A dynamo is an electrical generator that produces direct current with the use of a commutator. Today, the simpler alternator dominates large scale power generation, for efficiency, reliability, a dynamo has the disadvantages of a mechanical commutator. Also, converting alternating to direct current using power rectification devices is effective, the word dynamo was originally another name for an electrical generator, and still has some regional usage as a replacement for the word generator. The word dynamo was coined by Werner von Siemens in 1882, the original dynamo principle of W. Siemens meant only the direct current generators which use exclusively the self-excitation principle to generate DC power. The earlier DC generators which used permanent magnets were not considered dynamo electric machines, the invention of the Dynamo principle was a huge technological leap over the old traditional permanent magnet based DC generators. The discovery of the dynamo principle made the industrial scale electric power generation technically and economically feasible, a small electrical generator built into the hub of a bicycle wheel to power lights is called a hub dynamo, although these are invariably AC devices, and are actually magnetos. The electric dynamo uses rotating coils of wire and magnetic fields to convert mechanical rotation into a direct electric current through Faradays law of induction. A dynamo machine consists of a structure, called the stator, which provides a constant magnetic field. Due to Faradays law of induction the motion of the wire within the field creates an electromotive force which pushes on the electrons in the metal. The commutator is needed to direct current. When a loop of wire rotates in a field, the magnetic flux through it. However, in the days of electric experimentation, alternating current generally had no known use. The few uses for electricity, such as electroplating, used direct current provided by messy liquid batteries, dynamos were invented as a replacement for batteries. The commutator is essentially a rotary switch and it consists of a set of contacts mounted on the machines shaft, combined with graphite-block stationary contacts, called brushes, because the earliest such fixed contacts were metal brushes. The commutator reverses the connection of the windings to the circuit when the potential reverses, so instead of alternating current. The earliest dynamos used permanent magnets to create the magnetic field and these were referred to as magneto-electric machines or magnetos. However, researchers found that magnetic fields, and so more power. These were called dynamo-electric machines or dynamos, the field coils of the stator were originally separately excited by a separate, smaller, dynamo or magneto
The Faraday disk was the first electric generator. The horseshoe-shaped magnet (A) created a magnetic field through the disk (D). When the disk was turned, this induced an electric current radially outward from the center toward the rim. The current flowed out through the sliding spring contact m, through the external circuit, and back into the center of the disk through the axle.
Hippolyte Pixii's dynamo. The commutator is located on the shaft below the spinning magnet.