Why shading rings provided in armature core of ac contactor. Why not for dc contactor?

Why shading rings provided in armature core of ac contactor. Why not for dc contactor? 
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1 Answer

Answer :

Because the force developed is not steady in ac therefore contacts will chatter but if shading ring is used force developed becomes steady due to splitting of phases of flux, therefore contacts becomes bounce free and humming sound reduces.  
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