Through the previous article, we know what PWM pulse width modulation technology is, then in this article, we will talk about the advantages of PWM pulse width modulation technology when used in voltage-type inverters.
① The main circuit device for voltage control can be omitted, and the output voltage and output frequency can be adjusted with only one power control stage. The circuit is simple, which is conducive to miniaturization and cost reduction.
②The DC power supply can use an uncontrollable rectifier bridge, so that the power factor of the system grid has nothing to do with the inverter output voltage value.
③ Frequency and voltage can be adjusted at the same time, independent of the component parameters of the intermediate DC link, the dynamic response of the system is fast; fast current control can be realized, which is suitable for such as AC motor control (especially phasor control), high-performance transmission System is indispensable.
④It can eliminate or reduce low-order harmonics, and obtain better waveform improvement effects.
It is these advantages that make PWM technology occupy an absolute dominant position in today’s inverter field. With the practical application of fast semiconductor switching devices, PWM technology is rapidly popularized. However, the harmonic component near the carrier frequency of this technology has a greater impact on the overall characteristics, so how to select the carrier frequency and how to reduce the harmonics near its frequency has become the key to improving the performance of the PWM technology. Special attention should be paid to the influence of the dead zone on the waveform and stability of the dead zone set in order to avoid the series connection and short circuit of the upper and lower switch tubes of the same arm. In order to eliminate specific low-order harmonics at low frequencies, current-type inverters mostly use PWM control technology.
Figure 1 shows the basic circuit configuration diagram of a voltage-type single-phase PWM inverter. Among them, figure (a) is a single-phase half-bridge circuit, and figure (b) is a single-phase full-bridge circuit. The output voltage in Figure (a) is at S1, +E/2 when it is on, and -E/2 when S2 is on. The ratio of the output voltage to the interval of +E/2 and the interval of -E/2 varies with Change with time, so that the desired waveform can be obtained. The output voltage Uab in figure (b) is +E when S1 and S4 are on, and -E when S2 and S3 are on. In addition, when S1 and S3 or S2 and S4 are on, the load is short-circuited, which can be obtained The interval where the output voltage is zero. Figure 2 shows the basic circuit of a three-phase half-bridge PWM inverter. Through the combination of the on and off states of the switch tubes S1~S6, the output voltage can get three values of ±E and 0.
There are many kinds of control circuits for PWM inverters (that is, methods for generating PWM waveforms). (a), (b), and (c) in Figure 3 are three representative examples. Figure (a) compares the signal wave us and the carrier wave uc with a comparator, and determines the output state according to the magnitude of the amplitude. When the carrier frequency is sufficiently higher than the frequency of the signal wave (also called the modulation wave), it is generally called subharmonic modulation. The method in Figure (b) is to use a timer or counter to control the pulse width. The carrier pulse is used to trigger the counter, and the counter continues to count until the pulse width determined by the signal wave input is reached. Although the circuit modes of Figure (b) and Figure (a) are different, the pulse width is determined according to the signal wave, so as to obtain the pulse train synchronized with the carrier. Consider from this point, Figure (b) and Figure (a) ) Can be regarded as the same way. The way in Figure (c) is to store the pre-defined pulse width pattern in ROM, and then output it according to the carrier pulse. The decision of the pulse mode has nothing to do with the signal waveform, and the desired pulse width can be obtained by synchronizing with the carrier (clock). Therefore, the input signal that determines the pulse width is the averaged frequency command fref, voltage command uref, or the like. This method adds many functions to the modulation circuit, including the function of an oscillator (Voltage Controlled Oscillator, VCO) that converts the frequency command voltage into an AC signal. Its circuit structure is simple, and it has been widely used in digital general-purpose PWM inverters.
Figure 3 Generation method of PWM waveform