Four and Two Point Starter:
FOUR POINT STARTER
The difference between 3 point and 4 point starter is the connection of NVC. In three point starter, NVC and the field winding are connected is series.
In four point starter, NVC is connected across the supply through the fourth terminal called ‘N’. The tour terminals are L F. A and N.
Due to this, any change in the field current does not affect the performance of the NVC. Hence it is ensured that NVC produces a force which is enough to hold the handle in ‘RUN position. Hence a current through NVC is adjusted with the help of fixed resistance R. This resistance ‘R’ is connected in series with the NVC using fourth terminal ‘N’.
It does not provide high speed protection to the motor.
TWO POINT STARTER
In the case of a series motor, the flux does not remain constant and various with armature current which is exciting current Hence DC series motor should not be operated on no-load.
When T 0, at no load, the motor achieves high speed dangerously. Hence it should not be started without load.
The starting resistance is inserted in series with the field and armature. The NVC is connected in series with the armature. When the motor gets the supply, the handle is moved from OFF position to stud No. 1. Initially all the resistance all the resistance R…Rs are included in series. Then the starting resistance is gradually cut down when the stud is moved from 1 to RUN position and the motor gathers its speed.
The overload release coil is used to prevent the motor under overload conditions.
See More: Three Point Starter
See More: Working Principle of D.C Motor
See More: Working Principle of D.C Generator