In the previous applets the use of half-wave and full-wave rectifiers connected to a single phase voltage source have been investigated. The next applets investigate the rectification of 3-phase voltage sources. The most common rectifier used on 3-phase systems is the 6-diode or also known as the 6-pulse rectifier. It is known as a 6-pulse rectifier since it produces 6 ripple pulses on the DC output in one fundamental period. View the voltage at the output of the rectifier and before the inductor to see this. In this circuit, two diodes are connected to each phase to provide full-wave rectification on that phase. A capacitor is connected to the output to smooth the output voltage. The inductor is used to help smooth out the current that is drawn from the mains and to reduce the ripple voltage on the output. 

At any one time only two of the six diodes are conducting. The choice of the two diodes depends on which line-to-line voltage has the greatest value and the sign of the voltage. In this example for the positive half cycle of the phase R voltage, the voltage between phases R & S is greatest therefore the current flows through the top R phase diode and returns back through the bottom S phase diode. As the R voltage goes through its maximum, the line-to-line voltage between R & T phases becomes greater, so the current shifts from the bottom S diode to the bottom T diode. Each diode in the rectifier can conduct for 120deg of a fundamental cycle. 

If the inductor value is made very large, 35% is maximum in this case, the input current almost has a quasi-square waveform shape. This shape has the lowest level of harmonic distortion and the output DC voltage is almost constant. This is because there is a continual flow of current from the mains supply. However, it is not very practical as a 35% inductor would be very large, heavy and expensive. If instead the inductor value is made too small, then the current becomes very peaky and discontinuous. By reducing the inductor value to its minimum value and moving the time line to the peak of the blue phase voltage, it can be seen that there is no current flowing from the input and the entire load current is being supplied by the capacitor. Therefore in the real world, a design decision is made on the level of inductance placed on the DC side, i.e. trade off current distortion with more expensive inductors.

• Shift the operating point of the 3-phase rectifier by dragging the red line on the waveforms. As the line moves notice how the current commutates between the different diodes connected to each of the phases.
• The value of the DC side inductor can be adjusted by dragging the red bar. Notice how the inductor value changes the current waveform shape from a very peaky, discontinuous current to a more continuous, flatter current waveform shape. This also changes the harmonic composition of the current as seen in the spectrum.