Start-up monitoring of direct-on-line starting high-power synchronous machines with a real-time thermal model

High-power salient-pole synchronous motors utilized in high inertia compressor applications, e.g. in the air separation industry usually exhibit long starting times.During asynchronous starts with high run-up time, there is a high thermal load induced in the rotor and motor protection becomes more challenging. To prevent damage from the rotors a strict protection regime accounting for repetitive starts and necessary waiting times is required.State-of-the-art protection schemes often suffer from the fact, that they are based on stator-current-time characteristics, i.e. the critical rotor temperature is only accounted for indirectly.Here an alternative approach to protection is proposed, which relies on a real-time thermal model of the rotor supported by measurements of the rotor-surface temperature. The need for further external sensors is minimized with a new method, which determines speed directly from temperature measurements. With this approach a much more precise surveillance of critical rotor temperature is possible, resulting in enhanced protection with maximized utilization of rotor thermal capacity and minimized waiting times. Measurements are presented to underline the function of the protection scheme.