DC MOTOR STARTING METHODS
During the starting of the motor, the armature current will be high and it may damage the fuses, armature winding or commutator brush arrangement. There will be an increase in torque during the starting.
(torque is directly proportional to the armature current)
Due to this increase in torque, there will be a high centrifugal force and it could throw off the armature winding. And thus to avoid all these problems starters are used for the starting of the Dc machines.
There are many starters like 3 point starter, 4 point starter, thyristor starter…
While adding a starter the main difference between normal starting mode is adding an external resistance to the armature winding at the starting.
3 POINT STARTER
In this image there is a lever during the starting of the motor the lever is turned to the right and when the lever touches the point 1, armature winding will be connected to the resistance, the field winding will be connected across the supply. Thus during the starting of the motor full resistance is added in series to the armature and when the lever changes its position to higher position the resistance will cut off and the armature will be connected to the supply
Electromagnets hold the liver and if there is no supply then it will release the lever if the motor is overloaded beyond a certain value the electromagnet D will be activated and it short circuits the E electromagnet and then the lever will be released thus the motor turns off.
4 POINT STARTER
The major difference between 3 & 4 point starter is there is no voltage coil connected in series with field coil field will be directly connected to the supply as the lever comes in contact with the arc the voltage coil is connected with the current limiting resistance and this guarantees that a change of current in the shunt won’t affect the current to the coil which means that the spring doesn’t need to restore the lever position the electromagnet is sufficient.