Three point starter:
figure shows the schematic diagram of a three point starter for a shunt motor. It is so called because it has three terminals L, F and A. the starter consist of starting resistance divided into several sections and connected in series with the armature. The tapping points of the starting resistance are brought out to a number of studs. The three terminal L, F and A of the starter are connected respectively to the positive line terminal, shunt field terminal and armature terminal. The other terminal of the armature and shunt field windings are connected to the negative terminal of the supply .the no volt coil is connected in the shunt field circuit. One end of the handle is connected to the terminal L through the over load release coil. The other end of the handle moves against a spiral spring and make contact with each stud during starting operation cutting out more and more starting resistance as it passes over each stud in clockwise direction.
Working of three point starter:
1) Initially when a DC supply is switched ON with handle in the OFF position.
2) The handle is now moved clockwise to the first stud, the shunt field winding is directly connected across the supply while the whole starting resistance is inserted in series with the armature circuit.
3) As the handle is gradually moved over to the final stud, the starting resistance is cut out of the armature circuit in steps. The handle is now held magnetically by the no volt release coil which is energized by shunt field current.
4) If the supply voltage is suddenly interrupted or if the field excitation is accidently cut, the no volt release coil is demagnetized and the handle goes back to the OFF position under the pull of the spring. If no volt coil were not used, then in case of failure of supply. The handle would remain on the final stud. If then supply is restored, the motor will be directly connected across the supply, resulting in an excessive armature current.
5) If the motor is overloaded (or any fault occurs) it will draw excessive current from the supply. This current will increase the ampere turns of the overload release coil and pull the armature, thus short circuited the no volt release coil. The no volt coil is demagnetized and the handle is pulled to the OFF position by the spring .thus the motor is automatically disconnected from the supply
Limitations of the three point starter:
Unnecessary tripping of starter if a field regulator is connected in series with the field windings for speed control operation. This combination and magnetizing coil will carry same current if more and more resistance is added in field regulator then current in magnetizing coil is unable to hold the arm in attracted position and unnecessarily the arm will be released even if there is no over loading .this is avoided in four point starter.
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