## Working principle

A three-phase diode rectifier converts a three-phase AC voltage at the input to a DC voltage at the output. To show the working principle of the circuit the source and load inductances (L_{s} and L_{d}) are neglected for simplicity. The DC voltage is divided into six segments within one fundamental source period that corresponds to the different line-to-line source voltage combinations (V_{LL}). In each segment there is a minimum and maximum DC voltage:

- Minimum DC voltage: If one line-to-line voltage is zero, then the DC voltage is at a minimum of V
_{DC}= V_{LL}· sin(60°). - Maximum DC voltage: The DC voltage increases up to a maximum of V
_{DC}= V_{LL}, where two line-to-line voltages are equal.

In between the minimum and maximum DC voltages lies the average DC voltage that is given by: V_{DC,av} = V_{LL} · 3/pi. The ripple of the DC voltage occurs at 6-times the line frequency. For the six intervals the signs of the phase currents (I_{a},I_{b},I_{c}) are given by:

Phase interval | Sign of phase currents |
---|---|

0°<φ<60° | ( 0,-1, 1) |

60°<φ<120° | ( 1,-1, 0) |

120°<φ<180° | ( 1, 0,-1) |

180°<φ<240° | ( 0, 1,-1) |

240°<φ<300° | (-0, 1, 0) |

300°<φ<360° | (-1, 0, 1) |

### Influence of inductors

As with the single-phase diode rectifier, the inclusion of a load (L_{d}) and source inductance (L_{s}) leads to a current commutation interval between two diode pairs. The larger the source inductance, the more time it takes for the current to commutate. For example, after phase interval 1 (0°<φ<60°) the current commutates from diode pair D_{5}/D_{6} to D_{1}/D_{6}. During this interval V_{ca} remains equal to zero as D_{1} and D_{5} are both conducting, leading to a decreasing DC voltage. The drop of the DC voltage is proportional to the source inductance, i.e., ΔV_{out} ~ L_{s.}

### Experiments

- Change the source inductance from 0 μH to 50 μH and observe the increase of the current commutation interval as well as the load voltage drop.
- Check that a larger load inductance reduces the DC voltage ripple.