High Performance Control of AC Drives with Matlab / Simulink Models

5.6 Modified Direct Torque Control of Induction Machine with Constant Switching Frequency

From the discussion and simulation of Direct Torque Control of induction motors in previous sub-sessions, DTC has several major disadvantages such as [7]:

• problems during starting due to the absence of current controllers;
• operation in low-speed region due to high flux ripples;
• necessity of accurate estimation of stator flux and electromagnetic torque;
• changing switching frequency, because of hysteresis band-based control of torque; and
• high torque ripples in high-speed operation due to the utilization of zero voltage space vectors.

Variation of switching frequency in DTC is one of the major drawbacks in many industrial applications.

There have been many modifications suggested for constant switching frequency with DTC. These modifications include DTC with space vector modulation and deadbeat control for directly calculation of switching pattern to achieve constant switching period [24], DTC with predictive control, which calculates the optimal switching instant at each cycle on the basic of RMS torque-ripple equation during one switching period obtained from the instantaneous torque variation equation [9], and Direct Self Control with fuzzy controller for choosing switching states to reduce the flux and torque errors, while still achieving fast torque response and constant switching frequency [25]. However, these modifications have resulted in the increased sophistication of the control ...