Electric generator
In electricity generation, a generator is a device that converts mechanical energy to electrical energy for use in an external circuit. The source of mechanical energy may vary widely from a hand crank to an internal combustion engine. Generators provide nearly all of the power for electric power grids.
The reverse conversion of electrical energy into mechanical energy is done by an electric motor, and motors and generators have many similarities. Many motors can be mechanically driven to generate electricity and frequently make acceptable generators.
The operating principle of electromagnetic generators was discovered in the years of 1831–1832 by Michael Faraday. The principle, later called Faraday's law, is that an electromotive force is generated in an electrical conductor which encircles a varying magnetic flux.
He also built the first electromagnetic generator, called the Faraday disk, a type of homo polar generator, using a copper disc rotating between the poles of a horseshoe magnet. It produced a small DC voltage.
The Faraday disk was the first electric generator. The horseshoe-shaped magnet (A) created a magnetic field through the disk (D). When the disk was turned, this
1 | P a g e induced an electric current radially outward from the centre toward the rim. The current flowed out through the sliding spring contact m, through the external circuit, and back into the centre of the disk through the axle. AC GENERATOR
Through a series of discoveries, the dynamo was succeeded by many later inventions, especially the AC alternator which was capable of generating alternating current.
Alternating current generating systems were known in simple forms from Michael Faraday's original discovery of the magnetic induction of electric current. Faraday himself built an early alternator. His machine was a "rotating rectangle", whose operation was hetero polar - each active conductor passed successively through regions where the magnetic field was in opposite directions.
Large two-phase alternating current generators were built by a British electrician, J.E.H. Gordon, in 1882. The first public demonstration of an "alternator system" was given by William Stanley, Jr., an employee of Westinghouse Electric in 1886.
Sebastian de Ferranti established Ferranti, Thompson in 1882, to market his Ferranti-Thompson Alternator, invented with the help of renowned physicist Lord Kelvin. His early alternators produced frequencies between 100 and 300 Hz. Ferranti went on to design the Deptford Power Station for the London Electric Supply Corporation in 1887 using an alternating current system. On its completion in 1891, it was the first truly modern power station, supplying high- voltage AC power that was then "stepped down" for consumer use on each street. This basic system remains in use today around the world.
2 | P a g e Principle
A.C. generators or alternators (as they are usually called) operate on the same fundamental principles of electromagnetic induction as D.C. generators.
Alternating voltage may be generated by rotating a coil in the magnetic field or by rotating a magnetic field within a stationary coil. The value of the voltage generated depends on-
the number of turns in the coil. strength of the field. the speed at which the coil or magnetic field rotates.
As in the DC motor case, a current is passed through the coil, generating a torque on the coil. Since the current is alternating, the motor will run smoothly only at the frequency of the sine wave. It is called a synchronous motor. More
3 | P a g e common is the induction motor, where electric current is induced in the rotating coils rather than supplied to them directly.
One of the drawbacks of this kind of AC motor is the high current which must flow through the rotating contacts. Sparking and heating at those contacts can waste energy and shorten the lifetime of the motor. In common AC motors the magnetic field is produced by an electromagnet powered by the same AC voltage as the motor coil. The coils which produce the magnetic field are sometimes referred to as the "stator", while the coils and the solid core which rotates is called the "armature". In an AC motor the magnetic field is sinusoidal varying, just as the current in the coil varies.
The turning of a coil in a magnetic field produces motional emf in both sides of the coil which add. Since the component of the velocity perpendicular to the magnetic field changes sinusoidal with the rotation, the generated voltage is sinusoidal or AC. This process can be described in terms of Faraday's law when you see that the rotation of the coil continually changes the magnetic flux through the coil and therefore generates a voltage.
In 1891, Nikola Tesla patented a practical "high-frequency" alternator (which operated around 15 kHz). After 1891, poly phase alternators were introduced to supply currents of multiple differing phases. Later alternators were designed for varying alternating-current frequencies between sixteen and about one hundred hertz, for use with arc lighting, incandescent lighting and electric motors.
4 | P a g e Operation
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 at which the shaft rotates. 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. Traveler to Europe find other frequencies.
5 | P a g e Small Application
AC generators ranging in size 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.
Large Application
Larger AC generators are used for the same purposes as the smaller ones but also have other applications, including for use in oil and gas extraction, mining machinery and rail and marine transportation. Often these generators are packaged with an engine or turbine to be used as a motor-generator set. These applications require from 200 kilowatts to 18 megawatts. They can supply a significant proportion of a building's electrical power when used to replace much
6 | P a g e of the utility power in a computer or telecommunications facilities.
EXPRESSION FOR EMF IN A.C. GENERATOR
Let
N=number of turns
A=face area of turn
B=magnitude of magnetic field
=angle which normal makes with B
=angular velocity of coil
=NBA cos =NBA cos t
=-d/dt = -d/dt (NBA cos t) = NBAsint
7 | P a g e =0 sint
0=NBA
I=/R = 0 sint/R = I0 sint
Thank you
8 | P a g e 9 | P a g e