Transformer

[Riccardo_Tommasin]

Hi !
My name is Riccardo and I'm italian and this is my first post here...

I've been wondering: do transfomers with 2 indipendent primary windings and 1 secondary winding exist ? If yes, assuming the transformer is a 200VA and the load draws 100VA, if i connect only one primary would i have half the power ?? And if i connect them both does the power add up (50VA on the first primary and 50VA on the second primary )??
Thanks.

[ArthurDent]

Generally a dual primary transformer is so the 2 windings can be wired in series for 240 vac or in parallel for 120 vac but both windings are always used. If you use just one of the windings there will effectively be higher resistance on the primary side so if the secondary is drawing rated current, the secondary voltage will drop below the rated value and the primary will get hotter than intended so this isn't recommended.

If you do use both windings properly on a 200 va transformer and only draw 100 va on the secondary it will work properly and run a little cooler but the secondary voltage may be slightly higher with the lower than rated load.

[Zero999]

I would expect the secondary voltage to be lower, not higher, with the rated load, because of the increased volt drop, due to a higher series resistance and copper losses if only one primary winding is used.

Just one question: why do you want to do this?

[Riccardo_Tommasin]

I wanted to have two separate ac inputs maybe one from the grid and one from an external source (thinking of an pure sign wave inverter) to power the same load so that in the case that the inverter can't take all the load the grid helps by sharing it's power.
Don't know at all if it's possible though.

[Zero999]

I wanted to have two separate ac inputs maybe one from the grid and one from an external source (thinking of an pure sign wave inverter) to power the same load so that in the case that the inverter can't take all the load the grid helps by sharing it's power.
Don't know at all if it's possible though.

That won't work as you expect. The two primary coils will be coupled. If a voltage is applied to one of them, the same voltage will be induced in the other one, making everything connected to it live. A transformer will not enable you to switch between two AC sources. You need a switch.

[ArthurDent]

If you could connect each primary to a different source, a.c. is different than d.c. because you have to worry about frequency and phase as well as voltage. While it is theoretically possible to do what you want if you have zero phase difference, if the phase shifts (drifts), you could end up with the two sources 180 degrees out of phase and the transformer would basically be a dead short across both supplies.

Check this link: https://en.wikipedia.org/wiki/Synchroscope

[Zero999]

If you could connect each primary to a different source, a.c. is different than d.c. because you have to worry about frequency and phase as well as voltage. While it is theoretically possible to do what you want if you have zero phase difference, if the phase shifts (drifts), you could end up with the two sources 180 degrees out of phase and the transformer would basically be a dead short across both supplies.

Check this link: https://en.wikipedia.org/wiki/Synchroscope

Yes, that's true. Another thing to note is that a twin primary transformer won't have sufficient separation between the two primary windings, so the two sources can't be considered to be galvanically isolated. One might as well just connect them together with wire.

I think the original poster was asking if a transformer can be used as an ORing diode, but for AC and the answer to that is no.

[Riccardo_Tommasin]

Yes, that's what i was thinking.
Thanks.

[james_s]

Yes, that's true. Another thing to note is that a twin primary transformer won't have sufficient separation between the two primary windings, so the two sources can't be considered to be galvanically isolated. One might as well just connect them together with wire.

I think the original poster was asking if a transformer can be used as an ORing diode, but for AC and the answer to that is no.

Some of them will. I have a few transformers that have a sort of quad split bobbin with one primary on each side and a tapped secondary on each of the other sides. I used one in a nixie tube clock I built about 20 years ago, I was running the transformer far below rated VA so I used one of the primaries as a secondary, put it in series with one of the other secondaries to get the HV for the tubes. The remaining secondary I used to get the 5V supply for the logic.