Learning methods and common terms of rectifier circuits

Learning methods and common terms of rectifier circuits

The basic method of learning rectifier circuit

The key link of learning rectifier circuit is to analyze and study the working principle of rectifier circuit. That is, according to the structure of the circuit and the characteristics of the load, analyze the physical process of the turn-on and turn-off of each rectifier component in the circuit, so as to obtain a series of voltage and current waveforms, and derive the basic parameter relationship of the rectifier circuit from the analysis of the waveforms. Special attention should be paid to the influence of the structure of the circuit and the nature of the DC load on the rectifier circuit. Different loads have very different waveforms of the rectified output voltage and current. Therefore, the basic skill of waveform analysis should be mastered in the study.

1) The relationship between the ratio of the output DC average voltage of the rectifier circuit to the effective value of the AC input voltage and the trigger delay angle α.

2) Analysis and calculation of rectified output average current and RMS current.

3) Analyze the working conditions of the rectifier components, determine the maximum voltage and the maximum current that the components may withstand, and reasonably select the rated parameters of the components.

4) Understand and master the technical parameters and calculation methods commonly used in rectifier circuits, such as power, power factor, displacement factor, waveform distortion factor, ripple factor, etc. These parameters are specific evaluations of the technical performance indicators of a rectifier circuit.

5) Pay attention to the influence of the nature of the DC load on the rectifier circuit. Loads of different natures have a great influence on the voltage and current waveforms output by the rectifier circuit. The nature of the load is roughly divided into the following categories:

①Resistive load: such as electrolysis system, incandescent lamp and electric welding, it belongs to resistive load, which is characterized by the similar waveform shape of current and voltage, and both current and voltage are allowed to change abruptly.

②Inductive load: The excitation winding of various motors and the load with large reactor filter are all inductive loads. For the convenience of analysis, the resistance and inductance are usually separated and regarded as the load in the form of resistance string inductance. Its characteristic is that when the reactance value is much larger than the series resistance value, the load current waveform is easy to be continuous and flat.

③Capacitive load: The output end of the rectifier circuit is connected to a large capacitor filter, and its load is capacitive, that is, when the device is just turned on, a large capacitor charging current will flow, and the current waveform is in a peak shape, which is easy to damage the device; therefore, it is generally not appropriate to connect a large capacitor at the output end.

④Back EMF load: When the output end of the rectifier device charges the battery or supplies the DC motor as the power source, it is a back EMF load. Its characteristic is that only when the power supply voltage is greater than the back electromotive force, the device can be turned on, and the current waveform has large pulsation. In fact, there are few loads of a single nature, and the main and essential characteristics of specific loads should be highlighted in order to simplify the solution to the problem.

6) Pay attention to the combination of theoretical study and practical experience. It is necessary to pay attention to and strengthen the training of the experimental link, pay attention to participating in the design and debugging of the circuit or system, compare the theoretical analysis with the actual, and constantly practice and summarize, in order to effectively and effectively grasp the knowledge, and obtain the methods of analyzing and solving problems.

Common terms in rectifier circuits

1) Trigger delay angle α: The angle from when the thyristor starts to bear the forward voltage to being triggered and turned on is called the trigger delay angle α, and the trigger delay angle is also called the trigger delay angle or the control angle.

2) Conduction angle θ: The electrical angle that the thyristor conducts in one cycle is called the conduction angle. The conduction angle is related to the nature of the load.

3) Phase shift: Change the size of the trigger delay angle α, that is, change the phase of the trigger pulse ug, which is called phase shift.

4) Phase-shift control: The control method of changing the trigger delay angle α and adjusting the output voltage is called phase-shift control.

5) Phase shift range: the allowable adjustment range of the trigger delay angle α. When α changes from 0° to the maximum angle αmax, the corresponding rectified output voltage completes the maximum to minimum change. The phase shift range is related to the structure of the circuit and the nature of the load.

6) Synchronization: In order to stabilize the rectified output voltage, the trigger pulse signal and the thyristor anode voltage (ie the AC power supply voltage) must be coordinated in frequency and phase. This relationship of coordination and cooperation is called synchronization.

7) Natural commutation point: When all the controllable elements in the circuit are replaced by uncontrollable elements, the conductive transition point of each element is called the natural commutation point. By definition, in a single-phase circuit, the point of ωt=0° is the natural commutation point of the circuit; in a three-phase circuit, the point of ωt=30° is the natural commutation point of the circuit.

8) Phase commutation: At a certain ωt moment, the process of converting the conduction of one-phase thyristor to the conduction of another phase thyristor is called commutation.