DC Motors
Carbon brushes are preferable to copper brushes because _____

They have lower resistance
They reduce armature reaction
They have longer life
They reduce sparking

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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
This will result in excessive speed, possibly destroying armature due to excessive centrifugal stresses
Nothing will happen to motor

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