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

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Electrical Machines
The speed of a DC series motor is

 Proportional to the square of the armature current.
 Inversely proportional to the armature current.
 Proportional to the armature current.
 Proportional to field current.

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