About AC Generators

AC generators in a hydroelectric plant
AC generators in a hydroelectric plant (Image: Electricity Turbines image by steheap from Fotolia.com)

AC generators are electro-mechanical machines that use the turning of a rotor in a magnetic field to generate alternating current electricity. You use the output of an AC generator when you turn on a light or start your car. The power companyâ??s AC generators that power your house are at the large end of the size range. The alternator in your vehicle is at the small end.


In 1831 Michael Faraday discovered that by rotating a loop of copper wire between the poles of a permanent magnet he could produce an alternating electrical current (References 1). Faradayâ??s discovery of electromagnetic induction led to the developments and refinements of present-day AC generators.


AC generators generate electricity by the rotation of an armature in an electromagnetic field. The magnetic field is produced by permanent magnets or a direct current circuit in the stator windings. The voltage, current flow and frequency of the current are controlled by the strength of the magnetic field, the windings in the rotor, the poles in the stator and the speed the shaft rotates (References 2). For example, a small standby generator might produce 30 amps at 120 volts AC and 60 cycles per second (Hertz). Sixty Hertz is the standard frequency in the United States. Travelers to Europe find other frequencies.

Mechanical Power

The alternator in your car is powered by the engine. Power for your house is generated in most cases by steam, water or gas turbines. Steam is produced by the burning of coal, oil or natural gas, or from heat generated by radioactive decay in the case of nuclear power. Hydroelectric power is generated by the energy of water held by a dam. Gas turbines use the products of natural gas or oil combustion directly to turn a turbine.rnrnWind energy is used to power generators as are internal-combustion engines. These engines use gasoline, diesel fuel, propane or natural gas as fuel. Some AC generators are even driven by DC (direct current) electric motors in one form of motor-generator set (References 3).

Ac generator on a wind turbine
Ac generator on a wind turbine (Image: Wind turbine top maintenance 02.jpg image by Paul Dearden from Fotolia.com)

Small Applications

AC generators in sizes from three to 2,000 kilowatts and powered by engines operating on hydrocarbon fuels are used for portable generators for construction projects, as standby units to protect buildings from power outages, as the sole source of power at remote locations, and to limit power usage in utility systems that charge extra for use above a given quantity (References 4).

Large Applications

Larger AC generators are used for the same purposes as the smaller ones but also have other applications as in oil and gas extraction, mining machinery, rail and marine transportation. Often these generators are packaged with an engine or turbine to be used as motor-generator set. These applications require from 200 kilowatts to 18 megawatts. They can supply a significant proportion of a buildingâ??s electrical power when used to replace much of the utility power in a computer or telecommunications facility (References 5).

Locomotive powered by diesel engine driven AC generator
Locomotive powered by diesel engine driven AC generator (Image: locomotive image by Dawn Williams from Fotolia.com)

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