Soft starter working principle - Solutions - Huaqiang Electronic Network

The main circuit power device of the soft starter is SCR thyristor. The working principle of the soft starter is to control the output signal three-phase AC voltage by controlling the conduction angle of the trigger signal of the thyristor. The most common fault of the soft starter is that the main circuit thyristor is damaged, and the thermal effect of the short circuit current is the main cause of the damage of the thyristor. The thermal effect of the short-circuit current is represented by the I ² t value, and the current value during short-circuit and the duration of the short-circuit are important reference parameters for short-circuit protection. After the SCR triggers the conduction, there is no self-shutdown capability. If the thyristor is naturally switched off, the longest short-circuit shutdown delay time will reach half AC cycle, which is 10mS for the 50Hz power frequency grid. The short-circuit overcurrent withstand capability of the thyristor is generally not as strong, so the soft starter requires short-circuit overcurrent protection from an external device.