Electrical Generators & Areas of Application

In electricity generation, a generator is a device that converts motive power (mechanical Energy) into electrical power for use in external circuit. Sources of mechanical energy include steam turbines, gas turbines, water turbines, internal combustion engines and even hand cranks...

Photo: Abdul Aziz Abdo; commons.wikimedia.org

In electricity generation, a generator is a device that converts motive power (mechanical energy) into electrical power for use in external circuit. Sources of mechanical energy include steam turbines, gas turbines, water turbines, internal combustion engines and even hand cranks. The first electromagnetic generator, the Faraday disk was built in 1831 by British scientist Micheal Faraday. Generators provide nearly all of the power for electric power grids.

Alternating Current

Synchronous Generator or Alternators

These are rotating machines that rotate at a fixed speed fixed by the supply frequency and number of poles in the magnetic circuit. These convert mechanical power from the prime-mover to an AC electric power at a specific voltage and frequency. A synchronous generator rotates at constant speed called synchronous speed. Synchronous generators are usually three phase, because of several advantages of three phase generation, transmission and distribution.

• The cost of transmission is less than for the same voltage and power in a single phase system
• A three phase generator has a 180% greater capacity than a single phase generator of same physical size.
• Single-phase voltage and power is easily available from a three phase system by merely tapping any two of power leads.

Alternator consists of two parts rotor and stator. Stator is the stationary part which carry armature winding in which EMF is induced, output is taken from the stator. The rotor is the rotating part. It produces the main field flux. The rotor field winding may be supplied by DC generator whose shaft is coupled to the rotor shaft of same alternator. The power rating of this DC generator may vary from 2%-5% of the power rating of the same alternator. The rotor employed may be sailent type or cylindrical type. Sailent type rotor has field poles projected from the rotor surface. These type of rotors usually have large radius and short axial length. Sailent type of rotors are usually employed in low speed turbines due to their noisy operation in high speed applications. Usually, sailent type of rotor configuration is used in all hydro-generators. Cylindrical rotor has field poles distributed all over its outer periphery. About 66% of the rotor periphery is slotted and the unslotted regions represent field poles. This type of rotor has small diameter, but long axial length. Hence, it is suitable for high speed applications.

AC generator could employ either rotating armature or rotating field configuration, but selection of one over the other completely depends upon the size and capacity of alternator. Rotating armature AC generator are typically used in applications involving small amount of power. With large amount of power, larger amount of current may flow through the slip rings and brushes. It is difficult to and expensive to build slip rings and brushes to carry large amount of currents. Therefore, most large AC generators are rotating field generators.

Alternators are natural source of reactive power. Load current flows in stator circuit as EMF is induced in it due to rotating rotor magnetic field. As the current flows in armature winding located in stator, the armature field so produced may effect the main field and the effect is knows as armature reaction. This armature reaction may be magnetizing, de-magnetizing or cross magnetizing depending on the load power factor.

Magneto Hydrodynamic Generator
Image credit: skullsinthestars.com

Induction Generator

Induction AC motors may be used as generators, turning mechanical energy into electric current. Induction generators operate by mechanically turning their rotor faster than the synchronous speed, giving negative slip. A regular AC asynchronous motor usually can be used as a generator without any internal modifications. Induction generators are useful in applications such as mini hydro power plants, wind turbines, or in reducing high-pressure gas streams to lower pressure, because they can recover energy with relatively simple controls. They do not require an exciter circuit because the rotating magnetic field is provided by induction from the stator circuit. They also do not require speed governor equipment as they inherently operate at the connected grid frequency. To operate, an induction generator must be excited with a leading voltage; this is usually done by connection to an electrical grid, or sometimes they are self-excited by using phase correcting capacitors.

Linear Electric Generator

In the simplest form of linear electric generator, a sliding magnet moves back and forth through a solenoid – a spool of copper wire. An alternating current is induced in the loops of wire by Faraday’s law of induction each time the magnet slides through. This type of generator is used in the Faraday flashlight. Larger linear electricity generators are used in wave power schemes. Almost all wave energy devices proposed so far utilizes conventional high speeds, rotary generators to convert mechanical energy into electricity. Several imaginative solutions have been developed to accomplish the conversion of the wave’s bouncing motion to the rotary motion demanded by the generator. These power take off schemes are expensive both to construct and to maintain and they are often very vulnerable to extreme weather.

The main difference of a linear generator to conventional generator is that the motion of the rotor is linear which makes it possible to couple motion directly to the reciprocating, vertical motion of the waves which eliminates the need of the complex power take off scheme and gear boxes. The system consists of abuoy, floating on surface of the ocean connected with a row to the rotor i.e. a piston with permanent magnet the piston, in turn is moving in coil where electricity is induced.

Variable Speed Constant Frequency Generators

Many renewable energy efforts attempt to harvest natural sources of mechanical energy (wind, tides, etc.) to produce electricity. Because these sources fluctuate in power applied, standard generators using permanent magnets and fixed windings would deliver unregulated voltage and frequency. The overhead of regulation (whether before the generator via gear reduction or after generation by electrical means) is high in proportion to the naturally-derived energy available. New generator designs such as the asynchronous or induction singly-fed generator, the doubly fed generator, or the brushless wound-rotor doubly fed generator are seeing success in variable speed constant frequency applications such as wind turbines or other renewable energy technologies. These systems thus offer cost, reliability and efficiency benefits in certain use cases.

Direct Current

Homopolar Generator

A homopolar generator is a DC electrical generator comprising an electrically conductive disc or cylinder rotating in a plane perpendicular to a uniform static magnetic field. A potential difference is created between the center of the disc and the rim (or ends of the cylinder), the electrical polarity depending on the direction of rotation and the orientation of the field. It is also known as a unipolar generator, acyclic generator, disk dynamo, or Faraday disc. The voltage is typically low, on the order of a few volts in the case of small demonstration models, but large research generators can produce hundreds of volts, and some systems have multiple generators in series to produce an even larger voltage. They are unusual in that they can produce tremendous electric current, some more than a million amperes, because the homopolar generator can be made to have very low internal resistance.

MHD Generator

A magneto hydrodynamic generator directly extracts electric power from moving hot gases through a magnetic field without the use of rotating electromagnetic machinery. MHD generators were originally developed because the output of a plasma MHD generator is a flame, well able to heat the boilers of a steam power plant. The first practical design was the AVCO Mk. 25, developed in 1965. The U.S. government funded substantial development, culminating in a 25 MW demonstration plant in 1987.

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