In this paper a new method for Direct Torque Control (DTC) based on load angle control is developed. The use of simple equations to obtain the control algorithm makes it easy to understand and implement. Fixed switching frequency and low torque ripple are obtained using space vector modulation.
This control strategy overcomes the most important drawbacks of classic DTC. Results shows the feasibility of the proposed method, obtaining good speed control bandwidth while overcoming classic DTC drawbacks.
In recent years much research has been developed in order to find simpler control schemes for induction motors that meet the most demanding requirements, such as low torque ripple, low harmonic distortion or quick dynamic response.
Today Field Oriented Control (FOC) and Direct Torque Control (DTC) are considered the most important techniques to achieve high dynamic performance in AC machines. The use of current control loops in a rotating reference frame to decouple torque and flux control is the main characteristic of FOC.
The use of these controllers adds an additional time delay making torque dynamic response slower. Space vector modulation (SVM) has been frequently used in FOC to generate the gate pulses for the inverter semiconductors.
Classic DTC makes use of hysteresis comparators with torque and stator flux magnitude errors as inputs to decide which stator voltage vector is applied to motor terminals. The complex plane is divided in six sectors, and a switching table is designed to obtain the required vector based on the hysteresis comparators outputs.
Due to fast time constants of stator dynamics it is very difficult to keep machine torque between the hysteresis bands. This can be done either by increasing the sampling frequency as in , thus increasing switching frequency, commutation losses and computation requirements, or using multilevel power converters.
Source: Universidad Técnica Federico Santa María
Authors: Jose Rodrıguez, Jorge Pontt, Cesar Silva, Samir Kouro and Hernan Miranda