3 Phase Semiconverter | Power ELectronics | Lecture 61

Summary

In this video, the operation and analysis of a three-phase semicon converter circuit are explored. The script covers key concepts such as the difference between semicon converters and fully controlled rectifiers, the triggering sequences of thyristors and diodes, and how output voltage waveforms are generated at different triggering points (Alpha angles of 30°, 60°, and 90°). The discussion also includes how the line voltage waveforms influence the output and how they behave in the case of resistive and inductive loads. This provides a clear understanding of the circuit's behavior and operation.

Takeaways

• 😀 The three-phase semi-controlled converter circuit uses diodes in the lower legs instead of thyristors, which limits control to certain phases (T1, T3, T5).
• 😀 To analyze the circuit, line voltage waveforms are key. Understanding how to draw these waveforms is crucial for determining output voltage behavior.
• 😀 The circuit operates by triggering thyristors at 120° intervals (T1, T3, T5), and the output voltage waveform depends on the conduction sequence.
• 😀 For Alpha = 30°, the output voltage will initially follow the line voltage VAB, then VBC, and so on in a repeating cycle depending on the triggering sequence.
• 😀 When Alpha = 60°, the analysis starts at 90°, skipping the AB waveform and following the AC and BA waveforms instead due to reverse biasing of diodes.
• 😀 At Alpha = 90°, the output voltage follows the AC waveform but goes to zero before following the BA waveform, as the triggering sequence shifts.
• 😀 The use of diodes in the lower legs leads to a lack of control over the current compared to a fully controlled rectifier that uses thyristors.
• 😀 The output waveform becomes discontinuous for Alpha values greater than 60°, with periods of zero voltage observed between phases.
• 😀 The circuit behavior is similar to a fully controlled rectifier, but due to the diode configuration, it does not provide complete control over the output voltage.
• 😀 For inductive loads, the output voltage waveform remains the same, but the current may be continuous or discontinuous depending on the inductor's value.

Q & A

What is the main difference between a three-phase semicon converter and a fully controlled rectifier?

The main difference lies in the use of diodes in the lower legs of the three-phase semicon converter instead of thyristors. This limits the control over the output voltage, as the diodes can only allow current to flow in one direction, unlike thyristors which provide more control.

How does the circuit in a three-phase semicon converter operate?

The circuit operates by triggering the thyristors (T1, T3, T5) at specific intervals to control the conduction sequence. The diodes (D1, D2, D6) in the lower legs are triggered by the phase voltages, allowing current to flow and regulating the output voltage waveform.

What role does the triggering sequence play in the operation of the converter?

The triggering sequence determines which thyristor is triggered at specific intervals, ensuring the correct conduction path for current. This sequence influences the output voltage waveform, with each thyristor conducting for 120° of the cycle, creating a regular pattern of output voltages.

What happens when Alpha is equal to 30° in the circuit analysis?

At Alpha = 30°, the circuit starts triggering at 60° and follows the line voltage waveform VAB initially. The output voltage changes as different combinations of thyristors and diodes are triggered, leading to a cyclic sequence of output voltages such as VAB, VAC, VBC, and so on.

How does the output voltage behave when Alpha is equal to 60°?

At Alpha = 60°, the circuit will start following the VAC waveform. As the triggering continues, certain waveforms such as VAB and VBC are skipped, and the output follows different line voltage waveforms like VBA, depending on the phase relationships and the triggering sequence.

Why is the output voltage zero when Alpha is equal to 90°?

When Alpha = 90°, the triggering sequence skips the required thyristors, causing the output voltage to drop to zero. This happens because there is no trigger for the next thyristor (T3) at this phase angle, resulting in no current conduction until the next cycle.

What happens at higher Alpha angles, such as 90° and beyond, in terms of output voltage waveform?

As the Alpha angle increases, the output voltage waveform becomes discontinuous. This means that the output voltage will drop to zero for intervals where no triggering occurs, and the circuit's output is only generated when the correct thyristors are triggered at the appropriate phases.

How does the type of load (resistive or inductive) affect the output voltage waveform in the three-phase semicon converter?

The type of load affects the current behavior. In a resistive load, the output voltage waveform remains discontinuous or continuous depending on the triggering sequence. In an inductive load, the current may flow continuously or discontinuously, depending on the value of the inductance, but the output voltage waveform itself will generally remain the same.

What is the significance of the phase displacement between the three phases in the analysis of the output waveform?

The phase displacement of 120° between the three phases is crucial in determining the order in which the thyristors are triggered and how the diodes conduct. This displacement ensures a continuous flow of current through the converter, affecting the shape and behavior of the output voltage waveform.

Why do we skip certain line voltage waveforms like VAB and VBC at certain Alpha angles?

Certain line voltage waveforms are skipped due to the behavior of the diodes in the lower legs of the converter. At specific Alpha angles, the diodes reverse bias and prevent conduction along certain paths, leading to the skipping of waveforms like VAB and VBC, depending on the triggering sequence.