The starter which consists of three terminals is known as the three-point starter. The four-point starter consists of four terminals and is hence called the four-point starter.
The armature, field, and line are the terminals of the three-point starter. Whereas in the four-point starter along with the armature, field, and line terminal one additional terminal is added which connected the no voltage coil parallel with the shunt field winding.
In the three-point starter, the no-voltage coil (NVC) is connected in series with the field winding. Whereas in a four-point starter the no-voltage winding is connected in parallel with the field winding.
The three-point starter is suitable for starting the shunt and compound DC motors. The four-point starter is suitable for starting such DC motors that have variable speeds.
The no-voltage coil (NVC) is a type of relay used for switching off the motor when the supply is decreased or cut off. The NVC (no-volt trip coil) is also known as the under-voltage protection of the motor.