We need a starter to start a DC motor primarily due to the high starting current it draws. When a DC motor is first switched on, its back EMF (electromotive force) is zero. This means that the only resistance in the armature circuit is the armature resistance itself.

The starting current (I) can be calculated using Ohm's law:

I = V / R

where:

• I = starting current
• V = supply voltage
• R = armature resistance

Since the armature resistance is very low, the starting current can be significantly higher than the rated current of the motor. This high current can damage the motor windings and other components.

A starter helps to limit this high starting current by introducing a series resistance in the armature circuit. As the motor starts to rotate, the back EMF gradually increases, reducing the net voltage across the armature. The starter then gradually reduces the series resistance, allowing more current to flow through the armature.

By controlling the starting current in this way, the starter protects the motor from damage and ensures a smooth and efficient start-up.