transformers back to back - why won't work?

[hevans1944]

Interesting explanation as usual Hop, although I didn't understand that much of it! Can you address directly the question of what limits the output power in this step-down/step-up application?

The VA rating of the smallest transformer limits the maximum output power.

 

[KrisBlueNZ]

I spent several hours trying to find the answer to this question: what limits the power transfer capabilities of a real transformer? Bottom line appears to be losses creating unacceptable temperature rise.

So Adam was right in post #18 that losses and corresponding heat dissipation are the limiting factor?

I think I was thrown by his comment that excessive primary current would cause the core to saturate. That's not true, is it?

Pragmatically we know that more power transfer in a transformer requires more ferromagnetic material in the core, else electrical utilities would purchase much smaller transformers!

Yes, exactly. That's why I asked the question!

As for wiring transformers back-to-back: this does work. You should try to match the VA capabilities of the two transformers, and limit the power transfer to the transformer with the smallest VA specification.

Yes, two things are very clear.

1. Back-to-back transformers do work, although power will be lost due to inefficiencies, and
2. The output power is limited by the transformers, so even if the input supply (e.g. mains supply) can supply a lot of power, the power output of the back-to-back transformers is limited.

Sorry to be of so little help. Transformer design was not covered in my one power electrical engineering course, although I did find out how a three phase power source can create a constant amplitude rotating magnetic field. Fascinating, as Mr. Spock would say.

I liked how he would say "fascinating" without appearing to be very fascinated at all!

Thanks for looking it up. I was just curious really.

 

[Arouse1973]

I spent several hours trying to find the answer to this question: what limits the power transfer capabilities of a real transformer? Bottom line appears to be losses creating unacceptable temperature rise. Couldn't find anything, except a side-bar reference on Wikipedia, that relates winding e.m.f. to excitation frequency, core area, number of turns, and peak value of the magnetic field in the core, assuming a sinusoidal field. If you multiply both sides of this equation by the current in the winding... voila! you have a formula for power! Clearly power transfer depends on core area and length of the core (to obtain the induced magnetic field in the core), but where does magnetizing current play a role? All the stuff I found on magnetizing current says you want to keep it small, around one percent of the load current in the primary. Why? Why would you want or need any magnetizing current? More research is required.

Pragmatically we know that more power transfer in a transformer requires more ferromagnetic material in the core, else electrical utilities would purchase much smaller transformers!

As for wiring transformers back-to-back: this does work. You should try to match the VA capabilities of the two transformers, and limit the power transfer to the transformer with the smallest VA specification.

Sorry to be of so little help. Transformer design was not covered in my one power electrical engineering course, although I did find out how a three phase power source can create a constant amplitude rotating magnetic field. Fascinating, as Mr. Spock would say.

Hey Hop

Without any magnetizing current the transformer wouldn't work. This is the current in the primary with no load connected. This is what sets up the initial magnetic flux in the core and is related to the integral of the applied voltage. This is the one that will cause core saturation if set too high for the type of core.
Adam

 

[Arouse1973]

So Adam was right in post #18 that losses and corresponding heat dissipation are the limiting factor?

I think I was thrown by his comment that excessive primary current would cause the core to saturate. That's not true, is it?

Yes, exactly. That's why I asked the question!

Yes, two things are very clear.

1. Back-to-back transformers do work, although power will be lost due to inefficiencies, and
2. The output power is limited by the transformers, so even if the input supply (e.g. mains supply) can supply a lot of power, the power output of the back-to-back transformers is limited.

I liked how he would say "fascinating" without appearing to be very fascinated at all!

Thanks for looking it up. I was just curious really.

Sorry I should have been clearer. The core will saturate if there is too high a magnetizing current. This is the current with no load. The load you put on the output doesn't have much of an effect on this because of the cancelation of magnetic fluxes. The cancelation should just leave the mutual flux which is identical for both primary and secondary, hence mutual. But some of the flux reacts with just one of the winding and this is the leakage flux.
Adam

 

[hevans1944]

,,, This is the one that will cause core saturation if set too high for the type of core.
Adam

I am having a hard time understanding how the primary and secondary currents can produce canceling magnetic fields at arbitrary currents. Neglecting leakage flux, don't these currents require that the core be able to sustain them without saturation, even though the fields cancel? Else, why try to minimize the magnetizing current if not to leave some "headroom" for the real power transferring current? I hate non-mathematical hand-waving discussions, but must confess I don't really understand what is going on in the core of a transformer.

