RATING _______________________




Body 4

History and development 4

Direct current generators 7

Alternating current generators 8

Self-excitation 11

Specialized types of generator 11

Homopolar generator 11

Induction generator 12

Linear electric generator 12

Variable speed constant frequency generators 12

Conclusion 13

Key vocabulary 14



Generators are useful appliances that supply electrical power during a power outage and prevent discontinuity of daily activities or disruption of business operations. Generators are available in different electrical and physical configurations for use in different applications. An electric generator is a device that converts mechanical energy obtained from an external source into electrical energy as the output.

It is important to understand that a generator does not actually ‘create’ electrical energy. Instead, it uses the mechanical energy supplied to it to force the movement of electric charges present in the wire of its windings through an external electric circuit. This flow of electric charges constitutes the output electric current supplied by the generator.

Body History and development

Before the connection between magnetism and electricity was discovered, electrostatic generators were used. They operated on electrostatic principles. Such generators generated very high voltage and low current. They operated by using moving electrically charged belts, plates, and disks that carried charge to a high potential electrode. The charge was generated using either of two mechanisms: Electrostatic induction and the triboelectric effect. Because of their inefficiency and the difficulty of insulating machines that produced very high voltages, electrostatic generators had low power ratings, and were never used for generation of commercially significant quantities of electric power, even at the time of its development.

The operating principle of electromagnetic generators was discovered in the years of 1831–1832 by Michael Faraday. The principle, later called Faraday’s law, is that an electromotive force is generated in an electrical conductor which encircles a varying magnetic flux.

The first homopolar generator was also developed by Michael Faraday during his experiments in 1831. It is frequently called the Faraday disc Faraday wheel in his honor. It was the beginning of modern dynamos — that is, electrical generators which operate using a magnetic field. It was very inefficient and was not used as a practical power source, but it showed the possibility of generating electric power using magnetism, and led the way for commutated direct current dynamos and then alternating current alternators.

The Faraday disc was primarily inefficient due to counterflows of current. While current flow was induced directly underneath the magnet, the current would circulate backwards in regions outside the influence of the magnetic field. This counterflow limits the power output to the pickup wires, and induces waste heating of the copper disc. Later homopolar generators would solve this problem by using an array of magnets arranged around the disc perimeter to maintain a steady field around the circumference, and eliminate areas where counterflow could occur.


The Faraday disk was the first electric generator. The horseshoe-shaped magnet (A) created a magnetic field through the disk (D). When the disk was turned, this induced an electric current radially outward from the center toward the rim. The current flowed out through the sliding spring contact m, through the external circuit, and back into the center

This design was inefficient, due to self-cancelling counterflows of current in regions that were not under the influence of the magnetic field. While current was induced directly underneath the magnet, the current would circulate backwards in regions that were outside the influence of the magnetic field. This counterflow limited the power output to the pickup wires, and induced waste heating of the copper disc. Later homopolar generators would solve this problem by using an array of magnets arranged around the disc perimeter to maintain a steady field effect in one current-flow direction.

Another disadvantage was that the output voltage was very low, due to the single current path through the magnetic flux. Experimenters found that using multiple turns of wire in a coil could produce higher, more useful voltages. Since the output voltage is proportional to the number of turns, generators could be easily designed to produce any desired voltage by varying the number of turns. Wire windings became a basic feature of all subsequent generator designs.

Direct current generators


This large belt-driven high-current dynamo produced 310 amperes at 7 volts. Dynamos are no longer used due to the size and complexity of the commutator needed for high power applications.

The dynamo was the first electrical generator capable of delivering power for industry. The dynamo uses electromagnetic induction to convert mechanical rotation into direct current through the use of accumulator. An early dynamo was built by Hippolyte Pixii in 1832.

The modern dynamo, fit for use in industrial applications, was invented independently by Sir Charles Wheatstone, Werner von Siemens and Samuel Alfred Varley. Varley took out a patent on 24 December 1866, while Siemens and Wheatstone both announced their discoveries on 17 January 1867, the latter delivering a paper on his discovery to the Royal Society.

