Limitation of the load angle in a direct-torque-controlled synchronous machine drive

The basic direct torque control (DTC) principle is to rotate the flux linkage forward, if the torque must be increased, and reverse, if the forque must be decreased. The torque is, however, increased only up to a maximum torque of the machine, which corresponds to a load angle of about 90[degrees] in a synchronous machine. This paper presents a method to overcome the possible loss of synchronism when using DTC. This requires either that the rotor angle is measured or estimated. Both of these cases are considered. Simulation and laboratory results are presented to show the effectiveness of the presented method. Index Terms--Estimation, rotating machine stability, synchronous motor drives.