history
Alternating current is a form of electric energy, which uses alternating current to provide commercial electricity. One of the first practical books designed by William Stanley Jr. led to alternating current. His design is the early predecessor of modern transformer, called induction coil. From 188 1 to 1889, this system was conceived by nikola tesla, George Westinghouse, Lv Xian Gaurad, John Gibbs and Oliver Shalenger. According to Thomas Edison's discovery that it was first used in commercial power distribution, these systems overcame the limitations caused by the use of direct current.
The first long-distance transmission of alternating current took place in 189 1 near Telluride, Colorado, and it took place in Germany a few months later. Thomas Edison strongly advocated the use of direct current (DC), and there were many patented technologies, but eventually AC came into widespread use (see Tidal War). Steinmetz General Electric Company has solved many problems of power generation and transmission by alternating current.
Distribution and domestic power supply
Unlike DC, alternating current can rise or fall to another voltage through a transformer. In alternating current, higher voltage means higher power transmission efficiency. The improvement of efficiency is attributed to ohm's law, which points out that power loss depends on the current process in the command. The power loss in the command is attributed to the power flow and is described by the convention P = I 2 * R, which means that if the power flow is doubled, the power loss will be four times as large and so on.
By using a transformer, the voltage of electric power is transmitted and can "climb" to a relatively high voltage. Climbing is beneficial to long-distance transmission, because climbing means low current, so power loss is also low. Once the power supply reaches its destination, its voltage may be changed by other transformers and exit the stage, which is safe for domestic power supply.
Three-phase electronics is very common and is a more effective way to use commercial generators. Electric energy is rolled up in the magnetic field by rotating parts, and the capital cost is very high in the generator. However, it is relatively simple and efficient to include three different rollers on a single shaft instead of one. These rollers are located on the generator shaft, but they are completely separated, and the angle between them is 120 degrees. It is caused by the current signal waveform of 120 degrees outside each other, but it is also huge.
Three-phase distribution is widely used in industrial places, and single-phase distribution is widely used in home environment. A three-stage transformer can usually be several circuits, providing another stage to different sides of the power supply circuit.
The three-stage system is designed so that they are balanced in load, and if the load is balanced correctly, the tide will not flow from a neutral point of view. And the worst case of neutral power flow not exceeding the maximum phase power flow is loaded out of balance (linear). For the three-stage A 4 conductor system with low (normal) voltage, it is usually used by 1/3 to reduce the cable requirements and use a separate neutral wire for each stage. When entering the next stage of 3, transformers with triangular primary and satellite secondary are usually used, so the neutral wire on the power supply side is not needed.
For smaller customers (how little the installation changes according to the country and age), all three stages of larger installation only adopt single phase and neutral, and the motherboard adopts neutral. When choosing from the three-phase motherboard, the three-phase circuit may lead out (in some cases, it will lead out two phases (not to be confused with two phases)) and neutral lead out.
A phased system (single-phase center development is based on two hot 180 degrees, outside the phase) is useful when only one or two high-voltage phases are available, and only the part that may be easily supplied as a single phase under normal voltage is needed. This kind of system is common in almost all households in North America, and it is normal to connect large power equipment across hot.
The third wire (there should always be, but there are many wires that are older, incompatible or not installed in the third world) is usually connected between the corresponding electrical appliances in the house and the main electrical switch or fuse box. The third conductor is called a grounding conductor in Britain and most other English-speaking countries, but it is a grounding conductor in the United States. What happened before the grounding conductor on the motherboard changed, but according to their European names, there are three main possibilities TT (the customer grounding listed here is not connected to the basic neutral) Tn S (neutral and grounding are managed separately from the transformer star point of view) Tn C S (neutral and grounding are added to the water intake gate). Through the installation of Tn c, neutral and ground wires are added, but this method is not common and requires special procedures to ensure safety.
The system should be designed so that some form of fuse or crusher can be used as a safety system anywhere in the earth in a short time. The high grounding impedance of TT system means that residual current circuit breaker (RCD) must be used. In other grounding systems, this may be included in the normal overcurrent protection system. Rcd can still be used because they can resist the shortcomings of small earth, for example, through such a human system.
Frequency of communication by country
Most countries in the world standardize their power supply systems to one of two frequencies: 50 Hz or 60 Hz. 60 Hz countries, most of which are listed in the New World, are relatively short, but this does not mean that 60 Hz is not common. 60 Hz countries are: American Samoa, Antigua and Barbuda, Aruba, Bahamas, Belize, Bermuda, Canada, Cayman Islands, Colombia, Costa Rica, Cuba, Dominican Republic, El Salvador, French Polynesia, Guam, Guatemala, Guyana, Haiti, Honduras, South Korea, Liberia, Marshall Islands, Mexico, Rosia Island, Montserrat, Nicaragua and North China. Panama, Peru, Philippines, Puerto Rico, Saint Kitts and Nevis, Suriname, Taiwan Province Province, Trinidad and Tobago, Turks and Caicos Islands, United States, Venezuela, Virgin Islands (United States), Wake.
