In this study, direct torque control (DTC) of induction motor is evaluated based on space vector modulation (SVM). DTC is a method to control machine with utilizing torque and flux of motor controlled.
The torque and current ripple are occurred in the conventional DTC. Reason of undesired torque and current ripple is low number of voltage vectors applied to the motor controlled by the conventional DTC technique. SVM-DTC is a technique to reduce the ripple.
SVM techniques have several advantages that are offering better DC bus utilization, lower torque ripple, lower total harmonic distortion in the AC motor current, lower switching loss, and easier to implement in the digital systems. Simulation results from the classical and improved DTC are presented and compared. Result shows that the torque, flux linkage and stator current ripple are decreased with the improved DTC.
INTRODUCTION:
Induction Motors (IMs) are widely used in high performance drives. Its history is very extensive and also control is important in applications. There are many IMs in a number of industrial, commercial and domestic applications of variable speed drives. Since IMs demands well control performances: precise and quick torque and flux response, large torque at low speed, wide speed range, the drive control system is the most sensitive point of IMs.
DTC is method to control machine with utilizing torque and flux of motor controlled. The basic DTC scheme consists of two comparators having different features, switching table, Voltage Source Inverter (VSI), flux and torque estimation block and IM.Like a every control method has some advantages and disadvantages, DTC method has too.
Some of the advantages are lower parameters dependency, making the system more robust and easier implements and the disadvantages are difficult to control flux and torque at low speed, current and torque distortion during the change of the sector in d-q plane, variable switching frequency, a high sampling frequency needed for digital implementation of hysteresis controllers, high torque ripple.
The torque ripple generates noise and vibrations, causes errors in sensor less motor drives, and associated current ripples are in turn responsible for the EMI. The reason of the high current and torque ripple in DTC is the presence of hysteresis comparators together the limited number of available voltage vectors.
To reduce the torque ripple, the stator flux vector change, which is demanded to compensate the torque and flux errors, determination should be done and also any voltage vector should be produced by the control mechanism. If a higher number of voltage vectors than those used in conventional DTC is used, the favorable motor control can be obtained.
Because of complexity of power and control circuit, this approach is not satisfactory for low or medium power applications. One of the methods to increase the number of available vectors is an online modulation between active and null vectors.
Source: IJMER
Authors: Hanumanji Kantari
Space Vector Modulated – Direct Torque Controlled (DTC–SVM) Inverter – Fed Induction Motor Drive
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