Electric generator
An electric generator is any device capable of maintaining an electrical potential difference between two of its points (called poles, terminals or terminals) transforming mechanical energy into electrical energy. This transformation is achieved by the action of a magnetic field on the electrical conductors arranged on an armature (also called a stator). If a relative movement between the conductors and the field is produced mechanically, an electromotive force (EMF) will be generated. This system is based on Faraday's law.
Although the current generated is alternating current, it can be rectified to obtain a direct current. The attached diagram shows the induced current in a simple one-phase generator. Most alternating current generators are three phase.
The reverse process would be carried out by an electric motor, which transforms electrical energy into mechanical energy.
History
Before the connection between magnetism and electricity was discovered, electrostatic generators had already been invented. They worked on electrostatic principles, through the use of electrically charged ribbons, plates, and disks that carried the charge to a high-potential electrode. The charge was generated using one of two mechanisms, electrostatic induction or the triboelectric effect. Such generators produced very high voltages and low current. Due to their inefficiency and the difficulty in insulating the machines that produced such high voltages, electrostatic generators had low power levels and were never used for the generation of commercially significant amounts of electrical power. Its only practical applications consisted in powering the first X-ray tubes and, later, in some atomic particle accelerators.
Faraday disk generator
The operating principle of electromagnetic generators was discovered in the years 1831-1832 by Michael Faraday. The principle, later called Faraday's law, is that an electromotive force is generated in an electrical conductor that surrounds a changing magnetic flux.
He also built the first electromagnetic generator, called a Faraday disk; a type of homopolar generator, using a copper disc that rotates between the poles of a horseshoe magnet. A small continuous voltage is produced.
This design was inefficient due to self-cancelling backflows of current in regions of the disk not under the influence of the magnetic field. While the current was induced directly under the magnet, the current flowed backwards in the regions that were outside the influence of the magnetic field. This backflow limited the power output in the pickup cables and induced heating of the copper disk debris. Later homopolar generators would solve this problem by using a series of magnets arranged around the perimeter of the disk to maintain a stable field effect in one direction of current flow.
Another disadvantage was that the output voltage was very low, due to having only one current path through the magnetic flux. The experimenters found that using multiple turns of wire in a coil could produce higher and more useful voltages. Since the output voltage is proportional to the number of turns, generators could easily be designed to produce any desired voltage by varying the number of turns. Wire windings became a basic feature of all subsequent generator designs.
Jedlik and the phenomenon of self-excitation
Independently of Faraday, the Hungarian Ányos Jedlik began experimenting in 1827 with electromagnetic revolving devices which he called electromagnetic autorotors. In the prototype of the single-pole electric starter (completed between 1852 and 1854) both the stationary and rotating parts were electromagnetic. He was also the discoverer of the self-excitation principle of the dynamo, which superseded permanent magnet designs. He may also have formulated the concept of a dynamo in 1861 (before Siemens and Wheatstone did) but did not patent it because he thought he was not the first to realize it.
Direct current generators
A coil of wire rotating in a magnetic field produces a current that changes direction with each 180° rotation, an alternating current (AC). However, many early uses of electricity required direct current (DC). In the first practical electrical generators, called dynamos, AC was converted to DC with a commutator, a set of rotating switch contacts on the armature shaft. The commutator reversed the connection of the armature winding to the circuit every 180° of shaft rotation, creating a pulsating direct current. One of the first dynamos was built by Hippolyte Pixii in 1832.
The dynamo was the first electrical generator capable of delivering energy for industry. The 1844 Woolrich electric generator, now in the Thinktank, Birmingham Science Museum, was the first electric generator used in an industrial process. It was used by the Elkingtons firm for commercial electroplating.
The modern dynamo, suitable for use in industrial applications, was invented independently by Charles Wheatstone, Werner von Siemens, and Samuel Alfred Varley. Varley obtained a patent on December 24, 1866, while Siemens and Wheatstone announced their discoveries on January 17, 1867, with the latter submitting a paper on his discovery to the Royal Society.
The "dynamo-electric machine" (dynamo-electric machine) used self-powered electromagnetic field coils instead of permanent magnets to create the stator field. Wheatstone's design was similar to that of Siemens, with the difference that in the Siemens design the stator electromagnets were in series with the rotor, but in the Wheatstone design they were in parallel. The use of electromagnets instead of permanent magnets greatly increased the power output of the dynamo. and allowed the generation of high powers for the first time. This invention led directly to the first major industrial uses of electricity. For example, in the 1870s, Siemens used electromagnetic dynamos to power electric arc furnaces for the production of metals and other materials.
The dynamo machine that was developed consisted of a stationary structure, which provided the magnetic field, and a set of rotating windings that rotated within that field. In larger machines, the constant magnetic field is provided by one or more electromagnets, which are generally called field coils.
Large power generating dynamos are now rarely seen due to the almost universal use of alternating current for power distribution. Before the adoption of AC, very large DC dynamos were the only means of power generation and distribution. AC has come to dominate because of AC's ability to easily transform to and from very high voltages to allow low transmission losses over long distances.
Synchronous generators (alternating current generators)
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Thanks to a series of discoveries, the dynamo was followed by many later inventions, especially the AC alternator, which was capable of generating alternating current. It is commonly known as synchronous generator (SG). Synchronous machines are directly connected to the grid and must be correctly synchronized during startup. In addition, they are excited with a special control to improve the stability of the power system.
Alternating current generation 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 heteropolar: each active conductor passed successively through regions where the magnetic fields were in opposite directions.
