The timer controls conversion from star connection to delta connection. In this video we explain star delta connection in 3 phase motor terminal it is very important topic because we can easily make star delta starter but in motor terminal connection we have some.

A2a Wiring Diagram Of Star Delta Motor Starter Wiring Resources
3 phase motor star and delta connection. A star connection is a 4 wire system and a delta connection is a 3 wire system. Less commonly there is also an open delta connection where two single phase transformers are used to provide a three phase supply. Then once the motor reaches the required speed the motor is connected in through a delta connection. Therefore it needs a low number of turns hence saving in copper. This is similar to applying 380 volt to 220 v windings so clearly the motor would fail. In delta connection motor receives highest power output.
In star connection the phase voltage is low as 13 of the line voltage. Star and delta connections are the two types of connections in a 3 phase circuits. A star delta starter is the most commonly used method for the starting of a 3 phase induction motorin star delta starting an induction motor is connected in through a star connection throughout the starting period. If the motor is designed to run in star a 380v 3 phase power supply then it cannot be connected in delta on the same supply. Before going in to details of the star connection delta connection and comparing those two let us have a very. Delta or mesh connection δ system is also known as three phase three wire system 3 phase 3 wire and it is the most preferred system for ac power transmission while for distribution star connection is generally used.
A timer in star delta starter for a 3 phase motor is intended to do the move from star mode utilizing which the motor runs on a decreased voltage and current and produces less torque to the delta mode indispensible for running the motor at its full power utilizing. These are generally only used in emergency conditions as their efficiency is low when compared to delta delta closed delta systems which are used during standard operations. In delta connection the phase voltage is equal to the line voltage hence it needs more number of turns which increase the total cost.