Generators

Generators and electric motors would never have been invented if not for the discovery of the relationship between electricity and magnetism.

On 21 July 1820, a Dutch scientist called Hans Christian Oersted published the first important discovery. He found that an electric current produces a magnetic field.

On 17 October 1831, Michael Faraday found that if a conductor (for example a piece of copper wire) cuts through a magnetic field an electric current is produced in the conductor. If the conductor does not cut through a magnetic field, no electric current is produced.

How a generator works

A generator is a device that uses Faraday's discovery to change mechanical energy to electrical energy. The diagram below illustrates the operation of a simple generator.

1. In position 1, the sides of the coil AB and CD cut through the magnetic field producing an electric current.
2. In position 2, no current is produced because the sides AB and CD are moving parallel to the magnetic field and are therefore not cutting it.
3. In position 3, the sides AB and CD of the coil are cutting the magnetic field producing a current. However the current is now moving in the opposite direction to the current produced in when the coil was in position 1. (This is because the sides AB and CD are cutting the magnetic field in the opposite direction to when the coil was in position 1).

The graph in the diagram shows how the current changes. It is important to observe that the current reaches a peak each time it completes one-half revolution and the current keeps changing direction. This type of current is called an Alternating Current or AC.

The turbine spins the generator in the power station at 3000 rpm. This produces a 50 Hertz (Hz) alternating current. That is, the current changes direction 50 times per second. All household voltage in Australia is 240 Volts 50 Hertz.

The emf or voltage generated is determined by: Where: = magnetic field strength measured in teslas (T) N = the number of coils of wire A = the area of the coils measured in square metres (m 2 ) f = the frequency of revolution measured in Hertz (Hz) t = period of revolution measured in seconds (s) = electromotive force (emf or voltage)