Question

A 3-\(\phi\) fully controlled converter can be operated as a semi-converter, if
(a) One of the input ac supplies is disconnected and the thyristors connected to it are triggered at 180 degrees.
(b) One of the input ac supplies is disconnected and the thyristors connected to it are triggered at zero degrees.
(c) A free-wheeling diode can be connected across the load in the same direction of thyristors
(d) A free-wheeling diode can be connected across the load in the opposite direction of thyristors
Choose the correct answer

• A. \( \text{(a) and (c) are false} \)
• B. \( \text{(b) and (d) are false} \)
• C. \( \text{(b) and (c) are true} \)
• D. \( \text{(d) and (a) are true} \)

Hint:

To make any fully controlled converter act like a semi-converter: 1. Add a freewheeling diode across the output load to clamp negative voltage. 2. Or program a group of thyristors to fire at \(\alpha = 0^\circ\), turning them into standard diodes. This lets you quickly identify statements (b) and (c) as true.

Answer 1

The Correct Option is C

Concept: A three-phase fully controlled converter contains six thyristors. It normally operates as a two-quadrant converter, allowing the average output voltage to become negative when the firing angle $\alpha$ exceeds $90^\circ$. In contrast, a semi-converter is limited to single-quadrant operation because it cannot produce a negative output voltage. It achieves this using passive freewheeling action, which clamps any negative voltage spikes to zero. We can make a fully controlled converter behave like a semi-converter using two main modification methods:
• Adding a passive physical freewheeling diode directly across the output load terminals.
• Using a control technique known as asymmetric triggering or converter modification, where certain thyristors are held wide open ($\alpha = 0^\circ$) to act as uncontrolled diodes, sometimes combined with adjusting the input supply phases.

Step 1: Analyzing statements (c) and (d) regarding the freewheeling diode.
When a freewheeling diode is added across an inductive load, it must be reverse-biased during normal conduction cycles and forward-biased if the load voltage tries to reverse polarity.
• The diode anode must connect to the circuit ground/return line and its cathode must connect to the positive output terminal. This matches the forward current direction of the thyristors.
• Therefore, Statement (c) is True, and Statement (d) is False.

Step 2: Analyzing statements (a) and (b) regarding control modification.
To make a controlled thyristor branch act like an uncontrolled diode branch, its firing delay angle must be set to zero ($\alpha = 0^\circ$). This ensures it turns on as soon as it becomes forward-biased, exactly like a passive diode.
• Setting the firing angle to $180^\circ$ keeps the device turned off during the conduction cycle, which does not mimic semi-converter behavior.
• Therefore, Statement (b) is True, and Statement (a) is False.

Step 3: Selecting the matching choice.
Our analysis shows that statements (b) and (c) are true, which corresponds to option (3). Hence, the correct choice is option (3).