Method of detecting and modelling presence of shorted turns in DC field winding of cylindrical rotor synchronous machines using two airgap search coils

A method is described for detecting the loss of one or more turns in a concentric coil of the rotor field winding of a synchronous machine using two search coils in the airgap. This back-to-back method yields an EMF proportional to the rate of change of the `missing¿ flux linking the search coils, and is sufficiently sensitive for the coils to be positioned close to the stator surface. A two-dimensional linear analysis of the airgap field is used to model the method with good accuracy, and includes an estimate of the resulting unbalanced magnetic pull. A simple one-dimensional analysis predicts the EMF harmonics that are introduced by static or dynamic rotor eccentricity. Results from a small four-pole experimental machine are presented, including the effect of a known amount of static eccentricity. Calculations for a larger two-pole machine are also included, and a possibly useful method of fault location detection using the ratio of the magnitudes of only two harmonics taken from the search coil output is discussed.