The “dynamo-electric machine” employed self-powering electromagnetic field coils rather than permanent magnets to create the stator field. Wheatstone’s design was similar to Siemens’, with the difference that in the Siemens design the stator electromagnets were in series with the rotor, but in Wheatstone’s design they were in parallel. The use of electromagnets rather than permanent magnets greatly increased 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.

The dynamo machine that was developed consisted of a stationary structure, which provides the magnetic field, and a set of rotating windings which turn within that field. On larger machines the constant magnetic field is provided by one or more electromagnets, which are usually called field coils.


Large power generation dynamos are now rarely seen due to the now nearly universal use of alternating current for power distribution. Before the adoption of AC, very large direct-current dynamos were the only means of power generation and distribution. AC has come to dominate due to the ability of AC to be easily transformed to and from very high voltages to permit low losses over large distances.

Alternating current generators

Through a series of discoveries, the dynamo was succeeded by many later inventions, especially the AC alternator, which was capable of generating alternating current.

Alternating current generating systems were known in simple forms from Michael Faraday’s original discovery of the magnetic induction of electric current. Faraday himself built an early alternator. His machine was a “rotating rectangle”, whose operation was heteropolar – each active conductor passed successively through regions where the magnetic field was in opposite directions.

Large two-phase alternating current generators were built by a British electrician, J.E.H. Gordon, in 1882. The first public demonstration of an “alternator system” was given by William Stanley, Jr., an employee of Westinghouse Electric in 1886.

Sebastian Ziani de Ferranti established Ferranti, Thompson and Ince in 1882, to market his Ferranti-Thompson Alternator, invented with the help of renowned physicist Lord Kelvin. His early alternators produced frequencies between 100 and 300 Hz. Ferranti went on to design the Deptford Power Station for the London Electric Supply Corporation in 1887 using an alternating current system. On its completion in 1891, it was the first truly modern power station, supplying high-voltage AC power that was then “stepped down” for consumer use on each street. This basic system remains in use today around the world.


After 1891, polyphase alternators were introduced to supply currents of multiple differing phases. Later alternators were designed for varying alternating-current frequencies between sixteen and about one hundred hertz, for use with arc lighting, incandescent lighting and electric motors.


As the requirements for larger scale power generation increased, a new limitation rose: the magnetic fields available from permanent magnets. Diverting a small amount of the power generated by the generator to an electromagnetic field coil allowed the generator to produce substantially more power. This concept was dubbed self-excitation.

The field coils are connected in series or parallel with the armature winding. When the generator first starts to turn, the small amount of remanent magnetism present in the iron core provides a magnetic field to get it started, generating a small current in the armature. This flows through the field coils, creating a larger magnetic field which generates a larger armature current. This “bootstrap” process continues until the magnetic field in the core levels off due to saturation and the generator reaches a steady state power output.

Very large power station generators often utilize a separate smaller generator to excite the field coils of the larger. In the event of a severe widespread power outage where islanding of power stations has occurred, the stations may need to perform a black start to excite the fields of their largest generators, in order to restore customer power service.

Specialized types of generator Homopolar generator

A homopolar generator is a DC electrical generator comprising an electrically conductive disc or cylinder rotating in a plane perpendicular to a uniform static magnetic field. A potential difference is created between the center of the disc and the rim (or ends of the cylinder), the electrical polarity depending on the direction of rotation and the orientation of the field.

It is also known as a unipolar generatoracyclic generatordisk dynamo, or Faraday disc. The voltage is typically low, on the order of a few volts in the case of small demonstration models, but large research generators can produce hundreds of volts, and some systems have multiple generators in series to produce an even larger voltage. They are unusual in that they can produce tremendous electric current, some more than a million amperes, because the homopolar generator can be made to have very low internal resistance.

Induction generator

Some AC motors may be used as generators, turning mechanical energy into electric current. Induction generators operate by mechanically turning their rotor faster than the synchronous speed, giving negative slip. A regular AC asynchronous motor usually can be used as a generator, without any internal modifications. Induction generators are useful in applications such as minihydro power plants, wind turbines, or in reducing high-pressure gas streams to lower pressure, because they can recover energy with relatively simple controls.

To operate, an induction generator must be excited with a leading voltage; this is usually done by connection to an electrical grid, or sometimes they are self-excited by using phase correcting capacitors.

