Implications of boosting transformer voltage

[N Brock]

Hello all,

I am planning to use a transformer in the way described below. If you could please look over my work to see if I've missed anything it would be appreciated.

I have a dual primary transformer with 0-100-120.
The secondary is 27-0-27.

Wired "normally" in the US, after rectification and losses this would give 37v rails. However, if the US 120v mains are connected to the 100v primary tap, the secondaries become in effect 32-0-32 and the rails become 45v.

Before we started messing with anything the VA rating was (27v+27v)*10A = 540VA. This will be conserved after we change the voltages around so the new secondaries can be estimated to have a capacity of 8.4 A.

Does anyone know of a reason to not do this?

I know I have provided specific numbers and a picture, but feel free to comment on the method in general as well.

Thanks,
Nelson

 

[DUG]

Even without a load the flux density will be higher in the core.

Slightly more losses, slightly more heat.

To what extent I could not guess.

 

[DF96]

You are more likely to get core saturation, especially when the secondaries are unloaded or your mains happens to be a bit high in voltage. If you have a high quality transformer you might get away with it, but try it through a lamp limiter to limit the current in case you do get saturation.

 

[AndrewT]

Measure the primary current with secondary open circuit. A Variac is perfect for this. Use a 1r0 resistor in the mains lead. But make sure you can meausre the Vdrop without killing yourself.

Plot a graph of Ip vs Vp
You will see the current increasing dis-proportionally as the voltage rises.
By the time you get to your maximum US power generators voltage you may find that the primary current is so high that the transformer overheats. Maybe severely so.

In the UK we can use the series connected primaries and use any of the tappings, even different on the two windings to give effective input options of 200, 210, 220, 230 & 240Vac.

But in the US, you must use the primaries in parallel and then you MUST use the same tappings on both primaries. The options then become 100 & 120Vac.

I would not use a 540VA transformer @ 120/126Vac with 100Vac tappings. But you do know how safety conscious I am.

However, are you prepared to wire a 230Vac supply to your room? You have 230Vac at your distribution board.

Alternatively, wind on an extra pair of secondary windings. 20 Turns + 20Turns of dual 1.4mm diameter enameled copper wire will maintain your power/VA rating. Or 2mm wire but that is quite difficult to work with.