OP
MarkieSparkie
This thread is over 3 years old, but just in case students or apprentice are reading this...Dont quite agree with the resistances side of things. Obviously we are talking about impedence rather than resistance
And Xl = 2 Pi FL
the L is fixed for the coil, and the frequency hasnt changed. Therefore the Xl of the coil is the same, regardless of which V we apply to it. Obviously the I drawn will relate to the V applied, and the Xl of the coil will mean it will draw whatever I it needs. The coil MAY be able to handle this, it may not, depending on the VA rating of the tranny
It isnt 'automatically' going to blow!!!
This is utter tosh, most transformers have two or more windings (unless it's an autotransformer), lets consider the case in question. One primary winding and one secondary winding, the impedance of the windings in this case are likely to be different, because the inductive reactance and the coil resistance are different for each winding as they are wound from wire of differing CSA and number of turns wound.
The insulation of both windings will be rated and flash tested appropriately for 230Vac namely at 2300Vdc. If the primary and secondary are swopped so the former 12Vac secondary is supplied as a 230Vac primary, the former 230Vac primary now tries to become a 4408Vac secondary but the winding insulation quickly fails destroying the winding and possibly the other winding and the former their both wound on, until the point where the circuit protection breaks the primary supply.
If the "transformer" (really a switched mode voltage converter) was of the type used often for ELV lighting and fans. With 230Vac applied to the former 12Vac output of the converter, the output circuit protection devices would fail instantaneously due to over-voltage, rendering the "transformer" dead scrap.
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