About AC Generators

By David Weedmark

Updated September 13, 2019

About AC Generators
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Without the invention of the AC generator and its development over the past century or so, the world would be a very dark place indeed. Generators producing alternating current power our lights, our heating and cooling systems, our appliances and our computers. Originally invented by accident, AC generators were once deemed worthless and unusable and were then vilified for many years before they finally became the indispensable machines they are today.

An AC generator applies Faraday's law of electromagnetic induction to create an alternating current by rotating wire coils inside a magnetic field. Mechanical energy rotates the coils, usually from a motor powered by wind, water, diesel or other energy sources.

Alternating Current Compared to Direct Current

Alternating current, or AC, is the kind of electricity that is available from the outlets in your walls, while direct current, or DC, is made by batteries. Static electricity is also DC electricity, produced by rubbing silk and glass against each other or by running a plastic comb through your hair.

Direct current maintains the same polarity over time and always goes in the same direction, which is why the positive and negative terminals on a battery never change. The polarity or direction of alternating current changes several times each second, so the flow of electricity also changes several times each second. In the United States, AC power in your home alternates 60 times per second.

While many things in our homes and offices require AC current, many of the devices we power using an AC outlet actually use a DC current. This includes most electronics, including computers. A rectifier connected to the power cord does the conversion.

How Generators Work

Simply put, if you rotate a magnetic field around wire coils or rotate wire coils inside a magnetic field, you have created an AC generator. The physics behind this is described by Faraday's law of electromagnetic induction.

To understand how the magnetic field produces an alternating current, imagine a magnet, like the needle on a compass, spinning. At any point around the compass, the positive and negative poles of the magnet pass by as the magnet spins. The alternating magnetic field applied to a wire coil alternates the voltage polarity, producing a reversing current in the circuit.

DC generators are similar to AC generators, but they are more complex to engineer, are not as efficient and require much more maintenance than AC generators. A DC generator has its coil of wire mounted on the shaft, whereas an AC generator has the magnet there. As the coil spins, it comes into contact with carbon brushes mounted around the coil, turning the external circuit on and off in rhythm with the alternating magnetic poles so that only one polarity is sent to the external circuit.

The History of AC Generators

The history of AC generators is intertwined with the history of dynamos and DC electricity. The electric generator has its origins in the work of Michael Faraday and Joseph Henry, who discovered electromagnetic induction. The first AC generator was created accidentally by Hippolyte Pixii while he was inventing the first dynamo in 1832, which delivered pulses of DC electricity. After 1832, some important milestones in the development of generators included:

  • 1860: Antonio Pacinotti invented a dynamo that provided continuous DC power.
  • 1867: Werner Von Siemens and Charles Wheatstone invented a more powerful dynamo using a self-powered electromagnet.

  • 1871: Zenobe Gramme created the first commercially feasible dynamo by placing an iron core in the magnetic field, vastly increasing the power output.

  • 1878: The Ganz company created the first AC generators to be used in commercial operations in Budapest.

Until the late 1880s, early use of electricity was limited to direct current in the U.S. Thomas Edison had invested heavily in DC technology. However, DC power began to run into problems when more and more people wanted electricity in their houses and businesses. DC current could not easily be converted to different voltages, so power stations needed to be within a few city blocks of each customer. This would soon change thanks in large part to the work of Nikola Tesla.

1893 and the War of the Currents

In the late 19th century, Nikola Tesla, backed by George Westinghouse, had shown that unlike direct current, alternating current can be converted from one voltage to another using a transformer. So, power can be sent over long distances using a high voltage and then throttled down to a lower voltage to service customers.

Thomas Edison, however, had invested far too much money in DC infrastructure to make the switch to AC without a fight. Edison began a smear campaign to discredit AC power as being far too dangerous to use, and among other things, promoted an AC-powered electric chair as a new method of execution and electrocuted stray animals as public demonstrations of the danger of AC electricity.

Two events in 1893 marked the end of DC's reign. First, the Chicago World's Fair was illuminated by 100,000 electric lights powered by Tesla's alternating current, dazzling the millions of visitors who attended each night. Second, the Niagara Falls Power Company awarded Westinghouse the contract to generate electricity from Niagara Falls using Tesla's polyphase AC induction motors (a type of AC generator), which would soon provide power to Buffalo, New York and most of the eastern United States.

Types of Generators

Electrical generators all produce electrical power from kinetic energy, so it is common for them to be attached to a motor to move the generator's shaft. A diesel-powered motor turning a generator's shaft can be called a diesel generator, a diesel fuel generator set or a diesel fuel genset.

Most generators, including backup generators, are permanently installed where they are needed, while smaller, portable generators are used for things like camping trips or to power homes during a power outage. All AC generators can be categorized by the input energy source:

  • Fossil Fuels: These include gasoline generators, diesel generators, natural gas generators and propane generators.

  • Natural Energy: Also called green-power generation systems, these include solar generators and wind-powered generators. Hydroelectric generators, which capture the motion of flowing water, also fall into this category.

  • Existing Energy: Some generators capture excess energy that is already being produced by another machine or process, like in manufacturing plants. Hydraulic generators, for example, use the hydraulic pressure created by another system to power the electric generator. 

    Thermoelectric generators use the temperature difference between two conductors or semiconductors in a phenomenon called the Seebeck effect. 

Applications of AC Generators

In addition to their power sources, generators can be classified by their application in industry. Some of these include:

  • Power Plants: These generators produce electricity for an entire region, including cities, towns, homes and businesses.

  • Agriculture: A power take-off generator uses a tractor's driveshaft as an input energy source. This is an example of a portable generator.  

  • Aircraft: Large planes often use AC generators to provide power to onboard electrical systems. One type of system is called an aircraft ram air turbine generator, which harnesses air pressure created by the aircraft's flight to spin a turbine that is attached to a generator.

  • Airports: Aircraft starter generators and ground-support generators power aircraft when the engines shut down to operate onboard electrical systems.

  • Automotive Generators: Called alternators, these produce AC current that is transformed to DC current to run the vehicle's electronic systems. Vehicles with larger power requirements, like recreational vehicles, have larger generators.

  • Marine Generators: Using the ship's engines as an input energy source, these provide power to a ship's electrical systems.

  • Welding: Arc welding generators provide high output currents, measured in hundreds of amperes, to provide the electricity needed for arc welding equipment.