PWM rectifier working mode problem

shaorc

[Ask if you don't understand] In the following figure, both the single-phase full-bridge PWM rectifier in Figure b and Figure c are in the same switching mode. Figure b is easy to understand, the AC voltage e＞0, i(t)＞0, it is a simple rectifier, capacitor C stores energy. But in Figure c, the AC voltage e＞0, but i(t)＜0, the current is reversed, does it mean that this is the working mode of the inverter? The following figure is a three-phase PWM rectifier Figure f, which shows a working mode under the condition of ia>0, ib<0, ic<0. At this time, the power tube Va' of the lower bridge arm of phase A is turned on, the power tube Vb of the upper bridge arm of phase B is turned on, and the freewheeling diode VDc of the upper bridge arm of phase C is turned on. But I don't understand the flow direction of the current of phases a and c. At this time, ib comes out from the positive electrode of capacitor C through Vb and returns to the AC side of phase B. ia comes out from the AC side and passes through Va', and directly returns to the negative electrode of capacitor C? Does the current of phase ic return to ia through VDc?

maychang

"But in Figure C, the AC voltage e>0, but i(t)<0, the current is reversed, does it mean that this is the working mode of the inverter?" In Figure C, the direction of the instantaneous value of the AC voltage is opposite to the direction of the current, so the capacitor is discharging and the AC power supply is charging, that is, energy returns from the capacitor to the AC power supply.

shaorc

maychang posted on 2018-8-13 09:52 "But in Figure C, the AC voltage e＞0, but i(t)＜0, the current is reversed, does it mean that it is now in the working mode of the inverter?" Figure C...

This is clear, but is the topology still a rectifier at this time? Or is it an inverter at this time?

maychang

shaorc posted on 2018-8-13 14:14 This is understandable, but is this topology still a rectifier at this time? Or is it an inverter at this time?

"But is this topology still a rectifier at this time? Or is it an inverter at this time?" Since the energy flows from the DC side to the AC side, it doesn't matter if you call it an inverter at this time, but this period of time is very short, less than half an AC cycle.

shaorc

maychang posted on 2018-8-13 14:20 "But is this topology still a rectifier at this time? Or is it an inverter at this time?" Since the energy flows from the DC side to the AC side, what do you call it at this time...

This is understandable. In the working state of the rectifier, there is a period of time when the working mode is the same as the inverter. However, when comparing the topologies of the bridge controlled rectifier/inverter and the PWM rectifier/inverter, it is a bit strange. The following is a three-phase voltage type bridge inverter circuit and a PWM inverter circuit. They are almost identical. Their topological structures are exactly the same, and both flow through the MOS tube for inversion and flow through the diode for freewheeling. It is just a matter of the control signal of the power switch tube. This is easy to understand. However, the difference between the three-phase voltage bridge rectifier circuit and the PWM rectifier circuit is quite large. In the original non-PWM rectifier diagram, it is easy to understand the direction of current flow, and the rectification is also carried out through the power tube. However, in PWM rectification, although the power tube and the diode are anti-parallel, the rectification is completed by the diode. Instead, the power tube becomes a "freewheeling" tube. In addition, the DC side capacitor is reversely charged (appearing the inverter working mode in less than half an AC cycle as discussed before). Why is there such a big difference in the topology of whether to use PWM rectification?

maychang

shaorc posted on 2018-8-13 14:59 This is understandable. In the working state of the rectifier, there is a period of time when the working mode is the same as the inverter. However, the bridge controlled rectifier/inverter and P ...

"But if you look at the three-phase voltage type bridge rectifier circuit and the PWM rectifier circuit, the difference is huge." I did not find "three-phase voltage type bridge rectifier circuit" in the picture you posted. When we talk, it is best to clearly point out the object we are talking about. For example, in "Figure 7-7"..., or in "Figure 4-9"...

shaorc

maychang posted on 2018-8-13 15:11 "But if you look at the three-phase voltage-type bridge rectifier circuit and the PWM rectifier circuit, the difference is huge." I don't see the picture you posted...

Well, the three-phase full-bridge rectifier circuit, Figure 3-18, and the PWM rectifier circuit, Figure 7-32, are quite different from each other, and the difference is much greater than the difference between the inverter circuit, Figure 7-7 and Figure 7-9.

maychang

shaorc posted on 2018-8-13 16:41 Well, the three-phase full-bridge rectifier circuit Figure 3-18 and the PWM rectifier circuit, Figure 7-32, are quite different, much more so than the inverter Figure 7-7 and...

Figure 3-18 is not a "three-phase full-bridge rectifier circuit", but a "three-phase bridge fully controlled rectifier circuit". The difference between "three-phase full-bridge rectifier circuit" and "three-phase bridge fully controlled rectifier circuit" is huge. "Three-phase full-bridge rectifier circuit" uses six ordinary diodes, while "three-phase bridge fully controlled rectifier circuit" uses six unidirectional thyristors. There is also a three-phase bridge half-controlled rectifier circuit between the two, which uses three unidirectional thyristors and three ordinary diodes.

maychang

shaorc posted on 2018-8-13 16:41 Well, the three-phase full-bridge rectifier circuit Figure 3-18 and the PWM rectifier circuit, Figure 7-32, are quite different, compared to the inverting Figure 7-7 and...

The circuit in Figure 3-18 is reversible, which means it can be rectified or inverted. Energy can be transmitted from left to right or from right to left. Of course, for energy to be transmitted from right to left, the right side must be a DC power supply.

shaorc

maychang published on 2018-8-13 17:56 The circuit in Figure 3-18 is reversible, that is, it can be rectified or inverted. Energy can be transmitted from left to right or from right to left. When...

There was a question at the beginning of the post. I thought about it. See Figure f below, which shows a PWM rectifier working mode under the condition of ia>0, ib<0, ic>0. At this time, Va' of the lower bridge arm of phase A is turned on, Vb of the upper bridge arm of phase B is turned on, and VDc of the upper bridge arm of phase C is turned on. I drew the abc three-phase current myself. Is the flow direction of a yellow, b green, and c red correct? Is the current of phase c then fed into ia through VDc? The working mode at this time has both energy discharge from the DC side to the AC side, energy flow from the AC side to the DC side, and also a DC side load function. Is it that complicated?

maychang

shaorc posted on 2018-8-15 13:17 There is a question at the beginning of the post. I thought about it and saw the figure f below, which shows a PWM rectifier working mode under the condition of ia>0, ib0. This...

I don't know which book you posted the circuit diagram from. This circuit diagram must be accompanied by a three-phase voltage and current waveform diagram. If you want to know the current flow direction at a certain moment, a better way is to look at the voltage and current direction from the waveform diagram. It is much more convenient to look at the waveform diagram than to describe it in words.

shaorc

maychang posted on 2018-8-15 15:25 I don't know which book the circuit diagram you posted is from. This circuit diagram must be accompanied by a three-phase voltage and current waveform diagram. If you want to know the current flow direction at a certain moment...

It is Zhang Xing's "PWM Rectifier and Its Control" Machinery Industry Press P33 Although there is no current waveform diagram, I know the method you mentioned.

maychang

shaorc posted on 2018-8-16 16:37 It is Zhang Xing’s "PWM Rectifier and Its Control" Mechanical Industry Press P33. Although there is no current waveform diagram, I understand the method you mentioned

A textbook that does not provide voltage and current waveform diagrams is not a good textbook.

shaorc

maychang posted on 2018-8-16 17:50 A textbook that does not provide voltage and current waveform diagrams is not a good textbook.