In this paper, a simplified space vector pulse width modulation (SVPWM) method has been developed for three phase three-level voltage source inverter fed to direct torque controlled (DTC) induction motor drive. The space vector diagram of three-level inverter is simplified into two-level inverter. So the selection of switching sequences is done as conventional two-level SVPWM method.Where in conventional direct torque control (CDTC), the stator flux and torque are directly controlled by the selection of optimal switching modes. The selection is made to restrict the flux and torque errors in corresponding hysteresis bands. In spite of its fast torque response, it has more flux, torque and current ripples in steady state. To overcome the ripples in steady state, a space vector based pulse width modulation (SVPWM) methodology is proposed in this paper. The proposed SVPWM method reduces the computational burden and reduces the total harmonic distortion compared with 2-level one and the conventional one also. To strengthen the voice simulation is carried out and the corresponding results are presented.
Fig.1.Block diagram of proposed DTC drive.
EXPECTED SIMULATION RESULTS:
Fig 2 steady state plots of speed, torque, stator currents and stator flux for CDTC based IM drive at 1200 rpm.
Fig 3 Harmonic Spectrum of stator current along with THD.
Fig. 4 Simulation results of 2-level SVPWM based DTC: steady-state plots at 1200 rpm
Fig. 5 Harmonic Spectrum of stator current along with THD for 2-Level SVPWM based DTC-IM drive
Fig. 6 Starting transients in speed, torque, currents and flux for simplified SVPWM algorithm based 3-level inverter fed DTC-IM.
Fig. 7 Steady state plots of speed, torque, currents and flux for simplified SVPWM algorithm based 3-level inverter fed DTC-IM.
Fig. 8 Harmonic spectra of steady state line current for simplified SVPWM algorithm based 3-level inverter fed DTC-IM.
Fig. 9 transients during step change in load for simplified SVPWM algorithm based 3-level inverter fed DTC-IM: a 30 N-m load is applied at 0.5
Fig. 10 Phase and line voltages during a step change in load for simplified SVPWM algorithm based 3-level inverter fed DTC-IM: a 30 N-m load is applied at 0.5 sec.
Fig. 11 Transients in speed, torque, currents and flux during speed reversal for simplified SVPWM algorithm based 3-level inverter fed DTC-IM (speed is changed from 1200 rpm to -1200 rpm at 0.7 s)
Fig. 12Transients in speed, torque, currents and flux during speed reversal for simplified SVPWM algorithm based 3-level inverter fed DTC-IM (speed is changed from -1200 rpm to +1200 rpm at 1.35 s)
Fig. 13 Four-quadrant operation of proposed simplified SVPWM algorithm based 3-level inverter fed induction motor drive.
In this paper, a simplified SVPWM algorithm is presented for three-phase three-level inverter fed DTC drive. The proposed algorithm generates the switching pulses similar to a two-level inverter based SVPWM algorithm. Thus, the proposed algorithm reduces the complexity involved in the existing PWM algorithms. To validate the proposed PWM algorithm, numerical simulation studies have been carried out and results are presented. From the simulation results, it can be concluded that the three-level inverter fed DTC drive gives reduced steady state ripples and harmonic distortion.
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