Regarding the current parameter "10 A(20A)" marked on the electric energy meter, I would like to share with you what I have learned:
Calibration current and rated maximum current:
Calibration current (rated current): the basic current value marked on the table as the calculated load: Ib.
Rated maximum current: the watt-hour meter can work normally for a long time, and the error and temperature rise completely meet the requirements. Maximum current value: Imax.
Selection of technical parameters of electric energy meter
At present, the common technical parameters of civil electric energy meters in the market are the same: rated voltage 220 volts, frequency 50 Hz, and grade 2.0. The different parameters are the calibration current and the maximum rated current of the watt-hour meter. The so-called calibration current refers to the basic current that the electric energy meter can run normally for a long time. It is a parameter that determines the related characteristics of electric energy meter, and is expressed by Ib. Rated maximum current refers to the maximum current value that the electric energy meter can meet the precision specified in its manufacturing standard, and is expressed by Imax. So what kind of watt-hour meter to choose must be decided according to the load current. The upper limit of electric load shall not exceed the rated capacity of the electric energy meter, and the lower limit shall not be lower than the load current value specified by the allowable error of the electric energy meter.
For example, a user has a 40-watt and 60-watt electric lamp and a 1000-watt electric furnace. According to the calculation, IMAX = (40+60+1000)/220 = 5a.
Therefore, a single-phase common watt-hour meter with a maximum rated current of 5 A and a rated voltage of 220 V should be selected. However, in order to prevent users from adding new appliances in the future, it is suggested that users choose 5( 10) A wide load watt-hour meter, where 5 is the rated current and (10) is the maximum rated current.
(The following content is selected from the article uploaded by Zhao Zhikai in Jilin Physical QQ Group. )
On the significance of current calibration of 10(20)A watt-hour meter:
Although electronic watt-hour meters have appeared in recent years, inductive watt-hour meters are still widely used. The principle and application of inductive watt-hour meter are different from the most common magnetoelectric DC ammeter and voltmeter in middle school physics teaching. When the familiar knowledge of magnetoelectric DC ammeter and voltmeter is applied to the use of induction system watt-hour meter, there will often be some misunderstandings. The inductive watt-hour meter has no pointer, but drives the integral mechanism through the rotation of the turntable to display the sum of accumulated electricity consumption with time. It is an integrated instrument with no limit value and no over-limit problem. The voltage and current values marked on the electric energy meter are those of the connected load circuit, which are different from those marked on the electric appliance to determine the rated power of the electric appliance. The watt-hour meter itself consumes little electricity. The inductive watt-hour meter has great overload capacity, so there is no danger of burning as long as it is overloaded. According to the national standard, the electric energy meter is marked with two current values, such as 10 (20) A, where the reference number 10A is the basic current (symbol Ib), which is the current value to determine the relevant characteristics of the instrument, also called calibration current. (20) A in brackets is the rated maximum current, and the symbol Imax is the maximum current value that the instrument meets the precision specified in the standard. The current through the watt-hour meter can be as high as 2 times, 3 times and 4 times of its basic current, and some can be as high as 8 times, but it can't reach 2 times of the basic current value. That is to say, if the electric energy meter installed by the user has only one current value, such as 5A, it is only the basic current value, not the maximum allowable current. Generally speaking, this kind of watt-hour meter will not have problems when it is overloaded to 120%, which can meet the accurate measurement of watt-hour meter. ..... In physics teaching, there are often misunderstandings in the current selection of watt-hour meters, such as: "Watt-hour meters are marked with voltage and current, with voltage marked as rated voltage and current marked as maximum allowable current. A watt-hour meter marked' 220V5 A' can be used in household circuits with a maximum power of 220V×5A= 1 100W. If the total wattage of household appliances used at the same time exceeds this value, the calculation of the watt-hour meter will be inaccurate or even burned out. " This statement is unrealistic, and some common exercises and questions written on this basis are also inappropriate. Excerpted from junior high school physics thematic analysis series "Safe electricity use and home circuits" (People's Education Press, 2003 edition).
Let's introduce "Teacher Yang's statement from the Ministry of Education of Daan Normal School for everyone to learn".
