Adverse effects of starting-torque transients and high inrush currents in induction motors are typically mitigated by employing electronically controlled soft starting voltages through silicon controlled rectifiers (SCRs). However, the heat dissipation in the soft starter must be carefully managed in the design of motor drives. The objective of this study is to address the heat dissipation in the soft starter by implementing analytical solutions to the heat diffusion equations inside the soft starter. The transient analytical thermal model allows an estimation of the thermal system transfer function from the transient thermal impedance curve, and can be incorporated into a dynamic system model in order to determine the transient performance of a soft starter by evaluating the thyristor junction temperature for different switching time profiles, motor and load combinations, and “ON/OFF” cycles. Predictions from the model are validated by comparing against a coupled thermal and electrical model using a resistance/capacitance network approach.

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