Dynamo

"Dynamo Electric Machine" (end view, partly section, U.S. Patent 284,110)

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-currentalternator, 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 throughFaraday'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.

Historical milestones

Faraday's disc

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.

Siemens and Wheatstone dynamo (1867)

The first practical designs for a dynamo were announced independently and simultaneously by Dr. Werner Siemens and Charles Wheatstone. On January 17, 1867, Siemens announced to the Berlin academy a "dynamo-electric machine" (first use of the term) which employed a self-powering electromagnetic armature.^[4] On the same day that this invention was announced to the Royal Society Charles Wheatstone read a paper describing a similar design with the difference that in the Siemens design the armature was in series with the rotor, but in Wheatstone's design it was in parallel.^[5] The use of electromagnets rather than permanent magnets greatly increases the power output of a dynamo and enabled high power generation for the first time. This invention led directly to the first major industrial uses of electricity. For example, in the 1870s Siemens used electromagnetic dynamos to power electric arc furnaces for the production of metals and other materials.

Discovery of electric motor principles

While not originally designed for the purpose, it was discovered that a dynamo can act as anelectric 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 electric motor could be called either 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 commutators, 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 beween direct current (DC) and alternating current (AC), three phase and single phase power, 25 cycle AC and 60 cycle 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.

The technology of rotary converters was replaced in the early 20th century by vacuum tube circuits, 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