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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

50Hz equipment on 60Hz supply will usually operate with very few problems.

It is 60Hz equipment on 50Hz supplies that has problems of overheating primarily due to issues with the magnetic circuit.

When you will get in trouble is with synchronous motors.

Operating 60Hz transformers at 50Hz will cause the core to saturate, at which point the impedance drops to nil and the coil is essentially a short circuit. The transformer must thus be voltage derated by 20%. I had the misfortune of witnessing this first hand when some 60Hz transformers were mistakenly installed at a 50Hz site- the engineer that designed it swore up and down it would instantly explode into flames, but thankfully his calculations were just conservative enough to run. We recorded twice as much kVA going in as coming out, and even at light load with covers off and big fans blowing, the transformers were close to the max rated temperature.

Operating a 50Hz transformer at 60Hz will often cause excessive core hystersis and core heating due to the larger core and especially due to the wider core laminations, which provide much lower impedance to eddy currrents. I have not witnessed it, but another engineer I work with witnessed an oil transformer explode due to heat generated form this.

Dual-rated 50/60Hz transformers use thin 60Hz core laminations built up to a core size thick enough to support 50Hz at the rated voltage without saturation. They're generally less efficient than dedicated 50Hz or 60Hz, but are often cheaper as a single design and assembly line can provide equipmenet that works anywhere in the world.

This is the kind of equipment found in the Pacific theatre, especially Japan -- there's lots of 50 Hz there right outside the base.

Back in the day, rotary phase-converters for 400 Hz aircraft power were common as dust. They used additional poles within a motor-generator scheme to bump the frequency. It'd take the whole output of this XFMR to drive the load of a big aircraft while it's on the tarmac.

Now VFDs have made them out-dated.

Interesting to note the various tap options on the primary.