Enhanced estimation of the rotor position of MV-synchronous machines in the low speed range

In the past, plenty of identification methods have been proposed for the encoderless control of induction machines and of permanent magnet synchronous machines, including all important tasks of the encoderless operation, such as the identification of the initial rotor position, the low and the high speed operation of the drive. However, not all the available methods are suitable for the application on electrically excited synchronous machines (EESM) with damper winding, which are commonly used in high power, medium voltage (MV) applications. For a large class of EESM the known methods of encoderless control fail in delivering the correct estimation of the rotor position especially in the low speed range of operation. In the present work the problems related to the encoderless control of the EESM with conventional techniques are described and a new approach is introduced, which injects low frequency test signals to identify the rotor position. In the same way the model based methods are enhanced and a persistent operation at low and zero speed becomes possible. Measurement results on a conventional medium voltage inverter validate the applicability of the proposed scheme.