Synchronous Motors: Methods for Quantifying Rotor Condition

Synchronous motors play a key role in the operation of a large cement and aggregate facility. Advancement in operating practices tends toward predictive maintenance and increased time between planned outages. As end users extend the continuous duty cycle of synchronous motors to intervals of more than one year, the ability to accurately predict motor condition becomes critical. Although significant literature exists on stator testing, little information is available to quantify the life of a synchronous rotor. Synchronous motors with salient poles 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 (PF), and this additional rotor current increases heating, which accelerates the cumulative damage. This article 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.