What is the difference between firing angle control and pulse width modulation control in PWM inverters?
Firing angle control and pulse width modulation (PWM) control are two distinct methods used in power electronics for managing output power and waveform quality. They differ in their operation, applications, and performance characteristics.
Firing Angle Control
Definition:
Firing angle control is used in thyristor-based circuits (e.g., SCR inverters) to control the point in the AC waveform at which the thyristor is triggered to conduct. The firing angle (α) determines how much of the AC waveform is allowed to pass.
Key Features:
- Waveform Control: Adjusts the phase angle of the AC supply at which conduction begins.
- Output: Produces a chopped waveform with harmonics.
- Applications: Typically found in phase-controlled rectifiers, AC voltage controllers, and some older inverter designs.
How It Works:
- The conduction begins at a specific point in the AC waveform, delaying the rise of current until the firing angle is reached.
- The effective RMS value of the voltage or current is controlled by varying the firing angle.
Advantages:
- Simple to implement with SCR or thyristor-based designs.
- Suitable for high-power applications.
Disadvantages:
- Results in significant harmonic distortion.
- Limited control over output waveform shape.
Pulse Width Modulation (PWM) Control
Definition:
PWM control is a technique where the width (duration) of voltage pulses is modulated to create a desired average output voltage. PWM is commonly used in modern inverters to generate AC waveforms with reduced harmonics.
Key Features:
- Waveform Control: Outputs a series of pulses whose widths are adjusted to approximate a sinusoidal waveform.
- Output: Produces a waveform with lower total harmonic distortion (THD).
- Applications: Widely used in modern PWM inverters, motor drives, and renewable energy systems.
How It Works:
- High-frequency switching of transistors (like IGBTs or MOSFETs) modulates the pulse width.
- The pulse widths are adjusted based on the reference waveform (often a sine wave) to synthesize an AC output.
Advantages:
- Produces high-quality AC waveforms with low harmonic distortion.
- Offers precise control over output voltage and frequency.
- Efficient and compatible with digital control systems.
Disadvantages:
- Requires more sophisticated control circuitry and fast-switching devices.
- Higher switching losses compared to firing angle control.
Comparison of Firing Angle Control and PWM Control
| Aspect | Firing Angle Control | PWM Control |
|---|---|---|
| Waveform Quality | Poor, high harmonic distortion. | Superior, low harmonic distortion. |
| Control Mechanism | Adjusts conduction angle of thyristors. | Adjusts pulse width of switching devices. |
| Switching Devices | Thyristors (SCRs). | IGBTs, MOSFETs, or other high-speed devices. |
| Efficiency | Lower efficiency due to harmonic losses. | Higher efficiency in waveform synthesis. |
| Complexity | Simple, suitable for analog control. | Complex, often requires digital control. |
| Applications | Older rectifiers, voltage controllers. | Modern inverters, motor drives, and converters. |