Controlled rectifier with thyristors in Power Electronics - Step-by-Step Execution

Concept Flow - Controlled rectifier with thyristors

AC Input Voltage

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Thyristor Bridge Circuit

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Trigger Pulse to Thyristors

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Thyristor Conducts when Triggered

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Controlled DC Output Voltage

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Load Receives DC Power

The AC voltage is applied to a thyristor bridge. Trigger pulses control when thyristors conduct, allowing control of the DC output voltage supplied to the load.

Execution Sample

Power Electronics

AC voltage applied
Trigger pulse at angle alpha
Thyristor turns ON
Current flows through load
Output voltage controlled

Shows how triggering thyristors at different angles controls the output DC voltage from an AC source.

Analysis Table

Step	AC Voltage (Instant)	Trigger Pulse	Thyristor State	Load Current	Output Voltage
1	0 V (zero crossing)	No	OFF	0 A	0 V
2	50 V (rising)	No	OFF	0 A	0 V
3	100 V (peak)	Yes (alpha=30°)	ON	Current starts	Voltage follows AC from 30°
4	80 V (falling)	No	ON	Current flows	Voltage follows AC
5	0 V (zero crossing)	No	OFF (commutation)	Current stops	0 V
6	-50 V (negative half)	No	OFF	0 A	0 V
7	-100 V (negative peak)	Yes (alpha=210°)	ON	Current starts	Voltage follows AC from 210°
8	-80 V (rising)	No	ON	Current flows	Voltage follows AC
9	0 V (zero crossing)	No	OFF (commutation)	Current stops	0 V
10	Cycle repeats	Trigger pulses control conduction angle	ON/OFF accordingly	Load current controlled	Output voltage controlled
Exit	End of cycle	No trigger	OFF	0 A	0 V

💡 Cycle ends; thyristors turn OFF at zero crossing without trigger pulse.

State Tracker

Variable	Start	After Step 3	After Step 5	After Step 7	After Step 9	Final
AC Voltage	0 V	100 V	0 V	-100 V	0 V	Cycle repeats
Trigger Pulse	No	Yes (alpha=30°)	No	Yes (alpha=210°)	No	Controlled pulses
Thyristor State	OFF	ON	OFF	ON	OFF	Switches ON/OFF
Load Current	0 A	Starts	Stops	Starts	Stops	Controlled current
Output Voltage	0 V	Follows AC from 30°	0 V	Follows AC from 210°	0 V	Controlled DC output

Key Insights - 3 Insights

Why does the thyristor only conduct after the trigger pulse and not immediately when AC voltage rises?

Because the thyristor needs a trigger pulse to turn ON; without it, even if AC voltage is high, the thyristor stays OFF as shown in steps 2 and 3 of the execution_table.

What causes the thyristor to turn OFF at zero crossing of AC voltage?

At zero crossing, the current through the thyristor naturally falls to zero, causing it to turn OFF (commutation), as seen in steps 5 and 9.

How does changing the trigger angle alpha affect the output voltage?

Triggering later (larger alpha) delays conduction, reducing the average output voltage; triggering earlier increases output voltage, as shown by the conduction starting at 30° vs 210° in the table.

Visual Quiz - 3 Questions

Look at the execution_table at step 3. What is the thyristor state and why?

A ON because AC voltage reached peak

B OFF because AC voltage is not high enough

C ON because trigger pulse is applied at 30°

D OFF because trigger pulse is not applied

💡 Hint

Refer to step 3 where trigger pulse is Yes (alpha=30°) causing thyristor to turn ON.

At which step does the load current stop flowing and why?

A Step 3, because trigger pulse ends

B Step 5, because AC voltage crosses zero and thyristor turns OFF

C Step 7, because negative voltage starts

D Step 9, because trigger pulse is applied

💡 Hint

Check step 5 where load current stops as thyristor turns OFF at zero crossing.

If the trigger pulse angle alpha is increased, how does the output voltage change?

A Output voltage decreases

B Output voltage increases

C Output voltage stays the same

D Output voltage becomes negative

💡 Hint

Refer to key_moments and execution_table steps 3 and 7 showing conduction starting later reduces output voltage.

Concept Snapshot

Controlled rectifier uses thyristors to convert AC to DC.
Trigger pulses at angle alpha control when thyristors conduct.
Conduction starts at trigger angle, controlling output voltage.
Thyristors turn OFF at AC zero crossing (natural commutation).
Output voltage average depends on trigger angle.
Used for adjustable DC power supply.

Full Transcript

A controlled rectifier with thyristors converts AC voltage into a controlled DC voltage by using thyristors that conduct only when triggered. The AC input voltage is applied to a thyristor bridge. Trigger pulses at a specific angle alpha cause the thyristors to turn ON, allowing current to flow through the load. The conduction starts at the trigger angle and continues until the AC voltage crosses zero, where the thyristor naturally turns OFF. By changing the trigger angle, the average output voltage supplied to the load can be controlled. This process repeats every AC cycle, providing adjustable DC power. The execution table shows the AC voltage, trigger pulse presence, thyristor state, load current, and output voltage step-by-step through one cycle. Key moments clarify why thyristors need trigger pulses to conduct, why they turn OFF at zero crossing, and how trigger angle affects output voltage. The visual quiz tests understanding of these steps and concepts.

