DC Motors
For starting a dc motor a starter is required because

It limits the starting current to a safe value
It limits the speed of the motor
It starts the motor
None of the listed here

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DC Motors
If a D.C. shunt motor is working at no load and if shunt field circuit suddenly opens

This will result in excessive speed, possibly destroying armature due to excessive centrifugal stresses
This will make armature to take heavy current, possibly burning it
Motor will run at very slow speed
Nothing will happen to the motor

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