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Field Poles

  • Formerly the poles were cast integral with the yoke. This practice is still being followed for small machines. But in present day machines it is usual to use either a completely laminated pole, or solid steel poles with laminated pole shoes.
  • Laminated construction is necessary because of the pulsations of field strength that result when the notched armature rotor magnetic structure passes the pole shoe. Variations in field strength result in internal eddy currents being generated in a magnetic structure. These eddy currents cause losses; they may be largely prevented by having laminated magnetic structures. Laminated structures allow magnetic flux to pass along the length of the laminations, but do not allow electric eddy currents to pass across the structure from one lamination to another. The assembled stack of laminations is held together as a unit by appropriately placed rivets. The outer end of the laminated pole is curved to fit vel)’ closely into the inner surface of the main frame.
  • Fig. 3 shows the constructional details of a field pole. The pole shoe acts as a support to the field coils and spreads out the flux in the air gap and also being of larger cross-section reduces the reluctance of the magnetic path.

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