The soft-starter basically consists of an electronic device formed by thyristor-type bridges in the anti-parallel configuration, which are gradually driven by an electronic board, which is intended to control and optimize the starting current of three-phase AC electric motors . Its use is common in centrifugal pumps, fans and motors of high power, in which the application does not need the variation of speed.
The soft-stater controls the voltage across the motor through the power circuit, consisting of six SCRs, which can vary the firing angle of the motors, thus varying the optimized voltage applied to the motor.
In this way, it is possible to control the starting current of the motor, allowing a subtle starting, in English called soft start, so that it does not cause sudden electrical voltage drops in the mains, as in direct starters.
They usually work with the technology called by-pass, that after the motor starts and receives all the voltage of the network must be connected a contactor that replaces the thyristor modules, thus avoiding overheating them. https://www.mrosupply.com/electrical/transformers/884495_ae030050_acme/