How to use back to back transformers to get 250V AC

[thebadtall]

hello, i want to use back to back Transformers to get an ac 250v

grid voltage is 230. i will use a 220/12v transformer. the other transformer what secondary voltage should have in order to give 250 ?

10v will do the job ?
thanks

 

[jeffrey samuel]

Re: back to back transformers

that is based on the number of turns you take in the primary and the size of the core taken and not merely voltage source applied do specify the number of turns and then by transformer ratio we can predict the op voltage

 

[betwixt]

Re: back to back transformers

In theory, the voltage will be increased by 12/10 times so you would get 276V out. In reality, it will be somewhat less, especially uder load becasue you have the losses of two transformers instead of one in the path.

You might consider using an auto-transformer if it's safe to do so in your application or, accepting some losses, you can use a 230V to 20V transformr with it's primary across the 230V and secondary in series with it.

Brian.

 

[FvM]

Re: back to back transformers

Instead of two back-to-back tranformers, you can use a single step up auto transformer. A regular 230/20 V transformers can be used for the same purpose. Total transformer weight will about 1/20 for the same output power.

 

[thebadtall]

Re: back to back transformers

thank you about suggestin auto transformer , but i have already a 150va 12v transformer, and it would cost less to buy a 220/20v 150va transformer. plus, in future i will have plenty of transformers!

ok so i am using a 220/20v transformer right?

thank you

 

[betwixt]

Re: back to back transformers

Funny - most of us are trying to get rid of big bulky transformers but you are collecting them. :grin:

Yes, give it a try. Connect the 20V transformer across the 230V as you normally would so you get 20V out. Then connect the 20V in series with the 230V to give 250V. If you connect the secondary the wrong way around it will subtract 20 from 230 so if you see 210V instead, your wires are crossed.

Note that there may be slight phase differences between the incoming 230V and the 20V from the transformer so the voltages may not arithmetically add exactly, you will have to build it to see if it is adequate.

Brian.

 

[jeffrey samuel]

Re: back to back transformers

Note that there may be slight phase differences between the incoming 230V and the 20V from the transformer so the voltages may not arithmetically add exactly, you will have to build it to see if it is adequate.

Brian.

is this phase shift really significant

 

[betwixt]

Re: back to back transformers

Probably not and it would be difficult to quantify without a detailed transformer specification. If the transformer iis being followed by rectifiers it shouldn't make any difference, if you are relying on a clean wave shape there is a very small risk that the distortion might cause problems but you would have to experiment to find out.

What you are essentially building is an auto-transformer, if you consider the circuit topology, the primary and secondary windings are in series. One side is common to the input and output, the incoming AC is applied to the junction of the windings and the output is taken from the end of the two in series.

Brian.

 

[FvM]

Re: back to back transformers

is this phase shift really significant

No it isn't. The dominant phase shift will be caused by the leakage inductance related voltage drop. It's usually specified as percentage reactance (% X) number. 10 % percentage reactance means, that the voltage drop at the leakage inductance with rated current is 10 % of rated voltage. The effective output voltage drop calculation must however consider the 90° phase shift of reactant voltage drop for real load current.

 

[thebadtall]

ive made a lot projects since i posted this topic , using b2b transformers.
Yet I cannot realise how fast the time goes by!

the way i calculate is by multiplying the ratio of transformation of the second transformer , with the output voltage (of the secondary winding ) of the first transformer.

for instance we have a 220/24 back to back with a 220/18 . 220/18 = 12.2 . 24 x 12.2 = 292.8 V ac output

 

[FvM]

for instance we have a 220/24 back to back with a 220/18 . 220/18 = 12.2 . 24 x 12.2 = 292.8 V ac output

An example that shouldn't be implemented. Operating a transformer at 133% of it's rated voltage will bring it near to saturation, involving high magnetization currents and possibly overheat it.

 

[jeffrey samuel]

Loading a transformer at 100% of its load it self can cause very high level of damage if heat dissipated is not dealt with properly. Wonder what you did to tackle that issue here

 

[thebadtall]

the voltages mentioned were for reference.
in my projects i used either identical transformers for isolation or 220/7 with 220/9 1A, to get around 180v, and used 8 to 10ma out of them.
never had problem with that

once i wanted around 300v ac the back to back transformers were really hot and i bought a stepup transformer to complete the project