The internal wiring for Three point starter is shown in the fig below. The three terminals of the starter are marked A, F and L. One line is directly connected to point L which is further connected to the starting handle (arm) through the overload release. If motor has to be started, we need to close the main switch and move the starting handle (arm) toward the right. When the starting handle (arm) contacts stud 1, the circuit connects to the line. During the same instant, the complete external resistance R is applied to the armature in series.
The starting current drawn by motor armature = V/ (Ra+Rs)
(Rs stands for the starting resistance)
As the starting handle moves ahead, the starting resistance of the three point starter is slowly removed untill the starting handle comes to its running spot/location. At this location, complete external resistance is removed from the circuit. The starting handle is attached with a spring and it goes over all the (in this case, five) studs against the spring tension. This spring tension always tends to pull the arm to its initial position i.e. OFF contition. However, the arm is under magnetic attraction and held by an energized electromagnet in the running position till availability of the supply line. This electromagnet is also known as ‘Hold ON‘ coil.
When the starting handle moves from stud position 1 to 5, the field current still have to return through the cut off part of the external resistance. This causes a partial devaluation in Ish. This devaluation of Shunt current (Ish) is nullified with the help of a brass arc by connecting it to stud 1 (for this, refer the below figure).
If we take a look on the both above placed figures of three point starter, we can see that there are two protective devices used in the circuit. The basic function of the ‘Hold ON‘ coil is to hold/seize starter handle in its running position during normal running of motor. But if due to any reason the circuit gets opened, then the Hold ON coil gets de-energized. Thus the electromagnetic pull which keeps the starting handle to the ON position against spring tension reduces to zero and so the starting handle is released back to OFF condition.
The over current protection device contains an electromagnet which is provided with the main supply. If the current of the motor increases beyond a certain safe value, then the electromagnet energises and lifts the triangular iron piece. This in turn short circuits the electromagnet. By this process, the electromagnet de-energises and the starting handle (arm) is released. Thus, the arm return back to the initial OFF condition.
The protection of motors are also done by using thermal overload relays. In this method, the thermal overload relay consists of a bimetallic strip. The motor current heats this bimetallic strip at a same approx rate at which the motor is being heated. When the temperature crosses the set temperature range, this relay trips and removes the supply of the motor.