Transformers in series in the primary end and in parallel in the secondary?

Question

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.

Answer 1

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.

Answer 2

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...