Power Factor Improvement for controlled rectifiers

It shows the basic methods to improve power factor in controlled rectifiers.

1. Power Factor Improvement

2. Power Factor Power factor is the measure of evaluating how effectively the incoming electrical power is used in an electrical system. If the power factor is high, then we can say that more effectively the electric power is being used in an electrical system. Power factor represents the fraction of the total power that is used to do the useful work. The other fraction of electrical power is stored in the form of magnetic energy in an inductor or electrostatic energy in the capacitor.

3. Power Factor Improvement The power factor becomes low when the output voltage less than the supply voltage ( particularly when the firing angle is high ). The displacement angle ( The angle between fundamental component of alternating line current and phase to neutral voltage is known as displacement angle. ) between the supply voltage and supply current increases as the firing angle increases.  This will result in power factor decreases and lagging reactive power flows from load to supply side and power factor decreases. 

4. Power Factor Improvement Methods Extinction angle control • Symmetrical angle control • Pulse width modulation (PWM) control

5. 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.

6. Single Phase Semi Converter

7. Extinction angle control In extinction angle control, switch, S1 is turned on at ωt = 0 , and then turned off by forced commutation at ω = π−β . The switch, S2 is turned on at ωt = π, and then turned off at ω = 2π−β. The output voltage is controlled by varying the extinction angle, β. 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.

8. Output Voltage

9. Symmetrical angle control The switch, S1 is turned on at ω = (π−β) / 2 and then turned off at ω = ( π+β ) / 2 . The other switch, S2 is turned on at ω = (3π−β)/2 and then turned off at ω = (3π+β) / 2 . The output voltage is varied by varying conduction angle, β. The gate signals are generated by comparing half-sine waves with a dc signal as shown in Fig.

10. Gate Signal Generation

11. Output Voltage