Half-wave rectified usually means 1 diode per phase, and only works when the voltage is positive. EG, current travelling from the hot wire to the neutral. Full wave uses a 2nd diode to rectify power when the voltage is negative- EG, current travelling from the neutral to the hot. In ideal balanced 3 phase power, the 3 phases will always balance perfectly. In a 6-pulse (simple 3-phase/6-diode) power supply, even on a perfect ideal system, the switching drops a good chunk of the sine wave, making the neutral current cancellation imperfect, setting up semi-sinusoudal current at a higher frequency on the neutral in, IIRC, the 3rd and 5th harmonics.
Edit: er, I might be thinking of delta for 3rd and 5th harmonics. I'd have to hit the books back at the office to check- either way, the exact frequency of the harmonics isn't really important to this discussion!
[This message has been edited by SteveFehr (edited 07-31-2006).]
