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.
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Stoll et al. (1988) studied this question.