Hybrid random PWM algorithm for direct torque controlled induction motor drive for reduced harmonic distortion

The basic direct torque control has more torque, flux and current ripples in steady state, which results in acoustical noise and increased harmonic distortion. The standard space vector pulse width modulation (SVPWM) algorithm distributes the zero state time equally between the two possible zero voltage vectors. To reduce the complexity involved in the conventional space vector approach, the proposed pulse width modulation (PWM) algorithm uses instantaneous sampled reference phase voltages to calculate the actual switching times of the devices. The proposed PWM algorithm modifies the time duration of application of vector V₀ (000) by using a factor μ by which various switching sequences can be derived. By comparing the instantaneous ripple values in each sampling time interval, the suitable sequence is selected resulting in minimum current ripple. Thus, the proposed algorithm gives reduced harmonic distortion when compared with the SVPWM algorithm. As the zero state time is varied randomly according to the operating sequence, randomization effect will occur, which results in reduced dominating harmonics and hence reduced acoustical noise. The simulation results validate the proposed algorithm.