DC Motors
As there is no back e.m.f. at the instant of starting a D.C. motor, in order to prevent a heavy current from flowing though the armature circuit

A resistance is connected in series with armature
A high value resistor is connected across the field winding
Armature is temporarily open circuited
A resistance is connected parallel to the armature

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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
Nothing will happen to the motor
Motor will run at very slow speed

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