软启动器采用三相反并联晶闸管作为调压器，将其接入电源和电动机定子之间。这种电路如三相全控桥式整流电路，主电路图见图1。使用软启动器启动电动机时，晶闸管的输出电压逐渐增加，电动机逐渐加速，直到晶闸管全导通，电动机工作在额定电压的机械特性上，实现平滑启动，降低启动电流，避免启动过流跳闸。待电机达到额定转数时，启动过程结束，软启动器自动用旁路接触器取代已完成任务的晶闸管，为电动机正常运转提供额定电压，以降低晶闸管的热损耗，延长软启动器的使用寿命，提高其工作效率，又使电网避免了谐波污染。软启动器同时还提供软停车功能，软停车与软启动过程相反，电压逐渐降低，转数逐渐下降到零，避免自由停车引起的转矩冲击。软启动与软停车的电压曲线见 2 软启动器的选用 （1） 选型：目前市场上常见的软启动器有旁路型、无旁路型、节能型等。根据负载性质选择不同型号的软启动器。 旁路型：在电动机达到额定转数时，用旁路接触器取代已完成任务的软启动器，降低晶闸管的热损耗，提高其工作效率。也可以用一台软启动器去启动多台电动机。 无旁路型：晶闸管处于全导通状态，电动机工作于全压方式，忽略电压谐波分量，经常用于短时重复工作的电动机。 节能型：当电动机负荷较轻时，软启动器自动降低施加于电动机定子上的电压，减少电动机电流励磁分量，提高电动机功率因数。 （2） 选规格：根据电动机的标称功率，电流负载性质选择启动器，一般软启动器容量稍大于电动机工作电流，还应考虑保护功能是否完备，例如：缺相保护、短路保护、过载保护、逆序保护、过压保护、欠压保护等。
The soft starter adopts three-phase anti parallel thyristor as voltage regulator, which is connected between the power supply and the motor stator. This circuit, such as three-phase full controlled bridge rectifier circuit, the main circuit diagram is shown in figure 1. Start the motor using the soft starter, the thyristor output voltage increases, the motor accelerates gradually until the thyristor wholeconduction motor working in the mechanical properties of rated voltage, to achieve a smooth start, reduce the starting current, avoid starting over-current trip. When the motor reaches the rated speed, the startup process end, soft starter thyristor automatic bypass contactor with substituted completion pipe, provide the rated voltage of the normal operation of the motor, to reduce the heat loss of the thyristor soft starter, prolong the service life, improve the work efficiency, but also to avoid network harmonic pollution. The soft starter also provides soft stop function. The soft parking is opposite to the soft start process. The voltage decreases gradually and the revolution gradually drops to zero, avoiding the torque impact caused by free parking. The voltage curve of soft start and soft stop can be seen by the selection of 2 soft starter (1) type selection: the common soft starter on the market has bypass type, no bypass type, energy saving type, etc.. Different types of soft starters are selected according to the load characteristics. Bypass type: when the motor reaches the rated speed, the bypass contactor is used instead of the soft starter which has completed the task to reduce the heat loss of the thyristor and improve its working efficiency. You can also start multiple motors with a soft starter. No bypass type: thyristor in full conduction state, the motor operates in full voltage mode, ignoring the voltage harmonic component, often used for short time repetitive work of motor. Energy saving type: when the motor load is light, the soft starter automatically reduces the voltage applied to the motor stator, reduces the motor current exciting component, and improves the motor power factor. (2) selection specification: according to the nominal power of the motor current load selection starter, soft starter is generally slightly larger than the capacity of the working current of the motor, but also should consider the protection function is complete, such as: lack of protection, short-circuit protection, overload protection, reverse protection, overvoltage protection, undervoltage protection etc..
