A three-point starter is a device that helps in starting and running the shunt wound motor or compound wound DC motor.
The back EMF (Eb) plays a crucial role in governing the operation of motors.
The back EMF (Eb) is developed as motor armature starts to rotate in presence of magnetic field, so initially, back EMF (Eb) is zero.
For motors, V = Eb + IaRa. Since Initially Eb = O. so Ia =V/Ra
By above equation, we can say that starting current is dangerously high as Ra (armature resistance) is small.
Why we need starter on ship electrical system
To limit high starting current
To prevent motor from under voltage, no voltage and over voltage
Basic Construction of three-point starter
Consist of a variable resistance. having a number of contact points Off.188.8.131.52.5 called as studs.
There are three terminals in 3-point starter:
‘L’ Line terminal: (Connected to positive supply)
‘A’ Armature terminal. (Connected to armature winding)
‘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 of ship three-point stater
The handle is in OFF position:
When power is switched on to the motor. The handle is slowly moved to stud ‘1’ against the spring force, the following will happen:
Field winding will get supply via parallel path provided through stud 1 and NVC.
Entire starting resistance is in series with the armature and limits the starting current.
As the handle moves further to stud 2,3,4,5 and finally RUN position. It bypasses the starting resistance and the motor rotates at normal speed.
NVC coil holds the starter in RUN position so it is also called as Hold on the coil.
Functions of No Volt Coil (NVC)
The field winding is supplied through NVC and field current makes it an electromagnet
When the handle is at the RUN position, the soft iron piece on handle gets attracted by the magnetic force produced by NVC.
Whenever there is supply failure or field supply is broken then NVC loses its magnetism and unable to hold the handle. The spring action brings back the handle to OFF position.
NCV perform the similar action during low voltage condition and Save the device.
Main Functions of Ship’s Overload release (OLR)
The motor current is supplied through OLR coil, which makes it an electromagnet
Below the OLR coil, there is an arm which is fixed at its fulcrum or lying horizontally
At the end of the arm, a small triangular iron piece is fitted which is in the proximity of two ends of the shorting cable of NVC.
It is so designed that, till the full load current OLR coil magnetism and gravitational force are balanced and OLR is unable to lift the lever.
Whenever motor draws high current the magnetism of the OLR coil pull the arm and triangular piece of the arm shorts both point of NVC coil.
NVC coil loses its magnetism and leaves the handle. the handle than retracts back to OFF position because of spring action. The motor will stop.
Drawbacks of ship Three-Point Starter
To increase the speed of the motor field resistance can be increased.
This will reduce the amount of current through shunt field.
As NVC is connected in series with shunt field, this small current will reduce the strength of the electromagnet
This holding magnet may release the arm of the handle during normal operation of the motor and disconnect it from the supply.
This can be overcome by Four-Point Starters, where NVC ‘is connected in the parallel field.