Abstract: Concerning the limitation of the available detection methods for the common inter-turn short circuits in stator and field windings of synchronous generators, especially in the common turbo-generators, this paper puts forward a new type of air-gap search coils. Theoretical analysis shows that the open-circuit voltage of the novel search coil is zero under normal operation or external short circuits at machine terminals, for the special distribution and connection pattern. While asymmetrical faults occur in the windings, AC EMF could be induced in the search coils by the additional space harmonics of air-gap magnetic field, which do not exist under normal conditions. What’s more, in case of stator internal short circuits and field inter-turn short circuits, the open-circuit voltages of the search coils are obviously different in frequency. A series of experiments on a poly-phase model machine with rectifier load systems, where the novel search coils are installed, verifies the correctness of the theoretical analysis on the search coils. It is feasible to detect both the stator internal faults and the field inter-turn faults in generators according to the open-circuit voltages of the search coils, and distinguish them from each other by the voltage frequency.