In daily life, the sockets at home and the electricity meters on the streets are all AC. It seems that there is no intersection between AC and DC, just like two different fields. In fact, there are many and many connections between them.
Communication said: "I can turn into DC"
AC can be changed to DC. Our usual computers, mobile phones, etc. are all direct adapters connected to the AC outlets, and our mobile phones and computers run DC. Well, there are doubts. Since the power generation and transmission of the power grid are all using AC power, why should we turn it into DC power at this time?
The reason is this: AC is sinusoidal fluctuations on the time axis, rising from 0 to maximum, decreasing from maximum to 0, then gradually changing from 0 to negative, and then returning to 0. However, our mobile phones, computers and other electronic components work by recognizing high and low potentials. Because of their sinusoidal volatility, ACs generate high and low potentials, which cause conflicts with the logical judgment of electronic components. For example, a computer has a potential of 1 and a potential of 0, and the AC itself has a potential of zero crossing. Therefore, AC power cannot work for electronic components.
So, who is going to complete the work of converting AC to DC? The rectifier, as shown below, is a common rectifier product.
Figure 1 Common rectifier
When using an appliance such as a computer, we need to convert the AC to DC through a rectifier for use. For example, the washing machine we usually use, the drive motor uses AC power, and the control panel is the rectified DC power.
DC is not satisfied, I will change communication
So, is there a scene of DC change communication? The answer is yes. Inverter This device was born for DC conversion to AC.
Simply put, the inverter consists of an inverter bridge, control logic and filter circuits. Widely used in air conditioners, home theaters, electric grinding wheels, computers, televisions, etc. For example, car air conditioners require inverters to convert DC power in car batteries into AC power. The following picture is a 1000W inverter product.
Figure 2 1000W inverter
How to judge the quality of an inverter?
The inverter can be divided into a single-phase inverter and a three-phase inverter by distinguishing the output AC power. A single-phase inverter generates a single-phase AC after inversion, while a three-phase inverter outputs three-phase AC. How to judge whether the inverter is good or bad is mainly judged by the following criteria.
Conversion efficiency
Since the inverter itself also consumes power, one of the natural properties of the inverter is that it cannot completely convert 100% of the DC power into AC. However, conversion efficiency is a criterion for us to judge whether an inverter is good or bad. In the GB/T 20321 standard, it is stipulated that in the rated output state, the inverter whose output capacity is not more than 2kVA, the efficiency should be greater than or equal to 80%; for inverters larger than 2kVA, the efficiency should be greater than or equal to 85%. As shown in the figure below, the actual measurement of the inverter efficiency.
Figure 3 E6500 detection of inverter efficiency
2. Ripple voltage
Ripple voltage refers to the AC component in DC. The DC voltage itself should be a fixed voltage value. However, due to the incomplete filtering of the filter circuit in some inverters, AC components are mixed into the DC. Therefore, detecting the ripple voltage at the input is an effective means of detecting the quality of the inverter.
3. Output AC power quality
After the inverter converts the direct current into the alternating current output, the quality of the alternating current power is an important indicator for detecting the inverter. If there is a power quality problem in the communication at the output, it not only affects the efficiency of the inverter, but also reduces the service life of the inverter, and is more likely to bring safety risks to the user. For example, the AC output from the inverter often contains harmonics. The following figure shows the harmonic detection of the E6500 to AC.
Figure 4 E6500 detection harmonics
The mutual conversion between AC and DC is one of the root causes of power quality problems. For example, in the process of AC to DC, the output DC may entrain AC components, causing damage to electronic components. In the process of DC inverter to AC, the filtering is not complete, which will cause ripple voltage to the DC of the input terminal and affect the efficiency of the input terminal. At the same time, it will bring a lot of power quality problems to the communication at the output. Such as harmonics, imbalances, and so on.
Inverters and rectifiers are widely used in daily life applications, and DC exchanges are mutually converted, and power quality problems are often overlooked. These quality problems may cause property damage or even life safety (such as mobile phone charging explosion, auto air-conditioning, etc.). Therefore, the issue of power quality of electrical appliances should cause us to pay enough attention.
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