SCR Failure Mode in Soft-Starters Re-Examined: Controlled Experiments and Simulation

Across the line full-voltage starts of an induction motor produces locked-rotor current and potentially stressful mechanical acceleration while approaching synchronous speed. Reduced voltage starters of several types were introduced to limit the current applied to the motor to effectively lessen this effect and bring the motor shaft up to speed slower while simultaneously lowering sag voltage at the point-of-common coupling. The solid-state silicon controlled rectifier, or thyristor, controller adds stator voltage control options beyond the two-step reduced voltage starting methods and is known in industry as RVSS. However it has some known failure modes linked to the rate of change of current, di/dt, limitations in the SCR device physics where the potential cause has traditionally been linked to amount of load-side parasitic capacitance in the circuit. This paper will further examine how the source and load impedances impact laboratory experiments. Further computer simulations are performed with increased short-circuit available current for comparable results.