Analysis of the structure and characteristics of rectifier circuits in power electronics technology

1. Application of power electronics technology

Power electronics technology is an emerging technology. It is the intersection of three disciplines: electricity, electronics and control theory. It has become an indispensable professional basic course in the electrical automation major with strong professional foundation and close connection with production. This course reflects the control of weak current over strong current and has strong practicality. It can combine theory with practice and plays an important role in cultivating automation professionals. It includes the structure and classification of thyristors, overvoltage and overcurrent protection methods of thyristors, controlled rectifier circuits, thyristor active inverter circuits, thyristor passive inverter circuits, PWM control technology, AC voltage regulation, DC chopping and the working principle of frequency conversion circuits.

In power electronics technology, controlled rectifier circuit is a very important chapter. Rectifier circuit is a circuit that converts AC into DC, and its application is very wide. The speed regulation of various DC motors widely used in industry adopts power electronic devices; rectifier power electronics technology is also widely used in transportation tools such as electrified railways (electric locomotives, maglev trains, etc.), electric vehicles, airplanes, ships, elevators, etc.; various electronic devices such as the DC power supply used in program-controlled switches in communication equipment, the working power supply required by large computers, and the power supply inside microcomputers can all be powered by the DC power supply composed of rectifier circuits. It can be said that where there is power supply, there are power electronics equipment.

2. Rectification Circuit in Power Electronics Technology Course

Rectifier circuits can be divided into three types: uncontrolled, semi-controlled and fully controlled according to the components they consist of. The main rectifier circuits made of thyristor semiconductor devices are semi-controlled and fully-controlled. According to the circuit wiring method, they can be divided into bridge type and zero type rectifier circuits. According to the number of AC input phases, they can be divided into single-phase and multi-phase (mainly three-phase) rectifier circuits.

According to the law of students' learning and acceptance of knowledge, the knowledge points are expressed completely, accurately and concisely, and the principle knowledge is simplified, popularized and intuitive as much as possible. The author has conducted discussions and research in teaching. According to the circuit characteristics and load forms of the three forms of rectifier circuits, the main parameter calculations and main features are made into a single-phase rectifier circuit summary table (see Table 1) and a three-phase rectifier circuit summary table (see Table 2).

In the table, α is the control angle of the rectifier circuit, UFM, UKM are the maximum forward and reverse voltages that the thyristor can withstand, U2 is the effective value of the transformer secondary voltage, U4 is the average output voltage, I4 is the average output current, IT is the effective value of the thyristor current, and Cosθ is the circuit input and output power factor.

3. Summary of Rectification Circuits in Power Electronics Technology

Through the inductive table, students can master the following aspects of the rectifier circuit through comparative learning methods:

1. Requirements for pulse arrangement of various circuits

This is the most important point, because whether a rectifier circuit can work properly depends on whether its pulse arrangement is correct. For a single-phase circuit, one cycle is 360°. If the circuit is a half-wave, it is composed of one thyristor and a pulse is sent every 360°. If the circuit is a half-controlled bridge, it is composed of two thyristors and two diodes and a pulse is sent every 180° (360°÷2=180°). If the circuit is a fully controlled bridge, it is composed of four thyristors and works in two groups. Like the half-controlled bridge, a pulse is sent every 180° (note that "once" here means sending a pulse to two thyristors at the same time). For a three-phase rectifier circuit, the number of power supply phases is 3 times that of a single-phase, so the pulse arrangement is 1/3 times that of a single-phase. For example, a three-phase half-wave is a pulse sent every 120° (360°÷3＝120). A three-phase fully controlled bridge is composed of 6 thyristors, which are turned on in turn. It is necessary to ensure that two thyristors are turned on at the same time, so a pulse is sent every 60° (360°÷6=60°) and sent to two thyristors at the same time.

In the table, α is the control angle of the rectifier circuit, UFM, UKM are the maximum forward and reverse voltages that the transistor can withstand, U2 is the effective value of the transformer secondary voltage, I2 is the effective value of the transformer secondary current, Ud is the average output voltage, Id is the average output current, IT is the effective value of the transistor current, and θ is the conduction angle of the transistor.

(II) Calculation of the average output voltage of the rectifier circuit

The output voltage of the rectifier circuit refers to the average voltage of the circuit output. This parameter reflects the size of the circuit output. Usually, we choose the rectifier circuit based on this, so it is a very important parameter. To let students remember the calculation formula of the output rectifier voltage, it can be found from the table that for a single-phase rectifier circuit, whether it is a resistive load or an inductive load, its output voltage can be expressed as Ud=AU2(1+Cosα)/2, where A is the coefficient. If it is a single-phase half-wave, A=0.45, if it is a single-phase bridge type, A=0.9 (twice the half-wave), only the single-phase fully controlled bridge inductive load is a special case, and its output voltage is Ud=0.9u2Cosα. Similarly, for a three-phase rectifier circuit, when the Ud waveform is continuous (Ud waveform continuous means that there is a rectifier voltage output in one cycle, and Ud=0 does not appear), the output voltage Ud=AU2Cosα. A is the coefficient. When the circuit is half-wave, A=1.17. When the circuit is a full-controlled bridge, A=2.34 (twice the half-wave). Only the three-phase half-controlled bridge is a special case, and its output voltage is Ud=2.34U2(1+Cosα)/2.

(III) Calculation of average output current of rectifier circuit

Whether it is single-phase or three-phase, whether it is a resistive load or an inductive load, the output current of the rectifier circuit is
Id=Ud/Rd (Rd is the resistance value in the load).

(IV) Calculation of the maximum forward and reverse voltage that the thyristor can withstand

This parameter is an important parameter for selecting thyristors. It can be seen from the table that for a single-phase rectifier circuit, the maximum voltage that the thyristor can withstand is the peak value of the power phase voltage, that is, √2U2, while for a three-phase circuit, the maximum voltage that the thyristor can withstand is the peak value of the power line voltage, that is, √6U2: (because it is three-phase, the line voltage differs from the phase voltage by √3 times).

(V) Is the output voltage Ud waveform continuous?

As can be seen from Table 1, for a single-phase rectifier circuit, when the control angle of the thyristor α＞0°, the waveform of ud is discontinuous. Only for a single-phase fully controlled bridge inductive load, when α＞90°, the waveform of ud is discontinuous. As can be seen from Table 2, for a three-phase rectifier circuit, the three-phase half-wave circuit uses α＝30° as the dividing point for the output voltage waveform to be continuous or not; while the three-phase bridge rectifier circuit (including half-controlled bridge and fully controlled bridge) uses α＝60° as the dividing point for the output voltage waveform to be continuous or not.
The calculation rules of other parameters can also be found from the table, but due to limited space, they are not listed here one by one.

IV. Conclusion

This set of summary tables of single-phase and three-phase rectifier circuits can help students quickly master the structures of various rectifier circuits, circuit characteristics, and related calculation formulas under different loads, providing great help for better mastering the basic theoretical knowledge of power electronics technology.