Linear electric generator

In the simplest form of linear electric generator, a sliding magnet moves back and forth through a solenoid – a spool of copper wire. An alternating current is induced in the loops of wire by Faraday’s law of induction each time the magnet slides through. This type of generator is used in the Faraday flashlight. Larger linear electricity generators are used in wave power schemes.

Variable speed constant frequency generators

Many renewable energy efforts attempt to harvest natural sources of mechanical energy (wind, tides, etc.) to produce electricity. Because these sources fluctuate in power applied, standard generators using permanent magnets and fixed windings would deliver unregulated voltage and frequency.

New generator designs such as the asynchronous or induction singly-fed generator, the doubly-fed generator, or the brushless wound-rotor doubly fed generator are seeing success in variable speed constant frequency applications, such as wind turbines or other renewable energy technologies. These systems thus offer cost, reliability and efficiency benefits in certain use cases.


In electricity generation, a generator is a device that converts mechanical energy to electrical energy for use in an external circuit. The source of mechanical energy may vary widely from a hand crank to an internal combustion engine. Generators provide nearly all of the power for electric power grids.

The reverse conversion of electrical energy into mechanical energy is done by an electric motor, and motors and generators have many similarities. Many motors can be mechanically driven to generate electricity and frequently make acceptable generators.

Key vocabulary


Appliance – prietaisas

electric power – elektros energija

outage – (mašinos) darbo sustojimas

energy – energija

source – šaltinis

output – išeiga

wire – laidai

windings – apvijos

circuit – grandinė

current – srovė

connection – jungtis

magnetism – magnetizmas

electricity – elektra

Electrostatic – elektrostatinis

voltage – įtampa

electrically charged – elektros krūvis

Charge – įkrova

Electrode – elktrodas

Induction – indukcija

Inefficient- neefektyvus

Power – galia

Conductor – laidininkas

Flux – pastoviai kintamas

Magnetic – magnetinis

a magnetic field – magnetinis laukas

power source – enrgijos šaltinis

direct current – nuolatinė srovė

alternating current – kintama srovė

Counterflows – atgalinis srautas

Accumulator – akumuliatorius

high-current – aukšta srovė

coil – ritė

stator – statorius

rotor – rotorius

Alternator – kintamos srovės generatorius

Lighting – apšvietimas

Iron – geležis

Conductive – laidus

Cylinder – cilindras

Rim – rėmas

Disc – diskas

electrical polarity – elektrinis poliškumas

Field – laukas

Series- nuosekliai

Tremendous – didžiulis

Parallel – lygiagrečiai

loop – kilpa

mechanical power – mechaninė galia

generate – generuoti

permanent magnets – nuolatiniai magnetai

stator – statorius

air gap – oro tarpas

magnetic core – magnetinė šerdis

commutator – komutatorius

stationary brushes – stacionarus šepetėliai

field poles – lauko poliai

windings – apvijos

pole – polius

slip ring – slidimo žiedas

fixed-speed – fiksuoto greičio

variable-speed – kintamas greitis

universal motors – universalūs varikliai

electric utilities- elektros tiekimo paslaugos

electricity transmission- elektros energijos perdavimas

power station- elektrinė

generator- generatorius

kinetic energy- kinetinė energija

hydroelectric- hidroelektrinis

potential energy- potencinė energija

motion- judėjimas

utilise- naudoti

extracted energy- išgauta energija

energysaver- energija taupantis


signal- signalas

period- periodas

rheostat- reostatas

winding- apvija

earthing system-įžeminimas

condensed- kondensuotas

wave power- bangų energija

resistor- rezistorius

electric circuit- elektros grandinė

construction- konstrukcija

magnet- magnetas

diffusion- difuzija

conductor- laidininkas

voltage source- įtampos šaltinis

elements- elementai

switches- jungikliai

transformer- transformatorius

consumers- vartotojai

meter- matavimo prietaisas

electron tube- elektronų vamzdelis

frequency- dažnis

transfer energy- keisti energiją

thermal – šiluminis

terminal – gnybtas

positive – teigiamas

Resistor – rezistorius