Motor Starter

When the armature is stationary, there is no back EMF and due to the small resistance of the armature, a very high current will flow through it when directly connected to the mains. The current in some cases will be so high that it may damage the armature winding. Due to this reason, a starter is always used to start a motor. The starter is a variable resistance placed in series with the armature of the motor M.

The starting switch is so arranged that when it is first closed, there is a large resistance in the circuit and only a small current flow through the amature of the motor. As the motor gains speed, the resistance is gradually decreased by moving the handle H towards B. At full speed, the whole of the resistance is cut off and the handle is held in its position at B by the electromagnet F.


When the supply line fails or motor is disconnected from the mains, the electro-magnet is demagnetized and the handle goes back to its original position A with the help of a spring attached to it. When again the motor is to be started, it is started with the help of the starter as explained already. In small motor, no starter is used because they gain speed rapidly. A starter is used in large motors only which take long time to gain full speed.



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