The soft starter, connected in series between the power supply and the controlled motor, regulates the conduction angle of the internal thyristor. This allows the motor's input voltage to gradually increase from zero according to a preset function until it reaches full voltage at the end of the start-up phase. During this process, the motor's starting torque is smoothly increased, and its rotational speed rises progressively. Soft starters typically offer several starting modes, each designed for different application needs.
One of the simplest methods is the **ramp-up soft start**, where the thyristor's conduction angle is adjusted over time without current feedback control. However, this method can result in high inrush currents during startup, which may damage the thyristor and cause disturbances on the power grid. Due to these drawbacks, it is rarely used in real-world applications.
Another common mode is the **ramp constant current soft start**. In this approach, the starting current increases gradually at first and then stabilizes once it reaches a set value. This method provides better control over the motor's acceleration and is widely used, especially for fans and pumps where smooth and controlled starting is essential.
The **step start** method involves rapidly increasing the starting current to a predefined level in the shortest possible time. This allows for a quick start-up, making it ideal for applications that require fast response times.
Lastly, the **pulse shock start** technique briefly applies a high current pulse at the beginning of the start-up, followed by a linear rise to the set current level. This method is particularly useful for heavy-load applications where overcoming significant static friction is necessary.
Each of these methods has its own advantages and limitations, and the choice depends on the specific requirements of the motor and the system it is part of. Understanding these differences helps engineers select the most appropriate soft start solution for their application.
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