The basic concept of DTC of AC motor drives is to control both stator flux linkage and the electromagnetic torque of the machine simultaneously. Since a DTC-based drive system select the inverter switching states using switching table, neither current controllers nor pulse-width modulation (PWM) modulator is required, as shown in Fig. 1, thereby introducing fast torque dynamics response in comparison with the field oriented vector control technique.
However, this conventional DTC approach has some disadvantages such as; high torque ripple and variable switching frequency, which is varying with speed, load torque and the selected hysteresis bands.
On the other hand to reduce the torque ripple; the hysteresis torque and flux controller bands must be reduced to match the required torque performance, which requires reduction of the system sampling time and it is necessary to use a very fast processing controller.
Although the system sampling frequency can be increased in the conventional DTC the inverter switching frequency is still low, approximately less than one third of the sampling frequency.
The inverter switching frequency can be increased using a dithering signal, by adding a limited amplitude high frequency signal to the torque and flux error signals. Although the switching frequency is increased it is still variable for small error bands
However, this conventional DTC approach has some disadvantages such as; high torque ripple and variable switching frequency, which is varying with speed, load torque and the selected hysteresis bands.
On the other hand to reduce the torque ripple; the hysteresis torque and flux controller bands must be reduced to match the required torque performance, which requires reduction of the system sampling time and it is necessary to use a very fast processing controller.
Although the system sampling frequency can be increased in the conventional DTC the inverter switching frequency is still low, approximately less than one third of the sampling frequency.
The inverter switching frequency can be increased using a dithering signal, by adding a limited amplitude high frequency signal to the torque and flux error signals. Although the switching frequency is increased it is still variable for small error bands
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