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The Electric Generator

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In its most basic definition, an electric generator is a machine that produces electricity. Generators produce almost all of the electricity used by people. They supply the electric power that runs machines in factories, provide lighting, and operate appliances in the home. There are two main types of generators – alternating-current (AC) generators (sometimes called alternators) and direct-current (DC) generators (also known as dynamos). Both types of generator work on the same principles.

How A Generator Works (Basic Principles)

A generator does not create energy – “energy cannot be created or destroyed, only changed from one form to another” (Sir Isaac Newton). A generator changes mechanical energy into electric energy. Every generator must be driven by a machine that produces mechanic energy. For example, the large generators that provide electricity to Lae and the rest of PNG are driven by hydro power. The mechanical device that powers a generator is known as a prime mover. To obtain more electrical energy from a generator, the prime mover must supply more mechanical energy.

Generators produce electricity by means of a principle discovered independently by two physicists in 1831 – Michael Faraday of England and Joseph Henry of the United States. Faraday and Henry found that they could produce electricity in a coil of copper wire by moving the coil near a magnet. This process is called electromagnetic induction.

A simple generator (see Figure 1 below) consists of a single loop of wire turning in a magnetic field. Electricity is produced only while the loop is rotating. As the loop rotates, the two sides “cut” the magnetic lines of force. This induces electricity into the loop. One complete revolution of the loop through the lines of force is called a cycle. The frequency of the voltage or current, which is number of cycles in a second, is measured in units called hertz (one hertz equals one cycle per second). The rotating loop is connected to an outside circuit by either a pair of slip rings (in an AC generator) or a commutator (in a DC generator). These connections are the only mechanical differences between AC and DC generators.

Alternating Current (AC) Generators

The simple generator shown in Figure 1 produces alternating current, in which the electricity flows regularly in one direction and then the other, in a loop of wire. Slip rings and brushes on each end of the loop of wire allow the generator to send the current produced to the devices that use it.

How AC Generators Work

Practical AC generators are usually equipped with a small supporting generator called an exciter. The exciter supplies direct current for the electromagnets used to create the magnetic field in the generator. The armature of an AC generator consists of a copper wire wound in hundreds of coils around slots cut in an iron ore.

Uses of AC Generators

AC generators are the main generators in almost all electric power plants, meaning they provide the electricity we use every day. This is because alternating current can easily be increased or decreased (using a transformer). In this way, the voltage can be increased significantly to force the current over long distances, and then lowered when needed by household appliances etc. A Serbian engineer named Nicola Tesla was responsible for designing many of the devices today that use alternating current, including polyphase ac generators (generators with more than one phase), the electric motor, and transformer systems for changing the voltage of alternating current. Tesla’s inventions made it possible to generate electric current far from the place where the current is to be used.

Direct Current (DC) Generators

Direct current generators differ from AC generators in that a commutator is used instead of slip rings (see Figure 4). The commutator segments rotate with the loop, and make contact with two fixed brushes. When the loop turns, the commutator segments come alternately in contact with each brush. Because of this, the current flowing in each brush is always in the same direction. Therefore direct current flows from the generator to the outside electric circuit.

Uses of DC Generators

DC generators are used by the electroplating industry and factories that manufacture aluminium, chlorine and other industrial materials which need large amounts of direct current for their production. DC generators are also used in vehicles, trains and ships driven by diesel-electric motors.


§AC – alternating current – an electric current in which the electricity flows regularly in one direction and then the other, usually reversing 120 times per second.

§Armature – a coil of wire wound around an iron core, placed between opposite poles of a magnet in an electric motor or generator.

§Current – a flow of electricity through a wire, or the rate or amount of such a flow, usually measured in amperes.

§DC – direct current – a steady electric current that flows in one direction only.

§Electromagnet – a piece of soft metal that becomes a strong magnet when an electric current is passed through wire coiled around it.

§Electromagnetic Induction – the production of an electromotive force in a circuit by variation of the magnetic field with which the circuit is connected.

§Electromagnetism – magnetism produced by a current of electricity.

§Electromotive Force – the amount of energy derived from an electric source per unit of current passing through the source. Electromotive force is commonly measured in volts.

§Generator – a machine that changes mechanical energy into electrical energy and produces either direct or alternating current; also known as a dynamo.

§Magnetic Field – the space around a magnet in which its power of attraction is effective; space around an electric current in which the magnetic force of the current is felt.

§Prime Mover – the first agent that puts a machine in motion, such as wind or hydro-power.

§Voltage – the strength of electrical force, measured in volts.

§Watt – a unit of electric power equal to the flow of one ampere under the pressure of one volt, to one joule, or to 1/746 horsepower.


Nelson Physics – Updated Second Edition

Alan Storen & Ray Martine2000

IBM Word Book Encyclopedia 2000

IBM Inc2000

New Standard Encyclopedia – Vol 4


Physics 132 Notes : The Electric Generator

P. Rubin – Richmond University1999

Tiscali Reference Dictionaryhttp://www.tiscali.co.uk/reference


Physics Central – Generators



WVIC – How An Electric Generator Works



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