The frequency of alternating current refers to the number of periodic changes in its unit time, in Hertz (Hz), which is inversely related to the period. The frequency of alternating current in daily life is generally 50 Hz, and the frequency of alternating current involved in radio technology is generally large, reaching a measure of kilohertz (KHz) or even megahertz (MHz).
The peak of the sine-cosine alternating current corresponds to the amplitude, and the magnitude of the effective value is equivalent to the magnitude of the direct current that produces approximately Joule heat in the same time. The relationship between the peak value of the sine and cosine AC and the effective value is: 
For example, urban life electricity 220V represents an effective value, and its peak value is about 311V.
About modern AC applicationsThe AC power commonly used in China is generally 50Hz.
The electricity we use for electric lights, motors, etc. is all AC. In practice, the alternating current is indicated by the symbol "~".
The law of current changes with time, from which: three elements of sinusoidal alternating current: maximum (peak), period (frequency or angular frequency) and phase (initial phase). The basic problem to be discussed in AC is the current, voltage relationship and power (or energy) distribution in the circuit. Since the alternating current has a characteristic of changing with time, a series of characteristics different from the direct current circuit are produced. The components used in the AC circuit not only have resistance, but also have capacitive and inductive components. The components used are many, and the phenomena and laws are complicated. But basically follow the basic rules of Ampere's law. It is the test and difficulty of high school electricity.
According to the principle of Fourier series, the periodic function can be expanded into an infinite series composed of a sine function and a cosine function. Any non-harmonic AC can also be decomposed into a series of simple harmonic cosine AC synthesis.
Frequency cycle of alternating currentThe frequency is a physical quantity indicating how fast the alternating current changes with time. That is, the number of changes in alternating current per second is called frequency, and is indicated by the symbol f. Its unit is weekly / second, also known as Hertz commonly used "Hz", referred to as Zhou or He. For example, the mains is a 50-week alternating current, and its frequency is f=50 cycles/second. Thousands of weeks (kC) and megacycles (MC) are also available as units of frequency for higher frequencies.
1 thousand weeks (kC) = 103 weeks / sec, 1 megacycle (MC) = 103 thousand weeks (kC) = 106 weeks / sec
For example, the frequency of the signal transmitted by China's first artificial earth satellite is 20.009 megacycles, that is, it sends a change signal of 20.009 & TImes; 106 times per second. In the expression of the alternating current sinusoidal current, ω in i=Asin(ωt+φ) is called the angular frequency, and it is also a physical quantity reflecting the speed of alternating current with time. The relationship between the angular frequency and the frequency is ω = 2πf.
The speed of alternating current over time can also be described by the physical quantity of the period. The time required for the AC to change once is called the period and is indicated by the symbol T. The unit of the period is seconds. Obviously, the period and frequency are reciprocal to each other, ie 
It can be seen that the faster the alternating current changes with time, the higher the frequency f, the shorter the period T; conversely, the lower the frequency f, the longer the period T. λ=c/f
AC frequency monitoring methodGenerally, a frequency meter, an oscilloscope, and a digital multimeter (with Hz file) can be used to measure the frequency.
The frequency meter has the highest frequency detection accuracy and the largest range, but the function is single and the application is less.
The oscilloscope has an excellent detection frequency, but the volume is generally large, the price is relatively expensive, and the application is more.
Digital multimeters have good detection frequency, low price, rich functions and most applications.
In summary, if only low-frequency AC power is detected, the digital multimeter is the most cost-effective.
Measurement method: The digital multimeter file is located in the Hz file, and then the two test leads are respectively connected to the two ends of the signal source to read the signal source frequency value.
The determination of the frequency is related to the construction, materials, etc. of the generator, the motor, and the transformer.
The synchronous speed of a 50 Hz two-pole generator is 3000 rpm, and if the frequency doubles to 100 Hz, the synchronous speed will be 6000 rpm. Such high speeds will cause many problems in the manufacture of the generator, especially if the linear speed of the rotor surface is too high, which will greatly limit the increase in capacity. In addition, from the point of view of use, the frequency is too high, so that the reactance increases, the electromagnetic loss is large, and the amount of reactive power is increased. For example, in the case of a three-phase motor, the current is greatly reduced, and the output power and torque are also greatly reduced, which is not beneficial. In addition, if a lower frequency, such as 30 Hz, is used, the transformation efficiency is low, which will be detrimental to the voltage transformation and transmission of the alternating current.
The frequency of a modern power system is the frequency of a sinusoidal fundamental voltage generated by a synchronous generator in a power system. The frequency is a uniform operating parameter of the entire power system, and one power system has only one frequency. The power system in most of the European countries in the world and in the world is rated at 50 Hz. Most of the Americas are 60Hz. Most countries specify frequency deviations between ±0.1 and 0.3 Hz. In China, the frequency deviation of power systems above 3 million kW shall not exceed ±0.2 Hz; and the frequency deviation of small power systems below 3 million kW shall not exceed ±0.5 Hz. Because the operation of large units has strict requirements on the frequency deviation of power system, some countries also stipulate the frequency deviation of the operation mode of power system failure, which is generally specified between ±0.5±±1Hz. Exceeding the allowed frequency deviation, the large unit will trip, which is not conducive to the safe and stable operation of the system.
