软起动器和变频器是两种完全不同用途的产品。变频器是用于需要调速的地方，其输出不但改变电压而且同时改变频率；软起动器实际上是个调压器，用于电机起动时，输出只改变电压并没有改变频率。变频器具备所有软起动器功能，但它的价格比软起动器贵得多，结构也复杂得多。软起动器(软启动器)是一种集电机软起动、软停车、轻载节能和多种保护功能于一体的新颖电机控制装置，国外称为Soft Starter。软启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。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。
Soft starter and frequency converter are two kinds of products with different uses. The inverter is used for the need for speed place, its output voltage and frequency change not only change; the soft starter is actually a regulator for motor starting, output voltage and frequency change not only change. The inverter has all the soft starter functions, but its price is much more expensive than the soft starter, and the structure is much more complex. Soft starter (soft starter) is a new motor control device, which integrates soft start, soft stop, light load, energy saving and multiple protection functions. It is called Soft Starter abroad. The soft starter adopts the three-phase anti parallel thyristor as the voltage regulator, and connects it between the power supply and the motor stator. This circuit, such as three-phase full controlled bridge rectifier circuit. 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.
