Manual and Automatic Starters

Because the DC resistance of most motor armatures is low (0.05 to 0.5 ohm), and because the back EMF does not exist until the armature begins to turn, it is necessary to use an external starting resistance in series with the armature of a DC motor to keep the initial armature current to a safe value. As the armature begins to turn, back EMF increases; and, since the back EMF opposes the applied voltage, the armature current is reduced.

The external resistance in series with the armature is decreased or eliminated as the motor comes up to normal speed and full voltage is applied across the armature.

Controlling the starting resistance in a DC motor is accomplished either manually or by any of several automatic devices. The automatic devices are usually just switches controlled by motor speed sensors. Automatic starters are not covered in detail in this module.