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Firstly, the magnetic motive force distribution of the steam turbine generator excitation windings was analyzed, and the mathematic model of MMF was established. Then the change of MMF caused by rotor inter-turn short circuit was examined, and the MMF after inter-turn short circuit happens as the superposition of the normal MMF before inter-turn short circuit happens and the MMF produced by short circuit turns with opposite excitation current. As the former is completely symmetrical, only the opposite MMF produced by short turns was analyzed, and harmonic components with various orders were gained by Fourier analysis. The electromotive force induced in stator windings was got considering the air-gap eccentricity, and the theoretical deduction illustrate that the eccentricity has no influence on the induced electromotive force of the stator windings. The three-phase symmetrical currents of the stator produced a new revolving magnetic motive force, and induced harmonics in rotor windings, whose characteristic spectrum differs from that in normal conditions. The theory was validated by experiments at last and the method that employs harmonics detection to diagnose the inter-turn short circuit malfunction of the steam turbine generator was established.