How does a generator work 1

generator

Lexicon> Letter G> Generator

Definition: a machine for generating electrical energy from mechanical energy

More specific terms: synchronous generator, asynchronous generator, linear generator, outer pole generator, inner pole generator, direct current generator, alternating current generator, three-phase generator

English: generator

Categories: electrical energy, prime movers and power plants

Author: Dr. Rüdiger Paschotta

How to quote; suggest additional literature

Original creation: 04/26/2010; last change: 08.05.2021

URL: https://www.energie-lexikon.info/generator.html

An (electric) generator is a machine that can produce electrical energy from mechanical energy. Most generators are very similar in design to certain electric motors, and many electric machines can in fact also be used as motors or generators.

Often the term generator More broadly as a technical device that converts mechanical, chemical, thermal or electromagnetic energy directly into electrical energy. For example, a thermoelectric generator can generate electrical energy directly from heat. In this sense, photovoltaic cells are also generators. The remainder of this article applies to electromechanical generators, however.

Basic principle and types

The basic physical principle of the generator is electrical induction: a voltage is induced in an electrically conductive coil when the magnetic flux through the coil changes. This is achieved by moving a magnet against the coils. There are two different ways of doing this (apart from the less common principle of the linear generator), both of which are often used:

  • At the Outer pole generator the magnetic field is generated in the stator (the non-moving part of the generator) and electrical energy is generated by induction in the rotor. You must then z. B. be transmitted to the outside via sliding contacts with brushes, which is problematic at high power.
  • At the Internal pole generator the magnetic field is generated in the rotor and induction takes place in the stator. In the case of electrical excitation (see below), electrical energy often has to be transmitted via brushes, but to a much lesser extent, since the energy required for excitation is only a small fraction of the generator output. Alternatively, a small additional external pole exciter can be implemented in order to cover the power requirements of the rotor without brushes.

If the induction coil then also draws electrical current, i.e. electrical energy is generated by the generator, a counterforce is created that brakes the movement. The greater the electrical power drawn, the greater the mechanical drive power required. An electrically unloaded generator, on the other hand, hardly brakes the drive source. To estimate the required drive torque, you can simply divide the power drawn by the angular speed of the drive; the actual torque is slightly higher due to energy losses, among other things. due to friction and often also eddy currents as well as ohmic losses in the lines.

There are also electrostatic Generators that do not use magnetic fields. However, these are only used very rarely and are only suitable for very low levels of performance.

There are a large number of different types of generator that are adapted to the respective application. Depending on the design, a generator generates alternating current (possibly also in the form of three-phase current) or direct current. Direct current is obtained when the originally generated alternating voltage rectified which is done either internally by a commutator (electrical contacts that periodically reverse the connection direction of the rotor coil in the external pole generator) or by an external rectifier.

Synchronous and asynchronous generators

Some alternators and three-phase generators work synchronously, i. H. their speed is fixed by the frequency of the power grid into which they feed. For high achievements, these are almost exclusively used Synchronous generators used. However, smaller generators often work as Asynchronous generatorswhere there is a certain Speed ​​slip gives: They spin a little faster, especially when running at high power. This results in a certain loss of energy efficiency, especially with smaller generators. Another disadvantage of the asynchronous generator is a certain reactive power requirement. But it is particularly easy to build and robust. Synchronous generators are often designed in such a way that an adjustable reactive power can be generated.

Relationship between speed and number of poles

A high number of poles is necessary to operate a generator at low speeds.

With AC and three-phase generators (just as with motors) there is a more or less fixed relationship between the speed and the mains frequency, which, however, also depends on the number of poles (= 2 number of pole pairs): The speed of the magnetic field (Rotating field speed), which corresponds exactly to the speed of the rotor in the case of a synchronous machine, is the mains frequency divided by the number of pole pairs. For example, the minimum possible number of pole pairs 1 at 50 Hz mains frequency results in a rotating field speed of 50 / s = 3000 / min, i.e. H. 3000 revolutions per minute. With two pole pairs, the speed drops to 1500 revolutions per minute, while z. B. 2000 rpm are not achievable. Slow running generators, e.g. B. in hydropower plants and gearless wind turbines, must have a high number of poles. In contrast, turbo generators that are driven directly by turbines are usually two-pole or sometimes four-pole.

Permanent excitation and electrical excitation

Small generators (such as bicycle dynamos) are usually permanently excited, i. H. the magnets used are permanent magnets. For very large generators in power plants, in practice only electrical excitation (external excitation) comes into question, i. H. one uses electromagnets. A part of the generated electrical energy is therefore used for excitation, but this part can be quite small (with large generators well below 1% of the generated power), since the coils of the electromagnets have a low electrical resistance.

