Motor soft starter

At rest, the resistance is relatively low, so the starting or inrush current can be high if the full line voltage is applied to the motor.

[1] Another common side-effect, especially in residential installations, is voltage sag in the site's power supply created by the high inrush current, visible as flickering lights.

Electrical soft starters can use solid state devices to control the current flow and therefore the voltage applied to the motor.

If an air core is used for the series reactor then a very efficient and reliable soft starter can be designed which is suitable for all types of 3 phase induction motor [ synchronous / asynchronous ] ranging from 25 kW 415 V to 30 MW 11 kV.

Using an air core series reactor soft starter is very common practice for applications like pump, compressor, fan etc.

Some soft starters also include a "learning" process to automatically adapt the drive settings to the characteristics of a motor load, to reduce the power inrush requirement at the start.

In all systems, a soft start limits the inrush current and so improves stability of the power supply and reduces transient voltage drops that may affect other loads.