In this paper a new DTC scheme for interior permanent magnet synchronous machine (IPMSM) is proposed. The proposed DTC scheme having the advantages of low torque ripples as well as constant switching frequency.
The increment in the switching frequency requires neither an increase of the sampling frequency nor high frequency dithering signal injection. Here in the proposed scheme, a model based observer is used to generate the optimal reference voltage vector.
Furthermore, a well developed space vector modulation technique is applied to inverter control in the proposed scheme using a DSP-based hardware, thereby reducing the torque ripples and increasing the switching frequency which becomes constant. To illustrate the performances of the proposed control scheme, simulation and experimental results are presented.
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.
Author: F. A. Osman | A. M. Osheiba | F. M. El-Khouly | M. M. Khatter