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I know the basics of transformers and one thing I have never understood is the specification of primary and secondary voltages. By theory, the 2 parameters of importance is the transformation ratio and the VA rating right?. But then why do transformers have specified primary and secondary voltages?

I have a 120 VAC primary voltage (to 24VAC in sec.) transformer but my supply is 230VAC. So, Can I connect two such transformers in series (in the primary section) to make sure the specification voltage is not exceeded?

But what is the significance of these voltage specifications? Can I connect 230VAC directly and just make sure the current in the secondary output does not exceed the VA rating? Would that work?

The problem is I have two 120VAC to 24VAC transformers and my supply is 230VAC. So can I connect the primary in series and the secondary in parallel to achieve 230VAC to 24VAC transformation.

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The cores in the transformers do not behave linearly. In general you can use less than the specified voltage, but not (much) more. If you connected a 120VAC transformer to 230VAC you would get smoke because the transformer core would saturate, the effective inductance would be less, and the current would be limited only by the copper resistance in the winding. This is actually a useful 'feature' since the flux is limited by saturation the output voltage is also limited and the transformer will greatly attenuate large spikes on the primary.

If you use a 230VAC transformer on 120VAC and stick to the rated output current you'll have no problems with smoke, however the transformer will be heavier and more costly than the cost-optimized design (it will likely also be a bit more efficient than the cost-optimized design).

Putting transformer primaries in series (unlike secondaries) is not a great idea. It may work in some circumstances, though the tendency will be for one transformer to run warmer than normal, so I would not recommend it in general.

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  • \$\begingroup\$ Thats the part that I don't understand - effective inductance would be less and current limiting. Is it because of the non-linearity of the B-H curve? Since it is alternating current, how would connecting two transformers in series make one run warmer? (they are like two resistance in series and the voltage supplied should be divided right?) \$\endgroup\$ – Harsha Dec 29 '14 at 14:49
  • \$\begingroup\$ The resistance at the primary will depend on what is connected to the secondary. You have to think about the inductance and resistance of the coil but also look through the transformer to the secondary side to work out what is going on with the load. If one transformer has no load and another has a lot of load it will get pretty much the whole supply voltage on its primary. \$\endgroup\$ – Will Dec 29 '14 at 14:57
  • \$\begingroup\$ @Will In that case, if I connect two transformers in series in both the primary and secondary will it work? The Load gets evenly reflected onto both transformers since the transformers are identical? And how does effective inductance change? Isnt it an intrinsic quantity? \$\endgroup\$ – Harsha Dec 29 '14 at 15:03
  • \$\begingroup\$ The inductance is covered here: electronics.stackexchange.com/questions/79444/… - the inductance of the transformer coil is intrinsic but if you want to know what's going on with voltage/current you have to think about the current being coupled into the secondary too. I think you are right in theory but in practice transformers are not ideal so they won't be very well matched and you could have some problems. If possible it probably makes sense to use one big transformer instead of 2 small ones. \$\endgroup\$ – Will Dec 29 '14 at 15:16
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Don't forget also that the secondaries of the transformers have a 50/50 chance of being in or out of phase, depending on the direction both primary and secondary windings are connected...if series-connected in phase-opposition the result may be zero...

If you connect a 120v transformer to 240v, the output voltage will double to 48v, all other things being equal; the lower primary impedance will also double the input current of course. More than likely the core will saturate as mentioned previously, leading to a lower secondary voltage - a transformer only transforms when the flux in the core is changing - and some very nasty waveforms will result...

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