Soft starters and variable frequency drives are two different purpose products. VFD is for AC motor speed control, it's not only change the output voltage but also change the frequency; Soft starter is a regulator actually for motor starting, just changing the output voltage.
Simply so, why we are using soft starters?
A motor soft starter is a device used with AC electrical motors to temporarily reduce the load and torque in the power train and electric current surge of the motor during start-up.
What is the soft start?
A soft starter is a solid-state device that protects AC electric motors from damage caused by sudden influxes of power by limiting the large initial inrush of current associated with motor startup. They provide a gentle ramp up to full speed and are used only at startup (and stop, if equipped).
What is an SCR in a soft start?
SCRs in reduced voltage. soft starters. Application. A typical three-phase soft starter uses six SCRs oriented in an anti-parallel configuration to provide start and run control to industrial motors. The most common mode of SCR failure is the SCR shorting.