4 point starter working Construction

 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
FOUR-POINT STARTER:

    A four-point starter is used for starting shunt and compound motors.

    A motor can be run for a higher speed than the normal speed by reducing the field current. The reduced field current may not produce sufficient magnetic force to hold the handle in the 'ON' position. So the handle returns to the 'off' position. These are the disadvantages of a three-point starter. This drawback is overcome in Four point starter. To avoid this drawback, in a four-point starter the no-volt coil is arranged such that, NVR supply is independent of the shunt field current. The no-volt coil depends upon the supply voltage. Here the no-volt release coil in series with protective resistor 'Rp' is connected across the terminals L +, L -.

    There are four terminals namely,

4-point starter

  1. L + (Line plus)
  2. L - ( Line minus)
  3. A (Armature)
  4. F (field)
   In this starter, the resistor elements are mounted behind an insulating board. The tipping point of starting resistances is bought out to a number of studs. The handle of the starter is fixed to a point so as to be moved over the studs against spring tension.

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

   When the starter handle is moved towards rights, the resistor elements are cut out one by one, and the voltage applied to the armature increases step by step. When all resistances are cut out the handle is in the 'ON' position. A soft iron piece is attached to the handle. Now the handle is attracted by the No-colt release coil.  (Electromagnet)

NVR (NoVolt Release):

     It consists of an electromagnet (NVR). It is connected directly across the supply line through a protective resistor Rp. It holds the handle in the 'ON' position. In the case of failure of supply or the voltage or the voltage is very low, the no-volt gets de-energized and handle files back to the 'OFF' position. 

OLR (Overload Release):

     This also consists of an electromagnet. The electromagnet is energized by the line current. When 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 starter handle returns to the 'OFF' position. Thus the overload release protects the motor against overloads. 



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