Electrical Machines
The rotor speed of an induction motor is given by

  Ns = 120f /p × ( 1 - r).
  Nr = 120f / p × (1-s).
  Ns = 120f / p × ( 1 - s ).
None of these

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Electrical Machines
In a DC machine

 current and emf in armature are alternating.
 emf in armature conductors and at the terminals is alternating.
 current and emf at the terminals are alternating.
 current and emf in armature are unidirectional

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