Can an electrically excited generator be started without an external energy source?

When starting an electrically excited generator without an external energy source, there is in principle the problem that initially no energy is available to operate the exciter. However, at least a small residual magnetic field remains from earlier operation, which at least enables a small induction voltage. This now causes a small current through the excitation coil, so that the magnetic field and thus the induced voltage continue to increase. Ultimately, the generator can be started in a short time even without an external energy source. The basic principle described is called the dynamo-electric principle designated.

High-performance neodymium magnets also allow the implementation of larger, permanently excited generators.

In the meantime, generators with outputs of several megawatts, such as those used in particular in wind energy plants, are designed with permanent excitation. This is made possible by using high-performance neodymium magnets and enables both a compact design and (due to the particularly strong magnetic field) operation at very low speeds, so that even a gearbox can be dispensed with. Smaller versions of such generators are also used, for example, in vehicles with hybrid drives. Unfortunately, however, the extraction of neodymium in mines (currently largely in China) is a very polluting process, as the ore contains many other undesirable substances, some of which are very toxic and some of which are also radioactive. In addition, there is a risk of bottlenecks in the supply of rare earths such as neodymium. It should be noted, however, that mining activities could be designed to be more environmentally friendly in the future and that at the end of the service life of such a generator, the entire amount of the neodymium contained can be recycled, since this material is not consumed.

With high-temperature superconductors, relatively compact and very efficient generators can be built for use in ships, for example.

Another possibility is electrical excitation with superconducting coils, which has become practical with the development of high-temperature superconductors (HTS). In this case, there is no need for electrical energy for the excitation, since the current can flow through the coils without any resistance. However, this requires energy for the operation of refrigeration machines in order to cool the coils sufficiently so that superconductivity is achieved. Nevertheless, the energy efficiency can be quite high. In addition, this principle enables a particularly compact design.

Energy efficiency

Energy losses in a generator are mainly caused by the electrical resistance of the coils (ohmic losses, Copper losses) and by undesired eddy currents generated in iron cores (Iron losses), also through mechanical friction and air resistance and, in the case of larger generators, through the energy required for cooling equipment. Large stationary generators, however, achieve very high efficiencies of often more than 98% or even 99%. Generators with superconducting coils (see above), which are very efficient and can be made smaller, are now being developed primarily for supplying ship propulsion systems.

In principle, small generators can also have very high levels of efficiency. However, a compromise must often be made between efficiency and other aspects. For example, a car's alternator should be as light and compact as possible, and of course the costs (including material costs) often play an important role in optimization.

Applications of electrical generators

The main applications of electrical generators are:

The vast majority of electrical energy is generated with generators, especially turbo-generators.
  • In most power plants, electrical energy is generated from mechanical energy in one or more generators. This applies in particular to hydropower plants, wind turbines and all types of thermal power plants, regardless of whether the heat is produced by burning fuels or in a nuclear reactor. In the case of thermal power plants, turbo generators are practically always used.
  • In electric cars and vehicles with hybrid drive, the drive motor usually serves as a generator for the recovery of braking energy when braking (Recuperation) that can be used to charge the vehicle battery (accumulator). Similarly, most electric locomotives can use the engine as a generator and feed braking energy back into the catenary.
  • In vehicles with a combustion engine, an on-board alternator generates the necessary electrical energy, unless a larger generator is already available through a hybrid drive. Bicycles use a small dynamo of very low power, which today is often designed as a more efficient and reliable wheel hub dynamo.

Typical properties of electrical generators

Depending on the design, generators can meet a wide range of requirements:

  • Electrical outputs between less than one watt and significantly more than one gigawatt are possible.
  • Particularly large generators (e.g. in power plants with outputs of hundreds of megawatts) achieve very high efficiencies of sometimes more than 98%. A high degree of efficiency is possible over a wide range of outputs, i.e. also in partial load operation. The electrical voltage can be tens of kilovolts (but still well below that of high-voltage lines), and electrical currents of tens of kilo-amperes are common.
  • Depending on the design, a generator generates direct current, alternating current or three-phase current, and it can be driven at constant or variable speeds.
  • Many generators can also be operated as an electric motor. One then speaks more generally of Electrical machines.
  • The service life of a generator is usually very long (often many decades) as long as certain problematic operating conditions (e.g. severe overload, excessive speed or failure of the cooling) are avoided.

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See also: electric motor, electrical energy, mechanical energy, motor, turbo generator, alternator, starter generator
and other items in the electrical power, prime movers, and power plant categories