Controlled rectifier with thyristors in Power Electronics - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a controlled rectifier with thyristors?

A controlled rectifier with thyristors is an electronic device that converts alternating current (AC) to direct current (DC) and allows control over the output voltage by adjusting the firing angle of the thyristors.

beginner

How does the firing angle affect the output voltage in a controlled rectifier?

The firing angle is the delay time after the AC voltage zero crossing when the thyristor is triggered. Increasing the firing angle reduces the output voltage because the thyristor conducts for a shorter part of the AC cycle.

beginner

Name two common types of controlled rectifiers using thyristors.

Two common types are the single-phase half-wave controlled rectifier and the single-phase full-wave controlled rectifier (bridge type).

beginner

Why are thyristors used in controlled rectifiers instead of regular diodes?

Thyristors can be turned on at a controlled time during the AC cycle, allowing control of output voltage and power, whereas diodes conduct automatically when forward biased and cannot be controlled.

beginner

What is the main application of controlled rectifiers with thyristors?

They are mainly used in power supplies, motor speed control, and controlled DC voltage sources where adjustable output voltage is needed.

What does increasing the firing angle of a thyristor in a controlled rectifier do to the output voltage?

A Decreases the output voltage

B Increases the output voltage

C Has no effect on output voltage

D Turns off the rectifier completely

Increasing the firing angle delays the conduction start, reducing the time thyristor conducts and thus lowering output voltage.

Which device allows control over when it starts conducting in a controlled rectifier?

A Diode

B Thyristor

C Resistor

D Capacitor

Thyristors can be triggered at a chosen time, unlike diodes which conduct automatically.

What is the main purpose of a controlled rectifier?

A Convert AC to DC with adjustable output voltage

B Convert DC to AC

C Store electrical energy

D Increase AC frequency

Controlled rectifiers convert AC to DC and allow control of the output voltage.

Which of these is a common type of controlled rectifier?

A Three-phase inverter

B Step-up transformer

C Single-phase full-wave controlled rectifier

D Zener diode regulator

Single-phase full-wave controlled rectifiers are widely used controlled rectifier types.

In which application would you most likely find a controlled rectifier with thyristors?

A Battery

B Light bulb

C Mechanical switch

D Motor speed control

Controlled rectifiers are used to adjust voltage and power in motor speed control.

Explain how a controlled rectifier with thyristors controls the output voltage.

Think about when the thyristor starts conducting during the AC cycle.

Describe two common types of controlled rectifiers and their basic operation.

Consider how many thyristors are used and how they control current flow.

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