Question

in dc dynamo why we use split rings

Answer 1

slip rings are useful for DC motors where the current has to change direction every half revolution whereassplit rings are used to make constant current connection to a commutator, which is what you want for an ACmotor.
HOPE IT HELPS U

Answer 2

in DC dynamo,
the split rings acts as a commutator. with the split ring, the direction of current through the coil is reversed after each half rotation of the coil and thus the direction of couple rotating the coil remains unchanged and the coil continue student in the same direction.
hope it helps...