Transformers & frequency....

Group,

What's the story on transformer hertz ratings? I have a couple of
transformers connected in an open delta configuration to boost my
220V three phase to 440V for my motors that are not dual voltage. If
I were to input these transformers from a VFD, how would the variable
frequency (up to at least 120hz) affect them?

I'm asking this question because I notice all transformers seem to
have a hertz rating.

thanks,

Doug