- The output voltage waveform of a single phase full wave diode (uncontrolled) bridge converter (rectifier) fed from f = 50 Hz (fundamental) supply, contains harmonics of 2f = 100 Hz. So, it is necessary to filter out this and other harmonics from the output voltage to obtain dc component only.
- The harmonic frequency present in the output voltage waveforms of three-phase half-wave and full wave (bridge) diode converters, are 150 Hz (3f) and 300 Hz (6f) respectively. The higher the harmonic frequency, it is easier to filter it.
- For phase-controlled thyristor converters, the harmonic frequency remains same, but magnitudes vary, as the firing angle delay, α is changed. It may also be noted that the harmonics present in the output current waveforms of the converters with resistive (R) load, remain same.
- For simple filter, a capacitor (C) is connected in parallel across the output of the diode converters with resistive (R) load. The reactance of the capacitor should be low, such that harmonics currents pass through it. So, the harmonics in the output voltage decrease.
- The value of the capacitor chosen varies with the predominant harmonic frequency present. Thus, the capacitor of higher value is needed to filter lower harmonic frequency, say 100 Hz, whereas a lower value of C could be chosen for say, three phase converters.
- The function of the capacitor may also be explained in the following way. The voltage across the capacitor changes as per the input voltage, which is the output voltage of the converter, fed to it, and the capacitor voltage tries to stabilize at the overage value of the output voltage, as the capacitor voltage decreases, load resistance being connected across it.
- Same is the case with the filter used to reduce the harmonic content of the output current waveform for the above converters with resistive (R) load. Instead of a capacitor in parallel, an inductor (L) is connected in series with the load. The reactance of the inductor increases, thus reducing the harmonic component in the current waveform.
- Here, a smaller value of the inductor is needed to filter higher harmonics, for example a three-phase bridge converter. These are all simple cases, known to those, who have studied the circuit (network) theory. Also, by Faraday’s laws, induced voltage (emf) appears across the inductor, L, when the current through it changes, and the sign of it opposes the cause, thus opposing the changes in current.
- So, the current is not allowed to change much, as an inductor is placed in series with the load. In actual practice, a combination of L, C & R is needed to get an optimum filter needed to reduce or eliminate the harmonics in both output voltage and current waveforms.
Some of the common used filters are:
- Low Pass (L-C) Filter
- Two Stage Filter