Electronic – Why does a transformer’s ratio change

theorytransformer

I've connected various transformers to a circuit (one at a time) in reverse (power supplied to secondary) and I measured the voltages at both windings and found ratios very different from the ones announced by the manufacturers.

Isn't the ratio supposed to be maintained no matter the input voltage and/or the input side ?

Added:
I've also tried supplying power directly to the primary and measuring voltage at the secondary but I NEVER got the manufacturer's voltage and thus ratio:
Block VB 2.0/1/24 : 229>39,8 VAC
Block VB 2.0/1/18 : 229>30 (& 310mA = 9,3W !!)
Myrra 44127 2×6=12 : 229>10,6
Myrra 44131 2×18=36 : 229>45,4

How is this possible ?

Best Answer

The winding ratio of a transformer does not translate to a simple voltage step-up / step-down ratio.

The actual transfer function is affected by the transformer core material, the frequency of the signal used, winding quality, core design, environmental factors, and possible several other parameters as well.

In your specific experiment, here are the likely sources of significant variation from expectation:

  • Frequency of signal applied to the primary (which in this context is whichever side you applied a signal to). Soft ferrous core transformers transfer well at line frequencies (e.g. 50Hz), ferrite cores are good for intermediate frequencies, air core is good for radio frequency but pretty much useless at line frequency.
  • Waveform of signal applied: For anything but a pure sine wave, the transfer function would be different for the different harmonics making up the waveform, so output signal would have a complex relationship to the input, certainly not linear as you expect.
  • Method of measurement of signal: A multimeter or AC voltmeter is not ideal, an oscilloscope would be recommended.

If you could amend your question with more detail on how you conducted your experiment, a better response could be formed.