Electrical Machines
In a DC machine

 current and emf at the terminals are alternating.
 emf in armature conductors and at the terminals is alternating.
 current and emf in armature are unidirectional
 current and emf in armature are alternating.

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Electrical Machines
If the applied voltage of a certain transformer is increased by 50% and the frequency is reduced by 50%, the maximum flux density will (assuming that the magnetic circuit remains unsaturated)

 changes to three times the original value
 changes to 1.5 times the original value
 remain the same as the original
 changes to 0.5 times the original value

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