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Do transformers work the otherway around?

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drakejest:
Do transformers work the other way around?

Im specifically curious about AC to AC transformer those found in AC/DC supplies and its bigger brother the one powering your house and your neighbors. I assume when you provide power on the secondary side you get power out the primary side, as long as dont exceed the rating of transformers, it should work. But how about the big ones used to power homes or buildings? do they have special circuitry inside? I do think so, because that would mean if a home is installing for example solar system they would also have to change the transformer to export to the grid, which i dont usually hear "oh i cant export to the grid because the transformer in my place cant do that".

So do transformers work the other way around assuming the rating are being followed?

DavidAlfa:
Simply, yes, any transformer, any voltage, any power.
For example a 220V-24V 2KVA transformer:
Feed the primary with 10A 220V -> Get 24V 90A at the secondary.
Feed the secondary with 24V 90A -> Get 220V 10A at the primary.

Same for huge 110KVA or 1MVA transformers, or miniature 1VA ones.

IanB:

--- Quote from: drakejest on October 03, 2023, 03:19:54 pm ---So do transformers work the other way around assuming the rating are being followed?

--- End quote ---

Yes, but there is a small proviso to this. Suppose you have a distribution transformer that is designed for 11,000 volts on the primary and 240 volts on the secondary, and you consider connecting it backwards, and applying 240 volts to the secondary side.

Firstly, transformers change current as well as voltage. Let's suppose you try to take only 1 amp from the 11,000 volt side. This will draw 46 amps from the 240 volt side ( = 11,000 / 240 ). This is going to exceed the rating of any typical 240 volt circuit you have and trip the breaker.

Even worse will be the inrush current if you try to connect a 240 V supply to the low voltage side to begin with. The magnetizing current will be huge, and the transformer will look like a short circuit. It will instantly trip any circuit breaker. The only way to do it in a home experiment situation is to put a ballast in series with the transformer to limit the current.

TimFox:
Yes, an N:1 primary:secondary ratio transformer can be driven at the secondary to make an 1:N  ratio transformer.
For the actual voltage ratios you will get, you should consider that small transformers (what we used to call "filament transformers") that are specified from 120 V to 6.3 V will deliver 6.3 V at their rated current (perhaps 2 A), which means that they will deliver more (perhaps 10% more) into an open circuit.
The difference is due to the transformer loss (copper and core);  high-power transformers are more efficient, but low-power transformers would be heavier and costlier than reasonable for their use case if designed for fatter copper and more weight of iron.
Therefore, their actual turns ratio is not 120/6.3 = 19:1, but lower (perhaps 120/7 = 17:1 in that example).
Note that you cannot safely exceed the rated voltage on either winding (120 or 6.3 in this example), since the core saturates with higher voltage (at a given frequency).

Conrad Hoffman:
I remember decades ago the Ohio power company put on a safety demo at the Dayton Hamvention. They used standard "pole pigs," powering the secondary from 240 and putting 11 kV or so on lines between a couple short poles. They proceeded to draw arcs and fry hot dogs and give you an idea of what happens when you get too close to those lines. So yes, works both ways.

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