A soft starter is, as you would expect, a device which ensures a soft starting of a motor. A softstarter has different characteristics to the other starting methods. It has thyristors in the main circuit, and the motor voltage is regulated with a printed circuit board.
Soft starters are based on semiconductors. Via a power circuit and a control circuit, these semi-conductors reduce the initial motor voltage.
Frequency converters are designed for continuous feeding of motors, but they can also be used for start-up only.
The frequency converter makes it possible to use Low starting current because the motor can produce rated torque at rated current from zero to full speed. Frequency converters are becoming cheaper all the time.