DC Motors
Excessive sparking at brushes may be caused due to

Dirt on the commutator
Misaligment of machine
Worn bearings
Loose coupling

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

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