What exactly causes the phase shift in transformers ?

I know that vector groups basically determine the phase shift, and paralleling two transformers with different phase shifts is a genuine disaster.





Some phase shifts can occur due to difference in the dot place, but for Wye-Delta connection the phase shift is 30 degree even with same dot on the primary and secondary. Why ?|||http://www.allaboutcircuits.com/vol_2/ch…


http://forums.mikeholt.com/showthread.ph…


http://ecmweb.com/ops/electric_basics_tr… says:


"Three-phase transformers.


A basic 3-phase transformer consists of three sets of primary windings, one for each phase, and three sets of secondary windings wound on the same iron core. Separate single-phase transformers can be used and externally interconnected to yield the same results as a 3-phase unit.





The primary windings are connected in one of several ways. The two most common configurations are the delta, in which the polarity end of one winding is connected to the non-polarity end of the next, and the wye, in which all three non-polarity (or polarity) ends are connected together. The secondary windings are connected similarly. This means that a 3-phase transformer can have its primary and secondary windings connected the same (delta-delta or wye-wye), or differently (delta-wye or wye-delta). It's important to remember that the secondary voltage waveforms are in phase with the primary waveforms when the primary and secondary windings are connected the same way. This condition is called “no phase shift.” But when the primary and secondary windings are connected differently, the secondary voltage waveforms will differ from the corresponding primary voltage waveforms by 30 electrical degrees. This is called a 30∞ phase shift. When two transformers are connected in parallel, their phase shifts must be identical; if not, a short circuit will occur when the transformers are energized."|||Starrysk has explained the facts..

Phase shift is a serious matter. If you imagine a heavy table being lifted by four people, you recognize that they have to act synchronously, meaning same time. If one fellow tries lifting while others are idling, he cannot lift and will end up with muscle pull. When you have this at alternating current, or frequency, you have the phenomenon called phase shift. If the voltage induced in one winding is in phase with a voltage induced in another winding, they are in phase. At any time the voltage difference between them will be zero. So connecting them together will cause no problem. Indeed this is the way we want to share the power equally in each.

If they have opposite phase, the voltage difference will be maximum and they will face a short circuit situation. (Like connecting a 9V to another 9V battery, with terminals reversed!!).

Well phase shift can be 0 or 180 or even any value between these two. If there are two ac voltages with phase shift of 1 deg, the effective voltage difference between them will not be zero but

E.sin (1deg). So if they are shorted, additional current will flow but is limited due to finite ,non zero resistances of the sources.

So next time a table is to be lifted, encourage the lifters to be "in phase".

The 30 deg phase shift in Y-delta is incidental, and you have to bear with that. Do not connect their outputs (even if equal in magnitude) to another (say voltage applied to primary) unless you like fireworks. In any industry, that will be your last day.

Combining voltages is done by series addition and not by paralleling. So if you have two secondary outputs which are 180 degree out of phase, you are welsome to add them in series. However the sum will be the vector sum, which is zero. A 230V can be added to 50 V out of phase, to get 180V. You add in phase to get 280V. (See operation of buck boost regulators..). You can add a 90 phase shifted sine with an equal voltage at 0 degress to get 1.414V times a volatge which is at 45 degrees. Again vector sum. In this case of course, the voltage increases, and current is NOT SHARED by the sources. If the current has to be shared, the voltages have to be in phase.