Minimisation of flux droop in direct torque controlled induction motor drives

As a result of the uneven voltage vector contribution to the control of stator flux and torque in a direct torque controlled (DTC) induction machine, stator flux magnitude droops at every switching sector transition, particularly when the machine is running at low speed with a heavy load. A simple strategy rotates the conventional DTC switching sectors according to the states of the flux and torque hysteresis comparators and a gating signal. The applied voltage vector thereby maintains stator flux magnitude at the transition between switching sectors and hence improves the phase current without deteriorating the torque response. A thorough simulation analysis of stator flux droop is presented, and both simulation and experimental results confirm the effectiveness of the proposed strategy.