Ultrasonic dynamic air-gap monitoring system for large hydro-generators

The dynamic air-gap monitoring of hydro-generators consists of the distance measurement between rotor poles and stator while it is in operation. This work presents a new approach for the air-gap measurement performed by measuring the time of flight of an ultrasonic wave traveling between the stator and the rotor. The measuring system consists of two pairs of small-size rectangular (4.5 × 8.0 mm) piezocomposite ultrasonic transducers (420 kHz), operating in pitch-catch scheme, which were designed for allowing their installation in ventilation ducts from outside the hydro-generator stator. The sound velocity is corrected based on the measurement of air temperature close to the ultrasonic transducer. The effect of the air flow velocity in the region of the air gap is compensated by calculating the average of the time of flight in the air flow direction and in the opposite direction to the flow. The ultrasonic air-gap monitoring sensor was installed in the hydro-generator number 5 (UG05 - 170MW) of the hydroelectric power plant of CESP Ilha Solteira (Brazil). The air-gap sensor was tested on several conditions of machine operation showing a repeatability of 0.1-mm in the dynamic measurement of the air-gap.