Direct torque control with variable voltage duty ratio for inverter fed induction motor drives

Direct Torque Control (DTC) is a high performance control technique introduced for voltage inverter fed induction motor drives. This technique, based on selecting the optimum stator voltage vector of the inverter standard topology, makes the torque and stator flux reference values to be reached quickly, with a small number of changes in inverter switching states, resulting in a very good dynamic response. However, by this switching technique, based on hysteresis, large and small torque and flux errors are not distinguished, which cause a high torque and current ripple in motor steady state operation. To overcome this problem, this paper proposes a solution, which consists in the modulation of the nonzero voltage vector duration over a sampling period, according to the instant values of the torque and stator flux errors. The introduced duty ratio is calculated using a relation containing terms proportional to these errors. The presented results show the torque, flux and current ripple reduction obtained by using the proposed method. The main advantage is that it requires an insignificant additional computation, preserving the simplicity of the classical DTC.