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

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

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Electrical Machines
In dc machines, the field system has to be provided on stator unlike synchronous machines Where in it could be on any member, because

 it gives more uniform air-gap flux distribution
 it reduces field structure iron losses
 commutator action is not possible otherwise
 dc machines are comparatively low rating machines.

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Electrical Machines
In the 3-phase induction motor, the resultant flux is of a constant nature and is

 3/2 times maximum value of flux due to any phase
 equal to φm, where φm is the maximum flux due to any phase
 3 times maximum value of flux due to any phase
 1/2 times maximum value of flux due to any phase

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