Slip Rings:

Slip rings are electrical connections which are used to transfer power to and from the rotor of an AC generator (refer to Figure).  A slip ring consists of a circular conducting material which is linked to the rotor windings and insulated from the shaft.  A Brushes ride on the slip ring as the rotor rotates.  A electrical connection to the rotor is made through linkes to the brushes.

Slip rings are used in AC generators since the desired output of the generator is a sine wave. Within a DC generator, a commutator was used to gives an output whose current always flowed in the positive direction, as display in Figure. This is not must for an AC generator. Thus, an AC generator might use slip rings, that will permit the outcome current and voltage to oscillate by positive and negative values.  This oscillation of current and voltage takes the shape of a sine wave.

Figure - Comparison of DC and AC Generator Outputs