Power Factor Analysis of Single-Phase Bridge Rectifier Capacitor Filter Load

Abstract: The power factor of single-phase bridge rectifier capacitor filter load (i.e. nonlinear load) is quantitatively analyzed theoretically and compared with the transmission

Compare with the traditional concept of power factor.

Keywords: power factor, rectification and filtering

1 Introduction

Power factor has always been a concern in power systems. In recent years, with the widespread application of rectifier and filter circuits such as electronic ballasts and switching power supplies, and the increasing power, the impact of power factor has become more and more obvious in this field. Some practical circuits have appropriate correction circuits, but rarely involve quantitative analysis of power factor. This article analyzes this for reference in circuit design.

2 Traditional concept of power factor

In the past, due to the use of inductive or capacitive loads in power systems, there was a phase difference between the voltage and current in the AC circuit. The value was φ, and the power factor was defined as follows:

令：u=Umcosωt

Then: i=Imcos(ωt±φ）

It can be seen that when the phase difference between voltage and current is φ, and the waveforms are both cosine (or sine) waves, the power factor is only related to the phase difference φ.

3. Power factor of single-phase bridge rectifier capacitor filter load

The circuit of a single-phase bridge rectifier filter load is shown in Figure 1. Due to the effect of the capacitor in the rectifier filter circuit, the conduction time of each arm of the rectifier bridge is less than half a cycle, that is, the conduction angle is less than π, and its value is set to 2α, as shown in Figure 2. Since i is no longer a sine wave, the power factor cannot be expressed by cosφ.

Ignore the voltage drop during the capacitor discharge period. At this time, the capacitor can be equivalent to a voltage source. Assume that the voltage is E and the internal resistance is r, see Figure 3 and Figure 4. When ui>E, the current flowing through the rectifier bridge in half a cycle is usually Rr. Ignore the ui/R term, then the current flowing through the rectifier bridge is the top of the sine wave. The input voltage and current waveforms are shown in Figure 2.

Figure 1 Single-phase bridge rectifier capacitor filter circuit

Figure 2 Input voltage and current waveforms

Figure 3 Output voltage waveform

Figure 4 Filter capacitor is equivalent to a voltage source

As shown in Figure 5:

Figure 5 Relationship between current i and conduction angle 2α

4 Conclusion

As the conduction angle increases, the input power factor increases. When the conduction angle 2α is π, PF = 1. In practical applications, the conduction angle can be increased through circuit design to achieve the purpose of improving the input power factor.