Induction motor Direct Torque Control with improvement in low frequency harmonic content

In high power ac motor drives, power semiconductor devices are allowed to switch at a quite low frequency, which is generally limited to a few hundreds hertz. Therefore, high torque harmonic components in the 0-1000 Hz frequency range often arise, which may compromise the processes accuracy and the products quality. The asynchronous pulse width modulation (PWM) that is generally implemented in high performance control strategies further deteriorates the low frequency harmonic content. In this paper, a new induction motor control scheme is presented, starting from the theory of direct torque control. Assuming the electromagnetic torque and the stator flux vector as controlled variables and the stator voltage vector and the instantaneous switching frequency as input quantities, it is possible to obtain a synchronous PWM while preserving the high dynamics response. The new theory has been tested by making a comparison with a more conventional DTC scheme. The obtained results show an improvement in the torque harmonic spectrum especially in the low frequency region.