I ordered a second hand K30 from a small US Coffee Shop.It will come set at 120 V/60 Hz. Since Voltage in my country is 220 V/50 Hz, I would like to know if I could modify the K30 , after all it is manufactured in Germany , where the Voltage is 230 V/50 Hz. If not, I intend to use a dedicated 1,000 W Transformer.
I already know that the rotation speed will vary since it is frequency dependent. Just like a Turntable.

Do 10 hz make such a difference ?
I can't imagine that the grind speed of the MK will variate for more than +- 10% which doesn't really effects the result.
I only know transformers which change the voltage from 110v to 220v + the other way around and also 220v down to 12v dc.
So a normal transformer changing 110v to 220v will be fine.
I have many electrical gadgets here in Germany running with 220v and I just plug them in.

See it yourselves:

Type
Voltage/Frequency
K30 ES, K30 Vario
200-230V / 50/60Hz 240V / 50Hz 100-115V / 50/60Hz
55 dB
75 dB
16 A-slow
4A T (slow)
7A T (slow) S320%e.g.(4sec.on/16sec.off) (6 sec. on; 60 sec. off)*
W 240, D 320, H550 13 kg (28.7 lbs)
1,5 kg (3.3 lbs.)
Rotation
@ 50Hz 1420 rpm @ 60 Hz 1640 rpm
Sound Emission
Idle Operation Operation
Pre-Fuse
Overload Circuit Breaker 230-240V
110-115V
Max. Permissible Running Time
of the Grinder
Dimensions in mm
Weight
Hopper Capacity

I have the same setup in Australia (240V, 50Hz). My K30 also came from the US and thus contains a 110V motor. As these are analogue motors, they are made specifically for a certain voltage and unfortunately there is no simple switch. The grinder's manual on the Mahlkonig website even contains the different wiring diagrams for the two models.

The motor and burr assembly on the K30 is user-replaceable by undoing 4 screws and lifting the whole unit out of the case. You could buy a 220V replacement from Mahlkonig, but it might be expensive...

I also went with the option of using a transformer. The grinder is rated at 540W, so I ended up getting a transformer rated for 1000W continuous load (and 1450W peak load). Because motors can draw much higher currents on start-up than continuous operation, I didn't want to risk electrical issues with a transformer that was just powerful enough.

I've had this setup for about a year now (at home, ~2-3 double espressos per day) and it works very well. An added bonus of the transformer is that I can use many other kitchen appliances bought cheaply from the US

P.S. I forgot to mention that since the motor is built for a 60Hz electrical system, it will turn more slowly when used through a transformer in 50Hz countries. (Transformers cannot change frequency). No big deal though, and slower is better IMHO anyways. For comparison, it takes my grinder 4.2 - 4.6 seconds for 20g of coffee depending on the beans.