three phase transformer

[kolani]

A three phase transformer is constructed by winding three single phase transformers on a single core. These transformers are put into an enclosure which is then filled with dielectric oil. Now, if we construct a three phase transformer from three single phase transformers, but not on single core, and then put them into an enclosure, what would be drawback of this design vs. the first, single core design?

[pebe]

A 3 phase transformer is surely made by winding a transfomer for each phase on a separate core - total 3 cores - isn't it?

[jwax]

http://www.elec-toolbox.com/usefulinfo/xfmr-3ph.htm
Nice explanations!

[pebe]

I'm utterly confused. 3 phases on a 'SINGLE' core? Yet fig 1-6 shows 3 cores!<p>If 3 phases are all wound on a single core, then the core must link with the currents of all 3 phases - and they are 120degrees apart. So how does each of the secondaries get its correct induced current?

[Chris Smith]

Multi phase Transformers are all about the inter reactance between phases. <p>True three phase is commingled so that as one is on the rise, the last is on the fall, and the middle is at its peak. <p>No loss in reactance. <p>Separating three transformers will work, but a power loss is also had. <p>They commingle their fields to boost their energy like three solidly attached flywheels, and not like the loss of attaching three flywheels through a cog and chain.

[pebe]

This link gives more info. <p>http://www.allaboutcircuits.com/vol_2/chpt_10/6.html<p>It seems there are 3 cores - not 1.

[rshayes]

A three phase transformer can be wound on a single core. The laminations for this are different in that all three legs of the "E" are the same width. A primary and secondary are wound on each leg. In a single phase transformer, the outside legs are half the width of the center leg and the primary and secondary are placed on the center leg only.<p>The three phases never reach peak flux values at the same time. Often the flux direction is such that they cancel. This allows a smaller transformer since additional iron for the return path is not required. In a small transformer, the difference is a few ounces. When the transformer becomes several feet high, the difference in weight becomes tons instead of ounces, and thousands of dollars in material cost.

[pebe]

Thanks Steven. It makes sense now.

[Robert Reed]

And - to add to stephans reply , these 3 Ph. megawatt transformers used by utility companys are incredibly efficient. They run better than 98% efficientcy, with most of their losses only due to a very stingy magnetizing current.
They are so well designed and matched to the system, that they are just about transparent when analyzing power flow on one line diagrams.