The 3RH1921-1FA31 is a high-performance soft starter from the Siemens SIRIUS 3R series. It is used to smoothly start and stop three-phase AC induction motors by controlling the voltage applied to the motor, effectively reducing inrush current and mechanical stress during startup, thereby extending the service life of the motor and connected equipment. This model features an integrated bypass contactor. Once the motor reaches full speed, the thyristors within the soft starter are bypassed, and the motor is directly supplied via mechanical contacts, reducing losses and heat generation in the soft starter itself.