A four-point starter consists of four terminals A, B, C and D and two starting resistances S1 and S2. Compound motor has four terminals that are A1, A2, S1 and S2. When the starter is in the “Off” position, all four terminals are disconnected. When the starter is moved to the “Start” position, terminals B and C are connected to terminals S1 and S2 respectively, and terminals A1 and A2 are connected together. This connects the series field and the two stator terminals A1 and A2 in series with the starting resistances S1 and S2.
As the motor starts to run, the back emf is induced in the armature. This causes the current flow through the starting resistances to decrease, and eventually they are completely cut out of the circuit by means of moving the handle to the “Run” position. In this position, the series field and the two stator terminals A1 and A2 are directly connected to the supply. The shunt field is already connected across the supply terminals. Now the motor runs normally.
