Eb = V – Ia ra.
∴ Ia = V – Eb / ra.
At the time of starting from the rest there is no any back emf (Eb) in the armature. So a large current flows through the armature based on V / ra. This very large current blow out the fuses and before to that it will damage the commutator, commutator brushes and winding. To avoid this difficulties a proper resistance has to be introduce in series with the armature till the motor reaches it’s rated speed or till development of Eb in the armature to reduce the starting large current to safe value. This starting resistance is gradually cut out as the motor gains speed and the develops back emf (Eb) which regulates it’s speed and armature current. This can be achieved by the help of starter.
