3 Point Starter

 THREE POINT STARTER

 NECESSITY
  • To limit high starting current.
  • To prevent motor from under voltage, no voltage and over voltage.
CIRUIT DIAGRAM

             3 point starter


CONSTRUCTION

    It consist of a variable resistance having a number of contact points Off.1.2.3.4.5 called as Studs.
      There are three terminals  in 3-point starter:    
        
1)      ‘L’ Line terminal: (Connected to positive supply)      
2)      ‘A’ Armature terminal. (Connected to armature winding)  
3)      ‘F’ Field terminal (Connected to field winding)

  • Point L is further connected to electromagnet called OLR (overload release)  
  • The second point of OLR is connected to the starter handle.
  • The handle is free to move on other side towards RUN against the force of spring A  soft iron piece is attached to handle.
  • A parallel path is derived from stud 1 to No Voll coil (NVC) and other end of NVC is connected to ‘F’ terminal.
  • The Overload release (OLR) and No volt coil (NVC) are two protective devices.


OPERATION


    The handle of the 3 point starter can be moved from one stud to another stud (contact positions), and this increases the speed of the motor till gets the RUN position. There are three main points are considered in this position which includes the following.
  • The DC shunt motor gets the full speed
  • The voltage supply in the circuit is straight across both the motor’s windings.
  • The R-resistance is totally cut-out.
   The H-handle in the circuit is held in RUN condition with an electromagnet strengthened by an NVC (no volt trip coil). This NVC coil can be coupled in series with the motor field winding. In the incident turned OFF or dropped below a fixed value, then the NVC will get energized. By the act of S-spring, the handle-H is released as well as pulled back to the OFF condition.
    At first when a DC supply is turned ON by H-handle in the OFF position, then the handle will move clockwise direction to the Stud1. The winding of the shunt field is directly associated across the voltage supply as the total resistance, in the beginning, is included in series with the armature circuit.
  If the voltage supply is unexpectedly disrupted, then the no-volt discharge coil is demagnetized as well as the H-handle goes back to the OFF location in the pull of the spring.
   If the DC motor is overloaded, it will draw extreme current from the current supply, then it amplifies the ampere rotates off the excess release coil as well as pull the armature, therefore no-volt coil will be short-circuited. This coil is demagnetized as well as the H-handle is pulled near the OFF location by the S-spring. Hence the electric motor is automatically detached from the current supply.

DRAWBACKS

  • Under speed control by using field weakening method, this starter (3-point starter) is failed by turning OFF.
  • Since no extra resistance is present in series with the field winding speed regulation is not possible.
  • During a fault, condition circuit may get damaged as there is no extra protective resistance present.
  • In a three-point starter, NVC is linked directly with the shunt field and high resistance, so there is a chance that NVC is unable to operate if there is a decrease in current excessively.
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