Condition monitoring of synchronous motors based on measurement of circulating stator currents

Condition monitoring based on currents circulating between parallel stator windings of a synchronous motor has been demonstrated. Circulating currents arise from any asymmetries in the motor that influence the air-gap magnetic field such as an eccentric rotor or an inter-turn short circuit. A measurement system based on six individual current sensors has been designed to acquire current data on an industrial motor with high fidelity in order to get a better understanding of the sensitivity of the circulating currents to the different motor states. The selection of appropriate sensors as well as their dielectric layout is discussed in detail. The frequency analysis of the acquired current waveforms yields a rich and complex spectrum, proving the very high sensitivity of circulating currents even to small motor asymmetries.