The figure above shows that typical representation diagram of a 3 point starter for DC shunt motors with its protective devices. It contains 3 terminals namely L, Z, & A; hence named 3 point starter. The starter is made up of of starting resistances divided into many section and which are connected in series within the armature. The each tapping point on the starting resistances is carried out to a no. of studs. The starter 3 terminals L,Z & A are connected to the positive terminal of line, shunt field and armature terminal of motor respectively. The remaining terminal of the shunt and armature are connected to the negative line terminal. The No volt coil release is connected in series with field winding. The handle one end is connected to the L terminal by means of over load release coil. Then another end of handle travels against the twisting spring & make touching base with every single stud in the course of starting operation, tripping out the starting resistance as it moves above every stud in clockwise.
- Initially the DC supply is turned on with the handle is in OFF position.
- Now the handle is moved towards clockwise direction to the 1st stud. Once it contacts with the 1st stud, immediately the shunt field coil is connected to the supply, however the entire starting resistances is injected with armature circuit in series.
- As the handle moved gradually towards the final stud, so that the starting resistance is cut out step by step in armature circuit. And finally the handle is detained magnetically by the No volt coil release since it is energized by the filed winding.
- In case if the shunt field winding excitation is cut out by accident or else the supply is interrupted then the no volt coil release gets demagnetized and handle returned back to the original position under the influence of spring.
Note: If we were not used No volt coil release; then if the supply is cut off the handle would remain in the same position, causing an extreme current in armature.
- If any fault occurs on motor or overload, it will draw extreme current from the source. This current raise the ampere turns of OLR coil (over load relay) and pull the armature Coil, in consequence short circuiting the NVR coil (No volt relay coil). The NVR coil gets demagnetized and handle comes to the rest position under the influence of spring. Therefore the motor disconnected from the supply automatically.
In point starter, no volt relay coil is connected in series with field circuit; hence it carries shunt current in the field. When the speed control of DC motor through field regulator, it may be weakened the shunt field current to such extent the no volt coil release might not in a position to hold the starter handle in ON position. This might the motor disconnected from the source when it is not anticipated. This can be overcome by using the point starter.