A soft starter is a modern motor-control device combining motor soft start, soft stop, light-load energy saving, and multiple protection functions — known internationally as a Soft Starter. Its main composition is a set of three-phase anti-parallel thyristors and their electronic control circuit, connected in series between the supply and the controlled motor. By controlling the conduction angle of the thyristors in different ways, the motor’s input voltage varies as required, delivering different functions.
The soft starter uses three-phase anti-parallel thyristors as a voltage regulator, connected between the supply and the motor stator. The circuit resembles a three-phase fully controlled bridge rectifier.
When starting a motor with a soft starter, the thyristor output voltage rises gradually, the motor accelerates smoothly until the thyristors are fully conducting, and the motor then works on its rated-voltage mechanical characteristic — achieving a smooth start, reducing starting current, and avoiding overcurrent trips.
Once the motor reaches rated speed, the start is complete, and the soft starter automatically substitutes a bypass contactor for the thyristors that have done their job, supplying rated voltage for normal running. This reduces thyristor heat loss, extends the soft starter’s life, improves efficiency, and keeps the grid free of harmonic pollution. The soft starter also provides soft stop — the reverse of soft start: voltage falls gradually and speed decreases to zero, avoiding the torque shock of a coast stop.
