Definition: a machine for generating electrical energy from mechanical energy
More specific terms: synchronous generator, asynchronous generator, linear generator, external pole generator, internal pole generator, direct current generator, alternating current generator, three-phase generator
An (electrical) generator is a machine that can produce electrical energy from mechanical energy. By design, most generators strongly resemble certain electric motors, and many electric machines can indeed be used as motors or generators.
The term generator is often used in a broader sense than a technical device that converts mechanical, chemical, thermal or electromagnetic energy directly into electrical energy. For example, a thermoelectric generator can produce electrical energy directly from heat. Photovoltaic cells are also generators in this sense. However, the rest of this article applies to electromechanical generators.
Power Efficiency is the ratio of useful power output divided by the power input (total electrical power used).
Power efficiency principle and designs
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 frequently used:
With the out-of-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. It must then be transferred to the outside, e.g. via sliding contacts with brushes, which is problematic at high power levels.
With 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 again, but to a much lesser extent, as the energy required for excitation is only a small fraction of the generator output. Alternatively, a small additional external pole excitation machine can be implemented to cover the rotor's power requirement without brushes.
If electric current is then also taken from the induction coil, i.e. electric energy is generated by the generator, a counterforce is created which brakes the movement. The greater the electrical power taken, the greater the necessary mechanical drive power. In contrast, an electrically unloaded generator hardly brakes the drive source at all.
There are also electrostatic generators that do not use magnetic fields. However, these are very rarely used and are only suitable for very low power.
There is a wide range of different generator designs, adapted to the respective applications. Depending on the design, a generator produces alternating current (possibly also in the form of three-phase current) or direct current. Direct current is obtained by rectifying the originally generated alternating voltage, either internally through a commutator (electrical contacts which periodically reverse the direction of connection of the rotor coil in the external pole generator) or through an external rectifier.
Synchronous and asynchronous generators
Some AC and three-phase generators operate synchronously, i.e. their speed is fixed by the frequency of the power grid to which they feed. Such synchronous generators are used almost exclusively for high power. However, smaller generators often operate as asynchronous generators, where there is a certain speed slip: They turn a little faster, especially when operating at high power. This results in a certain loss of power efficiency, especially with smaller generators.
Another disadvantage of the asynchronous generator is a certain amount of reactive current. But in return it is particularly simple to build and robust. Synchronous generators are often designed so that an adjustable reactive power can be generated.
Relationship between speed and number of poles
A high number of poles is required to operate a generator at low speeds.
With alternating current and three-phase generators there is (just like with motors) 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 in the synchronous machine corresponds exactly to the speed of the rotor, 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. 3000 revolutions per minute. With two pole pairs, the speed drops to 1500 rpm, whereas 2000 rpm cannot be achieved.
Slow-running generators, e.g. in hydroelectric power stations and gearless wind turbines, must have a high number of poles. In contrast, turbogenerators 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.e. the magnets used are permanent magnets (permanent magnets). For very large generators in power stations, practically only electrical excitation (external excitation) is used, i.e. electromagnets are used.
A part of the electrical energy produced is thus used for excitation, but this part can be quite small (for large generators well below 1% of the power produced), as the coils of the electromagnets have a low electrical resistance.
When starting an electrically excited generator without an external energy source, there is in principle the problem that initially there is no energy available to operate the exciter. However, at least a small residual magnetic field remains from the previous operation, which at least allows a small induction voltage. This now causes a small current through the exciter coil, so that the magnetic field and thus the induced voltage continues to increase.
The generator can be started in a short time even without an external energy source. The basic principle described is known as the dynamoelectric principle.
In the meantime, generators with outputs of several megawatts, as used in particular in wind turbines, are also designed with permanent excitation. This is made possible by the use of high-performance neodymium magnets and allows 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 in vehicles with hybrid drive, for example. Unfortunately, however, the extraction of neodymium in mines (currently largely in China) is a very polluting process, as the ore contains many undesirable other substances, some of which are very toxic and some of which are radioactive. In addition, bottlenecks in the supply of rare earths such as neodymium threaten.
It should be noted, however, that mining activities could be made more environmentally friendly in the future and that at the end of the life of such a generator, the entire quantity of the neodymium contained can be recycled, as this material is not consumed.
Another possibility is electrical excitation with superconducting coils, which became practicable with the development of high-temperature superconductors (HTS). In this case, there is no need to use electrical energy for excitation, since the current can flow through the coils without any resistance. However, energy is required for the operation of refrigerating machines to cool the coils sufficiently to achieve superconductivity. Nevertheless, the power efficiency can be quite high. In addition, this principle enables a particularly compact design.
Energy losses in a generator are mainly caused by the electrical resistance of the coils (ohmic losses, copper losses) and by generated unwanted eddy currents in iron cores (iron losses), additionally also by mechanical friction and air resistance as well as in larger generators by the energy expenditure for cooling equipment.
However, large stationary generators achieve very high power efficiency of often more than 98 % or even 99 %. Generators with superconducting coils (see above), which are very power efficient and can be built smaller, are now being developed, especially for supplying ship propulsion systems.
Applications of electrical generators
The main applications of electric generators are:
- The vast majority of electrical energy is produced by generators, especially turbo generators.
- In most power plants, electrical energy is generated in one or more generators from mechanical energy. This is especially true for hydroelectric power plants, wind turbines and all types of thermal power plants, regardless of whether the heat is produced by burning fuel 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 (recuperation) during braking, which can be used to charge the vehicle battery (accumulator). Similarly, most electric locomotives can use the motor as a generator and feed braking energy back into the overhead line.
- In vehicles with combustion engines, an on-board alternator generates the necessary electrical energy, unless a larger generator is already available through a hybrid drive. In bicycles, a small dynamo of very low power is used, which today is often designed as a more power efficiency and reliable wheel hub dynamo.
Typical characteristics of electric generators
Generators can meet a wide range of requirements, depending on their design:
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 power efficiency of sometimes more than 98 %. A high degree of power efficiency is possible over a wide range of outputs, i.e. also in partial load operation.
Depending on the design, a generator produces 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 electric motors. One then often speaks more generally of electric 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 system) are avoided.