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3 point starter working Construction

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The armature resistance of the DC motor is also very small. Therefore if the rated normal voltage is applied to the armature terminals, the starting current would be dangerously high. This high current produces high mechanical stress and also heat, which may damage the winding. 

      Also, a large voltage drop in the supply line affects the other machine working in the same supply line affects the other machines working in the same supply line. Therefore limit the starting current to a safe value, the reduced voltage should be applied to the motor at the time of starting. It picks up speed and develops sufficient back emf, the normal rated voltage could be applied.

To order D.C shunt motors and compound of motors is following starters are used.
  1. 3 point starters
  2. 4 point starters

THREE-POINT STARTER:

     This starter is used to start a DC shunt motor & compound motor. This circuit diagram of a three-point starter connected to supply terminals through three terminals A, B, and C, is called a 3 point starter.

     The resistor elements are mounted on the insulation board. Tapping points of a starting resistance are brought out to a number of studs. The handle of the against spring tension.


     The starter handle is moved to the first stud, a reduced voltage is applied to an armature, due to drop resistor elements. Hence the starting current is limited to a  safe value. At the same time, full voltage is applied across the field this produces normal flux. 

     The handle is moved towards the right, the resistor elements are cut out one by one and the voltage applied to the armature increase a steep by step.

      When all resistance is cut out, the handle is in position, Now full voltage is applied across the armature terminals. A soft iron piece is attached to a handle. Now the handle is attracted by the volt release coil in the electromagnet.




Aconnected to either the positive or negative terminal of DC supply

B connected to either terminal of the Field winding.

C - connected to either terminal of Armature

NVR ( No Voltage Release):

     It consists of an electromagnet that is connected in series with the shunt field circuit and hence it carries a shunt position. In the case of failure of supply the voltage is very low, the no-volt coil gets de-energized and handles files back to an 'off' position. 

OLR (Over Load Release):

     The consists of an electromagnet. The electromagnets are energized by the line current. The load on the motor is increased, the magnetizing force produced by this coil is sufficient to lift the movable iron. The movable iron in turn short circuits the terminals of the No Volt Release. 

     Hence the no-volt coil is de-energized to starter handle returns to the 'OFF' position. Thus Over Load Release protects the motor against overloads.

Demerits:

     A motor can be run for high speed than normal speed by reducing the field current. The reduced field current may not produce enough magnetic force to hold the handle in the 'ON' position.

     The handle returns to the 'OFF' position. This is the disadvantage of a three-point starter. 

     It is eliminated in a four-point starter.


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