With a soft starter connected in series between the supply and the controlled motor, its internal thyristor conduction angle is controlled so the motor’s input voltage rises gradually from zero along a preset function until full voltage is applied at the end of starting — that is soft starting. During the process, starting torque and speed rise progressively. Soft starters generally offer the following starting methods.
(1) Ramp-voltage soft start. The simplest method, without current closed-loop control: it only adjusts the thyristor conduction angle as a fixed function of time. Its drawback is that, without current limiting, large surge currents during starting can damage the thyristors and disturb the grid — it is rarely used in practice.
(2) Ramp constant-current soft start. In the initial stage the starting current rises gradually; once it reaches the preset value it stays constant (stage t1 to t2) until starting ends. The current rise rate can be set according to the motor load: a faster rise means greater starting torque and shorter starting time. This is the most-used method, especially for fan and pump loads.
(3) Step start. At switch-on, the starting current reaches the set value in the shortest possible time — a step start. Adjusting the current setpoint achieves a fast-start effect.
(4) Pulse-kick start. At the beginning, the thyristors conduct with a large current for a very short time, then fall back and rise linearly along the original setting into constant-current starting. Rarely used for general loads; it suits heavy-duty starting that must overcome large static friction.
