When the armature rotates a half a revolution ‘A’ is positioned at the south pole “S” and ‘B’ positioned at the north pole “N”. As the magnetic field surrounding ‘A’ is in the anti-clockwise direction the magnetic field would be stronger under ‘A’ and above ‘B’, which would mean that the armature would now rotate in the opposite direction. The motor would never rotate.
To maintain rotation of the armature in the one direction the flow or current through the armature must be reversed when ‘A’ is at the “S” pole of the field coil.
A commutator is used with the motor to reverse the current flowing in the armature. Fig. “X” shows the black half of the armature at “N” and is connected via the segment of the commutator through the carbon brush to the battery negative terminal. In this position, the current flow through the armature produces a magnetic field of anti-clockwise direction through the black half ‘A’ of the armature and clock-wise through the white ‘B’. Interaction occurs and the loop rotates.
Fig. ‘Y’, the armature has moved through half a turn so the white section is now at “N”. The brush which connects to the negative side of the battery is now in contact with the white segment of the commutator. The current flowing in the armature is now reversed which means the direction of the lines of force of white is now anticlockwise and clockwise in the black “A”.
The interaction of the magnetic fields will rotate the armature in the same direction as when black “A” was at “N”. As the armature rotates therefore the magnetic field of the armature remains constant to the main field.
To increase the power of the motor many wire coils or loops are used in the armature tc maintain a constant push on the armature. Four poles are used two north and two south to produce a strong main field and are wound to give alternate N and S poles. Because some starters use 200 amps when operating four carbon brushes are fitted to distribute the current load.
Starter motors are usually series-wound motors because they produce their maximum torque at the beginning of their armature rotation.