Monitoring torsional vibrations of electro-mechanical systems using stator currents

A method is proposed to detect shaft torsional vibrations in AC motors and generators from their stator currents. This is achieved by extracting current components at specific frequencies which are the modulation products of the power system frequency and the vibrating frequency. A general closed form proof of this principle is given for both induction and synchronous machines, and the principle is verified quantitatively by digital simulation and experimental results. The effect of the machine operating point is also investigated. The method is applicable to torsional vibrations induced by different causes. When the oscillating torque is developing and the measured stator current is nonstationary, the Wavelet Transform can be used to extend the application of the proposed method to transient conditions.