Transformer voltage question

[Biff383]

  What are the pitfalls of running a transformer under its rated voltage?  I have noticed that if you put 120V thru a 480V to 120V  transformer you get an output around 26 or so volts. What parameters do we have to adjust when we do this?

[IanB]

The input/output voltage ratio is constant, it depends on the turns ratio. The transformer will have a specified primary voltage, operating frequency and power (or VA) rating.

If you deviate from the design spec (in permitted ways) you will have to re-rate the transformer. A search will provide rules of thumb for this.

Decreasing the input voltage is harmless. I think the only consequence is that you may no longer achieve the specified power handling.

[Biff383]

  That was what I thought. I didn't know anywhere to find how much I should derate . I'll look around a bit.

  Thanks.

[Zero999]

The secondary current rating of the secondary winding will be the same.