The starter is nothing but a variable resistance. Which is connected in a series with armatures winding in dc motor. Its main function is to reduce the starting current of the motor to its safe value.
Necessity of starter
When the motor is in off condition, the armature is stationary and the back EMF which is proportional to speed is also zero.
In Motor armature resistance is very low. If the rated voltage is applied to the armature, it will draw heavy current many times that of full load current. And there is more possibility of damaging the armature due to heavy starting current.
So, we need to limit the starting current to its safe value. This is possible by inserting a resistance in series with the armature at the time of starting for a short period.
As the motor gains rated speed, back EMF is built up in it, and then the starting resistance could be gradually cut off in step by step.
Basic Construction and operation
Figure shows a basic arrangement of starter. The variable Resistance ‘R’ is connected in series with armature circuit by keeping the moving arm in position ‘A’ at the time of starting.
And then it is moved towards position ‘B’ to disconnect the resistance ‘R’ when the motor is picked up its rated speed.
But such arrangement is purely manual and needs more concentration. For example, if the motor is running, the resistance ‘R’ will be disconnect. And the moving arm position will be at position ‘B’.
In case the input supply fails, the motor will stop but the moving arm will still be in position ‘B’. When the supply returns, as there is no resistance included in the armature circuit through ‘R’. So the armature may draw heavy current and may get damaged.
To prevent such a condition a device called starter is used in motor circuits with additional automatic insertion of resistance. It add the resistance at time of starting and the starters may protect the motor from overload and will switch off the motor, when supply fails.
These starters are classified according to the number of connecting terminals as follows,
Type of Starter
- Two-point starter
- Three-point starter
- Four-point starter