How to identify an old transformer windings «Soloelectronicos

the operation of transformers is based on the phenomenon of electromagnetic induction, whose mathematical explanation is summarized in the famous Maxwell equations which claim that applying an electromotive force in the primary or inductor winding, is produced by the electric current flowing through it» occurs induction of a magnetic flux in the iron core. according to the Faraday law, if such magnetic flux is variable, appears in the winding secondary or induced electromotive force. in this way, the primary circuit and the secondary circuit are coupled through a magnetic field.

the relationship between the induction electromotive force (Ep ), applied to the primary winding and the induced electromotive force ( is ), obtained in the secondary, is directly proportional to the number of turns of the windings primary (Np ) and secondary ( Ns)


the voltage induced in the secondary winding is directly dependent on the relationship between the number of turns of the primary and secondary winding and tension of the primary winding. D icha ratio is called ratio (m) and depends on the numbers of turns that have each. if the number of turns in the secondary is the double of the primary, the secondary will be double voltage qeu primary enel (seria a lift as is the case with the sde AT transfromadore tranfromador).

If you are a transformers guard for new projects or any other use and unknown which is their primary or secondary is really not too complicated to find out without any prior measurement, safely and quickly. Also of course also using a polymer can be find out more things.

for the test will use a configuration with an incandescent or halogen bulb in series with each of windings

as we do not know which is the primary or the secondary is can prove both safely as feeding the transformer with this lamp because this will cause both windings current limiter without damaging these

as we know if we put the mains voltage directly at the secondary of a transformer destruiríamos it, but with this configuration serial no nothing would happen as simply winding will be inductive resistance lighting lamp practically with all its intensity which is an indication that that secondary winding (less returns and therefore less resistance and so there is more light in the lamp connected in series with the)

Place if mains voltage in series with the other (primary) winding not nothing would happen not as if mplemente the coil of inductive resistance very little lamp lighting due to the greater number of turns therefore greater resistance and so there is much less brightness in the lamp connected in series with the (even we can measure the output voltage at the secondary will be proportional to the retrieved

summing up when connecting) either reeling with a bulb, three things can happen:

  • if the transformer is shorted (or has a shorted loop): lamp lights with all of its brilliance.
  • if the transformer is functional : the lamp lights up very little in the case of plugging in the primary and if we connect it to the secondary, which in both cases showed us that the transformer is working. Also in if feeding the primary if we maintain the connection, we can measure voltages output in the other (secondary) winding which logically will be below the nominal voltage but anyway and it will give us a good approximation of the output voltage
  • If this open transformer: will not turn on absolutely nothing lamp


 mounting series
as we see, with a little bit of practice can be identified even if the windings are of primary or secondary according to the intensity of the lamp.

with this method using a lamp of some 25 W can try up to 30 W transformers, but with 100 W bulbs could be testing transformer of up to 1000 W

physically there are some clues that can help us to distinguish the windings as for example, l to location and clearance of the terminals  , which we will define the type of winding:

  • separated more correspond to the primary (network) and tend to go up
  • L you closer together tend to be (s) output (s)

also the thickness of the wire t ambien is a clear winding:

  • and finer l : to winding entry
  • the thickest: winding output

subject to the measures, as we have seen that the number of coils increases the value of resistance, if we measure with a multimeter the resistance of the windings can tell us clearly what each winding:

  • lower resistances are secondary
  • the resistance high are for the primary
  • except for the auto-transformer there should be no continuity between primary and secondary
  • you can have several interconnected side for various voltages or not between them

! Eye with tension as in Europe is 220V and America usually 110V, which means that will burn it a transformer designed to work with 110V if we connect it to the 220 V mains! (in terms of frequency despite being different as in Europe it is in Europe 50 Hz and 60 Hz for the transformer is indifferent that value)

how to determine if a transformer is 110 or 220 v without burning it on the test? Because certain once the first will use a second transformer whose output is 110V

  • in the output of this connect the dubious transformer primary. If the dubious transformer is 110vac happened nothing since we will be feeding a 110vac with 110vac transformer and output will have the correct voltages.
  • if the dubious transformer 220 VAC and connect it to 110vac nor pass anything, only that output will get half of the rated voltage.

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