480V 3 phase to 120/240V single phase Xfmr

[GoldDigger]

....

I'm not so familiar with the Scott-T, does it successfully transform that load into a balanced 3-phase load? I guess power accounting again says it must, but I'm not sufficiently confident in my transformer theory to be sure.

Cheers, Wayne

The power on each phase of the incoming three phase supply (the Scott-T actually works in either direction, by the way) will be balanced only if the loads on the two output phases are equal.
Just as the load on a simple three phase transformer or bank must be balanced to produce a balanced draw on the supply.
The OP seems to be looking for a single output.

 

[tortuga]

The OP seems to be looking for a single output.

In post #8 he implies that there are several existing panels, so I dont think he really needs a single output.
Rather he just has a large amount of single phase loads:

And one more thing, all panels in the building are 120/240 so 3 phase 120/208 is not really an option ( is it an option? ) that being said we need appx 250 amp@480V

If he can balance the single phase on two 250A feeders I bet the scott-t would work nicely.

 

[wwhitney]

The flux in the core induced by a primary winding will be proportional to the integral of the applied voltage waveform across that primary winding. If the primary and secondary windings are tightly coupled and so we can ignore leakage flux, then the flux produced by a load current on a secondary winding will be cancelled by the corresponding current that flows through the primary, via Lenz's law. And so loading on a transformer secondary will not have a major effect on the flux within the core.

If there is a zero sequence current through the three secondary windings of a 3-leg core (in other words, a vector component of current that is the same in each winding) then that could cause problems because there is no return path for the zero sequence flux in a 3-leg core.

OK, I've figured out what I don't understand in the above. In the first paragraph, you are describing that in an idealized transformer, the magnetic flux in the core depends only on the applied primary voltages, not on the primary/secondary currents. That means that a 3-phase transformer can work with a 3-leg core whenever the 3 applied voltages sum to zero.

But in the second paragraph you are talking about the effects on the flux of zero-sequence current. So is that an effect that occurs only in the real, non-idealized case?

E.g. as the primary impedance is non-zero, the zero-sequence currents on the primary side cause the primary voltage to shift, and that shift means that it's no longer true that the sum of the primary voltages is zero. Which is a problem on a 3-leg core.

Thanks,
Wayne

 

[Jerry Solar]

Is 480V 3 phase to 120/240V single xfmr configuration available.

Does utility provide 1 ph, 2 wire, 480V, 400 amp service normally or needs special circumstances.

You can put a 2-pole circuit breaker in your 480V/3-phase panel and feed a single-phase 480V primary - 240/120V secondary transformer. Maybe I am late with this response, I haven't read all the postings.

 

[kwired]

You can put a 2-pole circuit breaker in your 480V/3-phase panel and feed a single-phase 480V primary - 240/120V secondary transformer. Maybe I am late with this response, I haven't read all the postings.

That is one way to do it.

You can just use two of three poles of a two pole breaker, or if you happen to have corner grounded delta system you could possibly have situation of one pole having overcurrent protection of some kind and other conductor is the grounded conductor - the transformer primary only cares about seeing ~480 volts across it's two input leads.