As we all know, direct current is constant, while alternating current is alternating. How is the conversion between DC and AC completed?
DC waveform and AC waveform
The conversion between DC and AC needs to control two characteristics, one is the direction of current and the other is the size of current. The equivalent circuit of the electric vehicle inverter circuit is shown in the following figure. We equate the power battery with the battery and simplify the motor as a load.
equivalent circuit
As can be seen from the figure below, the change of current direction can be realized by closing and opening the switch. Thereby changing the direction of the alternating current.
Current direction
Current direction
Alternating current has frequency. If the AC frequency required by the motor is 50Hz, it means that the above switch needs to complete 50 periodic changes within 1 second. However, there is actually no such switch. In fact, we use MOSET tube instead of switch. The highest frequency of MOSET tube can reach 1000KHz, which can meet the frequency requirements in the actual process.
Moset tube
Moset tube circuit
After solving the problem of current direction and frequency, how is the magnitude of direct current equivalent to alternating current? By turning off the photocell at quot and quot, whether there is current or not can be realized, thus outputting a square wave. When the switch is closed, the output value is constant, and when the switch is open, the output value is 0. As shown in the figure below, the green waveform is square wave.
Dc square wave
The red box in the picture above shows a square wave. It can be seen that the average value of the period varies with the ratio of high level to low level. Cyan waveform is the average of square waves in a period. It can be seen that the longer the constant time in a period, the higher the average value. Finally, the average waveform of the upper square wave becomes the lower waveform.
The current direction is changed by the combination of switches, and the waveform in the whole cycle is as follows.
It can be seen that through the combination of switches and the change of switching time, the square wave is combined into a waveform similar to a sine wave. If the cycle time of each square wave is reduced, the curve will be smoother and smoother, and the average square wave will be infinitely close to the sine wave. The effect of the average waveform of this prescription wave is equivalent to sine wave, which completes the inverter from DC to AC.
Finally, there is one more question. In the actual inverter process, how do we know the ratio of high level to low level in a square wave period? In the actual waveform modulation process, an electronic component called a comparator is needed, and the output signal of the comparator is used to control the switch of the MOSET tube. See the figure below for the equivalent circuit diagram. The big triangle in the figure is a comparator and the small triangle is a diode. The function of diode is to prevent the switches in the same branch from being turned on at the same time, resulting in short circuit.
Modulation circuit
Triangular wave and sine wave are input into comparator for comparison. See the figure below.
Comparison waveform
When the sine wave is smaller than the triangular wave, the comparator outputs 0; When sine wave is greater than triangle wave, comparator outputs 1. In this way, the square wave control signal matching the sine wave characteristics can be output and input to the MOSET tube. When the output is 1, it is controlled to turn on, and when the output is 0, it is controlled to turn off, so that the switch can be controlled to output the desired equivalent inverter waveform according to the signal, and finally the motor can be controlled to rotate, thus completing the whole inverter process.
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