Electrical Machines
The air gap between the yoke and armature is dc machine is kept very small

 to avoid locking of the armature.
 to achieve a stronger magnetic field.
All of these
 to avoid over heating.

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Electrical Machines
The synchronous motor can be started by

  coupling it with a DC compound motor.
  providing damper winding in its rotor circuit.
  any of the above methods.
  coupling it with a small AC induction motor called the pony motor.

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