The main historical and technical reasons have led to the popularity and use of large alternating current (AC) generators, while the relative lack of large direct current (DC) generators. Here are some explanations:
1.Transmission distance limitations: DC power generation faces large energy losses during transmission. DC current gradually decays as the transmission distance increases, requiring larger wires and more power to compensate for the energy loss. This was a serious problem in the early days of power delivery, as longer distances required more expensive wires. Alternating current can use transformers to increase voltage and reduce current, thereby reducing energy losses and transmission costs.
2.Generator design difficulty: The design of DC generators is relatively complex. DC generators require the use of rotating commutators, also known as rectifiers, to convert alternating current into DC current. This technical challenge has limited the development of large DC generators in the past. In contrast, alternators do not require the use of commutators, making them simpler and more reliable to design and manufacture.
3.Grid compatibility: AC is the commonly adopted grid standard. With the development of power systems, transmission and distribution infrastructure around the world is built on AC grids. AC power can be used to change the voltage up or down through transformers, from high-voltage transmission to low-voltage distribution. This unified grid structure makes the transmission, distribution and use of electrical energy more flexible and efficient.
Although there have been some developments in DC transmission technology in recent years, such as high-voltage direct current (HVDC) transmission systems, the use of large DC generators in the overall power system is still limited. This is mainly due to the transmission advantages of alternating current, the compatibility of grid facilities, and the maturity of alternating current technology. However, with the energy transition and the rapid development of renewable energy, the importance of DC transmission technology is gradually increasing, and the application of large-scale DC generators is likely to be more popularized and adopted in some specific fields.
Large alternating current (AC) generators and large direct current (DC) generators have some significant structural differences. Here are some of their key differences:
Structural features of large alternators:
1.Rotor structure: The rotor of an alternator is usually wound around a rotating shaft and rotated by mechanical energy to cut the magnetic field to generate induced electromotive force. Common types of alternators include synchronous generators and asynchronous (induction) generators, which have slightly different rotor structures.
2.Commutators: Alternators do not require the use of commutators as the current they themselves produce is AC. This reduces complexity and cost.
3.Stator structure: The stator is the immovable part that contains the coil and the core. In a synchronous generator, the coils on the stator move in synchrony with the magnetic field of the rotor. In an asynchronous generator, the coils on the stator generate an electric current by induction, without the need to synchronize with the rotor.
4.Brushes and slip rings: Some alternators, especially small synchronous generators, may use brushes and slip rings to supply current to the rotor. But in large generators, brushless designs are often used, eliminating the use of brushes and slip rings.
Structural features of large DC generators:
Commutator (Rectifier): A key component of a DC generator is the commutator, also known as a rectifier. This is used to convert the generated alternating current into direct current. A commutator usually consists of a series of commutators that are responsible for turning the alternating current into a unidirectional current.
Brushes and slip rings: Large DC generators typically use brushes and slip ring systems. The brushes are conductive carbon blocks that provide current to the rotor by coming into contact with the slip ring. This design allows the DC generator to maintain DC current during rotation.
Excitation system: DC generators typically require an excitation system to generate a magnetic field that cuts the rotor and generates electromotive force. The excitation system can be a separate part of the DC generator or a part of the DC generator itself.
In general, the structural differences between alternators and DC generators are mainly reflected in the way alternating current is handled and the mechanism by which the current is generated. These differences reflect their strengths and characteristics in different application scenarios.
The relatively small size of modern DC generators has to do with technological advancements and areas of application, and the reasons why they do not have as many volumes and types as alternators may be manifold:
1.Technological advancements: Advances in science and technology have enabled engineers to design more efficient and compact DC generators. Advanced materials science, manufacturing processes, and design methods enable DC generators to achieve higher power output in a smaller footprint. In addition, the miniaturization and integration of electronic components also contribute to the reduction of the size of the generator.
2.Changes in the field of application: DC generators are often used in more specific areas, such as electric motors, generator sets, wind turbines, etc. These applications can be more demanding in terms of volume and weight, so engineers are working to design more compact and lightweight DC generators to meet specific application needs.
3.Standardization: DC generators can be relatively standardized, as DC generators in most applications follow similar design codes and standards. This makes it easier for manufacturers to produce large quantities of standardized DC generators, reducing costs and increasing efficiency.
4.Limitations of scope of application: DC generators have a relatively narrow scope of application and are usually used in specific applications that require DC power, such as electric vehicles, ships, wind power generation, etc. In contrast, alternators have a wider range of applications, including household appliances, industrial machinery, power plants, etc., so alternators of different volumes and powers are required to meet different needs.
In general, the reason for the small size of modern DC generators is mainly due to the combined effect of factors such as technological progress, changes in application fields, standardization and limitations in the scope of application.
The replacement of large DC generators with modern large alternators in the past, as well as the development of DC generators to smaller DC generators, as well as the relatively variable alternator size, can be attributed to a series of technological and economic evolutions:
1.Transmission efficiency: DC generators need to use commutators (rectifiers) to convert alternating current to DC current. In the early power system, this commutator technology was relatively complex and introduced large energy losses. In contrast, alternating current can be raised or lowered by voltage through transformers, thereby reducing transmission losses. As a result, AC transmission is more efficient when transmitting power over long distances.
2.Grid standardization: The power system adopted alternating current as a standard in the early stages of development. The standardization of the power system has led to the widespread use of alternators, while the use of DC generators has gradually decreased.
3.Technological improvements: As power systems continue to evolve, alternator design and control techniques continue to improve, making them more efficient and reliable. This includes the use of advanced materials, cooling technology, digital control systems, and more.
4.Rise of renewable energy: With the rise of renewable energy, especially wind and solar, the current generated by these energy sources is usually direct. DC generators are more suitable for these applications because they do not require current conversion and are able to output DC power from renewable energy directly to the grid.
5.Increase in applications of small DC generators: With the development of electronics, small DC generators have become more practical in many applications, such as electric vehicles, computer hard drives, electronic devices, etc. These applications have created a demand for small, efficient, and controllable DC generators, prompting the development of related technologies.
For the variable volume of the alternator, this is mainly because the alternator is used in a variety of scenarios, from household electricity to industrial production, each application area has different requirements for the power, size and performance of the generator. As a result, the design of the alternator needs to be adapted to the specific use, resulting in the existence of alternators of different volumes and types.