Transformers in series for doubling the voltage

[neazoi]

Hello,
I have some identical transformers, intended for stepping down 220v to 6v.

In a reverse configuration, can I drive their 6V inputs in parallel and their 220V outputs in series so I can get 440v AC? (I am worrying about the HV insultion breakdown)

I have already read this article **broken link removed** about correct phasing, but I just need to make sure that the 440V will have no effect in the transformer windings, otherwise rated for 220v).
I also worry about the impedance mismatch from the first transformer secondary to the parrallel connected primaries of the other two transformers.


The attached schematic is what I am trying to do.
This might be usefull when a custom HV transformer is unavailable and a high voltage can be produced by more smaller widely available (and possibly cheaper) transformers.
Maybe more output transformers may be connected in series for higher voltages?

 

[FvM]

The transformers aren't designed to withstand 440V between winding and core or between input and output winding. In so far the circuit doesn't keep the insulation rating for all possible loads, e.g. grounding one output terminal.

On the other hand, mains transformers can handle some surge voltage without breakdown. So it won't fail immediately, but it's not what we consider safe operation.

 

[KlausST]

Hi,

It is possible, but expect less voltage. Especially with low power transformers.

Klaus

 

[Orson Cart]

If the transformers are identical, there will be no problems, mains transformers should withstand >1500Vac from pri to core, and >2500Vac from pri to sec. Just be careful what you apply the 440Vac to.
We use this approach to generate 2kV from isolating transformers with their pri's in // fed from a variac, and o/p's in series.

 

[neazoi]

If the transformers are identical, there will be no problems, mains transformers should withstand >1500Vac from pri to core, and >2500Vac from pri to sec. Just be careful what you apply the 440Vac to.
We use this approach to generate 2kV from isolating transformers with their pri's in // fed from a variac, and o/p's in series.

Yes, that is what I was thinking. Generating this HV required by tubes or small crts, using smaller readily available in local stores transformers.

 

[FvM]

If the transformers are identical, there will be no problems, mains transformers should withstand >1500Vac from pri to core, and >2500Vac from pri to sec.

Mmh. This are test voltages that the transformer has to withstand for 1 min. If you convert the test voltages for reinforced insulation, overvoltage category II to base insulation cat I, then you end up at 600 respectively 1000 VAC working voltage, which would be O.K. if the requirements for the category are met.

 

[Anna Conda]

FVM has a slight point, if you are going to operate this way for many hundreds of hours you need to know the long term volt withstand of the insulation, even opto-couplers has a time-voltage curve for max voltage stress that comes down with time a bit quicker than you might think...
Ideally one would test the transformers to see what the breakdown is (usually >3750Vac pri to sec and usually >2500Vac pri to core (earth) ) to gain an idea of the lifetime at elevated operating voltages.