Transformer primary voltage exceed the rated voltage

[PramodMunaweera]

What happens if transformer primary voltage exceed the rated voltage? Does it burn the transformer even when no load(or small load) is connected? (or what happens when no load is connected).
Ex: Transformer 230V/12V rating, Input voltage exceed 230V

 

[dorke]

You should define "exceeds".
It is normal for the input to have a "swing" of say 5% .
In this case nothing happens,nothing is burned ,the secondary voltage will "swing" in the same %.

If however,in the case that the transformer is not primary fused and the voltage exceeds significantly, the primary can be damaged.

 

[duke37]

If the input voltage is high enough to saturate the core, then the input current is only limited by the wire resistance and the transformer will soon be damaged or your house will burn down.

Another case where core saturation could be a problem is using a 60Hz transformer on 50Hz. Normally there is enough latitude to use the same voltage but it would be better to use a lower voltage.

A resistor in the primary circuit can drop some voltage at the expense of efficiency. If there is a low voltage spare winding, this can be employed to reduce the voltage applied to the primary.

 

[Colin Mitchell]

When you supply say 230v to a transformer, the magnetic flux produced by the current cuts all the turns of the primary and produces a back voltage of about 225v.
In effect, the transformer is only seeing a voltage of 5v and when combined with the resistance of the primary (about 50 ohms) only a small current flows and the transformer remain cool.
But as you increase the primary voltage, a point comes when the back voltage does not rise past say 270v when you are supplying 280v and the difference becomes 10v. If you supply 300v, the difference becomes 30v and the current increases enormously.