Three-point starter

The incoming power is indicated as L1 and L2. The components within the broken lines form the three-point starter. As the name implies there are only three connections to the starter. The connections to the armature are indicated as A1 and A2. The ends of the field (excitement) coil are indicated as F1 and F2. In order to control the speed, A field rheostat is connected in series with the shunt field. One side of the line is connected to the arm of the starter (represented by an arrow in the diagram). The arm is spring-loaded so , it will return to the "Off" position the not held at any other position.
ON the first step of the arm, full line voltage is applied across the shunt field. Since the field rheostat is normally set to minimum resistance, the speed of the motor will not be excessive; additionally, the motor will develop a large starting torque.
The starter also connects an electromagnet in series with the shunt field. ot will hold the arm in position when the arm makes contact with the magnet.
Meanwhile that voltage is applied to the shunt field, and the starting resistance limits the flow of current to the armature.
As the motor picks up speed counter-emf is built up, the arm is moved slowly to short.
Four-point starter
The four-point starter eliminates the drawback of the three-point starter. In addition to the same three points that were in use with the three-point starter, the other side of the line, L1, is the fourth point brought to the starter. when the arm is moved from the "Off" positionThe coil of the holding magnet is connected across the line. The holding magnet and starting resistors function identical as in the three-point starter.
The possibility of accidentally opening the field circuit is quite remote. The four-point starter provides the no-voltage protection to the motor. If the power fails, the motor is disconnected from the line.