Is it safe to use a 120V/60Hz rated espresso machine on a 100V/50Hz grid?
It will depend on the details of your coffee maker.
If all it does is boil the water into steam and let the steam pressure do the work, then it will not likely care about the frequency, or the voltage for that matter. if it has electronics in it, most electronics now use what are called &quot;Switch Mode Power Supplies&quot; that can accept any frequency at a wide range of voltages, because all it does is rectify it and drop it to low voltage DC (i.e. 24VDC). Giving the heater elements a higher than rated voltage will make them heat up higher and faster, resulting in their being on for less time. So the higher stress is generally offset by the lower duty cycle. But if you use it non-stop for hours on end, like in a shop, then it might die prematurely.
If on the other hand this is a &quot;pump&quot; type espresso machine, then it has a motorized or vibratory pump. If it is a small table top home-based machine, it will be a vibratory pump, basically a solenoid that extends and retracts a piston to pump the hot water. In either case,both will have solenoid valves at least. AC induction motors AND AC solenoids are both sensitive to the frequency. In the case of your situation, a solenoid designed for 60Hz but given 50Hz will over heat and fail prematurely. How long it will take is anyone's guess. If it is a commercial grade machine with an AC motor pump, the pump will turn 20% (5/6) slower, resulting in less pressure and poorer coffee quality, and it too will have solenoid valves that may fail prematurely. Most of the devices are going to accept +-10% voltage, so your transformer solution works in that case, but is cannot fix the frequency issue.
So again, it depends on the details of the coffee machine you are talking about.