What happens when input voltage is fed to the secondary winding of a transformer?

The transformer operates in reverse. In other Words, a step-down transformer becomes step-up, and vice-versa. Example: a transformer with a 480 volt primary and 120 volt secondary: Ratio = 4:1 (480/120=4) Fed in reverse - Ratio = 1:4 Apply 120 volts, output is 480. Note that if you accidentally reverse the transformer, still feeding it with 480 volts, the output would be 1920 volts (480 X 4 = 1920) - probably not a good thing!