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I have a transformer that has two separate primaries allowing one to wire them in series or parallel. If I were to wire them in parallel requiring 230vac, and I supply it with 230vac with a step-up transformer, what will happen to the secondary? will the voltage and/or the current double too?

You haven't given us the voltage ratings of the windings so I can only make assumptions based on usual practice. Normally if two primaries are supplied and each is rated, for example, 120vac you connect them in parallel if your supply is 120v and series if your supply is 240v. In either case the secondary output voltage is the same. If you supply 240v to the parallel connected primaries you will burn out the transformer. ( Don't forget you must get the polarity right.)

yes I am also agreed with Foxx's comment. make sure to get the polarity first. also if you have two 110v primary windings making it parallel will not work, if you connect the windings in correct polarity in series it can withstand 240V in primary but the problem is then your secondary winding may give higher voltage but your core will saturate due to high flux and you will experience a heavy heating some times. this is depending on your core size.

Here are the 4 ways to connect the transformers

I tried to upload the transformer with its windings but it would not accept it. Regardless, I have already tied the primaries correctly in polarity and found that I have doubled the output voltage on the secondaries (and probably halving the current). This is what I want. What does it mean to saturate the core? Saturate with what?

Note #1 the secondary windings can also be connected in series or parallel (again get the polarity right). In series, the two secondary voltages add and the current rating stays the same as for one winding. In parallel the voltage stays the same as for one winding but the current rating is doubled. Note #2 When you make an electromagnet with an iron core the magnetic field in the core is roughly proportional to winding current up to a limit called "saturation". Beyond this any energy you put into the magnet by raising the voltage, hence current is not stored in the magnetic field but is simply turned into heat and will burn up the winding.

Thanks. This has been helpful. So according to diagram #3 (which is how I want to use it), 120v in parallel becomes 240v out. What will happen to the current on the secondary? Is it under this condition that the windings become saturated and/or burn out?

Secondary current rating on #3 is simply the current rating of one transformer. As long as the applied voltage on any winding is at or below the rated voltage for that winding there is no worry about saturation. Circuits #3 and #4 are connected for 120v input and if you apply 240v the transformers will burn out. Circuits #1 and #2 are connected for 240v in and if you apply 120v at the input you will simply get half voltage at the output and no harm done.

So if I understand you, it is in the the application of the voltage on the primary that there is a potential of burn out. Specifically, if I want double the voltage on the secondary (and I do) while keeping the current as rated, I can wire the primaries in parallel and apply 120v to it to get 240v on the secondary without saturation or burnout. Correct?

And according to you diagram, I would get 240v by wiring the secondary in series. I could also get 120v if I wired the secondary in parallel.

Sounds like you've got it. Some of the nomenclature on this stuff may be a bit confusing but just remember the winding connected to the voltage source is called the primary and the winding connected to the load is called the secondary. I hope this helps.

Thanks for your help. I liked the clarity of your answers. Now I have another question related to this and I hope you can help. I would like to use this transformer with the 120v primary and produce 120v and 240v on the secondary with (I hope something simple) like the flip of a switch. Any suggestions?

You can get 120 and 240 output at the same time with no switching. See https://www.circuitlab.com/circuit/vt97q6tj27bn/120_240-reply/

Yes, I know about that. I 'll get back to that later. Here is where the plot thickens. I tested the output of the transformer using the various primary and secondary configurations from your diagrams. For the 240v configurations, I used a step up transformer (and a variac). Not half way up on the variac, the thermistor on the variac blew. Is it possible that the VA of the step up transformer was inadequate to the task? The VA for the power transformer is 740VA. Does that mean I need a setup up transformer that will handle 1000 watts?

You have to do some real detective work here. Are you using two separate 1:1 120v transformers or one transformer with four separate 120v windings? How are the polarities marked? Some manufacturers get creative in simplifying terminal arrangements and end up with something a bit arcane. You are using CircuitLab so why not model your setup, do some experimenting and make the model public so all can see it. In particular show where the variac is connected.

The difficulty with this site is that I am unable to upload the schematic of the windings which will answer your question perfectly. Now that I recall, you can find the schematic of the transformer here: https://www.surplussales.com/item/_TP/tn741-m390-cpe.html. The diagram is at the bottom left of the page after the description.

That reference gives a server error. Can't you look at the transformer information you have and manually reproduce it in CircuitLab like I did above?

Because of my unfamiliarity with using Circuitlab, as simple as it may seem to you, it is easier for me to do it this way. Please try again with this: https://www.surplussales.com/item/_TP/tn741-m390-cpe.html This like has worked successfully for me 2x. Just in case: It is a Tortran toroid transformer on Surplus Sales of Nebraska. It has multiple secondaries but I am mostly interested in the 390v/1.5A winding. Thanks.

I tried the new URL you gave and it works even though I can see no difference from the original you gave.---- In any case , the diagram for the transformer sheds new light on the original question It shows a transformer that has two separate 115v primaries allowing one to wire them in series or parallel. Original question.--- ("I have a transformer that has two separate primaries allowing one to wire them in series or parallel. If I were to wire them in parallel requiring 230vac, and I supply it with 230vac with a step-up transformer, what will happen to the secondary? will the voltage and/or the current double too?").--- So to give an answer for the original question: ----Don't do that You will burn out the transformer. ---There are no polarity marks as such but the way the diagram is drawn, to connect the two 115v windings in series connect RED to BRN and you can then feed 230v to the WH and ORG leads. and the output voltages and currents will then be as the diagram shows.---

If you want to use 115v feed connect WH to BRN and also connect RED to ORG. The 115v feed would then go to the WH/BRN connection and to the RED/ORG connection. And again, the output voltages and currents will be as the diagram shows.

Well, thank you again for your diligence, patience and clarity with me and this transformer. No matter what I tried, it did not seem to work for me. Now with your last suggestion, (WH to BRN and RED to ORG) with 115v feed gets 420v (as expected), but with 240v feed, I get double the voltage -- which is what I really want!! Yay. I also get a bit more hum from the step up transformer which means to me that I have reached its limits and that I have to get a larger step up transformer. Would you agree?

Given the parameters of this new setup and configuration, I would like to test it without building the whole circuit. Is there some way to determine the viability of the transformer with its two possible configurations under load without building the whole circuit?

The WH-BRN//RED-ORG connection is for 115v input only and must not be used with 230v..or you risk starting a fire. If you want to use 230v input use the RED-BRN connection. About the only option you have outside the transformer published rating is to feed it with 115v when connected for 230v and if you do this the outputs will all be half the published rating.

So are you saying that there is no way of doubling the voltage of the secondary to 800v regardless of the wiring of the primaries and the input voltage?

That's about it. I suppose you could string all the secondaries in series and get 670v but that's far outside the design of the transformer and the insulation might or might not stand up. And don't forget that you are working with potentially lethal voltages.

Given that the total VA rating is 670, what does it matter where or how that rating is achieved? Would doubling the voltage coming into the primaries not reduce by half the VA of the secondaries thus keeping the total VA the same?

One primary winding is rated 115v and you cannot go over that by more than a few percent no matter what the VA rating. If you attempt to do so the transformer primary winding simply draws more current without generating more magnetic flux in the iron so no coupling to the secondary and more heating. Of course you can connect two primaries in series and apply 230v but that is still 115v per winding so ok.