can we use step up transformer as step down without any change?????????
Lol...well if you reverse the definitions of the input and output sides, then I don't see why not
den y don we use it practically?////
Probably because it would be confusing to see one connected backwards.
They aren't used practically because they weren't designed for it.
With using a step up as a step down: You'll probably have a lot of resistance losses in the "primary." You might burn up the windings one the primary if you try to draw a lot of current.
Example 2: Using a wall-wart transformer backwards might cause the insulation to fail due to high voltages. Also, if it's unballasted (something to prevent it from drawing too much current), it might burn.
It's a yes and no type of answer.
You can certainly have power flow in either direction in a transformer, in which case the distinction between what is "primary" and what is "secondary" is quite arbitrary. However you must respect the voltage ratings of the respective windings so generally the answer is no, you cant just interchange primary and secondary while keeping the driving voltage unchanged.
As long as you respect the voltage and power ratings however it's no problem. For example if I had a 240V to 110V step down transformer (that say that I used to power 110V equipment in a country where the mains voltage was 240V) then I could certainly use that same transformer unmodified as a 110V to 240V step up transformer if I travelled to a location where the mains voltage was 110V.
BTW. It's not just winding resistance and insulation ratings that will cause you grief if you dont respect the voltage ratings. Saturation of the iron core and associated over current heating will become a problem even at fairly modest over voltages.
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