What is the Need of a Starter in DC Motors ?

Consider a dc motor as shown below. Voltage equation of the motor will be

       V=E_b+I_aR_a                                     ...... by  KVL

       Or

      I_a= (V-E_b)/R_a

At starting, when the motor is stationary i.e speed, N=0, there will be no back emf.

    E_b=(P ΦZN)/60A

    E_b=0

Therefore, I_a= V/R_a

Consequently, if the motor is directly  switched on to the mains, the armature will draw a heavy current because of small armature resistance, R_a.

For example, a 10 kw, 250-v shunt motor with armature resistance R_a=0.2 Ω if switched on directly, will draw 1250 A current, whereas the rated current is only 40A. Its more than 30 times. 

Such high current may cause problems as :-

1.  Burning of armature due to excessive heating (I_a²Ra).  
2.  Damaging commutator and brushes due to heavy sparking.
3.  High starting torque

                T_a=KΦI_a
                T_a ∝ I_a                       ..... Φ is practically constant in a dc                                                               shunt motor
            Thus quick acceleration may damage rotating parts of the                    motor.

         4.   Large dips in supply line to which motor is                                             connected, affecting working of other devices connected on               the same line.

In view of this, the armature current must be limited to a value that can be commuted safely. This is accomplished by inserting a variable resistance in series with the armature circuit. The resistance is gradually reduced as motor gains speed (and back emf, E_b starts appearing) and eventually cut out completely as motor gains speed.

This explains necessity of starter in dc motor and also highlights the importance of back emf.

Closely related concepts :-
  (1) Three Point Starter