3. Soft starter or frequency inverter?

A frequency inverter is often used to achieve a soft start for asynchronous motors / three-phase motors. Here, the speed can be slowly increased to the full value using a programmable ramp function. In contrast to reducing the voltage with a soft starter, the motor has a high torque even at low speeds when operated with a frequency inverter. The speed can therefore also be increased very slowly, e.g. to cope with large moments of inertia. The torque may briefly have higher values than during nominal operation so that starting under full load is also possible. This is important when starting compressors, for example, if they have to start up against pressure that is still present.
Another advantage of frequency inverters is the ability to vary the speed of the motor during operation and also to slowly reduce the speed or brake the motor in a controlled manner when switching off, depending on requirements. A frequency inverter is technically much better. If only a reduced starting current is required without a load, a soft starter has the advantage for cost reasons alone.