Influence of Excitation Resistance and Synchronization Conditions on the Transient Performance of a Synchronous Motor during Asynchronous Starting

Synchronous motors are widely used in industrial and power-system applications because of their high efficiency, controllable power factor, and excellent steady-state performance. However, their starting process remains a major challenge due to the high transient currents and torque oscillations that occur before synchronization. This study presents a numerical investigation of the asynchronous starting of a synchronous motor using the MATLAB/Simulink environment. A detailed simulation model of an 85 kVA, 400 V, 50 Hz synchronous motor was developed to evaluate its transient and steady-state behavior under different excitation resistance configurations. The influence of additional excitation resistances on excitation voltage, excitation current, electromagnetic torque, stator current, rotor speed, and direct- and quadrature-axis currents was analyzed. The results show that the insertion of an appropriate starting resistance significantly reduces transient current peaks and torque oscillations during startup. Furthermore, applying the excitation voltage when the rotor speed approaches synchronous speed improves synchronization stability and reduces electromechanical stresses. Among the investigated configurations, a starting resistance approximately five times greater than the field-winding resistance provides the most effective balance between voltage limitation and dynamic performance. The proposed simulation approach provides a practical tool for the analysis and optimization of synchronous motor starting strategies in industrial applications.