dynamo (from the Greek word dynamis; meaning power), originally another name for an electrical generator, generally 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. 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.

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. 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.

 

Historical milestones

Faraday’s disk

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 direct current.

Jedlik’s dynamo

A modern demonstration of Jedlik’s dynamo

Pixii’s dynamo

Pixii‘s dynamo. The commutator is located on the shaft below the spinning magnet

Pacinotti dynamo

dynamo, 1860

Siemens and Wheatstone dynamo (1867)

Gramme ring dynamo

Small Gramme dynamo, around 1878

Brush dynamo

Charles F. Brush assembled his first dynamo in the summer of 1876 using a horse-drawn treadmill to power it. U.S. Patent #189997 “Improvement in Magneto-Electric Machines” was issued April 24, 1877. Brush started with the basic Gramme design where the wire on the sides and interior of the ring were outside the effective zone of the field and too much heat was retained. To improve upon this design, his ring armature was shaped like a disc rather than the cylinder shape of the Gramme armature. The field electromagnets were positioned on the sides of the armature disc rather than around the circumference. There were four electromagnets, two with north pole shoes and two with south pole shoes. The like poles opposed each other, one on each side of the disc armature.[6] In 1881 one of The Brush Electric Company dynamos was reported to be; 89 inches long, 28 inches wide, and 36 inches in height, and weighs 4,800 pounds, and ran at a speed of about 700 revolutions per minute. It was believed to be the largest dynamo in the world at that time. Forty arc lights were fed by it, and it required 36 horse power.[7]

Discovery of electric motor principles

While not originally designed for the purpose, it was discovered that a dynamo can act as an electric motor when supplied with direct current from a battery or another dynamo. At an industrial exhibition in Vienna in 1873, Gramme noticed that the shaft of his dynamo began to spin when its terminals were accidentally connected to another dynamo producing electricity. Although this wasn’t the first demonstration of an electric motor, it was the first practical one. It was found that the same design features which make a dynamo efficient also make a motor efficient. The efficient Gramme design, with small magnetic air gaps and many coils of wire attached to a many-segmented commutator, also became the basis for the design of all practical DC motors.

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.

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 motor could also be called an electric motor though with the understanding that it could in principle operate as a generator.

Rotary converter development

After dynamos and motors were found to allow easy conversion back and forth between mechanical or electrical power, they were combined in devices called rotary converters, rotating machines whose purpose was not to provide mechanical power to loads but to convert one type of electric current into another, for example DC into AC. They were multi-field single-rotor devices with two or more sets of rotating contacts (either commutators or sliprings, as required), one to provide power to one set of armature windings to turn the device, and one or more attached to other windings to produce the output current.

The rotary converter can directly convert, internally, any type of electric power into any other. This includes converting between direct current (DC) and alternating current (AC), three phase and single phase power, 25 Hz AC and 60 Hz AC, or many different output voltages at the same time. The size and mass of the rotor was made large so that the rotor would act as a flywheel to help smooth out any sudden surges or dropouts in the applied power. Typical rotary converters were still used for the West Side IRT subway in Manhattan in the late 1960s, and possibly some years later. They were powered by 25 Hz AC, and provided DC at approx. 600 volts for the trains.

The technology of rotary converters was replaced in the early 20th century by mercury-vapor rectifiers, which were smaller, did not produce vibration and noise, and required less maintenance. The same conversion tasks are now performed by solid state power semiconductor devices.

Modern uses

Dynamos still have some uses in low power applications, particularly where low voltage DC is required, since an alternator with a semiconductorrectifier can be inefficient in these applications. Hand cranked dynamos are used in clockwork radios, hand powered flashlights, mobile phone rechargers, and other human powered equipment to recharge batteries.

Posted By

P.Aditya (2/4  ECE) MGIT

 

Courtesy::: http://www.wikipedia.org

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