Power Factor Improvement & Extinction Angle Control
Power Factor Improvement:
For phase-controlled operation in both single phase full wave half and full controlled bridge converters, the displacement factor (or power factor, which is lagging) decreases, as the average value of output voltage (Vdc) decreases, with the increase in firing angle delay, α. This is also applicable for both three phase half wave and full wave (bridge) converters. The three schemes used for power factor (pf) improvement are:
• Extinction angle control
• Symmetrical angle control
• Pulse width modulation (PWM) control
Extinction Angle Control:
- The circuit diagram of a single phase full wave half-controlled (semi) force-commutated bridge converter is shown in Fig. The thyristors, T1 & T2, are replaced by the switches, self-commutated devices, such as power transistor or equivalent.
- The power transistor is turned on by applying a signal at the base, and turned off by withdrawing the signal at the base. A gate turn-off thyristor (GTO) also may be used, in which case, it may be turned off by applying a short negative pulse to its gate, but is turned on by a short positive pulse, like a thyristor.
- In extinction angle control, switch, S1 is turned on at t0ω=, and then turned off by forced commutation at (tω=π−β. The switch, S2 is turned on at tω=π, and then turned off at (t2ω=π−β.
- The output voltage is controlled by varying the extinction angle, β shows the waveforms for input voltage, output voltage, input current, and the current through thyristor switches.
- The fundamental component of input current leads the input voltage, and the displacement factor (and power factor) is leading. This feature may be desirable to simulate a capacitive load, thus compensating the line voltage drops.
- This scheme of extinction angle control can also be used for single phase full wave full controlled bridge converter with four switches