 

[Y2KEDDIE]

Possibly the transformers in the video are of the "impedance protected" design such as wall-warts. Either by design or poor construction the windings will heat up and increase in resistance, limiting the current, as the load increases. With limited current the output will decrease in voltage and current. In a designed "impedance protected" device the current limiting windings will reach a safe limited temperature rise. In a poor design the current will be limited as it heats up but it may also burn up the transformer.

 

[Arouse1973]

I am having a hard time understanding how the primary and secondary currents can produce canceling magnetic fields at arbitrary currents. Neglecting leakage flux, don't these currents require that the core be able to sustain them without saturation, even though the fields cancel? Else, why try to minimize the magnetizing current if not to leave some "headroom" for the real power transferring current? I hate non-mathematical hand-waving discussions, but must confess I don't really understand what is going on in the core of a transformer.

Hi Hop

That's an interesting question. As I understand it when a load is connected to the secondary of the transformer a current appears through the load. This creates it's own magnetic flux which circulates in the core but in opposition to the original flux from the primary. This tries to reduce the flux in the primary, but in doing so causes the current in the primary to rise which counteracts this rise from the secondary. So the system flux balances out. Without this effect the transformers output would soon take a dive because there would be no extra current from the primary to supply the load.

Adam

 

[BobK]

But if the transformer is not 100% efficient (which it is not) the magnetic flux is not perfectly balanced between the secondary and the primary. Could this not eventually lead to saturation?

Consider this scenario. If there is no load on the secondary, the idle current in the primary is creating a certain magnetic flux, which, hopefully is lower than the saturation point. If we start loading the secondary, the primary current has to increase. But due to losses, the current x turns in the primary has to increase more than the current x turns in the secondary. So, though the current x turns in the secondary will balance out the portion of the current x turns in the primary that are transferring power to the secondary, they will not balance out the excess current needed due to other losses, both due to resistance and to magnetic effects. So we still should see in increase in magnetic flux after subtracting the two. If this exceeds the saturation point, the inductance falls, and current increases.

I can't say with any certainty that this effect would be the limiting factor, but theoretically, it seems to me that it could happen.

Bob

 

[Arouse1973]

Yes I think your right Bob. This should be taken into account when the transformer is designed. Of course a real transformer falls from ideal which is total cancelation and it is said an Ideal transformer with an ideal core shouldn't draw any current in the primary off load. I think a real transformer does a pretty good job at cancelation but as with everything losses are the limiting factor in a transformers efficiency. Other issues will have an effect, the main one I think is temperature rise.
Adam

 

[NuLED]

So... is there any consensus to the reason why the transformer, in the step-up part of this scenario, might be damaged?

 

[Arouse1973]

Well it makes sense that if a certain magnetic flux is produced when 220 V a.c is applied to the primary of the transformer which produces 18 V a.c then if you reverse it then it should produce the same. Does it? Well not far off.



But one thing that will be different is the inrush current to the second transformer. You see the primary of the transformer is designed to be connected to mains potential and deliver energy to the secondary. The core of the transformer was not designed to have a voltage connected across the secondary.

So take a 220 to 12 Volt transformer that has a primary resistance of 200R and a secondary resistance of 1R. Which one do you think is going to draw more current when a voltage is connected across it? Now obviously the secondary of the first transformer is not going to be able to supply 18A of current.

But I guess it's going to be quite high and produce an initial flux higher than would be recommended for the core. Not having calculated or ever measured this I can't give any figures but it sound reasonable.
Adam

 

[Scotophor]

IMO there is no way, if the two transformers are identical, for the output transformer to be damaged before the input transformer. The currents in the 18 volt windings must be identical, because they are in series. Because of losses, the output power from the second transformer's 120 volt winding will always be less than the input power to the first transformer's primary winding. Therefore, the input transformer will always be handling more power than the output transformer.

 

[Calmore]

I was successful in doing something similar, in order to power a small radio while on holiday!

110vac into the 110vac tap on a transformer, the 18vac output into the "output" of an identical transformer then the 210vac "input" taken to a mains socket, if that makes any sense?

Transformers were a couple of those flat. PCB mounting jobbies, with a centre tapped primary.

It worked fine, but the radio was drawing a miniscule amount of power. Didn't try it with anything more substantial.