Symmetrical Angle Control & Pulse Width Modulation Control
Symmetrical Angle Control:
- This control can be applied for the same half-controlled force commutated bridge converter with two switches, S1 and. The switch, S1 is turned on at ωt=(π−β)/2 and then turned off at ωt=(π β)/2 . The other switch, S2 is turned on at ωt=(3π−β)/2 and then turned off at ωt=(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. The half-sine waves can be obtained using a full wave diode (uncontrolled) bridge converter. The gate signals can also be generated by comparing triangular waves with a dc.
- In the second case, the conduction angle varies linearly with the dc signals, but in inverse ratio, i.e., when the dc signal is zero, full conduction (β=π) takes place, and the dc signal being same as the peak of the triangular reference signal, no conduction (β=0) takes place.
The fundamental component of input current is in phase with input voltage, and the displacement factor is unity (1.0). Therefore, the power factor is improved.
Pulse Width Modulation (PWM) Control:
- In Pulse Width Modulation (PWM) control, the converter switches are turned on and off several times during a half cycle and the output voltage are controlled by varying the width of pulses. The gate signals are generated by comparing a triangular wave with a dc. In this case, all the pulse widths obtained are equal.
- The lowest order harmonic can be eliminated or reduced by selecting the number of pulses per half cycle. However, increasing the number of pulses would also increase the magnitude of higher order harmonics, which could easily be filtered out. The earlier case of symmetrical angle control can be considered as single pulse PWM.
- The o/p voltage can be obtained in two steps (a) by considering only one pair of pulses such that, they have a phase difference of π, and (b) then by combining the effects of all the pairs of pulse.
- The equation for is (t) can be written as:
Fig: Pulse width modulation control