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Electromagnetic acoustic transducer for receiving longitudinal wave in the central hole of the wind turbine main shaft

For an operating wind turbine, the main shaft suffers severe structural damage. The surface breaking transverse crack threatens the safety of drive-train system directly. The crack examination method is available by diffracted waves in the central hole. In this research, an electromagnetic acoustic transducer (EMAT) for receiving diffracted longitudinal waves is proposed. Firstly, the converse piezomagnetic stress coefficients and magnetostriction coefficient were derived from the measured magnetic characteristic curve. The measured parameters were provided for sensor simulation. Considering the two coupling mechanisms, a receiving EMAT model was established for coil optimization. Secondly, the influence of materials, lift-off distance and cable type on sensor impedance would be analyzed in detail. Multiple factors were taken into account for impedance matching, and the signal amplitude was improved significantly. The angle divergence of the EMAT was also measured, showing the main lobe with 24°. The crack detection experiments were carried out on a shaft sample. The results show that the developed EMAT can achieve the reception of diffracted longitudinal waves, and the signals are distinguishable. The crack was evaluated simultaneously by the received diffracted waves, and the errors showed less than 3 mm.