# multiple primary transformer

#### wes

Joined Aug 24, 2007
242
Hi I was wondering if it is possible to have a transformer with two primary windings and one secondary? It's just a thought and I am not sure. I tried it in circuit program but it reads the voltage on the secondary as the voltage on just one primary winding which I set to 120V

I figured since there were two primarys with 120V applied to each that the secondary would read 240V as long as the primarys were in phase and not canceling out each others flux. The reason is because I believe that the flux created would be double that of just one primary winding and so the induced voltage on the secondary would be double that of one primary but equal to the total voltage of both primarys. It makes sense since the flux in the core is now double what would be created by just the single primary.

So yea, is it possible and is the program just doing it correctly and if it is not then why not? I am not sure what use this would have i just want to know if it is possible.

#### SgtWookie

Joined Jul 17, 2007
22,230
Transformers with dual primaries are more common than you would think. A single switch can convert the transformer from 115VAC input to 230VAC input; which makes it "close enough" to work here in the USA or in Europe where 220VAC mains is used.

The switch connects the primary windings in series in one position, and in parallel the other position.

#### wes

Joined Aug 24, 2007
242
Yes but I was wondering if you can have two unconnected primary's with 120V input and have a single secondary winding with 240V output, I guess sort of a step up transformer without increasing the secondary winding ratio to any one of the two primary's.

I would assume it should work given the primary's are exactly the same in every way including the phase so that the magnetic flux is doubled. Where one primary would induce 120 v onto the secondary, two primary's of with double the flux should induce double the voltage.

is it possible? if not why?

Joined Dec 26, 2010
2,148
Two primary sections in series and correctly phased* can be used to allow for an input voltage equal to that of the sum of the section voltages. *The "dotted" end of one winding section is connected to the "non-dotted" end of the other.

Two primary sections can only be used in parallel if they are of exactly the same voltage, and in this case they are connected dotted ends together. Here the effective input voltage is the same as if only one winding was used, but the safe current capacity may be improved. Connecting two windings in this way makes little difference to the primary inductance, nor to the secondary voltages except on load where the primary resistance is significant. This is a similar situation to having a single primary, but made with thicker wire.

Some commercial transformers are made with two primary sections each rated at about 115V. The two sections can be used in parallel for operation from domestic mains supplies in USA etc, or they can be connected in series if used on 230V mains, e.g. in Europe.

#### bertus

Joined Apr 5, 2008
22,231
Hello,

The following PDF might make things more clear:
transformers_mains.pdf

As adjuster stated, the connections with the DOT are the start of the windings.

Bertus

Joined Dec 26, 2010
2,148
No. Once you have one primary fed, connecting another equal primary to the same supply voltage does not do much except possibly help the safe VA rating. The point is that the flux does not get doubled, precisely because the windings are on the same core. The whole thing behaves rather like a primary with thicker wire, but the same number of turns.

#### wes

Joined Aug 24, 2007
242
So if you had two 120v primary windings Completely unconnected to each-other, That means no series or parallel connections, the magnetic field from each is the only connection, the voltage induced on the secondary winding would still be 120v but the current pull through each primary would be halved? is this correct.

this doesn't sound correct because the primary's that are UNCONNECTED are only connected through the magnetic field. It would seem that since there are two and the total flux is doubled the induced voltage on the secondary would be doubled to 240V because each primary was 120V

I have attached a picture to explain

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#### #12

Joined Nov 30, 2010
18,224
It's about turns ratio. Think this way for a moment: If you put the 2 primaries in series and put 120 volts across them, the output voltage would drop in half.

Putting 2 primaries in parallel does not increase the magnetic flux because it is limited by the core magnetizing properties, voltage, and turns. Two primaries in parallel just looks like fatter wire to a magnetic circuit.

#### wes

Joined Aug 24, 2007
242
okay I think I know where I was misunderstanding and wrong in describing it

You see I was saying a transformer with two primarys and and that secondary should have twice the voltage as the primary but that is obviously not the case with a normal transformer. like you said "(Putting 2 primaries in parallel does not increase the magnetic flux because it is limited by the core magnetizing properties, voltage, and turns.)"

Well I guess I was trying to say or describe was more along the lines of a pulse transformer. For example, 2 primary coils are pulsed at say 100v and they rise to a predetermined current. the magnetic flux generated is double that of a single primary is capable of because there are two coils and there fluxes are in phase, so they add. In this case I would assume that the secondary should have double the voltage of a single primary, so 200v. Again my reasoning is because the flux has been doubled. the turns are equal among all the coils.

So does this make any more sense or is it still wrong?

#### wes

Joined Aug 24, 2007
242
I guess I just want to know that if a magnetic flux of say a primary on a transfomer is doubled then wouldn't that induce double the voltage on the secondary. given all other conditions stay the same like turns, coil size, core properties, rate of change etc

#### #12

Joined Nov 30, 2010
18,224
The problem is, the only way to double the flux is to double the voltage, and you're back to turns ratio being dominant.

Joined Dec 26, 2010
2,148
Hi I was wondering if it is possible to have a transformer with two primary windings and one secondary? It's just a thought and I am not sure. I tried it in circuit program but it reads the voltage on the secondary as the voltage on just one primary winding which I set to 120V

I figured since there were two primarys with 120V applied to each that the secondary would read 240V as long as the primarys were in phase and not canceling out each others flux. The reason is because I believe that the flux created would be double that of just one primary winding and so the induced voltage on the secondary would be double that of one primary but equal to the total voltage of both primarys. It makes sense since the flux in the core is now double what would be created by just the single primary.

So yea, is it possible and is the program just doing it correctly and if it is not then why not? I am not sure what use this would have i just want to know if it is possible.
To return to your original question, yes, it is possible that the program is "just doing it correctly". The results with one or two primary sections each on the same voltage won't differ much. At least, that is what we expect for tightly-coupled transformers at moderate loadings, the sort of thing we associate with mains supplies.

Loosely-coupled devices like radio coils might seem to do something different if leakage inductance was significantly reduced by the extra section. Heavily (over)loaded transformers could also have drooping output voltages which should improve when the extra section was connected.

Perhaps the penny may drop if you imagine your double-primary transformer with only one primary section connected to the supply.
The other primary section will then behave as a secondary: assuming tight magnetic coupling, it with almost as much voltage induced in it as was applied to the other primary. Connecting up the second section to work as a true part of the primary therefore does not change much apart from ratings.