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

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

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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 alternating.
 current and emf in armature are unidirectional

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