why do we need a starter? to start a dc motor?

A starter is necessary to start a DC motor because it restricts initial high armature current that exists, as the value of starting back-EMF is zero.

During starting of a DC motor, a high current of up to 4-5 times the full load current occurs which is avoided by use of high resistance slowly cut out of the circuit.

This is done by using a starter which consists of a variable series resistance that is added to the armature of the motor.

It is used in order to reduce the starting voltage across the winding.

When the motor speeds up, the starter resistance decreases gradually and it will disconnect completely when the armature attains enough speed to generate the back EMF (that is the normal speed).

At full speed, the starter has no effect on the armature circuit of the motor.