DC motor starter is called 3 point starter bcoz only 3
terminal (L,A,F)are available from starter.These 3 terminal
L(line),A(armature),F(field) must b connected to supply
terminal,motar armatuer terminal,and shunt field terminal.
Actually Dc motor starter are classified as two point
starter,three point starter and four point starter.
Three point starter is used in case of Shunt motor(some
cases this can be used compound motor also)
Four point starter is mainly used in compound motor.
Two point starter is used in series motor in order to make
the motor to run in safe manner.
Three points denotes Line terminal.field terminal and
armature terminal in which three points are connected.
three points L,A F.
that is the main reason
the main purpose of using a three point starter is
to limit the starting armature current.......
This is because dc motor the initial voltage difference is
less so high current passes through the armature windings
which is not a desired factor.When we use a 3 point starter
it slowly induces the back emf in motor so that the voltage
difference is less and rate of current flow is less, so the
motor doesnt get spoiled