Dynamo

A dynamo is an electrical generator that creates direct current using a commutator. Dynamos were the first electrical generators capable of delivering power for industry, and the foundation upon which many other later electric-power conversion devices were based, including the electric motor, the alternating-current alternator, and the rotary converter. Today, the simpler alternator dominates large scale power generation, for efficiency, reliability and cost reasons. A dynamo has the disadvantages of a mechanical commutator. Also, converting alternating to direct current using power rectification devices (such as vacuum tubes or more recently via solid state technology) is effective and usually economical. A dynamo is an electrical generator that creates direct current using a commutator. Dynamos were the first electrical generators capable of delivering power for industry, and the foundation upon which many other later electric-power conversion devices were based, including the electric motor, the alternating-current alternator, and the rotary converter. Today, the simpler alternator dominates large scale power generation, for efficiency, reliability and cost reasons. A dynamo has the disadvantages of a mechanical commutator. Also, converting alternating to direct current using power rectification devices (such as vacuum tubes or more recently via solid state technology) is effective and usually economical. The word dynamo (from the Greek word dynamis (δύναμις), meaning force or power) 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 in 1831 by Michael Faraday, who utilized his invention toward making many discoveries in electricity (Faraday discovered electrical induction) and magnetism. The original 'dynamo principle' of Werner von Siemens referred only the direct current generators which use exclusively the self-excitation (self-induction) 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 (self-induction) was a huge technological leap over the old traditional permanent magnet based DC generators. The discovery of the dynamo principle made industrial scale electric power generation technically and economically feasible.After the invention of the alternator and that alternating current can be used as a power supply, the word dynamo became associated exclusively with the commutated direct current electric generator, while an AC electrical generator using either slip rings or rotor magnets would become known as an alternator. 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 pulsing direct electric current through Faraday's law of induction. A dynamo machine consists of a stationary structure, called the stator, which provides a constant magnetic field, and a set of rotating windings called the armature which turn within that field. Due to Faraday's law of induction the motion of the wire within the magnetic field creates an electromotive force which pushes on the electrons in the metal, creating an electric current in the wire. On small machines the constant magnetic field may be provided by one or more permanent magnets; larger machines have the constant magnetic field provided by one or more electromagnets, which are usually called field coils. The commutator is needed to produce direct current. When a loop of wire rotates in a magnetic field, the magnetic flux through it, and thus the potential induced in it, reverses with each half turn, generating an alternating current. However, in the early 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. It consists of a set of contacts mounted on the machine's 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 external circuit when the potential reverses, so instead of alternating current, a pulsing direct current is produced. The earliest dynamos used permanent magnets to create the magnetic field. These were referred to as 'magneto-electric machines' or magnetos. However, researchers found that stronger magnetic fields, and so more power, could be produced by using electromagnets (field coils) on the stator. 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. An important development by Wilde and Siemens was the discovery (by 1866) that a dynamo could also bootstrap itself to be self-excited, using current generated by the dynamo itself. This allowed the growth of a much more powerful field, thus far greater output power.