Learn about the firing angle, conduction angle, and extinction angle of SCR. Understand how these key parameters influence the operation, efficiency, and control of Silicon Controlled Rectifiers in various power electronics applications.
Firing Angle of SCR
The firing angle α (also known as the gate trigger angle) is the angle in the AC cycle at which the SCR is triggered or turned on. It is measured in degrees, with 0 degrees corresponding to the start of the positive half-cycle of the AC waveform.
By adjusting the firing angle, the amount of power delivered to the load can be controlled. A smaller firing angle results in more power being delivered because the SCR conducts for a larger portion of the AC cycle. Conversely, a larger firing angle means the SCR conducts for a shorter duration, delivering less power to the load.
The firing angle is a crucial factor in phase control techniques, where the phase angle of the AC supply is adjusted to control the power. This is commonly used in dimmer switches, motor speed controls, and heaters.
When the SCR is triggered at a specific firing angle, it remains off until that angle is reached in the AC cycle. After being triggered, it remains on for the rest of the half-cycle, allowing current to pass through.
The firing angle is typically controlled by a gating circuit that determines when to apply the gate pulse to the SCR. The timing of this pulse is usually calculated based on the desired power output and the characteristics of the AC supply.
The output waveform of an SCR-controlled circuit is a clipped version of the input AC waveform. The point at which the waveform is clipped (starts conducting) is determined by the firing angle. This results in a change in the RMS value of the output voltage, directly affecting the power delivered to the load.
Conduction Angle of SCR
The conduction angle of a Silicon Controlled Rectifier (SCR) refers to the portion of the AC cycle during which the SCR is in the conducting state.
The conduction angle is the angle during which the SCR conducts current in each AC cycle. It is measured in degrees and represents the period from the point where the SCR is triggered (firing angle) to the point where the current naturally falls to zero (usually at the end of the half-cycle).
The conduction angle is directly influenced by the firing angle. The later the SCR is triggered (higher firing angle), the shorter the conduction angle will be. Conversely, an earlier firing angle results in a longer conduction angle.
If the firing angle is α(alpha) degrees, the conduction angle β for an SCR in a half-wave rectification setup can be calculated as:
The conduction angle determines how much of the AC voltage is applied to the load. A larger conduction angle means the SCR is conducting for a longer portion of the cycle, resulting in higher power delivery to the load. A smaller conduction angle means less power is delivered.
The output voltage waveform of an SCR-controlled circuit shows a phase delay corresponding to the firing angle and a conducting portion that lasts for the duration of the conduction angle. This clipping of the waveform modifies the RMS value of the output voltage.
Control of the conduction angle is crucial in applications such as light dimmers, motor speed controls, and controlled rectifiers. It allows precise regulation of the power supplied to the load.
Extinction Angle of SCR
The extinction angle, also known as the turn-off angle or commutation angle, is a critical parameter in the operation of a Silicon Controlled Rectifier (SCR).
The extinction angle (γ) is the angle at which the SCR stops conducting after it has been triggered and conducting current. It is measured from the point where the SCR starts conducting (the firing angle) to the point where it stops conducting.
In an AC cycle, the SCR conducts from the firing angle (α) until the current through the SCR drops below the holding current level, usually at the end of the AC half-cycle. The extinction angle is thus dependent on the point in the AC cycle where the current naturally reaches zero.
If the conduction angle (β) is known, the extinction angle can be calculated as:
For a complete AC cycle, extinction angle γ will be within the range of 0 to 360 degrees.
The output waveform of an SCR-controlled circuit shows that the SCR conducts from the firing angle until the extinction angle, resulting in a clipped waveform that affects the RMS value of the output voltage. This clipping effect is used to control the power delivered to the load.
Conclusion
Understanding the firing angle, conduction angle, and extinction angle is fundamental to effectively utilizing Silicon Controlled Rectifiers (SCRs) in power electronics.