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An electrical generator is a device that produces electrical energy from a mechanical energy source using electromagnetic induction. The process is known as electricity generation.

Historic developments

Before the connection between magnetism and electricity was discovered, generators used electrostatic principles. The Wimshurst machine used electrostatic induction or "influence". The Van de Graaff generator uses either of two mechanisms:

* Charge transferred from a high-voltage electrode
* Charge created by the triboelectric effect using the separation of two insulators (the belt leaving the lower pulley)

Electrostatic generators are inefficient and are useful only for scientific experiments requiring high voltages.


In 1831-1832 Michael Faraday discovered that a potential difference is generated between the ends of an electrical conductor that moves perpendicular to a magnetic field. He also built the first electromagnetic generator called the Faraday disc, a type of homopolar generator, using a copper disc rotating between the poles of a horseshoe magnet. It produced a small direct current of electricity.


The dynamo was the first electrical generator capable of delivering power for industry, and is still the most important generator in use in the 21st century. The dynamo uses electromagnetic principles to convert mechanical rotation into an alternating electric current. It is the most common way to generate electrical energy for bicycle lighting.

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. Furthermore, 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.

Jedlik's dynamo

In 1827, 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.

Gramme dynamo

Both of these designs suffered from a similar problem: they induced "spikes" of current followed by none at all. Antonio Pacinotti, an Italian scientist, fixed this by replacing the spinning coil with a toroidal one, which he created by wrapping an iron 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. Various versions and improvements have been made since then, but the basic concept of a spinning endless loop of wire remains at the heart of all modern dynamos.

Permanent magnet generator


The generator creates an electric current, but does not create electric charge, which is already present in the conductive wire of its windings. It is somewhat analogous to a water pump, which creates a flow of water but does not create the water itself.

Other types of electrical generator exist, based on other electrical phenomena such as piezoelectricity, and magnetohydrodynamics. The construction of a dynamo is similar to that of an electric motor, and all common types of dynamos could work as motors. Also, all common types of electric motors could work as generators.

The generator rotor is turned by a device termed a prime mover, often a Diesel engine, steam turbine, water turbine or gas turbine coupled to the rotor shaft.


Early motor vehicles tended to use DC generators with regulators. These were not particularly reliable or efficient and have now been replaced by alternators with built-in rectifier circuits. These power the electrical systems on the vehicle and recharge the battery after starting. Rated output will typically be in the range 50-100 A at 12 V, depending on the forecast electrical load within the vehicle - some cars now have electrically-powered steering assistance and air conditioning, which places a high load on the electrical system. Commercial vehicles are more likely to use 24 V to give sufficient power at the starter motor to turn over a large diesel engine without the requirement for unreasonably thick cabling. Vehicle alternators do not use permanent magnets; they can achieve efficiencies of up to 90% over a wide speed range by control of the field voltage.

Some of the smallest generators commonly found are used to power bicycle lights. These tend to be 0.5 A permanent-magnet alternators, supplying 3-6 W at 6 V or 12 V. Being powered by the rider, efficiency is at a premium, so these may incorporate rare-earth magnets and be designed and manufactured with great precision. Nevertheless, the maximum efficiency is only around 60% for the best generators - 40% is more typical - due to the use of permanent magnets. A battery would be required in order to use a controllable electromagnetic field instead, and this is unacceptable due to its weight and bulk.

Aircraft have also switched from DC generators to alternators; these are typically powered by a takeoff from an engine.

Sailing yachts may use water or wind powered generator to trickle-charge the batteries. A small propeller, wind turbine or impeller is connected to a low-power alternator and rectifier to supply currents of up to 10 A at typical cruising speeds.


An engine-generator is the combination of an electrical generator and an engine mounted together to form a single piece of equipment. This combination is also called an engine-generator set or a genset. In many contexts, the engine is taken for granted and the combined unit is simply called a generator.

In addition to the engine and generator, engine-generators generally include a fuel tank, an engine speed regulator and a generator voltage regulator. Many units are equipped with a battery and electric starter. Standby power generating units often include an automatic starting system and a transfer switch to disconnect the load from the utility power source and connect it to the generator.

Engine-generators produce alternating current power that is used as a substitute for the power that might otherwise be purchased from a utility power station. The generator voltage (volts), frequency (Hz) and power (watts) ratings are selected to suit the load that will be connected. Both single-phase and three-phase models are available. There are only a few portable three-phase generator models available in the US. Most of the portable units available are single phase power only and most of the three-phase generators manufactured are large industrial type generators.

Engine-generators are available in a wide range of power ratings. These include small, hand-portable units that can supply several hundred watts of power, hand-cart mounted units, as pictured above, that can supply several thousand watts and stationary or trailer-mounted units that can supply over a million watts. The smaller units tend to use gasoline (petrol) as a fuel, and the larger ones have various fuel types, including diesel, natural gas and propane (liquid or gas).

Engine-generators are often used to supply electrical power in places where utility power is not available and in situations where power is needed only temporarily. Small generators are sometimes used to supply power tools at construction sites. Trailer-mounted generators supply power for lighting, amusement rides etc. for traveling carnivals.

Standby power generators are permanently installed and kept ready to supply power to critical loads during temporary interruptions of the utility power supply. Hospitals, communications service installations, sewage pumping stations and many other important facilities are equipped with standby power generators.

Small and medium generators are especially popular in third world countries to supplement grid power, which is often unreliable. Trailer-mounted generators can be towed to disaster areas where grid power has been temporarily disrupted.

The generator can also be driven by the human muscle power (for instance, in the field radio station equipment).

Mid-size stationary engine-generator

The mid-size stationary engine-generator pictured here is a 100 kVA set which produces 415 V at around 100 A per phase. It's powered by a 6.7 liter turbocharged Perkins Phaser 1000 Series engine, and consumes approximately 27 liters of fuel an hour, on a 400 liter tank. Stationary generators used in the US are used in size up to 2800 kW. These diesel engines are run in the UK on red diesel and rotate at 1500 rpm. This produces power at 50 Hz, which is the frequency used in the UK. In areas where the power frequency is 60 Hz (United States), generators rotate at 1800 rpm or another even multiple of 60.

Turbine generator

A microturbine (or miniature gas turbine) is a turbine designed for DARPA by M-Dot. Many model engineers relish the challenge of re-creating the grand engineering feats of today as tiny working models. Naturally, the idea of re-creating a powerful engine such as the jet fascinated hobbyists since the very first full-size engines were powered up by Hans von Ohain and Frank Whittle back in the 1930s.

generators, gas generators, turbine generators