Re: variacs

What you are thinking is true to a point, and that is a transformer rated for say 50-400Hz would operate within specifications over the whole frequency range because of the core material and winding design which minimize core and winding losses for the rated operating frequency range, However a transformer or variac designed for 400Hz would not work at 60Hz because its core and coil design result in an impedance too high for proper transformation to take place.Conversely, a transformer rated for 60Hz and operated at 400Hz would overheat and draw exessive current because its impedance is too low.