What is the purpose of slip rings in an alternator?

Slip rings in motors are employed to supply D.C excitation to the alternator's rotor. However, the requirement for a slip ring arrangement may be dropped in some excitation techniques.

While the rotor to which it must be fed is revolving, the D.C supply you produce after rectifying the alternator (stator) output is stationary. Therefore, you remove the rotor terminals and attach slip rings to their ends so that they revolve with the shaft. As the revolving slip rings pass over the brushes, a brush that receives the D.C supply keeps in touch with them and continuously feeds DC into the rotor winding.You may think of the slip ring and brush configuration as an interface between rotating rotor and the stationary DC supply.

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Excitor alternators are now used to produce DC excitation in situations where the field is stationary and the armature is spinning while linked to the main shaft. The main rotor windings are fed with the rectified induced emf that the excitor alternator's revolving armature creates. Since the entire system is placed on the rotor and rotates together with it, a slip ring arrangement is not necessary.