of the VSI, in order to keep the stator flux and torque with in the limits of the hysteresis bands. Despite its simplicity, CDTC is able to produce quick torque response and is robust with respect to motor parameter variations. Hence this control algorithm is increasingly being used in the industry and is considered to be the next generation motor control method [2]. However, the presence of torque and flux hysteresis controllers leads to variable switching frequency. Also CDTC has considerable ripple in torque, flux and current during steady state, which results in harmonics, power loss and incorrect speed estimation. In order to improve the performance of CDTC in terms of torque, flux and current ripple, discrete space vector modulation (DSVM) is proposed in [3]. DSVM can generate a higher number of voltage vectors than that used in CDTC, which allow a sensible reduction of torque and current ripple in all speed ranges. Only few schemes can produce the constant switching frequency operation with DTC. In [4][5], a voltage reference is generated based upon the errors of torque and flux and reference voltage is realized by using the principle of space vector pulsewidth modulation (SVPWM) to achieve the constant switching frequency operation. Also a substantial reduction in torqu...
