I found this on the net
http://ed-thelen.org/1401Project/TricksFor50-60cycles.html Common "Iron" Core Power Transformers the kind we have.
Executive Summary: Raising the applied frequency from 50 Hertz to 60 Hertz is just fine.
I am placing quotes around "Iron" because actually the metal is sheet steel with more than the usual amount of silicon to increase its resistivity. The sheets are insulated from each other with oxide to reduce losses and heating due to "circulating currents". For transformers designed for 50 Hertz, raising the applied frequency 20 percent to 60 Hertz is no big deal. :-)) Actually the "iron" losses will decrease a little due to reduced "iron" saturation and hysteresis loss. :-)) Losses due to "circulating currents" will increase a little.
Going the other way, from 60 Hertz design to 50 Hertz usage, is a *BIG DEAL*. The applied input voltage should be reduced to 5/6 name plate rating or bad things can/will happen. They need about 20 % more "iron" to work at 50 Hertz.
Reference - most any transformer book and also
http://www.sola-hevi-duty.com/products/powerconditioning/pdfs/opchars.pdf 
