Study on Power Limit principle of dormitory electric meter (Technical Research )----

Many people may have this experience. Four computers in the dormitory are on, the fan is on, and the fluorescent lamp is on.
Run, there is no problem, but no electric appliance is turned on, it is inserted into a small electric cup, the electric meter will trip
. If someone is using a computer, they can even hear crazy voices .... Why does this happen?
Symptom? The power supply of the computer is ~ The sum of the power of the four computers is absolutely above an electric cup,
What can I do if I have four computers working at the same time but cannot access a W electric cup? What is the difference between the two? Below
Let's take a look.

The most frequently used heating devices are hot and fast electric cups or rice cookers. Their working principle is
The current flows through the resistance wire, and the resistance wire heats up to boil water. For V power grids, this type of load is equivalent to a pure resistance
When connected to the power grid, those who have learned the circuit know that the ac v is added to the resistor, and the voltage waveforms and flows at both ends of the ac v are
The current waveform of the resistance is the same, that is, the difference between the two is 0. This type of load is called pure resistance negative.
.

What is the equivalent of a computer? We know there is a box-shaped iron box above the back of our computer chassis.
That is the power of the computer. This power supply is a load of the power grid. Computer Power Supply
It is a switching power supply (note that this is not a power supply with a switch). It is a non-linear load (also called a rectifying load)
. The principle of switching power supply is to first convert v @ 50Hz AC to high voltage DC, and then convert high voltage DC to High Voltage
The power supply efficiency is reduced to Low-Voltage High-Frequency AC by using a high-frequency transformer, and then converted to low-voltage DC output.
It is much higher than the traditional voltage regulator. The computer's switching power supply is considered as a V power grid load, which is at 22
The 0 V mains input is equivalent to a capacitive load. Although its voltage waveform is still a sine wave, its current
The waveform is distorted. It is no longer a regular sine wave, but a waveform close to the pulse wave (in fact, this nonlinear load
It is a vicious load that harms the power grid and brings higher harmonic to the power grid ).

The voltage and current waveforms of the two loads are shown in the figure below. How can an electric meter identify these two loads? There are many methods,
Both use single-chip microcomputer + AD converter to sample the voltage and current waveforms at the output end of V in real time, and then compile the corresponding
Program, through the algorithm to determine the proportion of the power of the two types of load, only limit the connection of pure resistance Load
Inbound, or detect the total current to limit the total power. Since this article is published in non-professional forums, I will not describe it in detail
Disconnected. If you are interested in specific algorithms, contact me and study them together.

For some electric meters that are widely used to monitor average power, use a rectification diode or a fast recovery diode (reverse
For safety reasons, the series must be in hot and fast connection,
This is equivalent to half-wave rectification, which reduces the voltage by half. Therefore, according to P = (U ^ 2)/R, the heat resistance of W is 50 euro.
MB ). It can be obtained that the average power has been reduced to 1/4 of the rated power, that is, 250 W, but in fact the instantaneous
The time power is the same as that before modification, but only the first half of the time after modification, average to the entire cycle
The average power is reduced. This method can only cheat the electric meter that monitors the average power, for the electricity that monitors the peak current
The table cannot escape its eyes.

The diode method is relatively simple, but the heating time will become very long because the power is only 1/4 of the original. Yes
Can I manually adjust the power? The answer is: yes. However, this solution is more complex than the diode solution.
The case is that the thyristor (also known as Thyristor) is used to change the conduction angle of the thyristor by adjusting the trigger pulse.
Controls the hot and fast conduction time in a cycle. The diode solution reduces the conduction time to a fixed value.
1/2. the thyristor solution can be understood as manually adjusting the conduction time to adjust the average heat rate.
Power. In this way, you can maximize the use of the load ceiling set by the electric meter, as long as the ratio of power adjustment to trip
The load limit is a little small. This reduces the heating time than the diode solution. However
The case is still flawed, that is, it cannot escape the monitoring of electric meters that detect peak current.

In order to prevent illegal use of hot and other electrical appliances, reduce fire hazards, but does not affect the use of computers and other electrical appliances
The electric meters installed in some apartments have been upgraded, and smart load identification devices are installed to determine whether there is high-power pure resistance heat source load access based on the current and voltage waveforms and the phase difference. So the dormitory can contain four computers.
Working at the same time, but cannot access a W electric cup phenomenon. For this smart electric meter, it is connected by a dual pole
No matter how you change the conduction time
The load type has not changed, so the current voltage waveform is still the phase difference is 0, it will still be recognized by this electric meter
.

Four computers are not tripped at the same time, which means that the usage of dormitory electric meters is not restricted. Previously analyzed
Because this type of load is a non-linear rectifying load, its current waveform is completely different from the pure resistance load.
. Some may ask why they are different? Aren't they all using electricity? Understanding the problem requires knowledge of the circuit
Knowledge and Power Electronics knowledge will not be explained in detail here. In general, as long as the input end has a rectifier bridge + capacitor +
High-frequency inverters (inverter DC to High-Frequency AC) can be considered as non-linear rectifying negative
. Such loads as computers and energy-saving lamps. They are not subject to the power limit of the electric meter's pure resistance load.
(Of course, the power cannot be too large. Generally, the electric meter has a total power limit ).

After such analysis, we can find that this smart electric meter is also defective, although it can distinguish between pure resistance and negative
Load and non-linear load, but it does not know whether the energy lost to the non-linear load is used for computer or heating.
So it is helpless for the following two situations:

1. After modification, the heat is fast. The pure resistance load is disguised as a non-linear load, and the electric meter monitoring is cheated.

2. Use an adjustable power heating product with an inverter device, such as an Induction Cooker (which can adjust the power) and a small micro
Furnace (small fire ~ Medium fire ).

The Induction Cooker is a new type of safe product. It has no open flames and uses high-frequency electromagnetic waves to generate eddy currents and fever for iron products,
Do not burn human body, adjustable power, temperature control circuit, can prevent overheating, power saving and safety. This avoids hot spots.
I think the school should not limit the use of this electrical appliance.

(Statement: I didn't want everyone to cook in the bedroom using this method ~)

The above two methods are actually the same. It should be noted that the first method should be changed by professionals.
Of course, if you want to learn electricity, you can design and analyze it yourself, but please pay attention to safety and be careful about electric shock.
, Leave as much threshold as possible for the choice of components, please be careful when using. For the second solution, I am ashamed of myself.
No tests have been conducted on Induction Cooker or microwave oven. It is feasible to analyze the principle only.
It is related to the specific power value. Please start from the minimum power. In addition, please use the famous brand products of the large factory. Please pay attention to the electromagnetic
The user manual of furnace and microwave oven generally requires the use of three-hole socket, good grounding, otherwise it may generate instantaneous
Fire, increase current, and cause danger.

The first scheme has been proved to be feasible, but the selection of component parameters must be reasonable. Otherwise, a large number
Current. The second solution seems like someone in the Forum said yes. I don't know if it is used in the dormitory. If it succeeds
, Please reply.

Again: please do not use illegal electrical appliances in the dormitory. Please comply with dormitory management regulations.
Security responsibilities of the apartment. This article only applies to power supply professionals.
If the electrical appliance is confiscated or the fire or other personal and property losses are caused by improper use, I shall not instigate or compensate
Any liability, such as compensation.