Abstract: Removal of the rotor of a large electrical machine is costly and time consuming, and in particular the outage time of a power plant, which is needed for the removal of a generator rotor, is very expensive and should be minimized. To avoid opening of the generator for inspection and maintenance instrument carriages can be used. A method allows the reliable guidance for an in-situ instrument carrier inside the annular gap of a generator and a corresponding instrument carrier. The method avoids the use of sensors and active control systems for the guidance of the instrument carrier. A passively self-guiding system and method takes advantage of the internal structure of a generator and uses the passive centering properties of magnets to guide the instrument carrier through the annular gap of a generator.