Rectiformer Windings

An one phase ac current that is double sided rectified (with 4 diodes) gives within one cycle 2 maximum current values (2 tops that are shifted over 180°)

A three phase ac current that is double sided rectified (with 6 diodes) gives in one cycle 6 maximum current values (6 tops that are shifted over 60°)

When we rectify this current, we get a much "smoother" DC current, because the time delay between a maximum in phase one and the start (taking over) of phase two is only 60 degree (one sixth of an AC cycle).

A hypothetical six phase ac current, would surely give an even more smoother DC current.

The problem is that we do not have a six phase supply.

BUT a transformer with one primary and two secondary windings (one secondary winding in delta and the other secondary winding in wye connected) gives a phase shift of 30 degree between the two secondary windings.

If we use these two supply's together, we can create a 6 phase power supply.

The results is a shorter time delay between a maximum in phase one from the delta winding and the start (taking over) of phase one in the wye winding.

If the two secondary windings are followed with a double sided rectifier (so in total one needs 12 diodes), the time delay between each maximum is only 30 degree (one twelfth of an AC cycle). We call this a 12 pulse rectifier.

There even exist 18 or 24 pulsed systems for making a very smooth DC current. This is done with multiple winding on a transformer.