Answer Posted / shashikant mahajan

At starting of motor it can't have back emf & winding resistance is very small, so armature winding takes 15-20 times more current than full load current due to which torque developed is proportional to Ia and which is also 15-20 times assuming shunt motor operation.Due to such a high torque,shaft & other accessories are subjected to mechanical stresses these stresses may cause permanent mechanical damage to the motor.
To restrict such high current at starting, the variable resistance(starter) connected in series with armature. at the beginning the entire resistance connected in series with armature & cutoff gradually with speed of motor.
i.e. starter works as a current limit device at starting of motor.

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