Soft starter VS variable frequency drive

Soft Starter reduces electric motor starting current to 2-4 times during motor start up, reduces the impact to power grid during motor start up, avoid the motor being burned out, and provide protection in motors running process.

Variable Frequency Drive allows the electric motor smooth start up, control startup current growing from zero to motor rated current, reduce impact to the power grid and avoid the motor being burned out, also provide protect in  motor running process. Besides these functions, the main function of variable frequency drive is adjusting the motor running speed according to actual operation conditions, to achieve energy saving effect.

So, from the function side, variable frequency drives are much better than soft starters.

One essential difference between a soft starter and a VFD in this regard is, that the VFD delivers "nearly" sinusoidal voltages (and currents) to the motor, which makes it possible to develop high starting torques during the acceleration, even higher than nominal full load torque, depending on the application, while a soft starter only supplies fractions of the basic waveform, which serves to reduce the current to the motor significantly, but still at the nominal frequency. This will reduce the available starting torque dramatically until the motor is up to around two-thirds of nominal speed, or maybe even higher.