Dynamo
From Wikipedia, the free encyclopedia
A dynamo, originally another name for an electrical generator, now means a 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. They are rarely used for power generation now because of the dominance of alternating current, the disadvantages of the commutator, and the ease of converting alternating to direct current using solid state methods.
The word still has some regional usage as a replacement for the word generator. In the UK British-English dialect, a small electrical generator built into the hub of a bicycle wheel to power lights is called a Hub dynamo.
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[edit] Description
The dynamo uses rotating coils of wire and magnetic fields to convert mechanical rotation into a pulsing direct electric current through Faraday's law. 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. 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 a set of contacts mounted on the machine's shaft, which 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.
[edit] Historical Milestones
[edit] Pixii's Dynamo
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 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. By adding a commutator, Pixii was able to convert the alternating current to direct current.
Unlike the Faraday disc, many turns of wire connected in series can be used in the windings of a dynamo. This allows the terminal voltage of the machine to be higher than a disc can produce, so that electrical energy can be delivered at a convenient and inexpensive high voltage.
The very low voltages of the Faraday disc require large amperages to provide power, and as amperage increases the mass of the current-carrying conductors must also increase. Since power is equal to the product of current and voltage, the high voltage of the dynamo could generate equivelant amounts of power with much lower amperage, reducing need for large and expensive electrical conductors.
[edit] Jedlik's dynamo
Main article: Jedlik's dynamo
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 (finished between 1852 and 1854) both the stationary and the revolving parts were electromagnetic. He formulated the concept of the dynamo at least 6 years before Siemens and Wheatstone. In essence the concept is that instead of permanent magnets, two electromagnets opposite to each other induce the magnetic field around the rotor.
[edit] Gramme ring dynamo
Main article Gramme dynamo
Both of these designs suffered from a similar problem: the electric current they produced consisted of "spikes" of current followed by none at all. Antonio Pacinotti, an Italian scientist, fixed this 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. This meant that some part of the coil was continually passing by the magnets, smoothing out the current.
Zénobe Gramme reinvented this design a few years later when designing the first commercial power plants, which operated in Paris in the 1870s. His design is now known as the Gramme dynamo.
Further improvements were made on the Gramme Ring, but the basic concept of a spinning endless loop of wire remains at the heart of all modern dynamos.
[edit] Discovery of electric motor principles
While not originally designed for the purpose, the dynamo was accidentally found to act as an electric motor when a freely spinning dynamo is attached to batteries or another spinning dynamo. Such situations became common as the need for larger and continuous amounts of power required multiple dynamos to be connected together in parallel for power transmission.
Large dynamos producing direct current were problematic in situations where two or more dynamos are working together and one has an engine running at a lower power than the other. The dynamo with the stronger engine will tend to drive the weaker as if it were a motor, against the rotation of the weaker engine. Such reverse-driving could feed back into the driving engine of a dynamo and cause a dangerous out of control reverse-spinning condition in the lower-power dynamo.
It was eventually determined that when several dynamos all feed the same power source all the dynamos must be locked into synchrony using a jackshaft interconnecting all engines and rotors to counter these imbalances.
[edit] Dynamo as Commutated DC Generator
After the discovery of the AC Generator and that alternating current can in fact be useful for something, the word dynamo became associated exclusively with the commutated DC electric generator, while an AC electrical generator using either slip rings or rotor magnets would become known as an alternator.
An AC electric motor using either slip rings or rotor magnets was referred to as a synchronous motor, and a commutated DC electric motor could be called either an electric motor though with the understanding that it could in priciple operate as a generator.
[edit] Rotary Converter Development
After dynamos were found to allow easy conversion back and forth between mechanical or electrical power, the new discovery was used to develop complex multi-field single-rotor devices with two or more commutators, with this known as a rotary converter. These special devices are usually never attached to an external mechanical load, but instead just spends all day spinning on its own.
The rotary converter can internally and directly convert any power source into any other, whether direct current into alternating current, 25 cycle AC into 60 cycle AC, or multiple different output currents at the same time. The size and mass of these was very large so that the rotor would act as a flywheel to help smooth out any sudden surges or dropouts.
The technology of rotary converters would go unsurpassed until the later development of vacuum tubes allowed purely electronic oscillators to be designed, without the need for a physically spinning rotor and commutators.
