Dynamo

A simple dynamo uses a split-ring commutator instead of two complete slip rings. Each half of the commutator is connected to one end of the rotating coil.

Neither an alternator nor a dynamo needs an electrical supply to start that current: the coil is turned by an external source of mechanical energy.

A dynamo can use the same basic arrangement of a coil rotating in a magnetic field, but it has a split-ring commutator instead of two separate slip rings. Each half of the ring is connected to one end of the coil. As the coil passes through each half-turn, the split ring swaps which coil end is connected to each external terminal.

A dynamo is adapted to give a direct output. The induced p.d. inside its generator still reverses every half-turn. A split-ring commutator swaps the connections to the external circuit at each half-turn, just when that reversal occurs.