Do transformers work the otherway around? - Page 1
Forum section: electronics / projects

author: drakejest
author_rank: Regular Contributor
timestamp: October 03, 2023, 03:19:54 pm
content: 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?

author: DavidAlfa
author_rank: Super Contributor
timestamp: October 03, 2023, 03:25:01 pm
content: 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.

author: IanB
author_rank: Super Contributor
timestamp: October 03, 2023, 03:53:58 pm
content: Quote from: drakejest on October 03, 2023, 03:19:54 pmYes, 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.

author: TimFox
author_rank: Super Contributor
timestamp: October 03, 2023, 03:56:34 pm
content: Yes, anN:1 primary:secondary ratio transformer can be driven at the secondary to make an 1:Nratio 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).

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: October 03, 2023, 04:10:17 pm
content: 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.

author: soldar
author_rank: Super Contributor
timestamp: October 03, 2023, 04:11:24 pm
content: Quote from: DavidAlfa on October 03, 2023, 03:25:01 pmIn spherical cow theoretical world yes. In practice... well, things start getting in the way.I have a 230 to 230 V isolation transformer for about 8 to 10 A. Connect the primary to 230 V and you get about 250 V on the unloaded secondary. This accounts for the drop when it is loaded.If you input 230 V to the secondary side you will get about 210 on the other side.To adjust for this the secondary has several taps you can choose from.

author: DavidAlfa
author_rank: Super Contributor
timestamp: October 03, 2023, 04:16:09 pm
content: Yes I know it's not always 1:1 conversion, depends on the load, etc.Where're not discussing that, neither powering a 10MVA tranformer with home mains, tripping the breaker.Stay on topic.

author: TimFox
author_rank: Super Contributor
timestamp: October 03, 2023, 04:18:08 pm
content: We are seeing a lot of griping about posts that clarify a point (perhaps minor) relevant to the original question.

author: coromonadalix
author_rank: Super Contributor
timestamp: October 03, 2023, 04:21:45 pm
content: depends on the case(s) / scenario(s) etc .. and how the people(s) do interpretation(s)and from the OP not telling / giving all the details lolllit's not 1:1 lolllDone that for x-formers under 1KW i must say loll

author: soldar
author_rank: Super Contributor
timestamp: October 03, 2023, 04:26:51 pm
content: Quote from: TimFox on October 03, 2023, 04:18:08 pmYes, I cannot see why what I wrote is not "on topic". More likely someone is having a bad day.I still think what I wrote is perfectly relevant to the question asked.Take my 230-230 transformer and connect it backwards and load it and you'll be lucky to get 180 V.

author: IanB
author_rank: Super Contributor
timestamp: October 03, 2023, 04:39:32 pm
content: Quote from: DavidAlfa on October 03, 2023, 04:16:09 pmThe OP specifically mentioned "the big transformers used to power homes or buildings". So I think discussion of those transformers is on topic.

author: TimFox
author_rank: Super Contributor
timestamp: October 03, 2023, 04:47:43 pm
content: Another clarification directly relevant to the original post, specifically "Im specifically curious about AC to AC transformer those found in AC/DC supplies" .In my example of a low-power "filament transformer", often used in AC-DC low-voltage supplies, that transformer was specified to deliver 6.3 V into a specified load (2 A) from 120 V, but would deliver 7 V with no load.In the vacuum-tube days, it was common to use the 6.3 V from the main transformer to power the tube heaters, and a separate low-power filament transformer run "backwards" from that supply into a rectifier for a low-current negative bias voltage.In my example, applying 6.3 V to the "6.3 V" winding on the small transformer would give 6.3 V x 17 = 108 V on the "120 V" winding, probably suitable for the bias voltage supply.

author: coppice
author_rank: Super Contributor
timestamp: October 03, 2023, 04:50:07 pm
content: Do transformers work in reverse? Yes. Are they as efficient in reverse? No. Some people make highly isolated supplies by cascading 2 transformers. If they want to use two transformers which are not 1:1, and use the same transformer both sides, they get rather spongy result. If they use a transformer built as a step down and one built as a step up the result will be a much better regulated output.

author: magic
author_rank: Super Contributor
timestamp: October 03, 2023, 04:58:08 pm
content: Quote from: IanB on October 03, 2023, 03:53:58 pmRight, magnetizing current will now flow in the secondary winding and it looks like it will be N times larger (N = turns ratio).The "magnetizing (passive) power" is still the same, but in the grid an HV circuit is more likely to be capable of providing it.Magnetizing current can hit huge peak if the transformer is switched on at a wrong time and the core saturates.Magnetizing current may eat into power dissipation budget of the secondary, while the primary is designed to cope with it.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 03, 2023, 05:08:25 pm
content: In principle they work also the other way around. A trap for beginners is that the nominal voltage ratio is not the same as the turns ratio, but is valid at nominal load. So the no load voltage ( closer to the turns ratio) is usually higher by a factor depending on the transformer size and build type. This could be some 20% for a 5 VA EI type and gets less with larger size and torroidial type.With very small transformers (e.g. <1 VA) things get a bit more tricky as the magnetizing current is rather large and thus usually more space is used for the primary and quite some voltage is lost due to the magentizing current. When used in reverse one may not be able to get much magnetization from the smaller windings and thus end with a significant (e.g. 1/4) lower power and voltage. E.g. a 230-12 V transformer may only want 6 V to drive the 12 V coil and may only give some 30-50 V out. These small transformers are far away from the ideal tranformer.Quote from: coppice on October 03, 2023, 04:50:07 pmI would not expect much difference with the votlage ratio. It is more a question of efficiency and coupling. Small transformer have low efficiency and thus give a spongy output. Larger ones (and also toroidial compared to EI) generally give a stiffer output. Some transformers (e.g. for microwave ovens and welding, some battery chargers) are made with extra added magnetic leakage / series inductance and this way give a spongy output even if large.

