Rotor magnetic pole non-roundness (RMPNR) is a prevalent mechanical failure encountered in large hydro-generators

potentially leading to severe unit vibrations and rotor sweeping

which can significantly impact the safe operation of the unit. An analysis of the air-gap flux density characteristics under various RMPNR conditions in hydro-generators is conducted through the air-gap permeance method in this paper. The fractional harmonic enhancement within the main flux of the hydro-generator is identified. Subsequently

a physical relationship and similarity of fault characteristics are revealed between the leakage flux of the stator core and the main flux

in accordance with the structural characteristics of hydro-generators. A measuring coil is arranged on the stator core positioning rib for capturing the leakage flux

with its fractional harmonics being utilized as the criterion for diagnosing such faults. Furthermore

a method for quantifying the fault severity based on the characteristic harmonic proportion is introduced. Finally

finite element simulations and experimental simulations are performed to validate the proposed method for measuring coils to detect RMPNR. The novel on-line detection method

which eliminates the need for installing sensors inside the generator

enables real-time assessment of RMPNR in hydro-generators and thus mitigates risk to the unit safe operation.