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AC power and DC power, and their mutual conversion

If asked: Where does the electricity in your home come from? Most people know the distribution box that is brought into your home from the outside. So is the electricity in the distribution box AC or DC? Most people know AC as a sine wave (very rarely DC). The distribution box divides several branch circuits to various rooms in the house to supply power for lights, TVs, refrigerators, washing machines, microwave ovens, air conditioners, induction cookers, etc., to meet people’s daily needs.

The distribution box and branch lines, and the AC outlets in the room constitute the AC distribution network of the home, which provides electricity of the same nature as the electricity accessed outside the house. Household electricity is different in different countries or regions. There are 220V50Hz, 240V50Hz, and 110V60Hz, etc., that is, there are two types of voltage: 110V and 220V (including 230V and 240V), and the frequency is 50Hz and 60Hz. two kinds. Different grades and types of electricity generally cannot be used directly, otherwise it will cause damage to electrical appliances and endanger personal safety.

Most of the household appliances now have printed circuit boards (PCBs), on which there are various electronic components, such as various integrated circuits.

These integrated circuits are powered by low-voltage direct current, and the electrical appliances in our home are connected The input is 220V50Hz or 110V/60Hz AC, which needs to be converted into DC of 24V, 12V, 5V, 3.3V and other voltages inside the appliance for use. In addition, the current common household photovoltaic power generation devices also need to convert the DC power generated by the photovoltaic panels on the roof into AC power and feed it back to the AC grid. These also tell us that alternating current and direct current can be converted into each other.

The conversion of the nature of the power source involves a series of complicated theories and devices, let us start with the most basic.

  1. Communication to Communication

The voltage of the electricity generated by the generator of the power plant is generally three-phase 10.5~18kV. After one rise, the voltage is three-phase 220kV to 330kV. After reaching the power consumption area, the voltage is reduced to three-phase 10kV to 18kV after one or two times. Kilovolts, step down to single-phase 220V or three-phase 380V before being connected to ordinary users. Here let’s ignore the frequency and talk about the phase of the alternating current. Theoretically, the large generator of the power plant is equipped with three sets of coils with the same number of turns and different orientations. The three sets of coils are distributed at an angle of 120° on the circumference of the generator stator. The magnetic field of the generator rotor rotates and cuts the three sets of coils in sequence.

The AC voltage of a certain frequency is generated respectively, and the midpoint connected by the three coils is the neutral point, the voltage generated by each winding is the phase voltage, and the relative voltage of the three unconnected terminals is the line voltage. According to the trigonometric function, it can be deduced that the difference between the line voltage and the phase voltage is 3 times the square root (1.732). The phase difference of the three single-phase electricity is 120°.

The amplitude change of alternating current requires the use of a power transformer. Using the principle of electromagnetic induction and the change of the turn ratio of different coils, the alternating current voltage can be raised and lowered with extremely low loss. This is the most efficient power conversion known so far.

  1. AC to DC

There are two ways to convert AC to DC. One is to use an AC motor to drive a DC generator, and the other is to rectify and filter AC power using unidirectional power electronic devices.

  1. DC to DC

We know that the known theoretical basis for high-efficiency power conversion is Faraday’s principle of electromagnetic induction, specifically using the magnetic field changes of transformers and inductors to realize power conversion. Since pure direct current does not produce effective magnetic field changes and cannot be directly converted by electromagnetic devices, we need to first chop the direct current into pulses through switching devices, then use magnetic devices to change the amplitude of the pulses, and finally can be rectified and filtered to generate the required the DC voltage.

  1. DC to AC

The method of direct current to sine wave AC is also chopping. The pulse is modulated according to the sine wave standard to generate a series of SPWM pulses, and then filtered to generate sine wave AC.

The above four basic conversion processes have been combined in a series to form various contemporary power conversion devices. And our new energy is also based on this.