This is a respresentation of how it would look like. The number of windings on both sides of the centre-tap are the same and they are all wound on the same core. If a voltage Vac is only applied across the lower primary winding, and the upper winding is open-circuited (with a switch) will the voltage on the free end of the upper winding keep increasing or is my understanding somehow wrong? The windings are in phase to each other so that they can be used as one big primary winding in certain use cases to reduce the transformer ratio.

 

No, the voltage between the pins you have labelled + and OC will be the same as the voltage between the pins you have labelled - and +.
The voltage (between + and OC) is proportional to the rate of change of flux in the core, and the rate of change of flux in the core is proportional to the voltage between + and -.

 

View attachment 304542
This is a respresentation of how it would look like. The number of windings on both sides of the centre-tap are the same and they are all wound on the same core. If a voltage Vac is only applied across the lower primary winding, and the upper winding is open-circuited (with a switch) will the voltage on the free end of the upper winding keep increasing or is my understanding somehow wrong? The windings are in phase to each other so that they can be used as one big primary winding in certain use cases to reduce the transformer ratio.

If all the windings have the same number of turns, the same series resistance, and the same inductance, then you should measure identical voltages across each of the windings. Where did you get the absurd notion that the voltage would increase without limit across the open winding?

 

What reasoning led you to the conclusion that it would keep increasing?

 

What reasoning led you to the conclusion that it would keep increasing?

What he said

 

If all the windings have the same number of turns, the same series resistance, and the same inductance, then you should measure identical voltages across each of the windings. Where did you get the absurd notion that the voltage would increase without limit across the open winding?

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I also thought that it would be the same, but a colleague kept insisting that I have to be careful about overvoltages because the upper end (OC) voltage to ground ref would keep increasing since the inductor stores energy and there is no current path to discharge the upper winding.
I got the same results as you from LTSpice, but I thought maybe there was something I was missing.

 

The two windings share the same core.
The changing flux in the core induces the same voltages in both windings (assuming they are the same number of turns)
The lower (driven) winding always has a current path, this allows the core's energy to charge and discharge through this path, resulting in a controlled sinusoidal flux in the core.
No crazy high voltages will be generated.

 

"Well, who ya gonna believe me or your own eyes?"

-- Chicolini (Chico Marx)​

 

The energy that goes into the inductor on one part of each cycle goes back to the grid in the other part. That is why a pure inductor does not draw any power.

 

"keep increasing", for what period of time?? As the voltage Vi, referenced to the end with the ground symbol keeps increasing, so will the voltage at Voc keep increasing. Then, as the voltage at Vi starts to fall, so will the voltage Voc fall. That is how an auto-transformer works.
Of course, the open circuit voltage will not increase to more than twice the value of Vi, presuming that the number of turns in the second half matches the number in the first half of the primary.
CURRENT TRANSFORMERS are a special case of transformers although technically they are still transformers.The situation there is that they are intended to operate under a heavy load, relative to the wire size and number of secondary turns. So an open-circuited current transformer secondary voltage will reach a value that may exceed the insulation capabilities. But still, the voltage ration should be very close to the turns ratio.