The 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" it was made by William Stanley, Jr., an employee of Westinghouse Electric in 1886.
Sebastian Ziani de Ferranti founded the company Ferranti, Thompson and Ince 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 Deptford Power Station for the London Electric Supply Corporation in 1887 using an alternating current system. Upon its completion in 1891, it was the first truly modern power station, supplying high-voltage AC power that was then 'stepped down' to low voltage. for consumer use on every street. This basic system is still in use today throughout the world.
After 1891, polyphase alternators were introduced to supply currents from multiple different phases. Later alternators were designed to vary alternating current frequencies between sixteen and approximately one hundred hertz, for use with arc lighting, incandescent lighting and electric motors.
Self-excitation
As the requirements for large-scale power generation increased, a new limitation emerged: the magnetic fields available from permanent magnets. Diverting a small amount of the power generated by the generator to an electromagnetic field coil allowed the generator to produce substantially more power. This concept was called self-excitation. Field coils were connected in series or parallel with the armature winding. When the generator began to rotate, the small amount of remaining magnetism present in the iron core provided a magnetic field to start it up, generating a small current in the armature. This flows through the field coils, creating a larger magnetic field that generates a larger armature current. This "booting" it continues until the magnetic field in the core is turned off due to saturation and the generator reaches a stable power output. Very large power station generators often used a separate smaller generator to drive the field coils of the larger ones. In the event of a widespread power outage, stations may need to perform a black start to energize the fields of their largest generators in order to restore customer power service.
Other electric current generation systems
Not only is it possible to obtain an electrical current from mechanical energy of rotation, but it is possible to do it with any other type of energy as a starting point. From this broader point of view, generators are classified into two fundamental types:
- Primary: They convert the energy of another nature that they receive or that of which they initially have, as alternators, dinamos, etc.
- Secondary: They deliver part of the electrical energy they have previously received, that is, they first receive energy from an electric current and store it in the form of some kind of energy. Subsequently, they transform again the energy stored in electrical energy. An example is rechargeable batteries or batteries.
Specific devices will be grouped according to the physical process that serves as their foundation.
Primary generators
The starting energy and the physical conversion process are shown schematically. Direct energy conversions have been considered in all cases. For example, hydrogen has chemical energy and can be converted directly into an electrical current in a fuel cell. It would also be its combustion with oxygen to release thermal energy, which could expand a gas, thus obtaining mechanical energy that would turn an alternator to finally obtain, by magnetic induction, the desired current.
| Power of departure | Physical process that converts such energy into electrical energy |
|---|---|
| Magnet-mechanical energy | They are the most frequent and treated as generic electric generators.
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| Chemical energy (without magnetic field intervention) | Electrochemical cells and their derivatives: electric batteries, batteries, fuel cells.
See your differences in electrochemical generators. |
| Electromagnetic radiation | Photoelectricity, as in the photovoltaic panel |
| Mechanical energy (without magnetic field intervention) |
|
| Thermal energy (without magnetic field intervention) | Thermoelectricity (seebeck effect) |
| Nuclear energy (without magnetic field intervention) | Radioisotope Thermoelectric Generator |
In most cases, the performance of the transformation is so low that it is preferable to do it in several stages. For example, converting nuclear energy into thermal energy, then into mechanical energy from a high-pressure gas that turns a turbine at high speed, to finally obtain, by electromagnetic induction, an alternating current in an alternator, the most important electrical generator. from a practical point of view as a source of electricity for almost all current uses.
Ideal Generators
From the theoretical point of view (circuit theory), there are two types of ideal generators:
* Voltage generator or voltage: an ideal voltage generator maintains a fixed voltage across its terminals regardless of the load resistance Rc that may be connected between they.
* Current generator or current: an ideal current generator maintains a constant current through the external circuit regardless of the resistance of the load that may be connected between them.
In (figure 1) you can see the simplest possible circuit, consisting of a constant voltage generator E connected to a load Rc and where the equation would be fulfilled:
E = I×Rc
The described generator has no real existence in practice, since it always has what has been conventionally called internal resistance, which, although it is not really a resistance, in most cases behaves as such.
In (figure 2) you can see the same circuit as before, but where the internal resistance of the generator is represented by a resistance Ri, in series with the generator, so the above equation becomes:
E = I×(Rc+Ri)
Thus, a real generator can be considered in many cases as an ideal voltage generator with an internal resistance in series, or as an ideal current generator in parallel with a resistance.
Components of a generator
In addition to the engine and generator, electric generators generally include a fuel supply, constant engine speed governor (governor) and generator voltage regulator, cooling and exhaust systems, and lubrication system.
Units greater than 1 kW of power often have a battery and an electric starter; In addition, very large units can be started with compressed air or either with an air-actuated starter or introduced directly into the engine cylinders to start engine rotation. Standby power generating units often include an automatic starting system and a transfer switch to disconnect the load from the utility power source when there is a power failure and connect it to the generator.
Electromotive force of a generator
A characteristic of each generator is its electromotive force (EMF), symbolized by the Greek letter epsilon (ε) and defined as the work that the generator performs to pass the unit of positive charge from the negative to the positive pole through the interior of the generator. generator.
The E.M.F. (ε) is measured in volts, and in the case of the circuit in Figure 2 it would be equal to the voltage E, while the potential difference between the points a and b, Va-b is dependent on the charge Rc.
The E.M.F. (ε) and the potential difference coincide in value in the absence of load, since in this case, being I = 0, there is no voltage drop in Ri and therefore Va-b = E.