In all the cases examined so far for 3-phase rectification it has been assumed that the voltage produced by all three phase has the same amplitude. However in the real world the voltage on all the 3-phases will not be exactly balanced. 

For the case where there is no percentage change in the R phase amplitude then all the 3-phase currents are balanced. By adjusting the amplitude of the R phase to be, say, 20% less than normal this now causes the other 2 phases to draw more current than the R phase. This is because the other phases have a higher voltage. Notice that the average output voltage has dropped significantly. There are also significant time periods where no current is flowing from the mains. 

For the case where the R phase has a voltage amplitude that is 20% higher than normal then the current peak in R phase is much higher and the current in phase T is reduced. For this case the output DC voltage has also risen with the R phase input voltage. 

Significant voltage unbalance can cause problems in the operation of the rectifier since the losses in the power diodes will not be equal and overheating may occur on specific devices. The level of imbalance in the voltage supply should be considered in any design.

• Increase and decrease the amplitude of a phase voltage by moving the dot on the uNR voltage arrow. This now causes imbalance between the line-to-line voltages. What does it do the phase voltages?
• Adjust the size of the 3 AC inductors by dragging the red bar on the inductors. What impact does this have for unbalanced supply voltages?
• By adjusting the time line you can see the path the current flows around in the rectifier. Notice the times in which the capacitor is being charged and discharged.