A fully controlled converter is regulated by adjusting the firing angle of the thyristors, which are semiconductor devices that act like electronically controlled switches.
Control Techniques
Here are the primary methods used to control a fully controlled converter:
1. Phase Control
- Principle: Adjusting the firing angle (α) of the thyristors directly controls the amount of voltage and current delivered to the load.
- Operation: By delaying the triggering of the thyristors, you can control the portion of the AC waveform that reaches the load.
- Advantages: Simple implementation, good for DC motor control.
- Disadvantages: Generates harmonics, limited control over output voltage.
2. Pulse Width Modulation (PWM)
- Principle: Chopping the DC input into pulses of varying width, thereby regulating the average output voltage.
- Operation: The thyristors are switched on and off rapidly, creating a series of pulses. The duty cycle (ratio of ON time to total time) determines the average output voltage.
- Advantages: Reduced harmonics, precise control over output voltage.
- Disadvantages: Requires more complex circuitry, can lead to higher switching losses.
Controlling the Firing Angle
Several methods are used to control the firing angle of the thyristors:
- Analog Control: Using potentiometers or other analog circuits to set the firing angle.
- Digital Control: Employing microcontrollers or digital signal processors (DSPs) to generate firing pulses.
- Feedback Control: Using sensors to monitor the output voltage or current and adjust the firing angle accordingly to maintain a desired setpoint.
Examples
- DC Motor Control: A fully controlled converter can be used to control the speed of a DC motor by varying the voltage applied to the armature.
- Battery Chargers: Fully controlled converters are essential for charging batteries, as they can regulate the charging current and voltage.
Conclusion
Controlling a fully controlled converter involves adjusting the firing angle of the thyristors using techniques like phase control or PWM. By manipulating the firing angle, you can regulate the output voltage and current, allowing for precise control of the load.
