How do I  maintain a portable induction cooker?
The upper surface is glass, so I clean it like a normal Ceran stove: A special ceran cleaner when a cloth isn't enough, a sharp blade for specks, a gentle abrasive (like Cif) for stubborn spots which won't get away. The sides of mine are alu, but I don't bother getting out a different cleaning agent for that, just continue wiping with whatever I used on the glass. 

I have never opened mine to clean the fan, and suspect that it will void the warranty. But if needed, I would handle it like a PC fan: open the case, use compressed air to blow it clean. Trying to wipe the fan could decenter it. It won't make the blades perfectly smooth, as the humid kitchen air tends to turn dust to clingy grime, but it should remove the bulk if the fan is too full to work efficiently.