Kuliah 7: Konverter Thyristor

Summary

This lecture delves into power electronics, focusing on thyristor-based converters. The speaker explains the difference between thyristors and diodes, highlighting their distinct characteristics and functions in a converter system. The discussion covers various types of converters, including single-phase and three-phase configurations, and how adjusting the firing angle of thyristors can control the output voltage. The role of inductors and freewheeling diodes in modifying current waveforms is also discussed. Additionally, harmonic effects, voltage drops, and the significance of load parameters in determining output voltage are explored. The lecture concludes with insights on the practical application of controlled rectifiers and their behavior in real circuits.

Takeaways

• 😀 Thyristors can be controlled and turned on at any time, whereas diodes automatically turn on when voltage is sufficient and off when the voltage is zero.
• 😀 A single-phase half-wave converter uses a thyristor, with the firing angle (α) determining when the thyristor is activated. The current waveform is discontinuous.
• 😀 The firing angle signal for the thyristor is generated using a sine wave input and a sawtooth wave generator system, which helps control the timing of activation.
• 😀 The height of the control signal (VC) affects the firing angle. Increasing VC raises the firing angle, while lowering it delays the activation.
• 😀 The addition of inductors and freewheeling diodes changes the current waveform, preventing negative voltage and ensuring smooth energy transfer.
• 😀 Harmonics in the output are introduced when using thyristors, with even-order harmonics being more dominant than odd-order ones.
• 😀 By adjusting the firing angle, the average output voltage of a converter can be controlled, with larger firing angles resulting in smaller average voltages.
• 😀 The average voltage of a single-phase converter is influenced not only by the firing angle but also by load parameters, such as the presence of inductors.
• 😀 The freewheeling diode eliminates negative voltage, improving the performance of the converter by providing a path for current to flow smoothly during negative voltage cycles.
• 😀 A converter can function as both a rectifier (AC to DC) and an inverter (DC to AC) depending on the firing angle, with higher firing angles operating in inverter mode.
• 😀 A three-phase thyristor converter introduces additional complexity but provides better voltage regulation and more controlled output, with a clear relationship between output voltage and the firing angle.

Q & A

What is the primary difference between thyristors and diodes?

Thyristors have three legs and can be activated at any time, allowing for control over when they turn on. In contrast, diodes cannot be controlled and automatically turn off when the voltage crosses zero.

How does the activation angle (Alpha) affect the current in a single-phase half-wave converter?

The activation angle (Alpha) determines when the thyristor turns on, causing the current to increase. After the activation, the current stops until the next activation. A higher Alpha value results in a smaller average output voltage.

What role does the fairing angle play in the functioning of a thyristor-based converter?

The fairing angle (Alpha) is critical for controlling the timing of thyristor activation. By adjusting the fairing angle, the voltage waveform can be controlled, influencing the output voltage and current in the converter.

What is the purpose of using a sawtooth signal in a thyristor-based converter system?

A sawtooth signal is used to compare with the AC input signal, helping to generate the activation signal for the thyristor. This enables precise control over when the thyristor is activated based on the intersection of the signals.

What effect does adding a freewheeling diode have on the output voltage in a thyristor-based converter?

Adding a freewheeling diode helps eliminate negative voltage, allowing the inductor energy to rotate through the diode. This results in a more stable output voltage and reduces the negative voltage that would otherwise appear.

How does the load impact the average output voltage in a thyristor converter?

The average output voltage is influenced by both the ignition angle (Alpha) and the load characteristics. For example, an inductive load will result in a voltage drop and reduced average output voltage due to the inductor's impact.

What happens when the ignition angle (Alpha) is set to 90° or higher in a thyristor converter?

When the ignition angle is set to 90° or higher, the converter operates in inverter mode, meaning it converts DC to AC. This is in contrast to when the angle is smaller, which typically results in rectification from AC to DC.

What type of harmonics are produced by thyristor-based converters, and how are they related to the input current?

Thyristor-based converters produce odd-order harmonics (such as 5th, 7th, 11th, etc.), which are due to the switching process. These harmonics are present in the input current and can affect the performance of the system.

Why is source inductance important in a thyristor converter circuit?

Source inductance affects the waveform of the current and voltage, and can alter the output characteristics of the converter. It can cause changes in the current waveform, making it less smooth and introducing additional harmonic components.

How does the average output voltage change in a three-phase thyristor converter?

In a three-phase thyristor converter, the average output voltage depends on the ignition angle (Alpha) and follows a specific equation involving the source voltage and the cosine of the ignition angle. The output is typically smoother compared to single-phase converters.