Can anyone tell me how the starter control the starting current in three
phase AC Motors?

The starter can? control the current. What it can do is connect the line voltage to the motor, allowing the motor to pull whatever current it requires to come up to full speed. If the motor has nothing attached to the shaft, it will draw very low amps at full speed. If the motor has a big load, like a forced draft fan with damper wide open, it will draw lots of current. If the fan bearings fail and the load becomes far greater than normal, the motor will draw enough current to overheat. The starter overload protection (if designed in and used) will then open those original contacts and disrupt the current.

Normally starting current of an induction motor is 6 to 7 times of the full load current. This is due to high
inertia of the rotor which draws more current require to move the rotor. So if a resistance is inserted in series with the circuit. For controlling inrush currents for starting only the resistance is gradually reduced. This would reduce the starting current.

There are a few types of starters for AC motor applications:
1. Rotor resistance starter - Is used only for slip rings
in squirrel cage wound rotors so external
resistance cant be added. In starting resistances are included to limit
the starting current.
2. Star delta starter-at the time starting the rotor is
connected as star and while it is in operation it switches to a delta connection.
It can be used for both slip and Squirrel cage IM.
3. Auto transformer starter- at the time starting by using an
auto transformer to reduce the initial voltage.
4. DOL (direct on line) starter - which doesnt reduce any staring current but
connects the motor terminals directly to the power supply.

Thanks dynasty, I was over-simplifying. I was not aware of all these reduced current starting methods, but I will do some reading up on these now. -JP