Necessity of Starter for D.C. Motor:
Armature current is given by equation Ia = (V−Eb)/Ra
i) If the motor is at standstill or rest, back emf Eb is zero (as Eb= ZNP/(60A), at start speed N is zero). This causes starting current Ia = V/Ra,which is large as armature resistance is usually low. This large starting current may damage armature winding due to heavy heating.
ii) Hence to limit the very high starting current, the starter is required.
iii) Once motor picks up the speed, the back emf Eb is induced in armature winding and armature current is limited to safe value. So starter is not required under running condition.
Types of D.C. motor starters:
i) Two point starter
ii)Three point starter
iii) Four point starter