3 Point Starter Working principle, Diagram

3 point starter is used for shunt and compound dc motor. it is called a three point starter because it has three terminals named L, F, and A. the terminal named L must be connected to either side i.e the positive or negative side of the DC supply. The terminal A must be connected to the one of motor armature terminal and F terminal to the shunt field terminal of the motor (any of two field terminals). The final connection must then be made from second-line terminals on the main switch to a junction of the remaining two armature and field terminals of motors.

3-Point Starter Diagram
3-Point Starter Diagram

Construction of 3-Point Starter

When the motor is at rest, a three-point starter handle "H" is placed at the "OFF" position by a strong spiral spring, the stud marked with OFF indicates the motor disconnected from the main supply.  and the starting resistance connected between studies 1, 2,3,4...6. the circuit diagram is shown in the above figure.

  • "L" line terminal connected to the supply
  • "A" Armature terminal connected to the armature winding
  • "F" Field terminal connected to the field winding of dc motor
  • "HC" Holding coil
  • "OR" Overload Release
  • "H" Handle
  • "M" Movable Soft Iron

Working Of Three point starter

For starting the motor handle "H" moved to the first stud indicated with 1. As soon as 3 point starter handle H touches stud 1. the shunt field and holding coil HC get connected in series across supply, On the other hand, the armature gets connected in series with entire starting resistance.

Since the current start flowing through both field and armature winding. The motor starts rotating. After attaining sufficient speed the handle H shifted to stud 2, Hence it cut down the resistance between stud 1 & stud 2. The movement of three point starter handle continued till the soft iron keeper touches to the holding magnet. During the shifting of the handle from stud 1 to toward the stud 6,  the armature current decreases, and the speed of the dc motor rises exponentially.

In the final position, the electromagnetic pull exerted by holding coil will be greater than the force exerted by the spiral spring. In this view, the handle "H" is held in ON position and the entire starting resistances are cut out. The holding coil HC is also called a NO-Volt release or low volt release.

Function of the No-Volt release Coil HC

  • In the case of power failure, electromagnet HC get demagnetize and the spiral spring bring back handle (H) to OFF position. In case of the power failure, the handle fails to return to the OFF position, The motor maybe get damaged if power restored because there would be no starting resistance in the circuit.

  • If the shunt field becomes open circuited accidentally HC gets demagnetize and handle H returns to OFF position because of spring pull. In case the starter handle not released, speed starts increasing with the reduction in field flux. the small residual flux result in highly dangerous speed and the motor might be get damaged.

Another protection device used in 3 point starter is the overload release which is connected in series with an armature circuit, Overload release  "OR" is a small electromagnet, In case of armature current exceeds a present value due to overload "OR" become more strong and attract the movable soft iron "M" which is pivoted at one end. 

As soft iron is attached to two terminals of HC coil a & b. Once OR attracted soft iron, terminal a & b of NO volt release coil get short circuited. In this manner HC coil get demagnetized and  Three point starter handle pulled back to OFF position by spiral spring and the motor gets disconnected from the supply.

The overcurrent on which overload release should operate can be adjusted by adjusting the distance between the electromagnet and movable soft iron M.

Disadvantages Of 3 point starter

  • Three point starter experience serious drawback for application of motor with a large variation of speed by adjusting the field rheostat.
  • If the speed above normal is to be required, the field current must be reduced. At a certain value of decrease field current, the electromagnetic pull of the holding coil (HC) becomes too weak than the spiral spring force, and handle return to the OFF position and motor stop.
  • So due to this application of  3 point starter are restricted and these problems overcome in 4 point starter.

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