Field Poles
In early stages or say in the case of small machines the poles were cast integral to the yoke.But nowadays we use completely laminated pole or solid steel poles with laminated pole shoes.
Why we use laminated field poles?
We use laminated field poles because the surface of armature is not uniform since it has got notches or slots.And so when it rotates through the field produced by the pole shoes there may be pulsations in the field and this varying field could produce an eddy current inside the field pole.So to avoid this we use laminated field poles.
How laminated field poles reduce eddy current ?
The laminated field poles allows only the eddy current to pass through the length of lamination not through the entire body of pole ie from lamination to another laminations.These lamination are held together by means of a rivet.The outerside of the laminations are curved to fit closely to the inner frame.
The field coils are supported by the pole shoe and it spread out the flux in the airgap and owing to its large cross-section it reduces the magnetic reluctance.
How the pole shoe is attached to the yoke?
Generally two methods are employed for attaching pole shoes to the yoke.In the case of small pole shoes,poles is drilled and tapped to receive pole bolts but for large size a circular or rectangular pole bar is fitted or passed through the pole.This pole bar is drilled and tapped and the pole bolts passing through laminations is screwed int o the tapped bar.
Written by arjun on September 13th, 2008 with
1 comment.
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