transformer

[chris54]

Hi and thankd for taking the time to reply. I have a question regarding using to idential trasformers either in series or parelle. Can this be done in either configuration. thanks

 

[crutschow]

You can connect the output is series to get a higher voltage, but not in parallel to get more current. If you connect them in parallel, the one with a slightly higher voltage (due to normal tolerances) will cause a circulating current to flow between the two windings. The current will be equal to the voltage difference divided by the sum of the two output winding resistances.

 

[chris54]

Thank you for your reply

 

[Diver300]

I would have said that if you have two identical transformers in parallel, with the primaries in parallel as well, that would usually be OK. There might be a problem with large transformers as they have better regulation, which means that their output drops less at full load.

If you are rectifying the output of the transformers, you can always put a rectifier on each transformer and parallel the outputs of the rectifiers. There will be no problem doing that.

It is certainly common practice to parallel two secondary windings on one transformer.

 

[Willbe]

I would have said that if you have two identical transformers in parallel, with the primaries in parallel as well, that would usually be OK. There might be a problem with large transformers as they have better regulation, which means that their output drops less at full load.

If you are rectifying the output of the transformers, you can always put a rectifier on each transformer and parallel the outputs of the rectifiers. There will be no problem doing that.

It is certainly common practice to parallel two secondary windings on one transformer.

And with a heavy load the windings will only source current into the load, not into each other.

If a secondary can source 1A without overheating, I guess it could also sink 1A without overheating.

 

[crutschow]

And with a heavy load the windings will only source current into the load, not into each other.

If a secondary can source 1A without overheating, I guess it could also sink 1A without overheating.

Any unbalance in voltage will cause an unbalance in the load distribution between the two transformers, potentially overloading one transformer.

It's ok to parallel two identical windings on a transformer. Although the transformer has some overall voltage tolerance between different transformers, the match between two identical windings on one transformer is much tighter, since they have the identical number of turns on a common core.

Don't quite understand your comment about sinking and source 1A. If 1A is the transformer rating and it's driving that current into the other transformer, then it wouldn't have any current left for a load.

That's not to say you couldn't get by by with paralleling two identical transformers, but it's not good engineering practice. As a test you could connect them together through an ammeter with no load and see what the circulating current is. If it's less than 5% or so of the transformer current rating then you're probably ok.

 

[Willbe]

That's not to say you couldn't get by by with paralleling two identical transformers, but it's not good engineering practice.

I finally bit the bullet and solved the two loop equations for this setup, for I1 and I2, and checked it with a spreadsheet.

For I1 loop consisting of V1, R1 and R3 and the I2 loop consisting of V2, R2 and R3, and defining A = R1+R3 and B=R2+R3, I1=(AV2-R3V1)/(BA-R3^2) and I1=(V1-I2R3)/A.

With these formulas you can check the currents for unequal V1 and V2 and add series current sharing resistors if necessary.