Understanding Why the Controlled Rectifier Output Voltage Remains Constant Between 30 and 90 Degrees

In electrical engineering, particularly in the design and operation of power supplies, the behavior of a controlled rectifier is a critical topic. Specifically, the firing angle of a controlled rectifier system is an important parameter that affects the output voltage. This article aims to explore the phenomenon where the controlled rectifier output voltage remains relatively constant when the firing angle is between 30 and 90 degrees.

Basics of a Controlled Rectifier

A controlled rectifier system, such as used in Silicon Controlled Rectifiers (SCRs), is designed to control the power transmission between the input power line and the output load by adjusting the firing angle. The firing angle is the point at which the rectifier begins to conduct the current from the power source to the load. This angle can be adjusted to change the output voltage and thus the power delivered to the load.

Typically, the peak charging of the output capacitor in a primitive SCR system occurs at the 90 degrees firing point, coinciding with the peak of the input power line voltage. As the firing angle increases beyond 90 degrees, the peak charging of the capacitor is reduced, leading to a decrease in the output voltage.

Why the Output Voltage Remains Constant Between 30 and 90 Degrees

The key to understanding why the controlled rectifier output voltage does not change significantly when the firing angle is between 30 and 90 degrees lies in the between-phase relationship of the sinusoidal waveform and the controlled operation of the rectifier.

When the firing angle is set to 30 degrees, it means that the transformer begins to conduct power earlier in relation to the peak of the input power line voltage. As the firing angle increases to 90 degrees, the conductor point shifts closer to the peak, making the phase difference more balanced and the average output voltage relatively constant. However, there is still a slight increase in the output voltage as the firing angle approaches 90 degrees because the capacitor has more time to charge.

This behavior is often observed in systems where the load is relatively light, and the output filter acts like a peak detector. The filter can smooth out the fluctuations and maintain a steady output voltage.

Impact of Power Factor

Power factor is a crucial concept in electrical systems, as it affects the efficiency and the overall performance of the rectifier. In systems with a power factor less than 1, due to inductance, the current lags behind the voltage. This can result in a phenomenon where the load can receive reverse voltage because of the current lag.

However, such conditions are less common in modern systems and are more applicable to older or less optimized designs. If the power factor is less than 1, the behavior of the controlled rectifier could be unpredictable, and the output voltage might not follow the expected pattern.

Conclusion

In summary, the controlled rectifier output voltage remains relatively constant between 30 and 90 degrees because the phase shift of the sinusoidal waveform and the controlled operation of the rectifier create a balanced charging of the output capacitor. This behavior is influenced by the load conditions and the overall power factor of the system. Further research and detailed analysis may be required to fully understand the specific impacts and variations in different scenarios.

Keywords: controlled rectifier, firing angle, power factor