The following countries have mixed power supply of 50 Hz and 60 Hz: Bahrain, Brazil (mainly 60 Hz) and Japan (60 Hz is used in the western region).
Most countries choose their TV standards to meet their short supply frequency. The NTSC standard was developed and operated at 60 Hz, while PAL and SECAM were designed at 50 Hz, but the 60 Hz version of PAL does not exist, that is, in the Brazilian Palm, high-resolution PAL and NTSC are provided with low flicker.
It is generally accepted that nikola tesla chooses 60 Hz as the lowest frequency, which will not cause visible flicker of street lighting. The origin of 50 Hz frequency used in other parts of the world remains to be discussed, but it seems to be a general electric meter standard with the structure of 1 2 5 10 of about 60hz.
Other frequencies are those commonly used in industry in the first half of the 20th century, and they are still in use today under special circumstances. 25 Hz power, many of which are formed in Niagara Falls, is used in Ontario and North America. Niagara Falls may still have a generator at about 25 Hz online. The design of low-speed motor with lower frequency relaxation can be built and transmitted more efficiently, but the reason is obvious flicker in lighting equipment.
Offshore and marine applications sometimes use 400Hz to gain various technical advantages.
16.67 Hz power supply is still used in some European railway systems, such as Sweden.
It is worth noting that electrical appliances powered by alternating current may emit a typical buzz to use the frequency of alternating current.
Ac voltage mathematics
Alternating current is usually associated with alternating voltage. The AC voltage v can be mathematically described as a function of time by the following equation:
nv(t) = A □sin( t)，n
over there
A is the height in volts (and peak voltage),
The angular frequency is radians per second, and
T is a few seconds later.
Because angular frequency is more beneficial to mathematicians than engineers, it is usually rewritten as:
nv(t) = A □sin(2 f t)，n
over there
F is the frequency in hertz.
The peak-to-peak value of AC voltage is defined as the difference between its positive and negative peak values. Because the maximum value of sin(x) is+1 and the minimum value is-1, the AC voltage swings between +A and -A. The peak-to-peak voltage is written as V page, so (+a)-(-a) = 2 □ a.
The amplitude of AC voltage can be expressed by root mean square (rms) value, and the unit is Vrms. For sinusoidal voltage:
nV rms = 2n
Vrms helps to calculate the power consumed by the load. If the V DC voltage provides some force P to the specified load, then if Vrms = VDC, the V AC voltage rms will provide the same average force P to the same load. Due to this fact, the effective value is a common method to measure the voltage in simple (power) systems.
To illustrate these concepts, consider the short-term use of 240V AC in the UK (it is worth noting that the current official voltage in the UK is 230V+10% -6%, but in many cases the voltage is still closer to 240V than 230V). It is so called because its root mean square value (at least nominal value) is 240 V, which means that it has the same thermal effect (height) as 240 V DC. We can modify the above equation:
nA = V rms□$ $ # 3069 & gt； 2 n
For our 240 V AC power supply, the peak voltage V page or a is therefore 240 V □ √2 = 339 V (about). The peak-to-peak value of 240 V AC is much larger: 2 □240 V □ √2 = 679 V (approximately. )
European countries (including Britain) now formally unify the power supply of 50Hz at 230v, but they make the tolerance band very wide at+10%. Some countries actually set stricter standards than this, such as 230V+10% in Britain, which is -6% of the old standard. The supply is the most up-to-date and does not need to be changed.
External connection
"AC/DC: What's the difference?" . Light, the United States experienced Edison's miracle. (Public Broadcasting Company)
AC/DC: In the alternator. Light, the United States experienced Edison's miracle. (Public Broadcasting Company)
Tony R. KuFalter, "Circuit Course: Capacity II-AC". March 8, 2003. (Design Science License)
Church Navier, C.R., "The concept of AC circuit". Superphysics.
"alternating current (AC)" Encyclopedia of magnetic particle inspection and nondestructive testing.
"Communication" mode program control service.
Hiob and Eric, "Applying trigonometry and infectious factors to alternating current". British Columbia Institute of Technology 2004.
"Introduction to AC and Transformer". Joint publication.
Wind Energy Reference Manual Part 4: Electricity. Danish Wind Industry Association 2003.
Chen Keelin, "Machining Alternating Current with Tools". JC physics 2002.
"measurement->; Communication. Mode program control service.
Williams, Travel "Central Bottle", "Understanding of alternating current, some power concepts".
Voltage, frequency, TV broadcasting system, radio broadcasting table, news country.