A soft starter is a solid state voltage controller designed to reduce the start current and start torque applied to an induction motor during start.
Soft starters reduce the start current and the start torque applied to an induction motor during start. The reduction in start current is achieved without any steps or transients and this reduces voltage interference and mechanical damage.
An induction motor has a very high Locked Rotor Current (LRC or LRI) and when the motor is full voltage or across theline started, it draws the locked rotor current initially. The start current then falls gradually until the motor reaches around 90% of full speed. This results in a severe overload current of 600% - 900% FLC for the duration of the start causing voltage droop on the supply. The start torque is also highenough to cause damage tosome planet and equipment.
The use of a soft starter reduces the start current down to a lower level and also reduces the start torque to minimise damage to the machanical driven load and to the electrical supply.
Soft starters can be used anywhere that there are induction motors that do not require their full voltage torque to start the driven load.
Where a very high start torque is required, the soft starter may not be the best option. A secondary resistance starter or a VFD may be a better option.