Abstract: Multi-phase brushless excitation system is the preferred excitation mode for large-capacity nuclear power units. Its rectifier diode is in high-speed rotating state. Diode faults frequently occur in actual operation. Accurate grasp of the characteristics of diode faults is the premise to achieve reliable and sensitive protection. Firstly, the analytical expressions of armature current of the exciter rotor are obtained by theoretical analysis when the rotating diode is in normal operation, one-tube open circuit and one-phase open circuit. Furthermore, based on the analysis of the excitation effect of armature reaction on the stator excitation winding, the harmonic characteristics of excitation current of exciter stator under three operation conditions are obtained. Finally, experiments are carried out on an eleven-phase model machine with load when diode is in normal operation, one tube is open-circuit and one phase is open-circuit. Experimental results of armature current and excitation current verify the correctness of the theoretical analysis. Experiments and theoretical analysis show that the stator excitation current of the exciter mainly contains DC component when the rotating diode is in normal operation, fundamental and harmonics components appear when one tube is open-circuit, and even harmonics components occur when one phase is open-circuit.