Correct Answer - Option 1 : It provides low reluctance path for flux.
Magnetic Frame or Yoke:
The outer frame or yoke serves a double purpose:
(i) It provides mechanical support for the poles and acts as a protecting cover for the whole machine and
(ii) It carries the magnetic flux produced by the poles.
In small generators where cheapness rather than weight is the main consideration, yokes are made of cast iron
For large machines usually cast steel or rolled steel is employed.
The modern process of forming the yoke consists of rolling a steel slab around a cylindrical mandrel and then welding it at the bottom.
The feet and the terminal box etc. are welded to the frame afterward. Such yokes possess sufficient mechanical strength and have high permeability
