This paper presents an experimental investigation into multi-diode open-circuit faults in the rotating rectifiers of brushless synchronous generators. Unlike single-diode faults, multi-diode faults introduce complex electrical behavior by reconfiguring conduction paths and altering rectifier topology. Representative fault scenarios, categorized from Class 3 to Class 8 are examined under no-load conditions to isolate excitation system dynamics. Key electrical quantities, including exciter armature currents (  -  ), rectifier output voltage ( ), main field current ( ), exciter field current ( ), and generator terminal voltage (  ), are evaluated. The results show that electrical characteristics are governed by the effective conduction paths and the resulting rectifier structure rather than the number of faulty diodes. Depending on the fault condition, the rectifier transitions move transitions from asymmetric three-phase rectification to single-phase full-wave or half-wave rectification. These structural transitions lead to amplitude reduction and the emergence of dominant frequency components, particularly at 60 Hz and 120 Hz. The findings provide a structural interpretation and electrical characteristic analysis of multi-diode faults.

Brushless excitation systems are widely used in marine synchronous generators due to their high reliability and reduced maintenance requirements. In these systems, the rotating rectifier converts the three-phase AC output of the exciter into DC current for the main field winding. However, faults in the rotating rectifier, particularly a single diode open-circuit fault, can degrade excitation performance without immediately triggering protective devices, making early detection difficult. This paper experimentally investigates the effects of a single rotating rectifier diode open-circuit fault on the excitation system and voltage formation of a brushless synchronous generator under no-load operating conditions. The no-load condition minimizes the influence of armature reaction and load current, allowing fault-induced excitation behavior to be clearly isolated. A brushless excitation system was implemented using three synchronous machines of identical rating, and a single diode open-circuit fault was intentionally introduced in the rotating rectifier. Excitation-related electrical quantities related to the excitation system, including DC excitation current and voltage, exciter armature currents, and generator terminal voltage, were measured and compared before and after the fault. Experimental results demonstrate that the single diode open-circuit fault causes reduction in the average excitation current and introduces low-frequency ripple components in the excitation current waveform while the terminal voltage reduction remains limited under no-load conditions. These results indicate that excitation-related electrical signals can serve as effective indicators for the detection of rotating rectifier diode faults in brushless synchronous generators.