Induced Voltages in single-phased Three-phase Xfmr Core
Induced Voltages in single-phased Three-phase Xfmr Core
(OP)
"Hypothetical" question:
Three-phase core-type transformer - assume five-legged core.
If only one primary phase winding is energized, how much voltage could be induced in the "unenergized" primary and secondary windings?
Thanks,
Dumb Dave
Three-phase core-type transformer - assume five-legged core.
If only one primary phase winding is energized, how much voltage could be induced in the "unenergized" primary and secondary windings?
Thanks,
Dumb Dave






RE: Induced Voltages in single-phased Three-phase Xfmr Core
Here's my reasoning. Consider rectangular wound cores. A 5-legged core would have 4 of these cores placed side to side.
First consider energizing the center phase winding. This means the winding goes around the window formed by 2 innermost rectangular cores. These cores also make up half the magnetic circuit for the other 2 windings. So you get 1 pu flux on the core for the center winding. This means that the other 2 windings get 0.5 pu flux, which leads to half voltage.
Next consider energizing an outer winding. As before, this leads to 0.5 pu voltage to the adjacent center winding. The other outer leg would have essentially no voltage since it has no flux, but would probably pick up some induced voltage from the center winding. Nothing big, close to 0.
RE: Induced Voltages in single-phased Three-phase Xfmr Core
Since reluctance is the same in the core, the voltages would be divided by 4, thus 0.25 pu.
Waiting for other answers...
RE: Induced Voltages in single-phased Three-phase Xfmr Core
RE: Induced Voltages in single-phased Three-phase Xfmr Core
RE: Induced Voltages in single-phased Three-phase Xfmr Core
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Induced Voltages in single-phased Three-phase Xfmr Core
Yes, that's the idea I keep coming back to.
But I've since learned that the feeder had a wye-delta transformer on it, so that changes the thinking a bit.
We were trying explain a low voltage condition after two phases were lost on the feeder. The low voltage was on one the "de-energized" phases.
Thanks,
Dave
RE: Induced Voltages in single-phased Three-phase Xfmr Core
Also, some utilities close the wye point to neutral connection when energizing with fused cutouts to minimize switching surges. The action of the wye delta bank is quite different depending on the wye point connection.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Induced Voltages in single-phased Three-phase Xfmr Core
RE: Induced Voltages in single-phased Three-phase Xfmr Core
Yes the wye of the wye-delta is probably grounded and tied to the neutral.
RE: Induced Voltages in single-phased Three-phase Xfmr Core
With the delta secondary, the energized secondary will be developing full voltage. The two unenergized transformer windings will be in series across the energized winding. With equal loading, the unenergized windings will each back feed 50% voltage to the primary lines. If there is a connected load, the voltages will be in inverse proportion to the loads.
Note: RUS, formerly REA recommends that the primary neutral of a wye delta bank be left floating.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Induced Voltages in single-phased Three-phase Xfmr Core
Bill,
That's basically what I was guessing for the wye-delta after I thought about it. It's just odd that I've never run across this problem before.
Cheers,
Dave
RE: Induced Voltages in single-phased Three-phase Xfmr Core
I may be mistaken, but I think the responses in this thread have missed the first step. I don't think it's easy to deenergize 2/3 primary phases.
If the primary is delta, then you have the phase-to-phase voltage applied accross parallel combination of one phase winding and two series phase windings. All windings have voltage applied (either full voltage or half voltage)
If the primary is wye, then you have phase to phase voltage applied accross two series phase windings. 2/3 phase windings have voltage applied.
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
RE: Induced Voltages in single-phased Three-phase Xfmr Core
The wye primary was fed by a 4-wire primary where two fuses blew upstream. So it was energized line-to-neutral on one phase of the primary.