Necessity Of A Starter

For D.C Motors When motor is at rest, then N=0 and due to Eb  is directly proportional to N, so 

 At begin E_b  =  zero

Thus armature current at the time of starting

 I_as  =  (V-0)/ R_a   =  V/ R_a

i.e. at begin armature, current can be measured by simply applying ohm's law, As the resistance of the armature winding is very small then the motor will take large amount of present at the time of starting i.e. if supply voltage is 220 V and armature resistance is 0.5 ?, then initializing current will be equal to 220/0.5  = 440A