A comparative study of field-oriented and direct-torque control of induction motors reference to shaft-sensorless control at low and zero-speed

In modern high performance ac drives, field oriented control (FOC) or direct torque control (DTC) schemes are applied to achieve high dynamic operation. Recently, efforts have been made to eliminate the machine shaft sensor in order to reduce costs and increase reliability. Although DTC is often designated as a sensorless scheme, it has similar problems to FOC when operated around zero flux frequency. In this paper, the performance of both schemes is investigated with special emphasis on operation around zero speed. In that operating range it is assumed nowadays that, without a shaft sensor, the flux position can only be determined by exploiting parasitic effects of the machine such as saturation or slotting. Sensorless schemes to estimate the spatial flux position control suffer from reduced accuracy in specific load ranges which degrades the performance of the drive when compared to operation with a shaft sensor. On a test stand FOC and DTC have been realized and the flux position calculated with the use of a speed sensor has been modified with different error functions to test the reaction of both schemes on erroneous flux input values.