Electrical Machines
The armature mmf of a DC machine has

  trapezoidal space distribution and is stationary in space.
  triangular space distribution and rotates at the speed of armature.
 stepped distribution and rotates at the speed of armature.
  triangular space distribution and is stationary in space.

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Electrical Machines
The 3 - Φ induction motors are, by a very considerable margin, the most widely used AC motors as

  they have maximum efficiency and maximum starting torque.
  these are cheaper in cost, simple and rugged in construction.
  they are capable of operation under range of leading and lagging pf.
  they have very good speed regulation.

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