DC Motors
Excessive sparking at brushes may be caused due to

Loose coupling
Worn bearings
Dirt on the commutator
Misaligment of machine

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

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

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