DC Motors
A series motor will over speed when

The armature circuit is opened.
Load is removed
The load is increase
The field is opened

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DC Motors
If a D.C. shunt motor is working at full load and if shunt field circuit suddenly opens

Nothing will happen to motor
This will result in excessive speed, possibly destroying armature due to excessive centrifugal stresses
Motor will come to stop
This will make armature to take heavy current, possibly burning it

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