A novel hybrid PWM algorithm for reduced common mode voltage in direct torque controlled induction motor drives

This paper presents a novel hybrid pulse width modulation (HPWM) technique for the reduction of common mode voltage (CMV) in direct torque controlled induction motor drives based on the concept of imaginary switching times. In the proposed approach, different active zero state PWM (AZPWM) sequences are considered in which the actual switching times are calculated based on the instantaneous values of phase voltages. So, it does not require sector identification and angle information. Moreover for the reduction of CMV, AZPWM sequences utilize active voltage vectors for composing the reference voltage vector instead of using zero voltage vectors, So that the CMV changes from +Vdc/6 or -Vdc/6 due to application of active voltage vectors. Though these AZPWM methods reduce the computational burden involved in calculation, they still suffer from steady state ripples in torque, flux and current. To reduce the ripples in steady state, a HPWM technique is developed in which stator flux ripple analysis is done for all the AZPWM sequences in terms of actual switching times, dc link voltage (Vdc), sampling time period (Ts). As AZPWM2, AZPWM3 exhibit same ripple characteristics only AZPWM3 is considered in this paper. Then by comparing AZPWM1, AZPWM3, AZPWM4 sequences with respect to each other sequences at various modulation indices the sequence with minimum flux ripple is obtained. This sequence is then fed to DTC based induction motor drive. As all the sectors are symmetric, the mean square flux ripple characteristics for a period of 60⁰ are plotted. To validate the proposed PWM algorithm, numerical simulation studies have been carried out using MATLAB-Simulink and results are presented and compared.