Electrical Machines
Induction motors normally use cast aluminium rotor because aluminium is

  cheaper in cost.
  of low resistivity.
  easy to cast owing to its low melting point and is easily available.
  lighter in weight.

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Electrical Machines
What is/are the necessary conditions for voltage build up in self excited generator?

All of these
 Poles should contain residual flux.
 Field winding should be properly connected to armature winding.
 Field winding resistance should be less than critical resistance.

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