Practice

1. What is the main function of a controlled rectifier with thyristors?

easy

A. To store electrical energy for later use

B. To control the output DC voltage by adjusting the firing angle

C. To convert DC voltage into AC voltage

D. To increase the frequency of the AC supply

Solution

1. Step 1: Understand the role of thyristors in controlled rectifiers

   Thyristors act as switches that can be turned on at specific times to control current flow.

2. Step 2: Relate firing angle to output voltage control

   By changing the firing angle, the time thyristors conduct in each AC cycle changes, adjusting the DC output voltage.

3. Final Answer:

   To control the output DC voltage by adjusting the firing angle -> Option B

4. Quick Check:

   Firing angle controls voltage = To control the output DC voltage by adjusting the firing angle [OK]

Hint: Remember: firing angle controls conduction time [OK]

Common Mistakes:

• Confusing rectifier with inverter
• Thinking thyristors store energy
• Assuming frequency changes in rectification

2. Which of the following is the correct symbol for a thyristor in circuit diagrams?

easy

A. A diode symbol with an additional gate terminal

B. A resistor symbol with an arrow

C. A capacitor symbol with two parallel lines

D. A simple switch symbol

Solution

1. Step 1: Recall the thyristor symbol basics

   Thyristors look like diodes but have an extra gate terminal to control conduction.

2. Step 2: Compare options to standard symbols

   The description of a diode symbol with an additional gate terminal matches the standard thyristor symbol, distinguishing it from resistors, capacitors, or switches.

3. Final Answer:

   A diode symbol with an additional gate terminal -> Option A

4. Quick Check:

   Thyristor symbol = diode + gate [OK]

Hint: Thyristor = diode + gate terminal [OK]

Common Mistakes:

• Confusing thyristor with resistor or capacitor symbols
• Ignoring the gate terminal in the symbol
• Choosing simple switch symbol

3. Consider a single-phase controlled rectifier with a firing angle of 60°. What happens to the output DC voltage compared to firing at 0°?

medium

A. Output voltage decreases

B. Output voltage becomes negative

C. Output voltage remains the same

D. Output voltage increases

Solution

1. Step 1: Understand firing angle effect on conduction

   At 0°, thyristors conduct immediately, giving maximum output voltage. At 60°, conduction starts later, reducing conduction time.

2. Step 2: Relate conduction time to output voltage

   Less conduction time means less average DC voltage output, so output voltage decreases.

3. Final Answer:

   Output voltage decreases -> Option A

4. Quick Check:

   Higher firing angle = lower voltage [OK]

Hint: Later firing angle means less voltage [OK]

Common Mistakes:

• Assuming voltage increases with firing angle
• Thinking output voltage stays constant
• Confusing negative voltage with delayed conduction

4. A controlled rectifier circuit is not producing any output voltage even though the AC supply is present. Which of the following is the most likely cause?

medium

A. The firing angle is set to zero degrees

B. The AC supply voltage is too high

C. The load is disconnected

D. Thyristors are not receiving the firing pulses

Solution

1. Step 1: Check the role of firing pulses

   Thyristors need firing pulses to start conducting; without them, no current flows.

2. Step 2: Analyze other options

   High supply voltage or zero firing angle would not stop output; disconnected load would cause no current but not zero voltage at output terminals.

3. Final Answer:

   Thyristors are not receiving the firing pulses -> Option D

4. Quick Check:

   No firing pulse = no conduction [OK]

Hint: No firing pulse means no output voltage [OK]

Common Mistakes:

• Assuming zero firing angle means no output
• Ignoring the need for firing pulses
• Confusing load disconnection with no voltage

5. In a three-phase controlled rectifier using thyristors, how does increasing the firing angle affect the output voltage ripple and average DC voltage?

hard

A. Ripple decreases and average DC voltage increases

B. Both ripple and average DC voltage increase

C. Ripple increases and average DC voltage decreases

D. Ripple remains constant and average DC voltage decreases

Solution

1. Step 1: Understand firing angle effect on conduction intervals

   Increasing firing angle delays thyristor conduction, reducing average DC voltage output.

2. Step 2: Analyze impact on voltage ripple

   Delayed conduction shortens conduction periods, causing larger gaps between pulses, which increases voltage ripple.

3. Final Answer:

   Ripple increases and average DC voltage decreases -> Option C

4. Quick Check:

   Higher firing angle = lower voltage + more ripple [OK]

Hint: More delay means less voltage, more ripple [OK]

Common Mistakes:

• Assuming ripple decreases with firing angle
• Thinking average voltage increases with delay
• Ignoring ripple changes with conduction timing