Rotor position identification in synchronous machines by using the excitation machine as a sensor

The control of any AC-machine demands the rotor or a flux position to describe the machine. The use of an encoder to solve this problem is possible but this reduces the total reliability of the drive. Encoderless methods can identify the required position by only evaluating the measured electrical quantities of the machine. Generally, additional test signals need to be injected into the machine in the low speed range to extract the desired information. However, in electrically excited medium voltage synchronous machines (EESM), most of the known encoderless solutions fail in the low speed range. In this paper, a novel approach to track the rotor position in a brushless excited EESM is introduced. As the excitation machine is mounted on the same shaft as the main synchronous machine, this approach focuses on the evaluation of the stator quantities of the excitation machines to identify the rotor position. In particular, the electrical asymmetry caused by the rotating rectifier is used to detect the rotor position without the need of injecting any additional test signals. However, like in the case of operation with an incremental encoder, the initial rotor position must be known to obtain the absolute position.