Improved Flux Estimation Technique for Direct Torque Control of Induction Motor Drives

In DTC drive stator flux is controllable variable so accurate flux estimation in a high-performance induction motor drive, is important to ensure proper drive operation and stability. Most of the flux estimation techniques proposed are based on the voltage model. The voltage model, on the other hand, is normally used in a high speed range, since at low speed, some problems arise. Practical implementation, even a small dc offset present in the back electromotive force (EMF) due to noise or measurement error inherently present in the current sensor, can cause the integrator to saturate. To overcome this, a low-pass (LP) filter is normally used in place of a pure integrator. The voltage only motor parameter used is the stator resistance, which can be measured quite accurately. These can be considered as an advantage of the voltage-mode and are also the reasons why the former is preferred over the latter in some industrial applications. However, the use of an LP filter in place of a pure integrator reduces the performance of the drive because of the phase and magnitude errors inherent in the LP filter as compared to the pure integrator, particularly at low speed. Attempts have been made to improve the estimated stator flux based on an LP filter as given by the proposed method used an adaptive control system which was based on the fact that the back EMF is orthogonal to the stator flux. The compensator is adapted for this condition. However, to implement the proposed system requires large processor resources and longer execution time for a slower processor. The implementation of adaptive Control will significantly increase the complexity of the control system; hence, it will outweigh the property of simple control structure inherent in DTC.