Electrical Machines
Open circuit in a transformer is prefered with

 rated transformer voltage
 rated transformer current
 high frequency supply
 direct current

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Electrical Machines
If the frequency of input voltage of a transformer is increased keeping the magnitude of voltage unchanged, then

 Ph will increase but Pe will decrease
 Ph will decrease but Pe will remain same
 Pe will increase but Ph will decrease
 both hysteresis loss(Ph) and eddy current loss(Pe) will increase

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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

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

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