A coupled thermal and electrical time-domain soft starter system model

A coupled thermal and electrical dynamic system model was developed to aid in determining the transient ratings of a medium voltage soft start motor starter for various motor and load combinations and cycles (start-run-stop). This model includes the medium voltage (4160 V) power circuit, the control algorithms (for a soft start scheme), the induction machine model, and a detailed transient thermal model. The formulation for the transient thermal model is based on the electrical analogy for thermal resistance, thermal capacitance and airflow. Soft starters can significantly attenuate the high torque transients and inrush currents that can occur with direct line starting of induction machines. The soft start scheme reduces the effective RMS voltage applied to the motor by controlling thyristors to switch "ON" at specific times during each half cycle. Average power thermal simulations and full time-domain simulations have been completed for the soft starter connected to a low voltage 200 hp induction machine. Experimental results provide verification for the thermal model and the dynamic electrical system. Additionally, a medium voltage 1000 hp induction machine was modeled.