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I see transformers being used quite commonly for AC power and in many places I see people saying that they are meant to be used for AC. The way I understand it, transformers use large inductors around a core and can step down and step up voltage with different ratios of coils. So won't these large inductors resist the AC current? Wouldn't it be much more efficient to use a DC current through the transformer coils and convert back to AC after?

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    \$\begingroup\$ Transformers ONLY work with AC. \$\endgroup\$
    – Finbarr
    Jul 10, 2023 at 22:23
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    \$\begingroup\$ @periblepsis You seem to have posted a good answer as 4 comments. This material may be lost in future. I strongly suggest that you copy the comments into a single answer. This should take minor diting and makes much better use of your input. \$\endgroup\$
    – Russell McMahon
    Jul 11, 2023 at 13:29
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    \$\begingroup\$ I commented because somebody flagged it BUT I agree with them - it is a very useful commentary. As long as it does not get downvotes (which it certainly shouldn't) then it is a useful post for the OP and (importantly) a long term post to have available for others. Do it !!! :-) \$\endgroup\$
    – Russell McMahon
    Jul 11, 2023 at 13:54
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    \$\begingroup\$ @periblepsis - Hi, Re: "Is it really something to write as an answer?" I will also say yes. It is useful in response to the overall question asked about "How can you use a transformer with an AC current?". Also, comments have a deliberately limited max length & have limited allowed uses. A long-form monologue, no matter how good, is not one of them :( A big indicator that you're bypassing the deliberate site limitations, is writing more than one consecutive comment. As Russell said, to have longevity (& searchability) please aim for answers. TY \$\endgroup\$
    – SamGibson
    Jul 11, 2023 at 14:10
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    \$\begingroup\$ @SamGibson Okay. I tried. With more room I added a little more. I'll delete the comments. \$\endgroup\$ Jul 11, 2023 at 14:50

4 Answers 4

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The whole point of a transformer is that it will resist AC current. It works by blocking net magnetisation from the current of primary and secondary coils, so taking into account the turn ratios of secondary and primary coils, magnetisation from the incoming current in the primary coil will be countered by that from the outgoing current in the secondary coil. An ideal transformer will not admit any net magnetisation to occur.

But it cannot properly resist DC, so DC will result in resistive heating of the primary coil, saturation of the core, a breakdown of the transformer principle and possibly even destruction through saturation and/or heat (once the core saturates, it does not inductively resist further net magnetisation current changes).

The way a transformer transfers energy is by making it very energy intensive to achieve any net magnetisation of the core, so by and large, the core will be left alone and incoming and outgoing current will be balanced along with the respective voltages. That is what makes the losses in a transformer small.

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Transformers cannot pass DC, only a changing magnetic field can pass through an inductor and a transformer is two inductors coupled to each other. so you need a switch to modulate the DC current to pass it through the transformer. Switching converters use transformers in this way and some AC/DC inverters.

Transformers are built for AC, a coil wire transfers current to magnetic energy and then back again, its one of the most efficient ways to be able to transfer energy while changing parameters (voltage/current). All electrical grids use transformers to be able to step up voltage up an down for better transmission.

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While it is true that a transformer, as an inductor, 'resists' AC current, it is also true that the inductive effect on the iron of the transformer is related to the DIFFERENCE current between the primary and secondary windings, not the sum. With the secondary open (unconnected) you don't want the primary to waste power, and that means you want it to take only small currents.

But, when the secondary draws current, that current is in the opposite sense to that of the primary, so the primary's inductance, to the extent that power is drawn, is not impeding the primary current as much. The effect of inductance of the transformer core of a M:N turns ratio transformer, is that the AC current sum, $$M\times I_{primary} - N \times I_{secondary}$$ is impeded, and that means that the secondary current is free to rise unimpeded, except that primary current in proportion is drawn.

The inductive reactance that acts to limit AC current thus only acts in a transformer to limit the energy-waste when the secondary isn't drawing current.

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You may already understand that increasing electric charge on a capacitor works up to a point. One can supply a specific DC current to a capacitor for only so long because, as the voltage rises from that fact, eventually the circuit no longer has access to the still-higher voltages needed to continue adding charge.

So, if the circuit depends entirely upon working in only one DC direction (sign) with a capacitor, and if it never discharges or otherwise allows the accumulating charge to reverse the sign of the current allowing a decline, then at some point everything must come to a screeching sudden halt.

Game over.

Consider perhaps this concept:

Imagine there exists something called magnetic charge as a dual of electric charge on capacitors, but now for inductors. In this case, these are often specified as Webers or, alternately, as volt-seconds.

(Names are important for communication. But the names we use for things aren't as important, when trying to understand a thing, as observing and understanding the thing itself.)

Thinking along these lines, if you apply a DC voltage that never changes sign, then over time these "volts times applied seconds" accumulate in only one direction and the circuit will then need to supply ever-increasing current.

So if the circuit depends entirely upon working in only one direction with an inductor and never allows the Webers to decline then at some point it must also stop working.

There is a small mental shift here, moving from capacitors to inductors, that may cause a momentary distress. Capacitor charge is like a static countable thing -- electrons or protons. Easy to understand and keep in mind. So when we discuss electric charge, there's little difficulty in teaching that even to a young child.

But Webers have time included. So it's not like marbles. In this case, more of them are created/needed the longer a voltage is applied to an inductor and this requires an increase in the current. (The number of Webers is directly proportional to the product of an inductance and its instantaneous current.)

This factor of time may offer a slight barrier. One needs to overcome that, if it is a problem. (Time is introduced, forced into the matter, because the speed of light imposes a time for interacting charges to interact.)

All this goes to say that while DC isn't strictly forbidden -- as an inductor will in fact respond to its application and do something about it -- it's generally not useful if it is never reversed and allowed to unwind/decline.

So you will find transformers used with DC -- but only if and when the DC is reversed repeatedly. Ham radio operators in the 1950's out of their cars would use a vibrator to cause the battery's DC to alternate across the transformer primary.

So DC works under the right circumstances. It has been used and will continue to be used. Just need to actively flip it every so often.

Flipping the DC, done just right, discharges accumulated Webers of one sign in to accumulate some of the opposite sign, before switching yet again to reverse the process.

The goal is that the net Webers hover around a usefully near-zero small value.

AC has a huge advantage: it automatically charges and discharges then reverses direction to charge the other way and then discharge. So it just happens as if by magic when using sinusoidal AC voltages.

No active devices or old-style vibrators needed then. Just plug and go.

There's the related slight problem that materials used in the core of transformers (those other than air or vacuum) do all have limitations with respect to the number of Webers per unit area (magnetic flux often specified in Teslas) they can support without seriously impacting the design inductance. So, at some point, DC when applied too long with one polarity across the inductor, the core material will cease to behave in the same way.

A new issue, then: core saturation. (But it's just a practical detail.)

This core saturation effect doesn't have to be avoided. It can be used and is used. (See this Wiki on the idea.) But it's more of a specialty area than mainstream.

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