As previously discussed, the star delta motor controller provides a changeable motor wiring configuration which is made possible by the sequential switching of magnetic contactors each wired accordingly with a star and a delta connection directly to the motor terminals.
When the motor terminal connection is switched to the delta mode after running in the star mode, a time gap exists in between this transition period wherein high inrush transient current generated from residual voltage is developed during the changeover delay. This intense current spikes can cause extreme over current to overload the motor or damage the magnetic contactors.
To prevent such an occurrence, the best way is to follow an orderly wiring configuration most suitable to achieve a reduced current peaks during the star to delta switching transition period. The delta wiring configuration U1-V2-L1, V1-W2-L2, W1-U2-L3 in Fig-1 achieves the best desirable result of reducing high inrush current upon switching of the magnetic starters from star (wye) to delta connection.
It is therefore a recommended practice in the field of industrial process automation control technology to follow a wiring configuration suitable for a reduced high changeover transient current. The best method to achieve a minimal inrush peak current during when the motor transitions from star to delta connection is the one in accordance to the conforming orderly configuration of the reference voltage's phase sequence with respect to the motor terminal connection illustrated accoridngly in Fig-1, Fig-2 and Fig-3.