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

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Electrical Machines
The residual magnetism of a self-excited dc generator is lost. To build up its emf again

 field winding must be excited by low voltage dc supply
 the field winding must be replaced
 the field winding connection must be reversed
 the armature connection must be reversed

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