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A dynamo is an electrical generator that produces direct current with the
use of 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 (vacuum
tube or more recently solid state) is effective and usually economic.
The word dynamo (from the Greek word dynamis; meaning power) was
originally another name for an electrical generator, and still has some
regional usage as a replacement for the word generator. 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
The 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
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stator, which provides a constant magnetic field, and a set of rotating windings called the armature which turn within
that field. The motion of the wire within the magnetic field causes the field to push 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 was needed to produce direct current. When a loop of wire rotates in a magnetic field, 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 first electric generator was invented by Michael Faraday in 1831, a
copper disk that rotated between the poles of a magnet. This was not a
dynamo because it did not use a commutator. However, Faraday's disk
generated very low voltage because of its single current path through the
magnetic field. Faraday and others found that higher, more useful
voltages could be produced by winding multiple turns of wire into a coil.
Wire windings can conveniently produce any voltage desired by changing
the number of turns, so they have been a feature of all subsequent
generator designs, requiring the invention of the commutator to produce
In 1827, Hungarian Anyos Jedlik started experimenting with electromagnetic rotating devices which he called
electromagnetic self-rotors. In the prototype of the single-pole electric starter, both the stationary and the revolving
parts were electromagnetic. He formulated the concept of the dynamo about six years before Siemens and
Wheatstone but did not patent it as he thought he was not the first to realize this. His dynamo used, instead of
permanent magnets, two electromagnets opposite to each other to induce the magnetic field around the rotor.
It was also the discovery of the principle of dynamo self-excitation.
The first dynamo based on Faraday's principles was built in 1832 by Hippolyte Pixii, a French instrument maker. It
used a permanent magnet which was rotated by a crank. The spinning magnet was positioned so that its north and
south poles passed by a piece of iron wrapped with insulated wire. Pixii found that the spinning magnet produced a
pulse of current in the wire each time a pole passed the coil. However, the north and south poles of the magnet
induced currents in opposite directions. To convert the alternating current to DC, Pixii invented a commutator, a
split metal cylinder on the shaft, with two springy metal contacts that pressed against it.
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Pixii's dynamo. The commutator is
located on the shaft below the
Pacinotti dynamo, 1860
These early designs had a problem: the electric current they produced consisted of a series of "spikes" or pulses of
current separated by none at all, resulting in a low average power output. As with electric motors of the period, the
designers did not fully realize the seriously detrimental effects of large air gaps in the magnetic circuit. Antonio
Pacinotti, an Italian physics professor, solved this problem around 1860 by replacing the spinning two-pole axial
coil with a multi-pole toroidal one, which he created by wrapping an iron ring with a continuous winding, connected
to the commutator at many equally spaced points around the ring; the commutator being divided into many
segments. This meant that some part of the coil was continually passing by the magnets, smoothing out the current.