Methods for quantifying rotor condition in synchronous motors

Synchronous motors play a key role in the operation of a large cement and aggregate facility. Advancement in the operating practices of the industry tends towards predictive maintenance and increasing time between planned outages. As end users extend the continuous duty-cycle of their synchronous motors to intervals above 1 year, the ability to accurately predict motor condition becomes critical. While significant literature exists on stator testing, comparatively little information is available to quantify the life of a synchronous rotor. Synchronous motors with salient poles will operate reliably for 25 years or more with proper care, but even the most durable motor will suffer rotor winding insulation breakdown after extended operation. As weak short-circuits in pole windings develop, increased excitation is required to maintain a given power factor and this additional rotor current increases heating, which accelerates the cumulative damage. This paper presents a selection of online and offline testing methods to provide the user with practical guidance on procedures and acceptance criteria to effectively quantify rotor turn to turn insulation. These methods provide a predictive tool to plan necessary repairs while maximizing the operational life of a synchronous motor.