A wide-speed high torque capability utilizing overmodulation strategy for direct torque control of induction machines

This paper presents a simple overmodulation method employed in direct torque control (DTC)-constant switching frequency (CSF) controller of induction machines. The proposed overmodulation method is utilized to extend a constant torque region and hence produces high torque capability in field weakening region with six-step operation. It is shown that the overmodulation operation using the DTC-CSF scheme can be established by controlling the stator flux locus from circular to the hexagonal shape. This is achieved by modifying the flux error status produced from flux hysteresis controller, before it is being fed to the look-up table. The main benefit in the proposed strategy is its simplicity and it can be implemented in many electric drive applications to obtain maximum torque capability for a wider speed range operation.