Soft starter and frequency inverter

The difference between line starting a motor and ramp starting the motor with a frequency inverter is that the motor/load does not pull the 6-7 times rated current of the motor, because the motor winding are not saturated with the full EMF produced to get the motor to synchronous speed it is ramped to it. If you are not trying to control the motors speed from process control then a soft start will serve the same purpose. The inverter main purpose is to control the V/F of the motor.

While you can use a frequency inverter to soft start a motor, it's expensive compared to a solid state starter if you intend to run the load a full speed all the time and there is no need for flow or speed control. A solid state starter achieves the same current limit and reduced water hammer effects by controlling just the voltage to the motor, typically using thyristors. Many soft starters have a contactor that bypasses the thyristor bridge once the motor gets to full speed.

You will have to adjust the ramp time on the inverter drive or the soft starter to over the force required to turn whatever the motor is turning, this can be accomplished with both devices. Soft starter is less expensive than frequency inverter, thus it has limitations.

If only required to limit the current during startup is not necessary to purchase a frequency inverter, the soft starter is your choice which is cheaper.