Further discussion on current parameters of watt-hour meter
Before that, I wrote an article about the parameters of electric energy meter on the internet of our school. In this paper, I take the current parameter "10(20)A" given in the eighth chapter of the eighth grade textbook of People's Education Press as an example to discuss. Because of the original mistakes in the textbooks, many of our teachers also made mistakes in teaching. Of course, this mistake is not the teacher's at all. Because the problem about the parameters of electric energy meter is too professional, we usually only care about its function and principle in teaching (the principle is only specially studied in college physics, because it involves the interaction of magnetic fields of current coil and voltage coil, the generation of induced current and the effect of magnetic field on induced current, and the damping effect of permanent magnet on rotating aluminum disk, etc. Junior high school students can't explain what the parameters of the watt-hour meter mean. In the last article, I only gave a general explanation: "10 A" is the calibration current of this watt-hour meter, and the calibration current (or basic current) is expressed by Ib. The size of "calibration current" Ib is related to the measurement accuracy of the watt-hour meter, and 20A in brackets represents the rated maximum current of the watt-hour meter. "Rated maximum current" is expressed in Imax. Of course, the greater the "calibration current", the greater the "rated maximum current" of the watt-hour meter. The relationship between them is 2 times, some electric meters are 4 times, such as "10(40)A", 3 times and 8 times. We only need to tell students that "basic current" or "calibration current" is an important parameter to mark the performance and measurement accuracy of electric energy meter, which determines the rated maximum current. Let's further discuss the specific meaning of "calibration current" (basic current).
"Calibration current" or "basic current", symbol Ib, is the current value that determines the working characteristics of the electric energy meter, including starting current, measurement accuracy and other indicators. The measurement accuracy of electrical instruments is always expressed by "grade", usually with grade 2 and 2.5 (the grade of electric meters is used to express the accuracy of electric meters. China stipulates that electric meters are divided into seven grades, namely 0. 1, 0.2, 0.5, 1.0, 1.5, 2.5, 5.0). The smaller the grade value. The higher the accuracy of the instrument. Usually, the grade of the electric meter used is marked on the dial of the electric meter. The level parameter "2" actually means that the maximum permissible error value of an electric meter (such as ammeter and voltmeter used by our students for experiments) is 2% of the range. (When measuring with an electric meter, the difference Δ x between the indicated value x of the electric meter and the measured actual value X0 is called the absolute error of electric meter measurement. The ratio of the absolute error value to the range Xn of the electric meter is expressed as a percentage, which is called the reference error en of the electric meter, that is, en =(δx/Xn)× 100%, and the meaning of "2" can be understood by pushing. It can be said that any instrument can only be said to be relatively accurate and it is impossible to eliminate the error, which is inevitable. In college physics, we also know that there is a "uncertainty principle". Take the household inductive 2.0 watt-hour meter as an example, the starting current (the minimum current that can make the turntable of the watt-hour meter rotate continuously) is 0.5%×Ib, and the measurement accuracy of the watt-hour meter is the highest under this current. So we can naturally understand what the current in brackets means! That is, the rated maximum current Imax is the maximum current allowed to pass through the watt-hour meter on the premise of meeting the current accuracy specified in the standard.
Nowadays, due to the improvement of our living standards, modern household appliances have gradually entered the family. The power of electrical appliances used in peacetime is small, and the power of electrical appliances used in a specific period is relatively large. In this way, the "basic current" of inductive watt-hour meters directly affects the accuracy of measurement, so now most places use digital watt-hour meters, and the purpose of more accurate measurement can be achieved by monitoring the current and voltage values in real time (sampling in a very short time). In addition, in order to disperse the peak power consumption, some big cities began to popularize smart electric meters, achieving the win-win goal of using high-power household appliances during low power consumption periods, which not only saved electricity bills, but also saved electricity for the country. From the perspective of environmental protection, energy saving and emission reduction, the country is also vigorously developing new energy sources including wind energy, solar energy and nuclear energy, and is also planning the construction of smart grid to achieve the best matching between power supply and consumption. As a physics teacher, students should be consciously educated in this respect in their usual teaching. The key is to organically combine education and teaching-related content in this area, rather than deliberately forcing irrigation. This is the art of teaching. I sincerely hope that all physics teachers will have the awareness of improving teaching art in their usual teaching. Let's stop this topic! Welcome physics teachers to contribute actively, so that more teachers can share your wonderful things on our school website.