author: macboy
author_rank: Super Contributor
timestamp: October 03, 2023, 05:14:53 pm
content: Quote from: soldar on October 03, 2023, 04:26:51 pmIt's important to note that your isolation transformer is not a 1:1 winding ratio. It is perhaps 10:11. In that case, connected backwards, it works perfectly fine as a 11:10 ratio transformer.

author: TimFox
author_rank: Super Contributor
timestamp: October 03, 2023, 05:18:08 pm
content: The original post did not mention winding or turns ratio, which are not given in the usual transformer catalogs.Transformers, including isolation transformers, that run at a substantial current are normally specified as an output voltage at a specified load current.

author: drakejest
author_rank: Regular Contributor
timestamp: October 03, 2023, 05:24:21 pm
content: Thank you for the answers ! so for the most part the transformers works both ways.I do have some follow up questions,- What happens when power is applied on both side?- If power is applied on the secondary and that power source can deliver more power than what the transformer can handle, will it backfeed to the house circuit and by extension the grid for the big transformers? I assume back feeding can only happen if i can get the voltage on the primary side higher than what is currently supplied is that correct? and since we are exceeding the transformers ratings , we are destroying the transformer.

author: TimFox
author_rank: Super Contributor
timestamp: October 03, 2023, 05:31:44 pm
content: 1. You applyvoltageto a transformer: thecurrentdrawn by the other winding (at the voltage induced there) gives you power at the load and power drawn from the source.2. Not a good idea generally to apply voltage to both sides of the transformer.3. When you apply voltage to something, the power available from the source is not directly relevant unless you load the source with too much current.You need to re-phrase your question about "back-feeding" the transformer.[In some applications, especially in measurements, you can replace "voltage" with "current", and "current" with "voltage" in this response: you then apply a current to the transformer primary and the current induced in the secondary's load determines the voltage and power at the load.]

author: IanB
author_rank: Super Contributor
timestamp: October 03, 2023, 05:45:19 pm
content: There are special circumstances where voltage will be applied to both sides of a transformer. For example, if there is a power generating plant feeding the grid. In this case the generator will be supplying voltage and power to the low voltage side of the grid transformer, and the grid will simultaneously be applying voltage to the high voltage side. Which way the power flows in this scenario is complicated, but it is carefully designed by electrical engineers to work in the expected way. And of course, the transformer itself is designed for the expected service.

author: JohanH
author_rank: Frequent Contributor
timestamp: October 03, 2023, 05:49:42 pm
content: Quote from: coppice on October 03, 2023, 04:50:07 pmI don't think there are any such differences in smaller transformers. It all depends on if they fit your application.If we are talking about large three-phase power distribution transformers, there might be some practical differences. Reverse connecting them is still done in some cases, from what I have understood. They do differ between step-up and step-down, but this is mainly due to which side has the voltage taps for voltage regulation (default on primary). There might also be a much greater inrush current when reverse feeding. This is because often the output winding is wound closest to the core and input winding on the outer layers. When reverse fed, the inner winding will be energized first and cause a larger inrush current (due to the proximity to the core). This could cause issues with breakers and fuses. But I'm not sure if these things matter with smaller transformers.

author: JohanH
author_rank: Frequent Contributor
timestamp: October 03, 2023, 05:57:16 pm
content: Quote from: drakejest on October 03, 2023, 05:24:21 pmThis happens if you produce your own electricity (e.g. with solar panels) and use less electricity than what you produce. The nearest transformer to your house will work both ways. The important thing is that the electricity produced must have its frequency synchronized to the electrical network. If you just connect a random device you will cause disturbances to the network.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 03, 2023, 06:08:23 pm
content: Transfering power to the other direction already can happen with a reactive load. The reactive power power flows both direction, usually 2 x every cycle.This needs a slightly higher than normal voltage at the driving side, but this usually not a problem with the transformer specs. The difference is usually small except for very small transformers. For the very small ones it may not be possible to reach the nominal voltage when operating the other way around.

author: coppice
author_rank: Super Contributor
timestamp: October 03, 2023, 06:24:45 pm
content: Quote from: JohanH on October 03, 2023, 05:49:42 pmAny time I've seen a small transformer used backwards the results aren't great.

author: oz2cpu
author_rank: Frequent Contributor
timestamp: October 04, 2023, 08:46:06 pm
content: >Any time I've seen a small transformer used backwards the results aren't great.The explain : all transformators got LOSS , it is compensated by their winding number ratio to make up for it.so if you take a transformator that is designed for 230V input and 23V output at x load..it will deliver that voltage at that load.. the winding ratio is NOT exactly 10:1this is very easy to reveal, when you use it backward , feed it 23V INTO its 23V winding.one should expect to get 230V out at the watt/load it was designed for..but you dont, you get a lot less voltage out, simply the winding ratio compensation only work when used the right way :-)

author: TimFox
author_rank: Super Contributor
timestamp: October 04, 2023, 09:37:14 pm
content: See my quantitative example a few replies up.

author: coppice
author_rank: Super Contributor
timestamp: October 04, 2023, 10:51:19 pm
content: Quote from: oz2cpu on October 04, 2023, 08:46:06 pmThat just means you don't get quite the voltage you expect, but you get poorer voltage regulation as well with most transformer constructions. That's because you aren't using the optimal pairing of the windings. There are exceptions. The oval transformers with the primary on one arm and the secondary on the other are very symmetrical in their behaviour, because the primary and secondary are really no different in their construction.