Question

Define Conduction angle and Firing angle.

Answer 1

In the context of electrical engineering, particularly when dealing with controlled rectifiers and phase-controlled circuits (such as those using thyristors or SCRs), two important terms often discussed are **conduction angle** and **firing angle**. Understanding these terms is crucial for analyzing and designing circuits involving power electronics. Let's explore each term in detail:

### Firing Angle (α)

**Definition:**
The **firing angle** (α) is the angle at which a thyristor or controlled rectifier is triggered or turned on during each half-cycle of the AC input voltage. It is measured from the zero crossing point of the AC waveform.

**Explanation:**
- In a typical AC waveform, the voltage rises from zero, reaches a peak, and then returns back to zero before becoming negative and repeating the cycle.
- The firing angle determines when the SCR will start conducting. By delaying the triggering of the SCR from the zero crossing point, we can control the amount of power delivered to the load.
- For instance, if the SCR is triggered at 30° (α = 30°), it will conduct until the next zero crossing (180° for half-wave circuits). Thus, the conduction starts at 30° and ends at 180°, and the delay introduces control over the output voltage and current.
- The firing angle can vary from 0° (fully conducting) to 180° (where the device does not conduct during the half cycle).

**Applications:**
- Used in light dimmers, motor speed controls, and heater controls, where varying the power output is necessary.

### Conduction Angle (β)

**Definition:**
The **conduction angle** (β) is the angular range during which the SCR or thyristor remains in the conducting state after it has been triggered. It is the difference between the angle at which the SCR is turned on (firing angle) and the angle at which it turns off (commutation).

**Explanation:**
- The conduction angle is calculated as:  
  \[
  β = 180° - α
  \]
  for a half-wave rectifier where the device conducts from the firing angle (α) until the next zero crossing at 180°.
- If the firing angle is set to 30°, the SCR conducts from 30° to 180°, which means the conduction angle is \( β = 180° - 30° = 150°\).
- The longer the conduction angle, the more power is delivered to the load. Conversely, a smaller conduction angle results in less power.
- In some configurations, such as full-wave rectifiers or multiple SCRs, the conduction angle may also refer to how long multiple SCRs conduct during a full cycle.

**Applications:**
- The conduction angle is crucial in applications requiring precise control of power, such as in industrial heating and variable frequency drives for motors.

### Relationship Between Firing Angle and Conduction Angle

The firing angle and conduction angle are inversely related. Increasing the firing angle results in a decrease in the conduction angle, meaning that less of the AC waveform is utilized to power the load. Conversely, a smaller firing angle allows for a longer conduction angle, delivering more power.

### Summary

- **Firing Angle (α):** The angle at which a controlled rectifier or thyristor is triggered during the AC cycle, controlling when it starts to conduct.
- **Conduction Angle (β):** The total angle of conduction after the SCR has been turned on, representing the time it remains in the conducting state until it is turned off.

These two angles are fundamental in power electronics, allowing engineers to design systems for efficient power control and management in various applications.

Answer 2

### Conduction Angle
The conduction angle, often denoted as \(\alpha_c\), refers to the duration in the electrical cycle during which a particular component, such as a diode or thyristor (SCR), conducts current. It is measured in degrees or radians and represents the portion of the cycle during which the current flows through the device. In a typical AC circuit, this angle starts when the device begins conducting and ends when the current falls back to zero or when another device takes over the conduction.

For example, in a full-wave rectifier, the conduction angle for each diode is typically 180 degrees because each diode conducts for half of the AC cycle.

### Firing Angle
The firing angle, denoted as \(\alpha\), is the angle at which a thyristor (such as an SCR) is triggered to start conducting. It is also measured in degrees or radians. The firing angle is crucial in controlling the output voltage in AC-DC converters (such as controlled rectifiers). By adjusting the firing angle, the point at which the SCR starts conducting can be delayed within the AC cycle, thereby controlling the average output voltage.

- **0 degrees firing angle**: The thyristor starts conducting at the beginning of the AC cycle, leading to maximum output voltage.
- **90 degrees firing angle**: The thyristor conducts at the peak of the AC waveform, leading to a reduced output voltage.
- **180 degrees firing angle**: The thyristor conducts just before the end of the positive half-cycle, resulting in minimal output voltage.

In summary:
- The **conduction angle** defines how long the current flows through the device in each cycle.
- The **firing angle** determines when in the AC cycle the device starts to conduct.