There are two very basic ways of determining the correct point at which the control gear should change the winding configuration from 'star' (starting mode) to 'delta' (running mode). Both methods determine the point at which the rotor has achieved it's maximum speed in star, and therefore the point at which the starter should change to delta.
One is by current measurement: Firstly, set the delta timer to it's maximum setting. Put a clip on (tongue tester) ammeter on one of the lines FEEDING the starter. Set a stopwatch going when you press the start button. Watch the ammeter - it will peak immediately on start up, then the current will drop off as the load accelerates. As soon as the current steadies off, stop the watch. At this point (approximately 85% full load speed) the motor can achieve nothing more by remaining in star, and this is the latest point in time that delta changeover should be made.
The other simple way to determine optimum change over time is to use a tachometer on the motor shaft (mechanical or optical). Again, use a stop watch. Observe the Tacho. Acceleration characteristics will vary dependent on the driven load, but the speed will settle out (again at approx. 85% full load speed. Stop the watch at this point. Set the timer to the time recorded on the stop watch.
In star Delta usually we consider that the load and motor torque are equal when the motor reach 75-85% of the speed. At this time you stop the star connection to go with delta connection, the time is around 30 to 50 milliseconds, to avoid short-circuit. But when the motor is connected in delta, at start you've got a temporary current peak due to back electromotive force.
So to avoid this you have different option, by these, you can delay of 1 or 2 seconds the time between star and delta, but it depend on inertia of the load, another solution is to put a resistor in series with the delta connection for 3 seconds to limit the current peak, another is to put the resistor directly in series with the motor at star disconnection.
These methods determine the LONGEST period a motor can be left in 'star'. High inertia loads should be allowed to reach this point.
Different applications i.e fan motors or centrifugal pump motors which run up to speed with closed dampers or valves can be switched into 'delta' much earlier, as the only load being accelerated is the physical mass of the impeller or blade.