In the power system, the power emitted by the generator is not balanced with the power consumed by the electrical equipment and the power transmitting equipment, which will cause the frequency of the power system to change. When the system load exceeds or falls below the output of the power plant, the system frequency will decrease or increase, and the change in power plant output will also cause the system frequency to change.
In addition, the frequency range of China's power grid is ±1Hz. Because the frequency adjustment inertia is large, the range is small and it is easy to cause the grid to oscillate. Those who have done temperature control or constant voltage should understand. Before the grid was connected to the grid, the frequency of the grid in Lanzhou was above 50.5 Hz, and that in Shanghai was around 49.5 Hz. The current large grid connection is conducive to grid frequency and voltage stability.
The higher the carrier frequency, the better the sine wave type and the less harmonics in the motor windings. However, the radiation interference energy increases and interferes with surrounding electrical equipment.
The difference in grid frequency depends on people's calculation habits. The large-scale power generation in the Americas was earlier. The calculation tools at that time were mainly English (12-digit) slide rule. For the convenience of calculation, the grid was used at 60 Hz and later. In hex data, 50Hz is more convenient.
About the voltage level, it is divided into two series of generator and motor. The voltage we often say is the motor voltage. It is the basic series, based on 220V. It is a level after each 1.414 and rounded. Except the inverter voltage; generator voltage Add 5% to the motor voltage of the same grade and round it. So there are only 230V or 400V generators and no 220V or 380V generators.
The special situation of the airport is: the airborne generator requires small size and light weight, only the frequency can be increased to meet the power requirements, so the corresponding airborne electrical equipment uses 400Hz, and the aircraft-related power supply needs 400 Hz! There is also a higher military.
The power supply on the aircraft uses 400Hz to reduce the size and weight, which is a complicated system engineering. The 400Hz of military and avionics mainly depends on the following points: 1. The generator or motor with high frequency is small in volume and weight due to high speed and small torque; 2. The power of the generator on the aircraft is taken from the aero engine. The speed is higher; 3, DC power equipment is more, the high frequency is beneficial to reduce the rectification ripple.
In the case of the same voltage, what is the difference in rectification efficiency between 50hz and 60hz and 400hz power supply?It is not necessary to use 100Hz or 120Hz because the frequency is too high, on the one hand, the transmission is difficult, and the understanding of the line inductance and the capacitive reactance of the inverter should be profound; on the other hand, the speed of the generator and the motor is too high or the number of poles is too high. It is not advisable. 400Hz power can not be transmitted over long distances. Users should specify the transmission distance and mode when ordering 400Hz generators. The rectification efficiency is also poor, but the ripple after rectification is small, the ripple frequency is high, and it is easy to handle.
If 50 Hz is put into a production line that requires 60 Hz, the speed of the AC motor is reduced, and (the motor speed is proportional to the frequency). The motor heats up, and it will burn for a long time. The control system generally passes through the rectification and switching power supplies and should be fine. Also look at the frequency sensitive devices. (big problem, consistent voltage level)
If you want to study the direct supply of 50Hz power to the production line that requires 60Hz power supply, mainly consider the electromagnetic characteristics of the electromagnetic device, such as motor, transformer, and then the sampling signal related to the power frequency. For the former, the method of research can find the electromagnetic expressions of these two devices, bringing 50Hz and 60Hz respectively, and some problems can be found. Xu Wu'an "Inductor Device Design and Calculation", Sichuan Science and Technology Press, 1985.08, 103 pages-106 of which mainly focuses on the design and calculation of high-frequency transformers, some of which can be extended to the motor. The latter need not be said to know.
For asynchronous motors, when the 50Hz power supply is supplied to the 60Hz load, the speed reduction is positive. The voltage should be reduced by 1/6 according to the motor nameplate voltage. At this time, the motor can run for a long time, and the torque and current are constant. Reduced by 1/6. If the voltage does not decrease, the magnetic circuit of the motor will be saturated, and the no-load current and no-load loss will increase a lot.
For inductors, the inductive reactance is reduced by 1/6. For the 60Hz dedicated contactor, it is changed to 50Hz, which is easy to trip. But currently it is generally 50/60Hz universal.
Developed countries also have 50HZ, such as most countries in Europe. Small countries also use 60HZ, such as Little Japan.
The UK's power supply standard is 50HZ single-phase 230V, three-phase 400V, which is close to the current standard in China. It is possible that the equipment purchased early will become a de facto standard;
In the old China, which was confusing, except for Taiwan’s long-term occupation and the influence of the United States, the power standard is 60HZ, and it is still possible to maintain the uniformity of power standards (it seems to have 110V power supply in the northeast when the Japanese is full).
Before liberation, China's various voltages and frequencies coexisted, mainly related to the standard of the country of production of power generation equipment. After liberation, China's use of the Soviet system, it has become what it is.
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